Graduate Theses and Dissertations Iowa State University Capstones, Theses andDissertations

2018

Transient electronics: Materials, mechanics, andapplicationsYuanfen ChenIowa State University

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Transient electronics: Materials, mechanics, and applications

by

Yuanfen Chen

A dissertation submitted to the graduate faculty

in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

Major: Mechanical Engineering

Program of Study Committee:

Reza Montazami, Major Professor

Nicole Nastaran Hashemi

Xinwei Wang

Liang Dong

Hantang Qin

The student author, whose presentation of the scholarship herein was approved by the

program of study committee, is solely responsible for the content of this dissertation. The

Graduate College will ensure this dissertation is globally accessible and will not permit

alterations after a degree is conferred.

Iowa State University

Ames, Iowa

2018

Copyright © Yuanfen Chen, 2018. All rights reserved.

ii

TABLE OF CONTENTS

Page

LIST OF FIGURES ............................................................................................................ v

LIST OF TABLES ............................................................................................................ vii

ACKNOWLEDGMENTS ............................................................................................... viii

ABSTRACT ....................................................................................................................... ix

CHAPTER 1. GENERAL INTRODUCTION ................................................................... 1 1.1 Motivation for Transient Electronics ....................................................................... 1 1.2 Literature Review of Transient Electronics ............................................................. 2

1.2.1 Transient Material ........................................................................................... 3 1.2.1.1 Transient substrate material .................................................................... 3 1.2.1.2 Active component material ..................................................................... 5 1.2.1.3. Encapsulation material ........................................................................... 6

1.2.2 Transiency Mechanism.................................................................................... 7 1.2.3 Manufacturing Technique ............................................................................... 8 1.2.4 Transient Device and Potential Application .................................................... 9

1.2.4.1 Biomedical and environmental sensors. ................................................ 10 1.2.4.2 Security devices .................................................................................... 10

1.3 Current Challenge of Transient Electronics and Our Approach ............................ 11 1.3.1 Understanding Interfacial Interactions of Multilayer Structure towards

Controlled Transiency ............................................................................................ 11 1.3.2 Transient Energy Storage Unit ...................................................................... 13 1.3.3 Intrinsic Soft All-Organic Transient Electronics........................................... 14

1.4 Dissertation Organization ...................................................................................... 16 Reference ..................................................................................................................... 17

CHAPTER 2. INTERFACIAL STRESS IN TRANSIENT LAYERED

STRUCTURES ................................................................................................................. 26 Abstract ........................................................................................................................ 26 2.1 Introduction............................................................................................................ 27 2.2 Materials and Methods .......................................................................................... 30

2.2.1 Substrate ........................................................................................................ 30 2.2.2 Electrode ........................................................................................................ 30 2.2.3 Mechanical Characterization ......................................................................... 30 2.2.4 Disintegration and Transiency....................................................................... 31 2.2.5 Analytical Method ......................................................................................... 31 2.2.6 Finite Element Analysis Predictive Modeling............................................... 32

2.3 Results and Discussion .......................................................................................... 33 2.3.1 Mechanical Properties of the Electrode......................................................... 33 2.2.2 Mechanical Properties of Swelling PVA....................................................... 33 2.2.3 Typical Von Mises Stress Distribution ......................................................... 34

iii

2.2.4 Von Mises Stress as a Function of Swelling Strain and Time. ..................... 36 2.2.5 Comparison between Computational, Experimental and Analytical

Result. ..................................................................................................................... 37 2.2.6 Stress/Strain Dependency on Electrode Thickness and Dimensions. ........... 39

2.4 Conclusion ............................................................................................................. 40 References.................................................................................................................... 41 Supplementary Information ......................................................................................... 44

CHAPTER 3. INTERFACIAL FRACTURE IN TRANSIENT LAYERED

STRUCTURES ................................................................................................................. 46 Abstract ........................................................................................................................ 46 3.1 Introduction............................................................................................................ 46 3.2 Materials and Methods .......................................................................................... 49

3.2.1 Materials and Sample Preparation ................................................................. 49 3.2.2 Mechanical Characterization of Electrode Layers ........................................ 50 3.2.3 PVA swelling strain and resultant electrode strain measurements................ 50 3.2.4 Characterization of Initial Defect Distribution, Crack Propagation, and

Resultant Fragment Size ......................................................................................... 50 3.2.5 FEM simulation ............................................................................................. 51

3.3 Results and Discussion .......................................................................................... 51 3.3.1. True Stress-Strain Curve of the Electrode Layer ......................................... 51 3.3.2 Surface Crack Predominated Failure ............................................................. 53 3.3.3 Crack Propagation and the Resultant Fragmentation .................................... 54

3.4 Conclusion ............................................................................................................. 58 References.................................................................................................................... 59 Supplementary Information ......................................................................................... 63

CHAPTER 4. MECHANICS OF INTERFACIAL BONDING IN DISSIMILAR

SOFT TRANSIENT MATERIALS AND ELECTRONICS ............................................ 65 Abstract ........................................................................................................................ 65 4.1 Introduction............................................................................................................ 65 4.2 Materials and Methods .......................................................................................... 68

4.2.1 Substrates....................................................................................................... 68 4.2.2 Conductive Ink .............................................................................................. 68 4.2.3 Pre-straining .................................................................................................. 69 4.2.4 Conductive Paths ........................................................................................... 69 4.2.5 Transiency ..................................................................................................... 69 4.2.6 Mechanical and Electrical Characterization .................................................. 69 4.2.7 Imaging and Microscopy ............................................................................... 70

4.3 Results and Discussion .......................................................................................... 70 4.3.1 Strain Dependence of Electrical Properties ................................................... 70 4.3.2 Predictive FEM Modeling ............................................................................. 73

4.3.2.1 Stress distribution and delamination ..................................................... 73 4.3.2.2 Cracking ................................................................................................ 75

4.3.3 Active Transiency.......................................................................................... 75 4.4 Conclusion ............................................................................................................. 77 References.................................................................................................................... 78

iv

CHAPTER 5. TRANSIENT LITHIUM-ION BATTERY ............................................... 82 Abstract ........................................................................................................................ 82 5.1 Introduction............................................................................................................ 82 5.2 Materials and Methods .......................................................................................... 84

5.2.1 Materials ........................................................................................................ 84 5.2.2 Substrate ........................................................................................................ 84 5.2.3 Active Material .............................................................................................. 85 5.2.4 Current Lead .................................................................................................. 85 5.2.5. Electrodes ..................................................................................................... 85 5.2.6 Packaging ...................................................................................................... 86 5.2.7 Electrolyte ..................................................................................................... 86 5.2.8 Electrochemical Testing ................................................................................ 86

5.3 Results and Discussion .......................................................................................... 86 5.3.1 Transiency of Single Electrode ..................................................................... 86 5.3.2 Transiency of Battery Cells ........................................................................... 89 5.3.3 Battery Performance ...................................................................................... 91

5.4 Conclusion ............................................................................................................. 95 References.................................................................................................................... 96 Supplementary Information ......................................................................................... 97

5.S.1 Electrochemical analysis ............................................................................... 97 5.S.2 Non-Swelling Substrate: ............................................................................... 98

CHAPTER 6. A SOFT TRANSIENT ALL-ORGANIC SENSOR .................................. 99 Abstract ........................................................................................................................ 99 6.1 Introduction............................................................................................................ 99 6.2 Materials and Methods ........................................................................................ 101

6.2.1 Materials ...................................................................................................... 101 5.2.2 Printing of PEDOT: PSS Conductive Patterns on PEO Substrate .............. 102 6.2.3 Mechanical Characterizations ..................................................................... 103 6.2.4 Electrical Characterizations ......................................................................... 103 6.2.5. Degradation ................................................................................................ 103

6.3 Results and Discussion ........................................................................................ 104 6.3.1 Printed All-Organic Electrodes ................................................................... 104 6.3.2 Strain - Electric Resistance Correlations ..................................................... 105

6.3.2.1 Static Stress ......................................................................................... 106 6.3.2.2 Dynamic Load ..................................................................................... 107

6.3.3 Transient Property ....................................................................................... 109 6.3.4 Epidermal Strain Sensor .............................................................................. 111

6.4 Conclusion ........................................................................................................... 112 References.................................................................................................................. 113

CHAPTER 7. GENERAL CONCLUSIONS AND FUTURE WORKS ........................ 118

v

LIST OF FIGURES

Page

Figure 2.1 Disintegration process and stress distribution of a typical electrode

consisting of electrochemically active materials deposited on a PVA

substrate ........................................................................................................ 29

Figure 2.2 a) Stress-Strain curve of soaked electrode .................................................... 35

Figure 2.3 a) Von Mises stress distribution ................................................................... 36

Figure 2.4 a) Total strain E11of electrode layer at different swelling strain of PVA

substrate ........................................................................................................ 37

Figure 2.5 a) Simulated Von Mises stress and critical swelling strain for electrodes

of different thicknesses laminated on 80 m substrate ................................ 40

Figure 3.1 a) Nominal (experimentally obtained) and true (theoretically driven)

stress-strain curves of the electrode coating ................................................. 53

Figure 3.2 a) Series image showing the surface cracking of the electrode layer ........... 54

Figure 3.3 a) Maximum principle strain distribution through an electrode, with

details shown around the crack ..................................................................... 56

Figure 3.4 a) Image of typical defect distribution, before (left) and after (right)

monochromic ................................................................................................ 57

Figure 3.S.1 Shrinkage of the fragments toward their centers .......................................... 63

Figure 4.1 Left: Optical digital microscope image of an extreme case of buckling ...... 71

Figure 4.2 a) Extent of delamination is directly related to the extent of pre-

straining. Buckles become less uniform on substrates pre-strained

beyond 20% .................................................................................................. 72

Figure 4.3 Stress distribution (MPa) through the substrate and printed pattern. ........... 73

Figure 4.4 Simulated delamination at the edge of a conductive spot ............................ 74

Figure 4.5 a) Original and b-c) processed images of defects through a polymer

thin-film ........................................................................................................ 74

vi

Figure 4.6 Elastic moduli of PVA-SBC films are inversely proportional to the

concentration of SBC up to 30%, above which the correlation becomes

directly proportional ..................................................................................... 76

Figure 4.7 PVA-SBC 10% film reached 82% transiency in a water-diluted

hydrochloric acid solution in 600 seconds. Bubbling of the film is

evident in the two middle images ................................................................. 76

Figure 5.1 a) Hydrolysis of PVA which results in swelling of the polymer

membrane ..................................................................................................... 87

Figure 5.2 a) Schematic representation of the cross section of the battery cell ............. 91

Figure 5.3 a) and b) discharging behavior of transient battery cells at different

current densities ............................................................................................ 93

Figure 5.S.1 cathode (group 1) fabricated on PVAc substrate .......................................... 98

Figure 6.1 a) Schematic of E-Jet printer ...................................................................... 105

Figure 6.2 Relative resistance changed as a function of static strain........................... 107

Figure 6.3 a) Resistance fluctuations of a polymer electrode for 200 m

displacement at 1 Hz, 0.1 Hz, and 0.05 Hz frequency, respectively .......... 108

Figure 6.4 a) Sequential images of transiency of all polymer electrode in water,

scale bars represent 10 mm ......................................................................... 110

Figure 6.5 a) Sequential images of transiency of all polymer electrode on a human

forearm skin, scale bars present 10 mm ..................................................... 112

vii

LIST OF TABLES

Page

Table 3.S.1 Predicted average piece area and the actual average piece area ................... 64

Table 5.1 Transiency test on samples with varying area density of the active

materials ....................................................................................................... 87

Table 5.S.1 Electronic component values calculated from the equivalent circuit

model ............................................................................................................ 98

viii

ACKNOWLEDGMENTS

First of all, I should give my sincere appreciation to Dr. Reza Montazami, my

major professor, for his constant support, instruction, encouragement, and inspiration. He

has provided sound advice guiding through my PhD study, including but not limited to

literature reviewing, research experimenting, paper writing, and research presenting.

Without his illuminating guidance, this thesis could not have reached its present form. Dr.

Montazami also has encouraged me to believe in myself and build confidence. His

astounding knowledge, passion, and diligence inspire me to pursue research as my

lifetime career. And he has set an excellent model as an advisor for me to follow in my

future. I truly cherish the opportunity to work with Dr. Montazami and to be his student.

Secondly, my hearty thanks go to my dissertation committee members: Dr. Nicole

Hashemi, Dr. Xinwei Wang, Dr. Liang Dong, and Dr. Hantang Qin, for their

encouragement, advice, and comments to widen my research from various perspectives.

I would also like to thank my labmates, Ruisi Zhang, Reihaneh Jamshidi, Dr.

Wangyujue Hong, and Dr. Abdallah Almomani, for their collaborative work, helpful

discussions, and supportive company in the lab. In addition, I would also like to thank

my friends, colleagues, the department faculties and staffs for making my time at Iowa

State University a wonderful experience.

Special thanks go to my beloved family. I would thank my parents, my brother,

and my sister for their unconditional love and support all through these years. My

heartfelt thanks go to my dear husband Jingyao Gai and cherished son Eric Gai, who are

always here to support me, and bring myriad of joys to my PhD journey.

ix

ABSTRACT

Transient electronics is an emerging field in materials science that has attracted

considerable attention from the scholar community in the last few years. The unique

attribute of transient technology is the capability to fully or partially disintegrate after a

predefined period of stable operation. Transient electronics have a wide range of potential

applications as biomedical implants, environmental sensors, and hardware-secured

devices. Biodegradable transient sensors could be dissolved and absorbed into a bio-

environment, eliminating complications associated with long-term presence of implanted

devices, or secondary surgery to extract implanted devices. Eco-friendly environmental

monitoring transient devices could be utilized to collect desired data, then degrade

naturally into the surrounding environment, reducing the recollection expenses, and

minimizing harmful waste. Self-deconstructing platforms could undergo disintegration

and physically remove sensitive information once transience is triggered. Recent

developments on transient materials, dissolution/disintegration mechanisms,

manufacturing techniques, structural designs, and transient energy storage devices have

advanced the functionality, performance, and applicability of transient electronics.

Further research on precisely controlled transiency, however, is needed to broaden the

applications of transient electronics. Since transient electronics are typically multilayer

thin-film structures, to achieve controlled transiency at device level, it is crucial to

understand the interfacial interactions among layers of dissimilar materials assembled on

one another to form complex transient devices.

This dissertation discusses materials, transiency mechanisms, and applications of

transient electronics. Firstly, interfacial interactions among layers of dissimilar materials

x

is systematically studied, revealing the mechanism of transiency achieved by swelling

induced disintegration. Following section reports a transient battery utilizing swelling

induced transiency as a proof-of-concept application. Lastly, the dissertation presents

materials, mechanical properties, and applications of all-organic soft transient electronics.

Firstly, to understand the underlying mechanism of swelling induced transiency,

we studied interfacial interactions of a particular case of polymeric substrate with lithium

titanate electrode coating layer. The structure is analogous to that of the anode in typical

lithium-ion batteries; yet, can be extended to more general cases of soft electronics. This

coordinated experimental-analytical-simulation study exhibited formation, accumulation

and propagation of swelling-induced stress and fracture through the membrane-coating

interface, when in transient mode. Swelling-induced stress as a function of electrode

thickness was studied; the analytical data and simulations were verified by experimental

results. Moreover, the fragment size of the electrode coating layer as a function of initial

defect prevalence and distribution was investigated. The average fragment size was

predicted using a combination of experimentally-determined initial defect distribution

and finite element method-obtained swelling strain – defect length curve. The predicted

average fragment size was found to be in good agreement with the experimental results.

Meanwhile, as an application of swelling induced transiency, a transient lithium-

ion battery based on polymeric constituents is presented. The battery takes advantage of a

close variation of the active materials used in conventional lithium-ion batteries and can

achieve and maintain a potential of > 2.5 V. All materials are deposited form polymer-

based emulsions and the transiency is achieved through a hybrid approach of redispersion

xi

of insoluble, and dissolution of soluble components in approximately 30 minutes. The

reported transient battery could be applied an onboard power for transient electronics.

In addition, a flexible all-organic transient sensor with potential to be applied as

epidermal sensor is reported. A conductive conjugated polymer electrode was printed

onto a water-soluble polymer substrate with an electrodehydrodynamic jetting printer.

The all-organic electrode is partially transient with supportive substrate dissolved

completely in water, while the conjugated polymer electrode remains intact. The intact

functional electrode layer formed conformal contact with human skin and was applied as

an epidermal strain sensor.

1

CHAPTER 1. GENERAL INTRODUCTION

1.1 Motivation for Transient Electronics

Transient electronics is a rapidly emerging technology in the last few years. Unlike

conventional electronics that are designed to function for decades, transient devices are

designed to operate over a typically short and well-defined time period, then

dissolve/disintegrate in a controlled fashion when exposed to stimuli. Transient electronics

have broad potential applications including biomedical devices, environmental monitors,

military and homeland security, to name a few examples. The most attracting and demanding

application of transient electronics is implantable electronics and edible sensors, such as

wound healing sensors and drug delivering platforms. These biocompatible and

bioresorbable transient electronics could be dissolved and absorbed within human body after

a predefined reliable operation. The transient characteristic would eliminate complications

(for instance, infections and transplant rejection) inherent in long-term presence of

implantable devices. Meanwhile, transient implant devices would bypass risks associated

with secondary surgery for implanted extraction. Transient devices with environmental

benign end byproducts could be applied in environmental monitoring and sensing. Theses

ecofriendly devices will reduce the expenses on recollection unwanted devices, as well as

minimize the unwanted waste. For application in military and hardware security area,

platforms that are capable of self-deconstruction will prevent recovery of sensitive data.

The advancement of new techniques in this area has broaden the functionalities and

applications of transient electronics. In the following section, a literature review will be

presented on the development of this new branch of electronics.

2

1.2 Literature Review of Transient Electronics

Transient electronics have received extensive and growing attentions from

researchers all over the world. Recent researches have explored and reported new transient

materials, triggering mechanisms, manufacturing techniques, transient energy storage units,

and system level devices. In initial work on transient electronics, devices are partially

transient: active non-resorbable components were supported by water-soluble polymer

membranes. (1-5)Following these initial steps, possibility of fully transient electronics are

examined: bioresorbable organic semiconductors, (6) physically transient silicon based

semiconductors, (1, 7-9) degradable dielectric (10-13), and water-soluble metal conductors .

(1, 14-17) are investigated and reported. At the same time, more stimuli for transiency have

been reported. The earliest and widely studied triggering signal is water and aqueous

solution. Later on, transiency initiated by light (18) and heat (19) are explored. In addition, a

wide range of manufacturing techniques have been applied in fabrication of transient devices.

In particular, transfer printing is developed for high performance transient electronics

incorporated with silicon-based semiconductors. Moreover, power sources for transient

electronics have advanced from external power supply to near field power receiver and

transient onboard power units, advancing the possibility of fully transient and fully functional

devices. Recent development of transient materials, mechanism, and manufacturing have lay

the foundation for system level transient devices, enable diverse functionalities and

applications. In this section, we will summarize the recent development of transient

electronics, with a focus on transient materials, triggering mechanisms, manufacturing

techniques, transient power units, and demonstrated transient devices.

3

1.2.1 Transient Material

In typical structural designs of transient electronics, functional components are

deposited/ printed on a thin film substrate, then encapsulated by a casing layer. Substrates are

used as supportive membrane for their favorable mechanical properties. Encapsulation layer

defines the reliable operation time of the device. All the constituent materials should be

capable to dissolve, disintegrate, or decompose once exposed to stimuli. Here, we present the

advancement on transient substrates, functional components, and encapsulation materials,

respectively.

1.2.1.1 Transient substrate material

Transient substrate plays a critically important role in the development of transient

electronics. For one thing, substrates provide mechanical supports for functional components.

For another, the transiency attribute of substrate could enable/ accelerate the

dissolution/disintegration of printed active components. In particular, for soft transient

electronics applications, intrinsic flexibility/ stretchability of substrate contributes to

accommodation of stress/ strain of the devices.

Most substrates reported so far are degradable polymer-based membranes. These

polymers would dissolve, disperse, or swell in water, enabled by hydrophilic groups in their

backbone. To our best collection, the first reported water-soluble biopolymer for transient

substrate is silk by Kim et al in 2009. (20) They deposited silicon nanomembrane circuits

onto silk substrate, then tested the electrical, bending, water dissolution, and toxicity of the

devices. Potential of silk membrane applied as substrate for implantable biomedical sensors

was demonstrated. The 25 m thick silk substrate dissolved completely in water in 3

minutes. Following by this study, a variety of transient electronics based on silk substrates

are reported. (1-5) Silk has been proven advantageous for its merits including robust

4

mechanical properties, biocompatibility, room processing temperature, and controllable

degradation rate. (21, 22)

Other polymers, especially water-soluble thermoplastics are also studied as transient

substrates. Examples include poly(L-lactide-co-glycolide) (PLGA), (6) polylactic acid

(PLA), (23) polycaprolactone (PCL), (15) polypropylene carbonate (PPC), (24) and

poly(vinyl alcohol) (PVA). (25-27) Bettinger et al. (6) fabricated field-effect transistors and

organic semiconductors onto PLGA membrane. The degradation of PLGA membrane took

approximately 70 days at low-pH environment. Our group (14) reported that transient rate of

PVA could be tuned by controlling the composite structure. The addition of sucrose would

accelerate the dissolution rate, while gelatin would decelerate the dissolution process. The

dissolution time of PVA composite substrate could be regulated ranging from minutes to

hours.

In addition, biodegradable elastomers are reported as soft substrates for

flexible/stretchable transient electronics. Examples includes Poly(glycerol-co-sebacate)

(PGS), (28) poly(caprolactone)-poly(glycerol sebacate) (PGS-PCL), (29) and Poly(1,8-

octanediol-cocitrate) (POC). (30) Rogers’ group (30) reported transient strain sensors based

on POC substrate. POC would dissolve in phosphate buffer solutions (PBS) solution within

few weeks.

Other biodegradable substrates reported including metal foils, (5) silicon wafer, (1, 4)

natural substrate (chitosan, rice paper, sodium alginate), (31, 32) and foodstuffs (rice paper,

cheese, charcoal, seaweed). (15, 33)

Besides water soluble substrates, polymers that decompose when subjected to light or

heat are also studied and reported. Moore’s group (18, 19, 34) presented a metastable

5

polymer cyclic poly (phthalaldehyde) (cPPA), which can be self-depolymerized back into

monomers triggered by heat or acid. They first reported an Ultraviolet (UV) light triggered

transient substrate by incorporation a photo-acid generator into the cPPA polymer matrix.

Later on, they presented a heat triggered transient electronic with double layer substrates: a

methanesulfonic acid/ wax layer would release acid when heated; and a cPPA layer would

disintegrate by the acid. Recently, they tailored the cPPA onset thermal depolymerization

temperature by removal of the latent Lewis acid catalyst and addition of radical inhibitors.

One trend for future research would be exploring more polymer composite designs

that incorporate internal stimuli to achieve self-transiency. The substrate can respond rapidly

to an external “signal”, and become transient rapidly by the internal stimuli.

1.2.1.2 Active component material

Active components determine the functionality and performance of transient

electronics. For a typical transient device, semiconductors, electrodes, interlayer connectors,

and dielectrics are the four major components. Compared to conventional electronics, the

ability to dissolve/disintegrate limits the materials applied in operative components in

transient electronics, thus limits the performance of the transient electronics. To achieve fully

transient and high-performance electronics, more functional constituent materials is needed.

Semiconductors are the most important component in transient sensors. Rogers’s

group first reported a physically transient form of silicon nanomembrane (30-300 nm) and

applied them as semiconductor for transient electronics. (1, 7-9) Later on, they explored more

dissolvable semiconducting materials including amorphous silicon, polycrystalline silicon,

germanium, silicon germanium alloy, indium-gallium-zinc oxide, and zinc oxide. (5, 35)

Biocompatibility of these semiconducting materials was confirmed by in vitro cytotoxicity

studies. The hydrolysis rate of silicon-based semiconductors depends on temperature, pH,

6

ionic concentration, and doping level of silicon. A nanomembrane of silicon with a thickness

of 70 nm would dissolve in water in approximately 10 days.(1) Besides silicon based high

performance semiconductor, bioresorbable organic semiconductors are also of interst. (2, 3)

Transient organic semiconductors have all the intrinsic advantagements of polymers, such as

low temperature processing, favarable mehcnaical property, and chemically altered

dissolution rate.

Dielectric materials are another essential constituent materials for active components.

Metal oxide and silicide such as magnesium oxide (MgO), magnesium difluoride (MgF2),

silicon dioxide (SiO2), and silicon nitride (Si3N4) are reported water-soluble and applied as

dielectrics for transient electronics. (10-13) Other dielectric materials including spin-on-glass

(SOG)(36), and egg albumen (37) are also investigated.

Electrodes and interlayer connectors for transient electronics are mainly metals.

Water-soluble metals such as Mg, Zn, W, Fe, and Mo, have been investigated as conductive

paths and connecting leads. (1, 14-17) Yin et al. (14) studied the dissolution mechanism of

water-soluble metals and found that the dissolution rate of each metal depends on ion and pH

of the aqueous solution, as well as the morphology and manufacturing technique of the metal

foils. The time requires for complete dissolution of metal nanomembrane varies from days to

weeks. In general, the dissolution rate for Mg and Zn are faster than that of W and Mo. Other

than water-soluble thin films, metal microparticles, such as silver paste, are also applied as

conductive electrodes for transient electronics. (38-40) Those conductive paths could

disintegrate and redisperse into water under interfacial stress from the swelling substrate.

1.2.1.3. Encapsulation material

Transient process follows two steps: the first one is the degradation of the

encapsulation; the second step is the dissolution/disintegration of the substrate and functional

7

elements. Thus, effective encapsulation layer is the key to precisely programmed stable

operation period of transient electronics upon subjected to triggering signals. Transient

materials that degrade by corrosion and hydrolysis and do not swell during dissolution

kinetics is attractive candidates for encapsulation material. Up to date, transient substrates

(1, 41, 42) including crystallized silk and PCL, dielectrics (1, 5, 12) such as MgO, MgF2,

SiO2, Si3N4, and silicate spin‐on‐glass (SOG) materials, have been applied as encapsulation

layers for transient electronics to define the reliable operation time of transient electronics.

Surface reactions dominate the dissolution kinetics of the encapsulation layer. To

extend a stable working time of transient electronics, layers of casing overcoats could be

repeated to increase the thickness of the encapsulation layer. (41) However, defects inside the

encapsulation overcoat provide path for diffusion, and increase the effective surface reaction

areas. As a result, defects reduce predefined lifetime of the transient electronics, and are

influential for precisely defined reliable functioning time of transient electronics. Researchers

have reported several techniques to fabricate effective casing layer and achieve a well-

designed lifetime of the transient electronics. The first method is to apply multiple different

encapsulation materials cooperatively to eliminate defects. Li et al. (16) analyzed the reactive

diffusion of biodegradable materials and presented that a well-structured encapsulation layer

with both MgO and silk, which could increase the operation time over hundreds of times.

The second approach is to apply advanced manufacturing techniques to reduce the defects.

Atomic layer deposition (ALD) has been reported an effective deposition technique to reduce

defects and improve the performance of the encapsulation. (43)

1.2.2 Transiency Mechanism

An extensive amount of demonstrated transient electronics are triggered by aqueous

solutions, especially devices targeted for biomedical applications. Constituents of transient

8

electronics, including substrate, active component, and encapsulation layer, are degradable in

aqueous solution through corrosion, dissolution, and hydrolysis. (1, 4, 8-10, 14, 16, 44-46)

Disintegration by internal mechanical stress is another way to achieve fast transiency. Our

group reported a hybrid approach towards transiency of the devices, which incorporates

physical redispersion of insoluble materials, and chemical dissolution of soluble ones. The

swelling induced interfacial stress of PVA substrate will lead to a fracture of the printed

active electrodes and enable a redispersion of the microparticles. (38, 47, 48) We also

analyzed and simulated the swelling- induced interfacial stress and fracture for a typical

electrodes. (49, 50)

In addition, researchers have investigated and reported other trigger signals including

light and heat. Presented by Moore’s group, low celling temperature polymer cPPA could be

disintegrated by elevated temperature or acid. Acid released from a photo-acid generator

under UV light or methanesulfonic acid released from melting wax expedite the

disintegration of cPPA. (18, 19) Recently, Kim et al. introduced a cyclododecane (CDD)

composite system in which sublimation under ambient conditions leads to mechanical

fragmentation and disintegration of active devices.(51)

1.2.3 Manufacturing Technique

The most common manufacturing technique for transient polymeric substrates is

solution casting. (4, 8-10, 14, 16, 44) Metallic and dielectric layers could then be printed onto

transient substrates. Stencil mask printing is widely used for fabricating electrodes of

transient electronics.(38, 47, 48) High resolution printing techniques, including electrode

spinning, electrohydrodynamic jet printing, and three-dimensional (3D) printing are also

reported. Electrode spinning is applied to fabricate fibrous polymeric substrate, (29) fabric

scaffold, (52) and membrane separator for transient lithium battery. (31) In a recent study

9

by Yoon et al., 3D technique was applied to control the structural parameters of the PVA

substrate, regulating the transient time of the PVA substrate in water. (27) In a study for all-

organic transient electronics by our group, electrohydrodynamic jet printing technique is

applied to print conductive conjugated polymer electrodes onto a transient substrate.

While metallic and dielectric layers could be printed onto transient polymeric

substrate, silicon semiconductors are usually fabricated using photolithography. Polymeric

substrate materials are sensitive to temperature and water, and usually could not remain

stable through the photolithography. To accommodate different manufacturing technique

required for different constituents in transient electronics, a novel fabrication schemes called

transfer printing is developed to integrate transient devices. (1, 53) Firstly, the active

components are manufactured using conventional photolithography. (54-56) Secondly, the

active components are transferred onto transient substrate by contact printing. (57, 58) PDMS

stamps could pick up nano/micro-scale features such as silicon nanomembrane or graphene

membrane, and place them onto desired locations. (58) Rogers’s group have presented

several transient devices fabricated using transfer printing technique. Active sensors are

fabricated through conventional photolithography on a bilayer of poly (methyl methacrylate)

(PMMA) and diluted polyimide (D-PI). Then an extra D-PI layer was cast on top of the

sensor. PMMA was then etched by acetone, and bottom layer of D-PI was removed by ion

etching. Active sensors was then pick up by PDMS stamp, and transfer printed onto transient

substrate. (30, 34)

1.2.4 Transient Device and Potential Application

Incorporating transient materials, mechanisms, and manufacturing techniques

reviewed in the previous sections, a variety of transient devices have been developed and

10

demonstrated, categorized into three application areas: biomedical implants, environmental

sensors, and secure memory devices.

1.2.4.1 Biomedical and environmental sensors.

Biomedical sensors and environmental monitors are the most compelling application

of transient electronics. Transient sensors ranging from electronic elements to system level

platforms have been demonstrated. Electronic components such as filed effect transistors, (9,

18) electrical double-layer transistors, (59) organic thin film transistors, (6) complementary

metal oxide semiconductors (CMOS), (15) logic circuits, (4) and electronically instrumented

stents, (60) are presented. System level platforms including temperature sensors, (1, 61, 62)

electrophysiological and pH sensors, (30) pressure sensors, (61, 63) hydration sensors, (15)

light emitting devices, (64) and multifunctional drug delivery scaffolds, (52, 65, 66) have

been reported.

A representative transient platform are biodegradable and stretchable

electrophysiological sensors presented by Roger’s group.(30) The EP sensor contains

semiconductors, transistors, Mg electrodes, and a biodegradable elastomer POC substrate.

This electrophysiological sensor can be reversibly stretched to strains within 30% with a

linear, elastic mechanical response, and achieved comparable electromyograms performance

compared to conventional gel-based electrodes. Recently, Feiner et al.(52) demonstrated

another transient platform. They presented a multifunctional degradable electronic scaffolds

which could on-line sense tissue function, provide stimulation to control contractility and

efficiently release drugs.

1.2.4.2 Security devices

Hardware-oriented secure devices are another attractive application area of transient

electronics. Researchers have reported a variety of secure memory devices that could

11

disintegrate once triggered, and physically remove sensitive data. A typical hardware security

device is switching memories. (11, 32, 67) Sun et al. (11) reported a physically transient

threshold switching device for security application. The device composes of water-soluble

metals and could be dissolved in water in 8 minutes. More recently, Pandey et al. (68)

reported a self-destruction device for hardware security. The transiency is triggered either by

an exothermal energy release that melts the surface of the microchips, or by a release of a

corrosive chemical agent that dissolves the surface of the chip.

1.3 Current Challenge of Transient Electronics and Our Approach

Recent development on transient materials, transiency mechanisms, manufacturing

techniques have advanced the functionality and performance of transient electronics. Yet, to

achieve precisely controlled transiency, on board powered transient electronics, as well as

soft transient electronics, further research on advanced materials, unconventional

dissolution/disintegration mechanisms, and innovated designs are demanding. (69-72)

1.3.1 Understanding Interfacial Interactions of Multilayer Structure towards

Controlled Transiency

Swelling-induced stress and fracture at the interface of functional component layer

and swelling substrate plays a key role in the fast physical disintegration of the device. To

better understand physical transiency, knowledge of formation, propagation, and

accumulation of swelling-induced fracture in such layered structures deems essential.

Swelling-induced stress in polymer has been studied and utilized to achieve coating

rupture for biomedical applications, in particular for drug delivery. (73-76) Peppas and his

colleagues measured the swelling-induced force of a cylindrical piece of polymer swelling in

a closed chamber with a flexible top cap. (77-79) The swelling of the polymer was

constrained in all directions except the top; the force measured at the top cap was considered

12

as the swelling force. The closed chamber method is feasible for measuring the swelling-

induced stress in encapsulated drug delivery systems, as swelling of the polymer is

constrained in all directions. Swelling of polymer substrate in layered structures, however, is

constrained only at the interface between the layers, rendering the closed chamber method

inappropriate.

Interfacial stress –not due to swelling– and the resulted fractures have been widely

studied for application in flexible –non-transient– electronics. Vlassak’s group investigated

the thickness effect on the ultimate strain of copper film deposited on polyimide substrate,

(80) and the thickness effect on the ultimate mode of stiff islands on deformable substrate.

(81) In another study, they calculated the interfacial fracture energy between SiNx islands and

polyimide substrate by using experimental debond length, and energy release rate - debond

length curve, obtained from FEM model. (82) Lu’s group has measured the adhesion strength

at the interface of MoS2 and polydimethylsiloxane by a buckle-based metrology. (83)

Swelling-induced stress has its unique features compared to the interface stress

mentioned above. First, the material property of the substrate is a function of swelling strain,

and swelling strain is a function of time. The materials’ properties as a function of time need

to be fully understood and applied in the stress analysis, which requires more complex

analysis. Second, the substrate swells spatially rather than being stretched uniaxially as is

considered in interfacial stress analysis.

In this dissertation, we analyzed the swelling-induced stress by analytical,

experimental and finite element method (FEM) modeling. The stress and strain of lithium-

titanate electrode coating sprayed onto poly (vinyl alcohol) (PVA) substrate as a function of

time are predicted. The dependency of stress on the electrode thickness and geometry is also

13

investigated to identify factors contributing to the reduced transiency rate in thick electrode

coatings. In addition, we studied the fracturing of such soft multilayer thin films under

similar conditions. The relationship between initial defect distribution and fragment size was

investigated and confirmed. The method presented here could be applied to analyze strain-

mismatch-induced fracture of soft multilayer thin films with dynamic material properties.

1.3.2 Transient Energy Storage Unit

To achieve fully transiency, and realize autonomous transient electronic devices, it is

essential to develop transient on board power unit. (69, 71) Thus far, there has been limited

efforts in design and construction of transient batteries, mainly due to the lack of soluble

proper materials Transient power sources reported so far including radio frequency power

transmitter, (13) mechanical energy harvesters, (5, 84) bio-cells, (28, 85, 86)

microsupercapacitors, (87) and galvanic battery cells. (42, 44, 88, 89) Kim et al. reported

edible water activated sodium batteries based on melanin electrodes where a potential of 0.6-

1.06 V and current of 5-20 μA, depending on design, was achieved. (28, 85) More recently,

Jeerapan et al. reported a fully edible biocell, providing a way for energy harvesting towards

ingestible biomedical devices. The biocell compose of food derived tissues and could output

an open circuit voltage (OCV) of 0.24 V. (86) An intrinsically transient battery capable of

environmental resorption was first reported by Yin et al. where Mg anode and biodegradable

metals (Fe, W or Mo) cathodes were used with a transient polymer casing; potentials ranging

from 0.45 V to 0.75 V (depending on cathode materials) were reported. (44)

To date, all reported transient batteries have shortcomings compare to their

conventional counterparts; uncompetitive potential, current, stability and shelf life are among

the top challenges in construction of a practical transient battery that can supply enough

power to run a common electric circuit. The low potential and power density in transient

14

batteries are mainly due to the use of non-optimal electrode materials because of their

solubility. One other significantly important limitations of transient batteries reported to date

is low transiency rate, which is a result of slow chemical reactions between the constituent

materials and the solvent. High transiency rates are specially anticipated in military and

hardware security applications.

To design a transient battery capable of supplying enough power and undergoing fast

transiency, new approaches toward the materials and structural design are necessary.

Lithium-ion battery technology is a well-established, mature and commercialized technology.

Recently, few transient lithium ion battery are reported. (31, 48, 90) Among the first few

demonstrations, our group presented a transient lithium battery that could output potential as

high as 2.6 V was reported. (48) The battery takes advantage of a close variation of the active

materials used in conventional Li-ion batteries and can achieve and maintain a potential of

>2.5 V. All materials are deposited form polymer-based emulsions and the transiency is

achieved through a hybrid approach of redispersion of insoluble, and dissolution of soluble

components in approximately 30 minutes.

1.3.3 Intrinsic Soft All-Organic Transient Electronics

To broaden the possibility of application in biomedical areas, transient electronics are

required to be highly flexible or stretchable to form a conformal contact with the bi-host, and

accommodate stress/strain. Thus far, soft transient electronics are fabricated by deposition of

an active inorganic functional layer on a soft organic substrate. These inorganic functional

materials, mostly metallic nano/micro-materials, form rigid and brittle layers; therefore, in

many cases the topology of the functional layer is designed such that it accommodates strain.

Such designs fall into two main categories. The first is to intentionally pre-strained the

substrate or/and the printed structure before regular use, thus surface waves and buckles

15

generated in the first cycle would accommodate subsequent cycles of stretching. (91-93) The

second is to laterally or topographically pattern the thin-films such that the global tensile

strains are converted to local bending strains. (94-97)

An alternative approach is to implement all-organic materials. The key concept, and

challenge, is to utilize functional polymers, which can accommodate strain through their

molecular structure and morphology, while (partially) maintaining their electrical properties.

(98) Application of intrinsically soft materials could simplify the fabrication, as well as

enhance mechanical compliance and robustness. (99, 100) In addition, the organic nature of

the all-polymer electronics contributes more advantages including low-cost, (101, 102)

oxide-free interfaces, (103) and tunability by synthesis. (104, 105) Soft all-organic

electronics that use intrinsically soft functional material have received increasing attention in

recent years. Liang and his colleagues reported an elastomeric polymer light emitting device

using a polyphenylenevinylene derivative as the emissive. (106) Lipomi’s group investigated

the effects of structural parameters of a series of poly (3-alkylthiophenes) on their

mechanical properties, (106) then demonstrated a stretchable organic solar cell which could

be conformally bonded to a hemispherical surface. (99) Bao’s group studied electronic and

morphological attributes of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate

(PEDOT: PSS) on stretchable poly-(dimethylsiloxane) (PDMS) substrates. (95) In another

study, they reported a highly stretchable and conductive polymer by adding a variety of

enhancers in PEDOT: PSS. (107) The application of intrinsically soft functional material in

soft transient electronics remain scarce. (103) Soft transient organic devices, if successful,

will not only simplify the fabrication process, but also enable more a programmable transient

rate, and more trigger signals.

16

In this dissertation, we reported the application of conductive organic conjugated

polymers for soft transient organic electronics printed using the electrohydrodynamic jet (E-

jet) printing technique. Mechanical, electronical, and transient properties of the all organic

electrode are carefully studied. Potential application of the soft transient all organic

electrodes reported as flexible strain/stress sensors are presented. In particular, potential

application of the pure active electrode layer (with substrate layer completely dissolved in

water) as ultrathin epidermal sensor are investigated.

1.4 Dissertation Organization

This dissertation starts with a comprehensive introduction for the recent development

of transient electronics. Following sections introduce our work on understanding the

underlying mechanism of transiency achieved by swelling induced disintegrations and its

associated applications. Interfacial stress and fracture for transient electronics with swelling

substrate are investigated and presented. A transient battery based swelling induced

transiency is then introduced as an example of application. Subsequent section presented an

example of all-organic transient electronics that could be applied as epidermal sensor. Final

section outlines a general conclusion and suggestions for future work.

Chapter 1 provides the background information on transient electronics. Moreover, a

comprehensive review of the existing researches and literatures are provided to lay a

foundation for the research presented in this dissertation.

Chapter 2 reports a fundamental study of interfacial stress in physically transient

layered structures. In this work the interfacial interactions of a particular case of polymeric

substrate with lithium titanate electrode coating layer was presented. Swelling-induced stress

as a function of electrode thickness was studied; the analytical data and simulations were

verified by experimental results.

17

Chapter 3 complements chapter 2. This chapter reports our investigation on the

interfacial bond failure and fracture in transient soft layered structures due to the swell-

induced interfacial stress.

Chapter 4 presents mechanics of interfacial bonding in dissimilar soft transient

materials and electronics. Soft transient electronics of polymeric substrates and silver-ink

electronics are studied for correlated mechanical-electrical properties.

Chapter 5 presents our studies on transient lithium-ion battery. The transiency of the

battery is achieved by a physical–chemical hybrid mechanism: water-soluble substrate

(chemically) and swelling induced fracture (physically) reported in chapter 3.

Chapter 6 presents development of a soft transient all-organic electronics fabricated

with electrohydrodynamic jet printing technique.

Chapter 7 presents the general conclusions and future works.

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26

CHAPTER 2. INTERFACIAL STRESS IN TRANSIENT LAYERED STRUCTURES

This chapter is based on a paper published in Advanced Materials Interfaces. 2017 April 7, 4

(7): 1601076

Yuanfen Chen, Reihaneh Jamshidi, Wangyujue Hong, Reza Montazami

Abstract

Transient materials is an emerging class of materials that are designed to undergo

disintegration at a predefined rate and manner. Transient materials are utilized in structures

and devices to enable device transiency, in particular transient electronics. While interfacial

stress in layered structures is well-studied and –to a large extent– well-understood, interfacial

stress in transient layered structures remains a challenging matter to study and understands.

This is solely because of the dynamic chemical and physical properties of the transient

materials as well as swelling-induced stress that is introduced to the interface due to the

mismatch in physical properties of the dissimilar materials forming the layers. In this work,

interfacial interactions of a particular case of polymeric substrate with lithium titanate

electrode coating layer is studied and reported. The structure is analogous to that of the anode

in a type of lithium-ion batteries; yet, can be extended to more general cases of soft

electronics. This coordinated experimental-analytical-simulation study exhibited formation,

accumulation and propagation of swelling-induced stress through the membrane-coating

interface, when in transient mode. Swelling-induced stress as a function of electrode

thickness was studied; the analytical data and simulations were verified by experimental

results. The stress analysis method could be extended to analyze interfacial stress in a wide

range of layered structures with dynamic properties.

27

2.1 Introduction

Transient electronics have become a rapidly developing technology. The new class of

devices are designed to dissolve/disintegrate in a fast and controlled manner, after they serve

their purpose. (1) Recently, researchers have studied a variety of transient components

including substrates, (2-9) electronic components, (2, 3, 5, 6, 10-12) system level devices (3,

4, 7), and batteries. (13-16)

While in most transient devices the transiency is achieved through chemical

dissolution/interactions between the device and a solvent (typically water), in a recent study

we introduced for the first time the concept of hybrid physical/chemical transiency for a

layered structure consisting of electrochemically active material printed on a polymer

substrate. (16) In this approach physical disintegration is utilized as well as chemical

dissolution. Swelling-induced stress at the interface of electrochemically active layer and the

swelling substrate (caused by swelling of the polymer substrate) plays a key role in the fast

physical disintegration of the device. To better understand physical transiency, knowledge of

formation, propagation, and accumulation of swelling-induced stress in such layered

structures deems essential.

Swelling-induced stress in polymer has been studied and utilized to achieve coating

rupture for biomedical applications, in particular for drug delivery. (17-20) Peppas and his

colleagues measured the swelling-induced force of a cylindrical piece of polymer swelling in

a closed chamber with a flexible top cap. (21-23) The swelling of the polymer was

constrained in all directions except the top; the force measured at the top cap was considered

as the swelling force. The closed chamber method is feasible for measuring the swelling-

induced stress in encapsulated drug delivery systems, as swelling of the polymer is

constrained in all directions. Swelling of polymer substrate in layered structures, however, is

28

constrained only at the interface between the layers, rendering the closed chamber method

inappropriate.

Swelling-induced stress in layered structures, e.g. soft electronics, can be

characterized by investigating interfacial stress between coating and substrate, as shown in

Figure 2.1. When swelling, the coating undergoes tensile stress while the polymer substrate

is experiencing compression stress at the interface. The magnitude of stress increases as the

swelling strain of the polymer substrate increases. When the swelling-induced stress exceeds

the ultimate stress, the electrode coating disintegrates.

Interfacial stress –not due to swelling– and the resulted fractures have been widely

studied for application in flexible –non-transient– electronics. Vlassak’s group investigated

the thickness effect on the ultimate strain of copper film deposited on polyimide substrate,

(24) and the thickness effect on the ultimate mode of stiff islands on deformable substrate.

(25) In another study, they calculated the interfacial fracture energy between SiNx islands and

polyimide substrate by using experimental debond length, and energy release rate - debond

length curve, obtained from FEM model. (26) Lu’s group has measured the adhesion strength

at the interface of MoS2 and polydimethylsiloxane by a buckle-based metrology. (27)

Swelling-induced stress has its unique features compared to the interface stress

mentioned above. First, the material property of the substrate is a function of swelling strain,

and swelling strain is a function of time. The materials’ properties as a function of time need

to be fully understood and applied in the stress analysis, which requires more complex

analysis. Second, the substrate swells spatially rather than being stretched uniaxially as is

considered in interfacial stress analysis.

29

Figure 2.1 Disintegration process and stress distribution of a typical electrode consisting of

electrochemically active materials deposited on a PVA substrate. Top view is an actual

photograph while the side view images are schematic illustrations where σc and σs are the

swelling induced stress of the coating layer and the substrate layer, respectively.

In this work, we analyzed the swelling-induced stress by analytical, experimental and

finite element method (FEM) modeling. The stress and strain of lithium-titanate electrode

coating sprayed onto poly (vinyl alcohol) (PVA) substrate as a function of time are predicted.

The dependency of stress on the electrode thickness and geometry is also investigated to

identify factors contributing to the reduced transiency rate in thick electrode coatings. PVA is

slightly crosslinked, therefore hydration will cause it to swell considerably before falling

apart; moreover, PVA can be synthesized at different molecular weights and hydrolyzations,

each with unique physical properties. In this work, as described in the experimental section,

we have used a single grade of PVA for all the experimental work which the same as one

used in our previous studies that are mentioned here for comparison. Additionally, chemical

interactions, e.g. hydrogen bonding, at the interface are probable. In our computational and

analytical model with did not consider such interactions as their contribution to the outcome

is speculated to be inconsiderable. A more detailed polymer-particle interaction model can be

obtained through more specialized molecular dynamic tools utilizing the first principles

polymer reference interaction site model (PRISM) theory. (28-30)

30

2.2 Materials and Methods

2.2.1 Substrate

Poly (vinyl alcohol) (PVA) (Mw: 61,000 g mol−1, 98.0–98.8 mol% hydrolyzed,

Sigma Aldrich) was used as received. 1.0 g PVA, 0.1 g sucrose, and 50 L of 1 M aqueous

HCl solution were added to 20 mL of DI water. The solution was then stirred at 70 ℃ for 4

hours, cooled down to ambient temperature, cast onto a plastic mold, and let dry in ambient

conditions for 24 hours; the dried PVA substrate was then carefully peeled off of the mold.

The resultant PVA films had thickness of 80 ± 5 m.

2.2.2 Electrode

Lithium-titanate (LTO) powder (Li4Ti5O12, MTI Corp.), Super-P carbon (MTI

Corporation), and PVA were mixed at weight ratio of 5:1:1 in DI water. The mixture was

then stirred for 2 hours to obtain a homogeneous slurry. The slurry was then sprayed onto the

PVA substrate to a thickness of approximately 24 m and area of 0.33 cm2 to obtain a

substrate-mounted electrode; or, sprayed onto an aluminum foil substrate to a thickness of

100 m, dried and peeled off to obtain a free-standing electrode.

2.2.3 Mechanical Characterization

Free-standing electrodes were characterized for their Elastic moduli and yield

strength using a dynamic mechanical analyzer (DMA) (Mettler Toledo, DMA-1). Elastic

modulus of the swelled pristine (without electrode coating) PVA substrate was measured as a

function of time. Pristine PVA substrates were immersed in water for 0 – 80 seconds (10

second intervals), and then characterized for their mechanical properties using DMA.

Swelling of PVA substrates was monitored and recorded using an optical imaging charged-

coupled device at 30 fps; consequently, sequential images were processed to obtain swelling

31

strain; which was then paired with the corresponding stress data for any given time. Swelling

strain was defined as 𝜀 = (𝐿𝑡 − 𝐿0) 𝐿0⁄ , where L0 is the length at time t = 0 (before

immersion), and Lt is the length at time t = 40 seconds (immersion time).

2.2.4 Disintegration and Transiency

Substrate-mounted electrode and pristine PVA substrate were immersed into water to

measure the total strain in x direction E11 varying with swelling strain. The substrate-

mounted electrode was immersed 2 seconds earlier than the pristine PVA substrate, to

account for the obstruction of water penetration into the PVA substrate from the electrode

coating side. The E11 of the electrode coating and the corresponding swelling strain of the

PVA substrate were then compared to the computational results.

2.2.5 Analytical Method

According to the Laminate Theory, the in-plane forces and moments could be

calculated by the mid-plane strains and curvatures:

{𝑁 + 𝑁𝑠

𝑀 + 𝑀𝑠}𝑥𝑦

= [𝐴 𝐵𝐵 𝐷

] {𝜀0

𝑘}

𝑥𝑦 Equation 2.1

where {N}xy and {M}xy are the external in-plane forces and moments, respectively;

{Ns}xy and {Ms}xy are the swelling-induced in-plane forces and moments, respectively; [A],

[B], and [D] are in-plane stiffness, bending stiffness coupling, and bending stiffness,

respectively; and {ε0}xy and {κ}xy are the strain and curvatures of the mid-plane, respectively.

More information regarding each component is provided in the supplementary information

(Equation 2.S.1 – Equation 2.S.9).

Stress and strain of each layer, as a function of thickness, can be deduced from the

following equations:

{𝜀(𝑧)}𝑥𝑦 = {𝜀0}𝑥𝑦 + 𝑧{𝑘}𝑥𝑦 Equation 2.2

32

{𝜎𝑐(𝑧)}𝑥𝑦 = [𝑄𝑐]{𝜀(𝑧)}𝑥𝑦 Equation 2.3

{𝜎𝑠(𝑧)}𝑥𝑦 = [𝑄𝑠]({𝜀(𝑧)}𝑥𝑦 − {𝜀𝑠}𝑥𝑦) Equation 2.4

where subscript c and s represent electrode coating and PVA substrate, respectively.

Q is the stiffness matrix; and {εs}xy is the swelling strain matrix for pristine PVA substrate.

Details of the {ε(z)}xy, {σc(z)}xy, { σs (z)}xy, [Qc], [Qs]and {εs}xy are provided in the supporting

information (Equation 2.S.1 – Equation 2.S.11).

2.2.6 Finite Element Analysis Predictive Modeling

Generation and distribution of swelling-induced stress throughout substrate-mounted

electrode structure were simulated by development of a FEA predictive model on ABAQUS

platform. Swelling behavior was analogized with thermal expansion in ABAQUS platform to

assist its simulation. Swelling behavior of the PVA substrate in electrode-substrate system

was assumed to be similar to that of a pristine PVA substrate, when a hindering factor due to

the electrode coating on one surface was taken into account. Experimental preliminary

studies suggested an approximately 2-second lag in swelling of electrode-coated PVA

substrate. The 2-second lag was integrated with FEA modeling. An electrode-substrate

system consisting of a 3.57 × 2.94 mm2 substrate of 80 µm thickness and a 3.15 × 2.52 mm2

electrode coating of 24 µm thickness was analyzed. The geometry parameters of the model

were identical to those of the actual (experimental) sample electrode. The rectangular

electrode-substrate system has two lines of symmetry; thus, only one quadrant of the system

was modeled and the results were analyzed and extended to the other three quadrants,

following the laws of symmetry. Symmetry surface boundary condition was applied on the

left and front surfaces of the electrode and substrate; fixed point condition was applied on the

lower left corner of the electrode layer. The electrode and substrate were meshed separately.

33

The electrode was simulated as a linear elastic material for up to 42 kPa of the Von Mises

stress and as an elastic-plastic material thereafter. The PVA substrate, however, was

simulated as a linear elastic material with elastic modulus as a function of the swelling strain.

2.3 Results and Discussion

2.3.1 Mechanical Properties of the Electrode

Knowledge of the mechanical properties of the electrode layer is essential for the

stress analysis. From the stress-strain curve (Figure 2.2a), we deduced elastic modulus and

yield stress of the electrode layer to be 2.82 MPa and 42 kPa, respectively. Necking occurred

when the nominal stress reached 60.6 kPa in the uniaxial stress-strain test (at true stress of 62

kPa). Stress analysis was performed using the measured mechanical properties and Poisson’s

ratio of 0.3, a typical value for similar materials. (31, 32)

2.2.2 Mechanical Properties of Swelling PVA

Mechanical properties of swelling PVA membranes were measured for up to 80

seconds of exposure to water, at 10-second intervals, Figure 2.2b. Swelling rate appeared to

be slow for the first 40 seconds of exposure to water, which is in agreement with our

previously reported analysis of mechanics of PVA swelling and degradation. (33) Local

rearrangement of molecules occurs during the first stage of swelling; thus, change in volume

is minimal. During the second stage, as the water intake increases, long range migration of

small molecules occurs which consequently results in a rapid increase in the volume of the

membrane. (34) Presented in Figure 2.2c is the elastic modulus of PVA membrane as a

function of swelling strain. The elastic modulus exhibited a sharp decrease within the first

2% of swelling strain; this is mainly an attribute of local rearrangement of the molecules that

is due to the water uptake and penetration throughout the polymer chains. Beyond the 2%

swelling strain, the rate of change of elastic modulus became significantly slower. It is

34

insinuated that the PVA membrane turns into hydrogel at the 2% swelling strain threshold.

Elastic modulus of hydrogels is reversely proportional to the water content. (35) In one study,

Horkay et al. have reported that the elastic modulus of swelling gel is proportional to

polymer concentration in the gel by a power factor of 2.25. (36) Assuming that the polymer

content doesn’t change when swelling, the polymer concentration in the gel becomes a

function of the swelling strain (Equation 2.S.15). Beyond 2% swelling, the experimentally

measured elastic moduli and Horkay’s model were in good agreement with one another,

Figure 2.2c, confirming the above-mentioned speculation; as well as our methodology for

measurement of elastic moduli. Typical stress-strain curve of swelling PVA substrate is

shown in Figure 2.2d. In this particular case, the substrate was immersed in water for 40

seconds, resulting a swelling strain of 17.4%. A slight increase in the slope of the stress-

strain curve was observed; it is speculated that the change occurred because the water content

of the substrate decreased during the period of measurement due to evaporation. The slope at

the initial region was considered for calculation of the elastic modulus of the substrate.

Poisson’s ratio of 0.43 was used for swelling PVA substrate, as suggested by Urayama and

his colleagues. (37)

2.2.3 Typical Von Mises Stress Distribution

The disintegration of the electrode layer is highly dependent on the Von Mises stress

distribution. Typical Von Mises stress, consequent to swelling of the substrate (membrane),

in an electrode-membrane system was modeled. Stress distribution appeared to be uniform

across the x-y plane and varied only throughout the thickness (z-axis), Figure 2.3a. The

highest stress buildup occurred at the electrode-membrane interface; which is due to the

highest swelling mismatch, while lowest stress was around the neutral plane. The bottom

surface of the membrane exhibited a relatively higher Von Mises stress compared to the

35

neutral plane, resulting in higher mechanical strain and causing the whole structure to bend

inward while swelling.

Figure 2.2 a) Stress-Strain curve of soaked electrode; b) Swelling strain and elastic modulus

of PVA substrate changed with immersion time; c) Experimentally measured and

theoretically predicted elastic modulus-swelling strain curves; d) Typical stress-strain curve

of the PVA substrate measured after 40 seconds of exposure to water and reaching a swelling

strain of 17.4%.

36

Figure 2.3 a) Von Mises stress distribution; b) Average Von Mises stress at different

swelling strain; c) Von Mises stress and strain of electrode along the x-axis as a function of

time.

2.2.4 Von Mises Stress as a Function of Swelling Strain and Time.

The swelling strain of the substrate is the cause of –and directly proportional to– the

stress buildup in the electrode layer; thus, it is anticipated that the Von Mises stress will

increase as the swelling strain of the substrate increases. Presented in Figure 2.3b is the

volume-average Von Mises stresses of the electrode coating and the substrate plotted as

functions of swelling strain of the substrate. The thicknesses of the electrode layer and the

substrate were 24 m and 80 m, respectively. It was observed that the stress increased

almost linearly for both the electrode layer and the substrate up to the 2% swelling strain

mark. Stress in the electrode layer increased faster than that in the substrate, because the

37

elastic modulus of the PVA substrate decreased as swelling strain increased. As the electrode

started yielding, the Von Mises stress in both electrode layer and PVA substrate leveled off.

The average Von Mises stress and strain as a function of time are also of interest.

From Figure 2.3c, it was observed that both the average Von Mises stress and total strain in

the x direction (E11) of the electrode coating increased slowly up to 42 seconds of exposure

to water, and increased with a sharper slope thereafter, corresponding to the fast swelling of

the PVA substrate. The average Von Mises stress in the electrode layer exceeds the yield

stress a few seconds after the fast swelling of PVA substrate begins (at t = 42 seconds in

Figure 2.3c). The time at which the electrode starts to disintegrate highly depends on the

time at which the PVA substrate starts the fast swelling (e.g. t = 42 seconds in this study).

Figure 2.4 a) Total strain E11of electrode layer at different swelling strain of PVA

substrate (0.1% error bars); b) Simulation and analytically obtained stress along the x-axis

thorough the thickness of the structure, σxx(z), for a constant swelling strain of 1.28%; gray

shade represents the electrode layer and yellow shade represents the substrate.

2.2.5 Comparison between Computational, Experimental and Analytical Result.

Presented in Figure 2.4a are the experimentally measured E11 values for the

electrode coating plotted as a function of swelling strain of the substrate, fitted with the

simulated data with high confidence. The experimentally measured E11 appeared to be

slightly lower than the numerical values for swelling strains lower than 2%, when the

38

experimental and computational data are in good agreement with one another beyond the 2%

strain mark. The slight mismatch is believed to be caused by two factors: experimental errors,

and our assumption that the properties of the electrode are unchanged and constant regardless

of the immersion time in water. The Young’s modulus of the electrode was taken as constant

value once the electrode was wet. The 2.82 MPa was obtained after immersing the electrode

in water for 40 seconds. The PVA binder in the electrode also interacts with water, which

make the elastic modulus of the electrode –to some extent– depended on time. Consequently,

the elastic modulus of the swelled electrode is slightly higher than 2.82 MPa, which results in

a lower experimental E11 compared to computational value. The good fit between the

experimental and simulated data suggests that the conjecture of homogenous swelling is

realistic.

The agreement between computational (i.e. simulation) and analytical (i.e. classical

laminate theory) methods was verified by comparing the stress along the x-axis thorough the

thickness of the structure, σxx(z), for a constant swelling strain of 1.28%. As evident from

Figure 2.4b, the analytical and computational data are in good agreement; especially for the

electrode coating. The slight mismatch, close to the surfaces of the substrate, however, could

be due to the relatively large thickness of the substrate. The classical laminate theory is most

applicable and accurate for multilayer thin-films of few tens of micron thicknesses. Although

the 80 m thick substrate studied here is still within the defined limits of the classical

laminate theory, it is at the very upper limits of the thickness range. The theoretical analysis

shown here, however, could be a reliable and feasible method to obtain fast estimations on

the interfacial stress between coating and substrate layers of multilayer structures.

39

2.2.6 Stress/Strain Dependency on Electrode Thickness and Dimensions.

Swelling stress and strain is due to diffusion of solution (i.e. water in this study)

molecules into the layers of the structure. Therefore, thickness of each layer as well as the

overall dimensions of the structure are deemed influential on stress distribution at the

interface of layers and through the structure. In a previous study, (38) we qualitatively

demonstrated the influence of electrode thickness on structures disintegration. It was

observed that the beyond a threshold thickness, although the structure swells, the swelling

stress is not enough the physically breakdown the electrode layer; moreover, PVA does not

dissolve due to the lack of hydration. The effect of (increased) thickness can be due to three

main factors: 1) thicker layers require higher swelling stress for average Von Mises stress to

reach the critical value; 2) stress distribution –and consequently swelling strain– in thick

electrodes is not homogeneous; and 3) thicker electrodes have more uniform morphology

with less manufacturing defects, which translates into less stress accumulation and crack

formation points. Here, factors 1 and 2, the dependence of Von Mises stress and critical

swelling strain (swelling strain at which the average Von Mises stress reaches the yield

stress) on electrode thickness are investigated through computational means.

Von Mises stress and swelling strain were simulated for structures of different

electrode thicknesses (18 m to 42 m) and constant substrate thickness of 80 m. The

simulation data suggest that as the thickness of the electrode increases, although both Von

Mises stress and critical swelling strain increase, their rates of increase are not the same. As

demonstrated in Figure 2.5a, the critical swelling strain increases at a faster rate for thicker

electrodes, causing the difference between the two values non-linearly increase as the

40

electrode thickness is increased. Consequently, the swelling stress needed to yield the

electrode increase non-linearly with thickness.

Figure 2.5 a) Simulated Von Mises stress and critical swelling strain for electrodes of

different thicknesses laminated on 80 m substrate; b) Simulated Von Mises stress for

structures of constant thickness and different length-width ratios.

The influence of electrode’s dimensions on stress distribution was studied by varying

the length-width ratio of the electrodes from 2 to 0.5 while the thickness was kept constant.

Within the limits of this study, no correlation between the electrode’s dimensions and stress

distribution was observed (Figure 2.5b).

2.4 Conclusion

The swelling-induced stress is a major contributor to disintegration of transient

layered structures of dissimilar materials. Stress analysis of soft coating of lithium titanate

sprayed on PVA as the swelling substrate was studied by theoretical, experimental and finite

element modeling. Physical properties of the swelling PVA substrate as a function of time is

fully characterized and time dependence stress-strain profile at the interface is presented. The

computational data are in good agreement with the experimental results and the analytical

solution. The computational model was also utilized to study the influence of thickness and

41

geometry of the electrode layer on stress distribution through the structure. It was concluded

that the critical swelling strain and stress inhomogeneity increase as the thickness of the

electrode layer is increased. The relatively high critical swelling strain, and stress difference

at the interface of the thick electrode layers found to hinder disintegration while the

geometry, within the limits investigated in this study, does not appear to affect the stress

distribution. The computational model could be utilized to provide optimum design

parameters for physically transient electronics, including thickness ratios, electrode

geometry, critical swelling strain, and disintegration rate. The stress analysis method could

be extended to analyze interfacial stress distribution in a wide range of layered structures

with dynamic properties and help to better understand the physics of such dynamic

structures.

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Supplementary Information

{𝑁}𝑥𝑦 = [

𝑁𝑥𝑥

𝑁𝑦𝑦

𝑁𝑥𝑦

] = [000

] Equation 2.S.1

{𝑀}𝑥𝑦 = [

𝑀𝑥𝑥

𝑀𝑦𝑦

𝑀𝑥𝑦

] = [000

] Equation 2.S.2

{𝑁𝑠}𝑥𝑦 = [

𝑁𝑠𝑥𝑥

𝑁𝑠𝑦𝑦

𝑁𝑠𝑥𝑦

] = [(𝐴11 + 𝐴12) × ε𝑠

(𝐴12 + 𝐴22) × ε𝑠

0

] Equation 2.S.3

{𝑀𝑠}𝑥𝑦 = [

𝑀𝑠𝑥𝑥

𝑀𝑠𝑦𝑦

𝑀𝑠𝑥𝑦

] = [(𝐵11 + 𝐵12) × ε𝑠

(𝐵12 + 𝐵22) × ε𝑠

0

] Equation 2.S.4

[𝐴] = ∑ [𝑄]𝑘41 (𝑧𝑘 − 𝑧𝑘−1) Equation 2.S.5

[𝐵] =1

2∑ [𝑄]𝑘4

1 (𝑧𝑘2 − 𝑧𝑘−1

2) Equation 2.S.6

[𝐷] =1

3∑ [𝑄]𝑘4

1 (𝑧𝑘3 − 𝑧𝑘−1

3) Equation 2.S.7

{𝜀0}𝑥𝑦 = [𝜀𝑥𝑥0 𝜀𝑦𝑦

0 𝜀𝑥𝑦0 ]

𝑇= [

𝜕𝑢0

𝜕𝑥

𝜕𝑣0

𝜕𝑥

𝜕𝑢0

𝜕𝑦+

𝜕𝑣0

𝜕𝑥]

𝑇 Equation 2.S.8

45

{𝑘}𝑥𝑦 = [𝑘𝑥𝑥 𝑘𝑦𝑦 𝑘𝑥𝑦]𝑇

= [−𝜕2𝑤

𝜕𝑥2 −𝜕2𝑤

𝜕𝑥2 − 2𝜕2𝑤

𝜕𝑥𝜕𝑦 ]

𝑇

Equation 2.S.9

[𝑄𝑐] =𝐸𝑐

1−𝑣𝑐2 [

1 𝑣𝑐 0𝑣𝑐 1 00 0 1 − 𝑣𝑐

] Equation 2.S.10

[𝑄𝑠] =𝐸𝑠

1−𝑣𝑠2 [

1 𝑣𝑠 0𝑣𝑠 1 00 0 1 − 𝑣𝑠

] Equation 2.S.11

𝐸𝑐 = 2.82MPa, 𝑣𝑐 = 0.3, 𝐸𝑠 = (30 − 1265.625 × ε𝑠)MPa, 𝑣𝑠 = 0.43.

For εs=1.28 %, thickness of electrode coating is 24 m, thickness of the PVA

substrate is 80 m. Mid-plane is 28 m down the interface.

{𝜀(𝑧)}𝑥𝑦 = [0.0119 − 45.0239 × 𝑧0.0119 − 45.0239 × 𝑧

0] Equation 2.S.12

{𝜎𝑐(𝑧)}𝑥𝑦 = 4.028 × [0.0119 − 45.0239 × 𝑧0.0119 − 45.0239 × 𝑧

0] Equation 2.S.13

{𝜎𝑠(𝑧)}𝑥𝑦 = 24.210 × [0.0119 − 0.0128 − 45.0239 × 𝑧0.0119 − 0.0128 − 45.0239 × 𝑧

0] Equation 2.S.14

𝐶𝑒 =𝑚𝑝

𝑚𝑝+((1+𝜀)3−1)×𝑉𝑝×𝜌𝑤=

𝜌𝑝

𝜌𝑝+((1+𝜀)3−1)×𝜌𝑤 Equation 2.S.15

Ce is the equilibrium polymer concentration of the gel, weight ratio.

mp: dry polymer mass

ε: swelling strain

Vp: dry polymer volume

w: density of water

p: density of polymer

46

CHAPTER 3. INTERFACIAL FRACTURE IN TRANSIENT LAYERED

STRUCTURES

This chapter is based on a paper published in Advanced Engineering Materials. 2017 August

1; 19(8): 1700139.

Yuanfen Chen, Reihaneh Jamshidi, Wangyujue Hong, Nicole N. Hashemi, Reza Montazami

Abstract

Soft multilayer thin films have broad application in transient electronics. Strain-

mismatch-induced fracture is one way of achieving disintegration of electronics coated on

soft polymer substrate. Here, we studied swelling-mismatch-induced fragmentation of

physically transient electrodes with dynamic material properties. The fragment size of the

electrode coating layer as a function of initial defect prevalence and distribution was

investigated. The average fragment size was predicted using a combination of

experimentally-determined initial defect distribution and FEM-obtained swelling strain –

defect length curve. The predicted average fragment size was found to be in good agreement

with the experimental results. It was suggested by the findings that only large defects initiate

fragmentation; this concept can be used to control disintegration of physically transient soft

multilayer thin films by means of materials and microstructure design. The presented method

can be generalized to study fragmentation of soft multilayer structures.

3.1 Introduction

Multilayer structures consisting of dissimilar materials are pervasive, and have

attracted considerable interest and attention, since they are relevant to diverse engineering

applications including soft electronics, stimuli responsive morphing structures, (1) batteries,

(2) supercapacitors, (3) sensors, (4) actuators, (5-7) and many others. Besides fracture caused

47

by external forces, such as contact load, (8-10) cyclic load, (11, 12) and impact, (13) failures

of multilayer structures can result from strain mismatch of the constituent layers: problems

include thermal expansion mismatch, (14) hydro swelling mismatch, (15) strain gradient

caused by non-uniform electric field, (16, 17) and many others. (18, 19)The strain-mismatch-

induced fracture could occur in the form of layer delamination, surface cracking, or

combination of both, depending on the layers’ interfacial bonding strength, mechanism, and

material properties. Extensive research has studied the strain-mismatch-induced failure of

hard material multilayer structures, such as interfacial delamination of thermal barrier

coatings (TBC), (20-26) thermal fatigue of transistors, (27) cracking of ceramic actuators,

(16) and supercapacitor. (28) Fracture of hard coating printed on soft substrate has also

received considerable interest. Lu and colleagues found that the failure of polymer-supported

un-annealed nanocrystalline metal films was concomitant with grain growth, strain

localization, and interfacial debonding. (18) In another study, they found that the failure

mode of stiff islands on deformable substrate, either channeling cracks or debonding,

depends on the island size and thickness. (19) However, strain-mismatch-induced fracture of

soft multilayer thin films (soft coating sprayed or printed onto soft substrate) is rarely

reported. (29-31)

Soft multilayer thin films are widely applied in transient electronics and swelling-

mismatch-induced fragmentation of the coating layer speeds up the re-dispersion/dissolution

process. The initial flaw distribution (or pre-crack distribution) plays a key role in the

fracture of the material, affecting the fracture strength, final fragment size, and fatigue life. In

real applications, obtaining the initial defect distribution is not always possible. In addition,

the distribution of flaw size and location in real cases could complicate the strength analysis.

48

Some researchers have simulated brittle fracture with controlled pre-existing flaws, such as

flaws with designed length and location, (32) or uniformly distributed flaws.(33) Other

researchers predicted the fatigue life of bulk metals using equivalent initial flaw size (EIFS)

distribution. The EIFS was obtained using either the back-extrapolation method,(34-38) or

the Kitagawa–Takahashi diagram. (39, 40) In some applications, such as multilayer thin

films, the initial flaw distribution in the coating can be estimated if the substrate is

transparent. Again, strength analysis is rather complicated with defect distribution in real

coating, but a fragment size analysis of surface cracking based on real initial defect

distribution is feasible. Surface spalling of coating from swelling strain mismatch is essential

for the fast disintegration of physically transient electronics, and the fragment size will affect

the subsequent redistribution into the fluid. (15) To the best of our knowledge, few studies

have investigated how the actual initial defect distribution affects the fragment size of the

material, especially the soft coating in multilayer thin films. (41-43) Controlled

fragmentation is possible if the relation between the initial defect distribution and the

fragment size is well understood.

Stress and fracture analysis of soft multilayer thin films resulting from strain

mismatch could be challenging because of the dynamic material properties of the substrate

and coating. In our group’s previous study, (44) we investigated the interfacial stress of the

physically transient soft multilayer thin films caused by swelling mismatch, incorporating the

dynamic material property of the swelling substrate. Here, we have studied the fracturing of

such soft multilayer thin films under similar conditions. Results are presented for a

prototypical electrode (lithium titanate electrode active material printed on poly(vinyl

alcohol) (PVA) substrate), whose coating could disintegrate when the substrate swells

49

beyond some limit in water. (15) The relationship between initial defect distribution and

fragment size was investigated and confirmed. The method presented here could be applied

to analyze strain-mismatch-induced fracture of soft multilayer thin films with dynamic

material properties.

3.2 Materials and Methods

3.2.1 Materials and Sample Preparation

Poly(vinyl alcohol)substrates with printed electrodes were fabricated according to

our previous work. (15) Briefly, to fabricate substrates, PVA (Mw: 61,000 g mol−1, 98.0 -

98.8% hydrolyzed, Sigma Aldrich) was used as received. 1.0 g PVA, 0.1 g sucrose, and 50

µL of 1 M aqueous hydrochloric acid solution were added to 20 mL of DI (Deionized) water.

The solution was then stirred at 70 ℃ for 4 hours, cooled down to ambient temperature, then

cast onto a plastic mold, and allowed to dry in ambient conditions for 24 hours. The dried

PVA substrate was then carefully peeled off the mold; the resultant PVA films had thickness

of 80 5 m. To fabricate electrodes, Li4Ti5O12 (LTO) powder (MTI Corporation), Super P

carbon (MTI Corporation), and PVA were mixed at weight ratio of 5:1:1 in DI water. The

mixture was then stirred for 2 hours to create a homogeneous slurry. Then, the slurry was

printed on the PVA substrate at a thickness range of 24 - 40 m to get prototypical electrodes

of different thickness; or printed on an aluminum foil substrate to a thickness of 100 m,

dried and peeled off to obtain free-standing electrodes. A G25 (0.2 mm tip) precision gravity

feed nozzle at constant pressure of 20 psi was used along with vinyl masks to spray-print the

electrodes.

50

3.2.2 Mechanical Characterization of Electrode Layers

The nominal stress-strain curves for the free-standing electrodes were obtained using

a dynamic mechanical analyzer (DMA) (Mettler Toledo, DMA-1). A tensile mode with force

ranges from 0 – 0.15 N at the rate of 0.03 N min-1 was applied at room temperature. The

sample is 10 mm in length, 2.5 mm in width, and 0.08 mm in thickness.

3.2.3 PVA swelling strain and resultant electrode strain measurements

Pristine PVA substrate and prototypical electrodes were immersed into water to

measure the total strain in x-axis E11 of the electrode coating as a function of swelling strain.

The prototypical electrode was immersed 2 seconds earlier than the pristine PVA substrate,

to account for the delay in water penetration into the PVA substrate from the electrode

coating side. The E11 of the electrode and the corresponding swelling strain of the PVA

substrate was compared to the simulation result.

3.2.4 Characterization of Initial Defect Distribution, Crack Propagation, and Resultant

Fragment Size

An optical microscope (AmScope) equipped with a charge-coupled device (CCD)

was used to take images of the electrodes before, during, and after immersion. These images

were used to obtain defect distribution, crack propagation, and fragment size, respectively.

Images before and during immersion were taken with a 10 lens for high magnification,

while images during and after immersion were taken by 4 lens to increase field of view.

MATLAB’s image processing module, with a red channel threshold of 0.3, was then used to

convert digital images to black-and-white images for further processing. White areas of the

image were indicative of defects or cracking, while black areas represented more uniform,

defect-free coating. Defect size and prevalence, as well as fragment size, were then

determined from the monochrome images using MATLAB.

51

3.2.5 FEM simulation

An FEM (finite element method) model was developed in ABAQUS to simulate

substrate swelling in a substrate-electrode layered structure, and to predict the consequent

strain on the electrode layer, as well as the initiation and propagation of cracks throughout

the electrode layer. 2D symmetry (about x- and y-axes) of the structures allows for

simplification of the model to quarter symmetric. The PVA substrate and the electrode

coating were connected by cohesive contact. The variation of the PVA substrate properties as

a function of swelling strain and true stress-strain curve of the electrode layer were applied

for the analysis. A defect-free sample was assumed to obtain the strain of the electrode layer

as a function of PVA swelling strain and verify the true stress-strain curve. An electrode

layer with through-thickness defects was applied for simulation of crack initiation and

propagation. The defect length ranged from 2.5 to 35 m. The crack initiated when the max

principle strain of the electrode layer reached 17%. Critical swelling strain (swelling strain at

which crack starts to propagate) as a function of defect length was recorded.

3.3 Results and Discussion

3.3.1. True Stress-Strain Curve of the Electrode Layer

True stress-strain curve of the electrode layer is necessary for simulation of crack

propagation. To obtain the true stress-strain curve of the electrode layer, Hollomon’s

constitutive law (Equation 3.1) was applied; where σt and εt are the true stress and true

strain, respectively, K is the strength coefficient, and n is the strain-hardening exponent.

𝜎𝑡 = 𝐾𝜀𝑡𝑛 Equation 3.1

According to the Considère criterion (45) , the necking occurs when the true strain

reaches the strain-hardening exponent. Then strain-hardening exponent, n, can be obtained

from Equation 3.2. To make the stress curve, as deduced from Equation 3.1, to intersect the

52

true stress-strain curve when necking initiate, the strength coefficient K is defined as in

Equation 3.3:

𝑛 = ln(1 + 𝜀𝑒) Equation 3.2

𝐾 =𝜎𝑒(1+𝜀𝑒)

[ln(1+𝜀𝑒)]ln(1+𝜀𝑒) Equation 3.3

where εe and σe are the nominal strain and normal stress at the initiation of necking,

respectively.

The nominal stress-strain curves for the free-standing electrodes were obtained using

a dynamic mechanical analyzer. Based on the nominal stress-strain data, Hollomon’s

equation was then applied to calculate the true stress-strain curve. The nominal stress-strain

curves and the true stress-strain curve are shown in Figure 3.1a, where n=0.02882 and

K=0.06912. True strain beyond 5% is extrapolated for studying the crack initiation. As

shown by the plot, stress increases almost linearly with the true strain after necking.

To verify the true stress-strain curve in Figure 3.1a, the curve was applied in a FEM

model to obtain the swelling induced strain of the electrode layer as a function of the PVA

swelling strain, εs, then the simulation outcomes were compared and verified by the

experimental result. In the simulation, the PVA substrate was set to swell up to 10%, and the

corresponding strain of the electrode layer was recorded. For experimental result, the

prototypical electrodes were immersed into water to measure the total strain in x-axis E11 of

the electrode coating as a function of swelling strain. As shown in Figure 3.1b, the

simulation and the experimental results agree. For εs < 3%, the experimentally-obtained

values of the electrode strain were found to be slightly lower than those obtained from

simulation. This mismatch could be due to the slight inward bending of the electrode upon

exposure to water. The agreement of the experimental and simulation results validates the

53

true stress-strain curve of the electrode coating. The true stress-strain curve was applied in

the FEM simulation discussed in the following section.

Figure 3.1 a) Nominal (experimentally obtained) and true (theoretically driven) stress-strain

curves of the electrode coating; b) Swelling strain of the substrate and the resultant strain on

the electrode layer (along the x-axis).

3.3.2 Surface Crack Predominated Failure

To characterize the fracturing of the electrode, a series of images of the electrode

during submersion were taken and analyzed. As shown in Figure 3.2a, surface cracking was

found to be the predominant fracture form of the prototypical electrode. When the PVA

substrate swelled, cracks initiated from the pre-existing defects in the electrode layer. These

cracks propagated and merged, resulting in small electrode pieces. The fragments are then re-

dispersed in water when the PVA substrate was almost dissolved. No obvious interfacial

delamination was observed. The predominant surface failure could be because of the

relatively strong interfacial bonding compared to that inside the electrode coating, as

illustrated in Figure 3.2b. When the electrode slurry was sprayed onto the PVA substrate, the

solvent (i.e. water) in the slurry wetted the surface of the PVA substrate. Once the multilayer

thin film dried, exclusive bonding formed between the PVA substrate and the electrode’s

active material at the interface. PVA is also used as the binder insider the electrode layer,

54

thus that the active material inside the electrode layer was also bonded by PVA. However,

the concentration of PVA inside the electrode layer was much less compared to that at the

interface; thus, the bonding inside the coating layer is weaker. As a result, spalling of the

coating occurs instead of interfacial delamination.

Figure 3.2 a) Series image showing the surface cracking of the electrode layer (images are

taken at 10-second intervals). A few defects are circled to show the propagation of the initial

defects. b) Schematic showing the bonding inside and between electrode and PVA substrate

3.3.3 Crack Propagation and the Resultant Fragmentation

The size of the electrode coating layer fragments that result from surface cracking

affect the subsequent redistribution of the active electrode material into the fluid. The

following sections will analyze the fragment size that results from the surface cracking and

its dependency on the initial defect distribution.

Studies have demonstrated that the stress and energy required to initiate cracks from

defects is inversely proportional to the defect length (46-48). Griffith first reported that the

maximum tension concentration at a crack tip is proportional to the square root of the length

of the crack. On a later study, Rivlin and Thomas reported that the tearing energy at the

55

incision of elastomer is proportional to the crack diameter. These findings suggest that larger

defects require a smaller swelling strain to start propagate compared to smaller defects. To

understand the correlation between the defect length and the critical swelling strain, straight-

line, through-thickness defects with different lengths were simulated in ABAQUS using

XFEM (extended finite element method). The simulated maximum principle strain and Von

Mises stress for defects of 10 m length at the moment the crack starts to propagate (t = 0.00

s) are presented in Figure 3.3a and Figure 3.3b, respectively. Insets show magnified views

of the crack and stress accumulation around the crack tip. This simulation aimed to gain

insight on the general trend of critical swelling strain as a function of defect length. Figure

3.3c plots critical swelling strain as a function of defect length. The maximum defect length

in the electrode coating was about 35 m, which would start crack propagation at 5.86% of

swelling strain. The critical swelling strain was almost identical for defect lengths ranging

from 35 m to 25 m. As the defect length decreased, down to 10 m, the critical swelling

strain increased slowly. When the defect length decreased further, the critical swelling strain

increased with a much sharper slope than before. Defects with length of 2.5 m would start

crack propagation at 12% of swelling strain. The critical swelling strain – defect length curve

(Figure 3.3c) appears to show that critical swelling strain for large defects is much smaller

than for small defects. The experimental and simulation data suggest that as large defects

propagate, they merge with small defects before small defects start to propagate

independently. One large defect has one crack path, while small defects contribute to the

zigzag pattern of the large defect crack path. Thus, the number of large defects define the

number of fragments. Here, suggested by the experimental data, the large defects are defined

as those longer than 5.5 m, whose critical swelling strain for crack initiation is 1.5 times of

56

the largest defect length’s. The 5.5 m threshold was obtained from the critical swelling

strain – defect length curve. The large-defect-effect conclusion is verified in the following

paragraph.

Figure 3.3 a) Maximum principle strain distribution through an electrode, with details

shown around the crack; b) Von Mises stress distribution, with details shown around the

crack; c) critical swelling strain – defect length curve.

57

Figure 3.4 a) Image of typical defect distribution, before (left) and after (right)

monochromic; b) Image of typical fragments, before (left) and after (right) monochromic; c)

Predicted fragment area vs. experimental fragment area.

To confirm the large-defect-effect conclusion, the predicted average area of the

fragments based on the large-defect-effect was compared with the actual average area. As

shown in Figure 3.4a, images of the electrode coating were analyzed using MATLAB to get

the number and length (maximum length) of the through-thickness defects. The number of

defects longer than the 5.5 m large-defect threshold was applied in calculation of the

predicated average area. Large defects constitute approximately one third of the total number

of defects. The calculation method and further details are provided in the supplementary

document, Equation 3.S.1 (in the Supplementary Information section). Similarly, images of

the fragments were analyzed to get their actual average area (Figure 3.4b, Equation 3.S.1,

and Figure 3.S.1, Equation 3.S.1 and Figure 3.S.1 are in the Supplementary Information

section). Figure 3.4a was taken at 10× magnification to estimate the defect length, while

Figure 3.4b was taken at 4× magnification for a wider field of view. The comparison of the

predicted average area and the actual average area is shown in Figure 3.4c; detailed data

could be found in supporting document Table 3.S.1. Fragments with areas under 0.1 mm2 are

58

from very thin electrode layer, while areas above 0.4 mm2 are from relatively thicker

electrode layer. The linear fitting of the log-log data, using Origin, resulted in a slope value

of 1.049, and an R-Square of 0.993. The linear relationship between the predicted average

area and the actual average area supports the large-defect-effect that the number of large

defects defines the number of fragments. Small defects merge during the propagation of large

defects, so fragments have uneven edges. Fragment size is determined by the initial defect

distribution, especially that of large defects.

3.4 Conclusion

Surface cracking is the predominant fracture form in the prototypical soft electronics.

In this study, the correlation between the initial defect distribution and predictions of the

fragment size in layered structures is established. The agreement of the predicted average

fragment area and the actual average fragment area led to the “large-defect-effect”

conclusion, suggesting that the number of large defects (length >5.5 m) dictates the number

of fragments, and therefore the average fragment area. Cracks initiate at large defects much

earlier than at small defects; thus, small defects typically merge into the propagating large

defects, rather than initiating cracks independently. The conclusion that the preexisting

characteristics of the layers can be used as a means to determine and control the mechanics of

fragmentation of the layered structure as a whole, can be applied to design, tuning, and

programming transiency and disintegration of printed layered soft electronics.

59

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63

Supplementary Information

𝑝𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑟𝑒𝑎 =1.08 𝑚𝑚2

��

4×2

Equation 3.S.1

Where 1.08 mm2 is the actual area of the image, �� is the average number of large

defects (length higher than 5 pixel, which is 5.5 m) of all the images taken for the same

electrode coating in different place. Since each piece is formed by an average of four cracks,

and every crack is shared by two pieces, ��

4 × 2 represents the number of fragments.

𝑎𝑐𝑡𝑢𝑎𝑙 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑟𝑒𝑎 =𝑡𝑜𝑡𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑙𝑙 𝑡ℎ𝑒 𝑝𝑖𝑒𝑐𝑒𝑠 𝑖𝑛 𝑜𝑛𝑒 𝑖𝑚𝑎𝑔𝑒

𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑖𝑒𝑐𝑒𝑠 Equation 3.S.2

The method to get the total number of the pieces in Equation 3.S.1 is shown in

Figure 3.S.1. The fragments are shrunk two pixels inward (toward the center) for

convenience in counting them. This ensures that two pieces that were originally connected by

a narrow bridge would be counted as two independent pieces, e.g. the pieces in the red circle

in Figure 3.S.1. The pieces at the edge of the image would be taken as half pieces when

counted.

Figure 3.S.1 Shrinkage of the fragments toward their centers

64

Table 3.S.1 Predicted average piece area and the actual average piece area

Sample

number 1 2 3 4 5 6 7 8 9 10 11

Predicted

size

(mm2)

0.0267 0.0339 0.0446 0.0498 0.0559 0.0589 0.0682 0.0848 0.454 0.513 1.231

Actual

size

(mm2)

0.0287 0.0389 0.0411 0.0500 0.0455 0.0607 0.0657 0.0761 0.485 0.564 1.556

65

CHAPTER 4. MECHANICS OF INTERFACIAL BONDING IN DISSIMILAR

SOFT TRANSIENT MATERIALS AND ELECTRONICS

This chapter is based on a paper published in MRS Advances. 2016 Jan: 1-1.

Reihaneh Jamshidi, Yuanfen Chen, Kathryn White, Nicole Moehring, and Reza Montazami

Abstract

Soft transient electronics of polymeric substrates and silver-ink electronics are studied

for correlated mechanical-electrical properties. Experimental and predictive finite element

analysis are used to understand, explain and predict delamination, cracking, buckling, and

failure of printed conductive components of such systems. An active transient polymer

system consisting of poly (vinyl alcohol) and sodium bicarbonate is introduced that results in

byproducts (alkaline and bubbles) when undergoing transiency. These byproducts are

facilitated to control and expedite transiency of the electronic components based on

redispersion of metallic nano/micro materials. Complete mechanical and electrical

characterization of such systems is reported.

4.1 Introduction

Recent advances in design of materials has enabled design and synthesis of transient

materials and structures capable of maintaining stable mechanical and electrical properties

for a desired and preset amount of time; and, undergo fast and complete degradation and

deconstruction once transiency is triggered. Transient devices have several potential uses,

including those in biomedical devices and military applications. With ever increasing

demand for wearable gadgets, for personal or medical applications, soft electronics and soft

complex structures have become more common. There have been substantial research efforts

focused on soft electronic (1, 2) such as batteries (3) and epidermal electronics (4), ionic

66

materials (5-10) and soft actuators (11-14); and considerable studies on transient materials

and electronics (15-18). Soft transient electronics is a relatively new field of science and

technology emerged from combinations of transient materials science and soft electronics

technology. One major concern with soft transient electronics, however, is the typical

mismatch in mechanical properties of polymeric substrate and metallic electronic

components. Such mismatch is very likely to result in failure of the electronic components

under mechanical strain. Polymeric substrates coated with metallic conductive inks are

considered polymer-metal systems. Polymer-metal interactions in majority are rare and thus

difficult to engineer and control. There have been efforts to elucidate physical and chemical

features of polymer-metal interactions to gain an understanding of the bonding (19-22) and

behavior of such systems under stress (14, 23, 24). However, most efforts have been focused

on polymer-bulk metal interactions rather than metals deposited from solution such as

metallic inks. (25, 26) Polymers typically have low surface tension, which is a hindering

factor for formation of bonds. (27, 28) Polymer surface modification through chemical

reactions (29-31) and plasma treatment (32-37) have been studied and reported to improve

polymer-metal bonding. An area of exploration pursued by few researchers involves placing

a circuit on a pre-strained, elastomer substrate. (38-41) The conductive, connecting paths

may take form in a number of shapes from a straight to wavy/serpentine and zigzagged.

Positive results are reported from this process, noting increased stretchability of the

connecting, conductive paths with little change in resistance.

Initial studies on transient electronics were focused on passive transiency where the

substrate and electronics (organic or inorganic) both undergo chemical reactions (typically

hydration) with the solvent (typically water or phosphate buffered saline (PBS)) and dissolve

67

over time. (42, 43) Two major concerns with such systems are 1) slow dissolution of metals,

and 2) very limited domain of soluble metals. Previously we have introduced transient

electronic systems based on redispersion of particulate electronic components, (44) this

approach provides new opportunities for inclusion of non-soluble metals in design of

transient electronics. Solvent-triggered transiency is very common and strongly dependent on

chemical and physical interactions between the solvent and the device, as well as those

within the device itself, among its constituent components. Such interactions can be utilized

as a means to program and control the mechanics and extent of transiency in complex

transient electronics. We have investigated transiency dependence on substrate (24) and

electronic components (23); mechanics of transiency of prototypical transient circuits have

demonstrated strong dependence on the transiency characteristics of the substrate. To

overcome slow transiency rate of metals, integration of active transiency approach is

suggested. Active transient materials undergo multiple interactions with the solvent and

produce byproducts that facilitate dissolution and/or re-dispersion of materials.

In this work, we have studied mechanical-electrical correlations in pre-strained

flexible electronics and have quantified the effect of pre-straining on the lifespan and failure

of such systems. We have developed a predictive finite element analysis (FEA) model to

understand stress distribution throughout soft electronics, and to predict location, extent and

probability of delamination, buckling, cracking and crack penetration in such systems. We

have also designed, implemented, and fully characterized a material system that exhibits

active transiency by undergoing secondary reactions in acidic solvents. This system produces

micro-bubbles when triggered, bubbles expedite transiency of the system by facilitating

redispersion of conductive materials.

68

4.2 Materials and Methods

All materials were used as received, solutions were prepared fresh for each set of

experiments. Experimental results are an average of multiple (at least three) trials.

4.2.1 Substrates

Polydimethylsiloxane (PDMS) elastomer kit (Sylgard 184) was obtained from Dow

Corning and used according to curing and casting guidelines provided by the manufacturer.

Briefly, the elastomer and crosslinker were mixed at 10:1 ratio and stirred vigorously. The

mixture was then poured into the casting molds and left undisturbed for 30 minutes to degas

completely. Molds were then heated at 85 °C for 20 minutes to fully crosslink. The cured

PDMS films of 0.6 mm thickness were then peeled off and cut into strips of desired

dimensions.

Transient films were prepared by dissolving 1 g of Poly (vinyl alcohol) (PVA) (Mw:

61,000 g mol−1, 98.0–98.8 mol% hydrolyzed) (Sigma Aldrich) in 20 ml of DI water; and

adding various amounts (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 g) of sodium bicarbonate

(NaHCO3) (SBC) to obtain desired concentrations. The mixture was then stirred at 140 °C

for 3 hours, cooled to room temperature, and then casted into molds and let dry over 48

hours. The dried PVA-SBC films of approximately 100 µm thickness were then carefully

peeled off of the molds, stored under dry environment, and cut into desired dimensions upon

use.

4.2.2 Conductive Ink

Fast drying silver paint (Ted Pella) and acetone were mixed at 1:1 ratio. The

dispersion was then shaken on a vortex for several hours to obtain a uniformly dispersed

conductive ink.

69

4.2.3 Pre-straining

The PDMS films were mounted on two micro-positioners using in-house made clips.

The micro-positioners were initially 4 inches apart. Straining was obtained by precisely

changing the distance between the micro-positioners.

4.2.4 Conductive Paths

To fabricate conductive paths on the substrates, conductive ink was doctor bladed

over a 70 μm thick vinyl stencil mask of desired pattern attached to the substrate. The mask

was removed before the ink fully dries. Resulted conductive patterns also measured to have

the same 70 μm thickness as the mask.

4.2.5 Transiency

Transiency of the films was measured by placing each sample inside an aluminum-

mesh enclosure and submerging it into a water-diluted HCl solution for 30, 180, 300, and 600

seconds. The mass ratio of the films before and after submersion were measured and

recorded to determine the extent of transiency. Accounting for solvent intake, to accurately

measure the mass of the films after submersion, they were left to dry for a minimum of 24

hours prior to weighting.

4.2.6 Mechanical and Electrical Characterization

Mechanical properties of all samples were measured, monitored and recorded using a

dynamic mechanical analyzer (Mettler Toledo, DMA-1) in tensile testing mode. To study

electrical properties, clamps were modified and isolated from electrical contact with the

conductive paths on the samples to prevent short-circuit and instrument error. Electrical

properties were monitored and recorded through a potentiostat (Princeton Applied Research,

VersaSTAT 4) equipped with low amp module.

70

4.2.7 Imaging and Microscopy

Imaging was conducted using a benchtop scanning electron microscopy (Joel. JCM-

6000 NeoScope) and a handheld high-magnification digital microscope (Dino-lite, 5MP

Premier 400×).

4.3 Results and Discussion

4.3.1 Strain Dependence of Electrical Properties

Mismatch in mechanical properties of the polymeric membrane and inorganic

conductive paths results in fracture or delamination of the conductive paths which

consequently causes the flexible device to fail after few or several mechanical cycles. In our

previous work (44) we have investigated limitations posed by this issue. In the current study

we have further investigated the delamination concept and have quantified some of the

factors that hinder performance of flexible devices. In this particular study we have used

PDMS membranes as the substrate instead of our usual PVA-based membranes. This

selection of materials is mainly due to the high elasticity of PDMS that allows testing of the

extreme mechanical conditions. PVA, on the other hand, is used for transiency-oriented

studies.

Straining a stretchable device (substrate and conductive paths) results in formation of

micro cracks, some of which are recovered upon removal of stress; (44) one approach to

minimize formation of cracks is to deposit electronic components on pre-strained substrates.

This approach has some limitations and drawbacks that we have studied in this work. The

major drawback is delamination of the conductive structures and formation of buckles.

Presented in Figure 4.1 are optical and SEM micrographs of buckled and delaminated

conductive structures deposited on PDMS substrates.

71

Figure 4.1 Left: Optical digital microscope image of an extreme case of buckling. Silver-ink

pattern (500 m wide) has delaminated from a polymeric substrate. Right: Scanning electron

microscopy image of delamination resulting from 5% pre-straining.

Buckles are likely to form along the printed conductive paths because the silver-ink

does not adhere strongly to the PDMS surface. As compressive forces are applied to the

conductive path (removal of pre-strain) delamination occurs and buckles form. The

amplitude of these buckles depends on the extent of pre-straining; average height of

delamination vs. pre-straining is shown in Figure 4.2a. When pre-straining is small, the

resulting buckles have small amplitudes and smooth undulation. These buckles remain fairly

uniform, occurring solely at the peaks and troughs of the conductive path until pre-straining

reaches 20%. At this threshold, the buckles gather height and become less uniform. Since the

magnitude of the compression force increases when the PDMS is pre-strained more, more

substrate delamination occurs. The portions of the printed silver-ink between the peaks of

the arcs begin to delaminate as well, creating wider and taller buckles. Therefore, there is a

direct correlation between the extent of pre-straining and the height of the buckles. The more

the substrate is pre-strained, the greater the compression force on the conductive patterns and

the greater the average buckle height are.

72

Micro-cracking of the conductive path arises from a buildup of stress in a localized

area and typically only occur when the substrate is strained past its pre-strained length. When

the PDMS substrate is strained to a threshold length at which the micro-cracks penetrate

across the printed silver-ink pattern and prevent electrical conductivity, the resistance of the

system becomes infinite. This threshold length is the same as pre-straining extent for samples

pre-strained beyond 5%. Electrical properties were monitored for pre-strained samples and

were used as a means to determine and quantify failure. For samples pre-strained by 0% and

5%, conductivity remains for few millimeters beyond the pre-strained length (4% and 2%

respectively); this phenomenon may be due to the presence of less initial stress in the system.

The conductive patterns withstand repeated mechanical cycling between strained and relaxed

states within this threshold range and maintain a constant electrical conductivity. Number of

cycles before failure of the system decreases as the pre-straining is increased, mainly due to

the higher stress experienced in each cycle. Correlation between pre-straining and cycling is

demonstrated in Figure 4.2b.

Figure 4.2 a) Extent of delamination is directly related to the extent of pre-straining.

Buckles become less uniform on substrates pre-strained beyond 20%; b) Pre-strained systems

are more vulnerable to mechanical cycling. Each sample was cycled to its pre-strained strain

until the electrical conductivity was spiked to infinite.

73

4.3.2 Predictive FEM Modeling

4.3.2.1 Stress distribution and delamination

To better understand and study delamination and buckling in flexible electronic

systems, an FEM model was developed and used to predict stress distribution and

delamination throughout the structure. A cohesive contact was assumed to form between the

substrate and the printed electronic component that results in formation of an elastic layer

between the two. The elastic layer is very thin compared to the substrate and conductive

pattern. The thickness of the printed electronic component, in form of a spherical cap, is

assumed to be 3 m in this model; the thickness of the thin elastic layer is assumed to be 3

nm. The elastic modulus of the thin elastic layer is usually some value between the elastic

moduli of the substrate and the printed ink and was estimated, based on preliminary

measurements, to be 0.1 GPa. Presented in Figure 4.3 is the FEM modeling of stress

distribution throughout a typical structure, a membrane and a conductive-ink spot.

Figure 4.3 Stress distribution (MPa) through the substrate and printed pattern.

Delamination of conductive paths from the substrate was calculated through

analytical and numerical modeling, based on the stress distribution throughout the system.

Presented in Figure 4.4 is the simulation of delamination at the very edge of a conductive

spot on the substrate. Mechanical characteristics of substrate and conductive coating were

deduced from experiment and used in development of the FEM model.

74

Figure 4.4 Simulated delamination at the edge of a conductive spot. Stress distribution (top)

is matched with the extent of delamination (bottom). Node numbers correspond to the mesh

at the edge of the model.

Figure 4.5 a) Original and b-c) processed images of defects through a polymer thin-film; d)

Stress around the defect points is increased significantly. Only stress above 0.35 MPa is

shown in the simulation.

75

4.3.2.2 Cracking

Polymer thin-films have several nano/micro defects that can influence stress gradient

through the thin-film. These defects influence and initiate crack formation throughout the

conductive patterns. Considering particulate structure of conductive patterns which are

deposited from conductive inks, cracks initiated at these defects can somewhat easily

penetrate throughout the conductive structure. Analysis of such structures, using optical

imaging (Figure 4.5a) and image processing (Figure 4.5b-c), we have modeled stress

accumulation on the conductive film. As shown in Figure 6d the stress accumulates around

the defects which consequently initiates cracks and results in transiency of the conductive

film. Size of defect is a defining factor on whether or not the defect results in a crack; and,

the size of resultant pieces after crack penetration.

4.3.3 Active Transiency

Material systems with active transiency mechanism were designed and implemented.

The most basic form of transiency, passive transiency, is based on direct chemical

interactions between the transient system and the solvent. Active transiency mechanism,

however, takes advantage of a more complex transiency approach in which the basic system-

solvent interactions result in byproducts which are effective in controlling and expediting the

transiency rate. In this work we have designed a materials system which results in an alkaline

environment upon transiency. The basic environment could help control transiency of the

electronic components of the system. The silver-ink used in this study contains nano/micro-

silver particles held together by a polymer resin. The transiency of such system is mainly

based on redispersion of the nano/micro-materials and dissolution of the resin. We have

discussed this multistep transiency in our recent studies. (23, 44)

76

PVA-SBC substrates of varying compositions with silver-ink patters printed on them

were studied for their transiency behavior. Inclusion of SBC in the composite enables a

secondary set of reactions between the solvent and substrate that mainly result in bubbling of

the substrate while decomposing. The bubble formation is extremely helpful with transiency

of the silver-ink component. Moreover, integration of SBC filler into PVA matrix results in a

general decrease in the films’ elastic moduli. Presented in Figure 4.6 is the influence of the

SBC content on overall elastic moduli of the substrates. It was observed that the elastic

modulus decreases with SBC concentration up to approximately 30%. For concentrations

above 30% the elastic modulus tends to increase.

Figure 4.6 Elastic moduli of PVA-SBC films are inversely proportional to the concentration

of SBC up to 30%, above which the correlation becomes directly proportional.

Figure 4.7 PVA-SBC 10% film reached 82% transiency in a water-diluted hydrochloric acid

solution in 600 seconds. Bubbling of the film is evident in the two middle images.

77

To study active transiency on PVA-SBC systems, they were submerged in diluted

solution of hydrochloric acid for an allotted time period as noted in the experimental section

above. Upon submersion, the films underwent rapid degradation and significant dissolution

within a 600-second time period. Increasing SBC concentration to up to 20% substantially

increased the rate and extent of transiency. Samples with 10 and 20% of SBC exhibited a

transiency time constant of 470 seconds that is significantly faster than the 0% control

sample with time constant of 530 seconds. Other samples either did not reach the transiency

threshold (24) of 63.2% within the 600 seconds or reached it at time constants close to 600

seconds. Both 10 and 20% samples reached 82% transiency in 600 seconds, when the control

reached 76% and all other samples reached below 75%. Analysis of transiency data suggests

that 1) active transiency mechanism is proven effective, and 2) the optimum concentration of

SBC filler in PVA matrix is 10-20% which provides faster transiency and low elastic

modulus; both of which are desirable in most flexible, stretchable, and transient electronics.

Figure 4.1depicts a demo of a PVA-SBC 10% system dissolved in a water-diluted

hydrochloric acid solution. It can be seen in the figure, that the rate of degradation of the

circuit is dependent on the degradation of the substrate. The circuit does not begin to break

apart until the substrate begins to bubble and swell.

4.4 Conclusion

In the present work we have studied mechanical and electrical properties of soft

transient electronics. Correlations between mechanical strain, electrical properties, and

failure of soft electronics is studied in detail when electronic components are fabricated on

pre-strained substrates, with attention to mechanical cycling and extent of pre-straining.

Predictive FEA model was used to predict stress distribution and delamination in such

systems. Active transiency of such systems is investigated using a polymer composite that

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exhibits secondary reactions (bubbling and change of pH) when undergoing transiency. The

secondary reaction was designed to facilitate redispersion of metal nano/micro-particles in

the silver conductive ink. It was concluded that a PVA-SBC circuit with silver-ink

electronics containing 10-20% SBC exhibits fastest (time constant of 470 seconds) and most

extensive (82%) transiency.

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CHAPTER 5. TRANSIENT LITHIUM-ION BATTERY

This chapter is based on a paper published in Journal of Polymer Science Part B. 2016 Oct

15; 54(20):2021-7.

Yuanfen Chen, Reihaneh Jamshidi, Simge Çınar, Emma Gallegos, Nastaran Hashemi, Reza

Montazami

Abstract

Transient Li-ion battery based on polymeric constituents is presented, exhibiting two-

fold increase in the potential and approximately three orders of magnitude faster transiency

rate compare to other transient systems reported in the literature. The battery takes advantage

of a close variation of the active materials used in conventional Li-ion batteries and can

achieve and maintain a potential of >2.5 V. All materials are deposited form polymer-based

emulsions and the transiency is achieved through a hybrid approach of redispersion of

insoluble, and dissolution of soluble components in approximately 30 minutes. The presented

proof of concept has paramount potentials in military and hardware security applications.

5.1 Introduction

Unlike conventional electronics that are designed to last for extensive periods of time,

a key and unique attribute of transient electronics is to operate over a typically short and

well-defined period; and undergo fast and, ideally, complete self-deconstruction and vanish

when transiency is triggered. Transient electronics have a wide range of potential

applications including those in healthcare, biomedical devices, environmental

sensing/monitoring, green electronics, military and homeland security, to name a few

examples. In the very recent years, researchers have developed a wide range of transient

83

electronic devices capable of performing a variety of functions and responsive to a variety of

triggering mechanisms including exposure to light, (1) heat, (2) or solvent (often aqueous).

(3-7) What is common among all these reported transient devices is the need for an external

power source. The power is either supplied by inducting coils from a very close distance,(8)

or from external power supplies. (2) To realize autonomous transient electronic devices,

similar to conventional ones, a transient battery is essential. Thus far, there has been limited

efforts in design and construction of transient batteries, mainly due to the lack of soluble

proper materials. Jimbo et al. reported swallowable batteries based on Zn and Pt electrodes

and ceramic porous separator; maximum potential of 0.42 V and current of 2.41 mA were

achieved. (9) More recently Kim et al. reported edible water activated sodium batteries based

on melanin electrodes where a potential of 0.6-1.06 V and current of 5-20 μA, depending on

design, was achieved. (10, 11) An intrinsically transient battery capable of environmental

resorption was first reported by Yin et al. where Mg anode and biodegradable metals (Fe, W

or Mo) cathodes were used with a transient polymer casing; potentials ranging from 0.45 V

to 0.75 V (depending on cathode materials) were reported. (12) To date, all reported transient

batteries have shortcomings compare to their conventional counterparts; uncompetitive

potential, current, stability and shelf life are among the top challenges in construction of a

practical transient battery that can supply enough power to run a common electric circuit.

The low potential and power density in transient batteries are mainly due to the use of non-

optimal electrode materials because of their solubility. One other significantly important

limitations of transient batteries reported to date is low transiency rate, which is a result of

slow chemical reactions between the constituent materials and the solvent. High transiency

rates are specially anticipated in military and hardware security applications.

84

To design a transient battery capable of supplying enough power and undergoing fast

transiency, new approaches toward the materials and structural design are necessary.

Lithium-ion battery technology is a well-established, mature and commercialized technology;

here, we are integrating a variation of this technology with our new approach toward

transient electronics, which is ultimately based on a hybrid approach of physical redispersion

of insoluble materials, and chemical dissolution of soluble ones. In our previous studies, we

have reported electrical (13) and mechanical (14) properties of such hybrid systems. The

transient primary Li-ion batteries we report here incorporate this technology and are consist

of electrodes comprise of active materials similar to those in conventional Li-ion batteries

while swelling force of the substrate/casing materials is utilized to introduce transiency

through defragmentation and redispersion of the active materials.

5.2 Materials and Methods

5.2.1 Materials

Polyvinyl alcohol (PVA) (Mw: 61,000 g mol−1, 98.0–98.8 % hydrolysis), sucrose,

lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC) and dimethyl carbonate

(DMC) were purchased form Sigma Aldrich (Sigma Aldrich, MO, USA) and used as

received. LiCoO2, (LCO) powder, Li4Ti5O12 (LTO) powder and carbon black were purchased

from MTI (MTI Corporation, CA, USA) and used as received. Commercially available short

fiber cellulose-based tissue was used as separator. Silver paint was purchased form Ted Pella

(Ted Pella, inc. CA, USA) and diluted with acetone (1:1 vol).

5.2.2 Substrate

1.0 g PVA, 0.1 g sucrose, and 50 µL of 1 M aqueous HCl solution were added to 20

mL of DI water. The solution was stirred at 70°C for 4 hours, then cooled down to room

temperature and stirred for 2 additional hours. The clear solution was then casted onto a

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plastic mold (with14 mm × 1.2 mm × 0.1 mm grooves for current leads) and dried at ambient

conditions for 24 hours. The dried PVA film was peeled off of the mold carefully and used as

the substrate for cathode and anode.

5.2.3 Active Material

To prepare Cathode active material 1.0 g PVA was dissolved in 20 mL DI water and

stirred at 70°C for 4 hours. 5.15 g LCO powder and 1.0 g carbon black were added to the

PVA solution and stirred for 2 hours to get uniform emulsion. Anode active material was

prepared in the same manner with the exact same ratios and procedure as cathode, where

LCO was substitute by LTO.

5.2.4 Current Lead

1 g PVA was dissolved in 20 mL DI water as described above, carbon black was

added at 3:2 wt. ratio and stirred for 2 hours to obtain a uniform emulsion. Silver paint was

diluted with acetone (1:1 vol. ratio). Current lead at anode was fabricated by spraying 60 μm

thick layer of silver in the groove of PVA substrate; then, an approximately 50 μm thick layer

of carbon black-PVA emulsion was spray coated on top of the silver layer at slightly wider (~

500 μm) area to cover the silver coating. Stencils were used at both steps. Current lead at

cathode was fabricated by spraying a 110 μm thick layer of carbon black-PVA emulsion

through a stencil in the groove of PVA substrate.

5.2.5. Electrodes

Approximately 25 μm thick layers of anode and cathode active materials were spray

coated through stencils over the current leads on PVA substrates to form the electrodes. The

areas covered by active materials were smaller than the substrate area to allow PVA edge for

packing.

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5.2.6 Packaging

Cathode, anode and cellulose-based porous membrane were brought together to form

a stack. Uncoated edges of PVA substrates were dampened slightly to bond and seal the stack

to form a battery cell.

5.2.7 Electrolyte

1 M solution of LiPF6 in 1:1 mixture of EC: DMC was used as the electrolyte.

Electrolyte (25 μL) was injected into the packed battery cell by a syringe and pierced area

was sealed. All steps involving electrolyte were carried out in a glovebox under nitrogen

environment.

5.2.8 Electrochemical Testing

The electrochemical testing and impedance measurements were carried out on a

VersaSTAT-4 potentiostat (Princeton Applied Research). The battery was charged and

discharged at 2.7 V and 1.0 V, respectively, and discharge current densities of 10, 20 and 50

μA cm-2. The internal resistance of the battery cell was measured at frequencies between

1.0E5 Hz and 0.1 Hz and a potential difference (ΔV) of 10 mV. The Z-view software was

used to obtain the equivalent electric circuit and fit experimental data.

5.3 Results and Discussion

5.3.1 Transiency of Single Electrode

Due to its ease of control over transiency rate and fabrication, PVA and PVA

composites were used as binder, substrate and casing materials. Abundance of hydroxyl

groups in PVA allows high equilibrium swelling indices, up to 153% in water. (15)

Hydrolysis results in disentanglement of polymer chains and leads to swelling of the polymer

membrane (Figure 5.1a).

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Table 5.1 Transiency test on samples with varying area density of the active materials

Group 1 Group 2 Group 3

Fragment Average Area ~0.77 mm2 ~1.82mm2 ~4.4 mm2

Electrode-Substrate

Delamination

Yes No No

Outcome Transiency Sample curled Sample curled

Figure 5.1 a) Hydrolysis of PVA which results in swelling of the polymer membrane, PVA

repeat unit is identified on the top left of the figure; b) half reactions take place at the

electrodes; c) swelling and d) curling of electrodes with high area density layers of active

materials, deconstruction and transiency did not occur; e) sequential images of transience of a

cathode electrode with low area density layer of active materials, the swelling force is large

enough to deconstruct the electrode; f) dissolution of soluble and redispersion of insoluble

constituent materials leads to full transiency, the yellow arrow points at a scale bar drawn in

the background. Note that scale-bar on (f) is different.

88

In PVA-sucrose composites, presence of fast-dissolving sucrose expedites the process

by opening up voids in the structure that enhance penetration of water. Subsequently, PVA

chains are eventually decomposed and the membrane dissolves in the solvent. Lithium cobalt

oxide, LiCoO2, (LCO) and Li4Ti5O12 (LTO) were used as the ultimate cathode and anode

active materials, respectively. Cathode and anode half reactions(16) are shown in Figure

5.1b. PVA and carbon black were used as binder and filler in LCO and LTO to form water-

deconstructible nanostructures. Presence of PVA and carbon black in LCO and LTO

increases uniformity of the electrodes while also enhancing redispersion of the materials

when triggered; on the other hand, efficiency of the electrodes drops due to the lack of

electron conductivity in PVA. Active materials (LCO/LTO-PVA-carbon black aqueous

emulsion) were deposited via spray coating over a stencil mask. Systematic studies of

electrode components revealed that the thickness of active materials coating is the defining

factor on whether or not the electrode fully disintegrates, where no dependence on the

thickness of the PVA-sucrose substrate was observed. Cathode electrodes with active

materials area densities ranging from 1 mg cm-2 to 2.5 mg cm-2 (categorized in three groups:

1) 1.2 ± 0.1, 2) 1.6 ± 0.1 and 3) 2.3 ± 0.2 mg cm-2) were fabricated on PVA:sucrose (10:1 wt

ratio) substrates of thicknesses ranging from 30 μm to 80 μm (categorized in six groups, 10

μm increments). PVA at this particular composition was selected for its fast and controllable

transience rate based on our previous work. (17) Regardless of the thickness of substrate,

when exposed to the solvent, electrodes with active materials’ area densities of 1.5 mg cm-2

or higher only curled and/or swelled but did not disintegrate nor exhibit transiency (Figure

5.1c and d). For active materials area densities of 1.3 mg cm-2 or lower (group 1), the

electrodes swelled, and the swelling force was large enough to break and deconstruct the

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whole electrode, regardless of the thickness of the substrate. As the solvent molecules

penetrate through the substrate, swelling force is generated and increased to a point large

enough to physically break the active materials layer (Figure 5.1e). Swelling of PVA and the

associated force which breaks the active materials into pieces are essential to the transiency

of the electrode; an identical sample of active materials (same area density) on polyvinyl

acetate substrate (does not swell but dissolve in water) only curled and did not break or

exhibit transiency (Figure 5.S.1 in the supplementary Information section). After initial

swelling, the substrate is eventually dissolved and resorbed into the aqueous environment;

subsequently, PVA binder in the remaining pieces of the active materials also dissolves

which enhances redispersion and transiency of the active materials in the solvent (Figure

5.1f). Summarized in Table 5.1 are the experimental data collected and outcomes for variety

of cathodes structures. Results revealed that electrodes with small area density are essential

for construction of a transient battery. Similar study on anodes resulted in comparable

conclusions.

5.3.2 Transiency of Battery Cells

Single cell batteries were constructed by fabrication of current leads and active

materials on PVA-based substrates; substrates were featured by grooves to hold current

leads. A cellulose-based porous membrane (short fibers thus water decomposable) was used

as the separator and electrolyte holder between the electrodes; and, battery was formed and

sealed by attaching the uncoated margins of the PVA-based electrodes together. Electrolyte

was then injected into the separator and the hole was sealed. Presented in Figure 5.2a is a

schematic representation of cross section of a single cell battery. Electrodes were fabricated

with characteristics of group 1 electrodes as presented in Table 5.1. Carbon black and silver

paint were used for fabrication of current leads; while the silver paint has higher electrical

90

conductivity, only carbon black was used as the cathode current lead to prevent oxidation of

silver in cathode when under applied potential (charging). The single cell batteries were

charged and discharged couple of times before the transiency testing to ensure proper

functionality of the battery. Heat generated during this process strengthen the bonding among

the materials and slightly hindered transiency rate compare to that of single -unheated-

electrodes. Thus, although active materials used in this study are most suitable for secondary

cell batteries, we report the constructed batteries as primary cell that require an initial

charging and can undergo few charging/discharging cycles in practice. Yet, it must be noted

that cycling the cell will result in chemical reactions among constituent elements that hinder

transiency.

To evaluate transiency of the single cell batteries (Figure 5.2b), DI water at room

temperature was used as a trigger (solvent). As expected from single electrode experiments,

the single cell swelled, and the electrodes cracked moments after exposure to the solvent; yet,

it took a longer time for substrate to dissolve and active materials to redisperse.

Demonstrated in Figure 5.2c are sequential images of the transience of the battery. The

substrate swells first witch results in physical deconstruction of the electrodes. Initially the

current leads are less impacted as the thickness of those areas is greater than that of the rest

of the electrode. The anode cracked into larger pieces while the cathode deconstructed into

much smaller ones (see Figure 5.2c at 5.5 minutes). Eventually the substrate dissolved

completely and electrode remains were released to roam freely in the solvent. Dissolution of

the PVA binder in the active materials resulted in dispersion of milli/micrometer size

particles in the solvent. After approximately 30 minutes, most of the battery was either

dissolved or redispersed. Remains were left for another 16.5 hours to redisperse; although the

91

remaining pieces got smaller over time, the change was incremental between the 1st and 17th

hours. The overall transience behavior was similar to that of the single electrodes yet

expanded over a longer period of time. The extended time was mainly due to 1) increase of

the overall thickness of the battery compare to single electrode; and 2) induced bonding due

to the heat generated during charging.

Figure 5.2 a) Schematic representation of the cross section of the battery cell; b) a transient

battery cell; c) sequential images of transience of a battery cell, the yellow arrows point at

scale bars drawn in the background. Note that scale-bars on the last two images are different.

5.3.3 Battery Performance

The performance of the single cell transient batteries was evaluated at different

discharge current densities of 10, 20 and 50 μA cm-2; and, cut-off voltages of 2.7 V (charge)

and 1.0 V (discharge). As demonstrated in Figure 5.3a, the battery has comparable

92

performances at different discharge current densities. The discharge voltages start at

approximately 2.6 V, slowly decrease to 1.8 V, and then drop to 1.0 V at sharp slopes. The

battery cell had the highest discharging specific capacity (~2.27 mAh g-1) and efficiency

(12.5%), and total capacity of 0.571 μAh when discharged at 20 μA cm-2. Typically, the

discharging capacity decreases as the discharging current increases; in our study, however,

the battery has higher discharging specific capacity at 20 μA cm-2 compared to 10 μAcm-2

mainly because of the relatively high internal resistance of the battery. At 50 μA cm-2

discharging current density the reaction time is deemed to be the limiting factor for higher

discharging specific current; thus, the 20 μA cm-2 discharging current density exhibited the

highest discharging specific capacity for this cell configuration. Shown in Figure 5.3b is the

discharging potential as a function of time.

The electrochemical reactions occurring at the electrodes/electrolyte interfaces

determine the electrochemical performance of the battery. To evaluate the parameters

involved in the electrochemical processes, the electrochemical impedance of the battery was

measured over a wide frequency range (Figure 5.3c,d); an equivalent circuit was introduced

based on Randles cell model (18) to fit the data of electrochemical impedance Nyquist plot.

The impedance at low frequency range (between 0.1 and 1.0 Hz) is between 10 and 2.5 k

(Figure 5.3d).

93

Figure 5.3 a) and b) discharging behavior of transient battery cells at different current

densities; c) and d) electrochemical impedance of a battery cell and the equivalent electric

circuit (c-inset); e) a battery cell connected to a digital multimeter, and f) transient battery

cell utilized to power a calculator (battery and solar panel are disconnected from the

calculator’s circuit)

94

The Nyquist plot presented in Figure 5.3c consists of a depressed semicircle at high

frequency, which is an indication of non-uniform electrode/electrolyte interfaces, and a linear

spike at low frequency. The equivalent circuit (Figure 5.3c, inset) consists of two resistors:

Rs corresponding to the ohmic resistance at high frequency and Rct corresponds to the charge

transfer resistance of the electrodes, a Warburg element (W) which accounts for the diffusion

reactance at low frequencies, and a constant phase element (CPE) corresponding to the double

layer charging (Cdl) at the porous electrode/electrolyte interfaces; CPE is used instead of an

ideal capacitor because of the depressed shape of the Nyquist semicircle. (19) Results suggest

an Rs value of approximately 242 , which includes the electrolyte resistance as well as the

ohmic resistance of the battery cell and an Rct of approximately 327 . The relatively large

value of Rct is mainly due to the poor connection between the electrodes and electrolyte

layer. While both non-uniform interfaces and poor connections between the battery

components are responsible for relatively poor electrochemical performance of the presented

battery cell, they are not fundamental issues and could be improved by utilization of

advanced manufacturing techniques.

Shown in Figure 5.3e is a battery cell connected to a digital multi-meter, showing

2.53 V potential that double the potential of transient electrodes reported in the literature. (9-

12) A single battery cell is capable to supply enough power to power a basic calculator for a

short period of time; the threshold voltage to power the calculator is ~1 V. Presented in

Figure 5.3f is a battery cell powering a four-function calculator. The battery and solar cell of

the calculator were disconnected from the device’s circuit and the power was only supplied

by the transient battery. The battery cell was able to power the calculator for approximately

15 minutes before the screen started to fade. Fabricating electrodes with higher area density

95

or connecting several battery cells in parallel can significantly improve the performance of

the battery for higher power consuming applications; yet, higher area density electrodes take

longer time to deconstruct. Also, as discussed earlier, more advanced fabrication techniques

can be used to improve the overall efficiency and performance of battery cells, while

transiency rate can be controlled via optimization of the nano/microstructure of electrodes

and substrate.

5.4 Conclusion

Transient Li-ion battery was fabricated and tested for the transient property and the

electrochemical performance to investigate the battery’s potential to be an onboard power

source for transient electronics. The results showed that electrode layer sprayed onto PVA

film can disintegrate when the PVA film swelled in water. The disintegration of the electrode

layer was mainly determined by the area density of the electrode layer. The transient property

of the battery was similar to that of the single electrode. The disintegration time of the

transient Li-ion battery was designed to be within one day, a timescale much smaller than

that of the previous designed battery. The output potential of the transient Li-ion battery was

designed to be between 1.0 V-2.5 V. Based on the literature previously discussed, single

battery output voltage lower than 1.1 V. The relatively high output potential enables the

transient Li-ion battery to drive transient electronics that operating at high voltage. With a

fast disintegration time and high output voltage, the transient Li-ion battery reported here is a

promising candidate for on-board transient energy storage element that can be applied in the

non-biological field such as environmental sensor.

96

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Transience of Metastable Poly(phthalaldehyde) for Transient Electronics. Advanced

Materials. 2014;26(45):7637-42.

2. Park CW, Kang S-K, Hernandez HL, Kaitz JA, Wie DS, Shin J, et al. Thermally

Triggered Degradation of Transient Electronic Devices. Advanced Materials.

2015;27(25):3783-8.

3. Hwang SW, Tao H, Kim DH, Cheng H, Song JK, Rill E, et al. A physically transient

form of silicon electronics. Science. 2012;337(6102):1640-4.

4. Hwang SW, Kim DH, Tao H, Kim Ti, Kim S, Yu KJ, et al. Materials and Fabrication

Processes for Transient and Bioresorbable High‐Performance Electronics. Advanced

Functional Materials. 2013;23(33):4087-93.

5. Hwang SW, Park G, Cheng H, Song JK, Kang SK, Yin L, et al. 25th Anniversary

Article: Materials for High‐Performance Biodegradable Semiconductor Devices. Advanced

Materials. 2014;26(13):1992-2000.

6. Irimia-Vladu M. “Green” electronics: biodegradable and biocompatible materials and

devices for sustainable future. Chemical Society Reviews. 2014;43(2):588-610.

7. Irimia-Vladu M, Głowacki ED, Voss G, Bauer S, Sariciftci NS. Green and

biodegradable electronics. Materials Today. 2012;15(7):340-6.

8. Hwang SW, Huang X, Seo JH, Song JK, Kim S, Hage‐Ali S, et al. Materials for

bioresorbable radio frequency electronics. Advanced Materials. 2013;25(26):3526-31.

9. Jimbo H, Miki N. Gastric-fluid-utilizing micro battery for micro medical devices.

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10. Kim YJ, Chun S-E, Whitacre J, Bettinger CJ. Self-deployable current sources

fabricated from edible materials. Journal of Materials Chemistry B. 2013;1(31):3781-8.

11. Kim YJ, Wu W, Chun S-E, Whitacre JF, Bettinger CJ. Biologically derived melanin

electrodes in aqueous sodium-ion energy storage devices. Proceedings of the National

Academy of Sciences. 2013;110(52):20912-7.

12. Yin L, Huang X, Xu H, Zhang Y, Lam J, Cheng J, et al. Materials, Designs, and

Operational Characteristics for Fully Biodegradable Primary Batteries. Advanced Materials.

2014;26(23):5.

13. Jamshidi R, Çinar S, Chen Y, Hashemi N, Montazami R. Transient bioelectronics:

Electronic properties of silver microparticle-based circuits on polymeric substrates subjected

to mechanical load. Journal of Polymer Science Part B: Polymer Physics. 2015;53(22):1603-

10.

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14. Çınar S, Jamshidi R, Chen Y, Hashemi N, Montazami R. Study of mechanics of

physically transient electronics: A step toward controlled transiency. Journal of Polymer

Science Part B: Polymer Physics. 2016;54(4):517-24.

15. Krzeminski J, Molisak-tolwinska H. The Structure of Water-Swollen Poly(Vinyl

Alcohol) and the Swelling Mechanism. Journal of Macromolecular Science: Part A -

Chemistry. 1991;28(3-4):413-29.

16. Aldon L, Kubiak P, Womes M, Jumas J, Olivier-Fourcade J, Tirado J, et al. Chemical

and electrochemical Li-insertion into the Li4Ti5O12 spinel. Chemistry of Materials.

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Physically Transient Insulating Materials as a Potential Platform for Transient Electronics

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18. Randles JEB. Kinetics of rapid electrode reactions. Discussions of the faraday

society. 1947;1:11-9.

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1999;3(7-8):397-405.

Supplementary Information

5.S.1 Electrochemical analysis

The CPE was calculated by Equation 5.S.1, where Q is the adjustable parameter, and

ω is the angular frequency. CPE acts as an ideal capacitor for n = 1, for the system reported

here, n was calculated to be 0.76. The reason, as mentioned in the article, could be the

unsmooth electrodes/electrolyte interfaces. The Warburg impedance was calculated by

Equation 5.S.2, where R is the resistance, τ is the time constant, and p is the exponent to

describe the capacitive character of the impedance; p was calculated to be 0.68, which

suggests that the diffusion process is a finite length process, not an ideal infinite length

process. Summarized in Error! Reference source not found. are calculated values of

equivalent circuit components.

𝑍 =1

𝑄(𝑖𝜔)𝑛 Equation 5.S.1

98

𝑊 = 𝑅𝐶𝑜𝑡ℎ (𝑖𝜔𝜏)

(𝑖𝜔𝜏)𝑝 Equation 5.S.2

Table 5.S.1 Electronic component values calculated from the equivalent circuit model

Rs [] Rct [] R [] τ p Q [-1] n

242.2 326.8 8766 0.53 0.68 9.17E-7 0.76

5.S.2 Non-Swelling Substrate:

Polyvinyl acetate (PVAc) was casted onto a plastic mold and dried in ambient

condition to form a thin-film of approximately 100 μm thickness. Dry PVAc composite film

was peeled off of the mold and used as the substrate. Active materials, similar to that in

group 1, was deposited on the films and the film was soaked in DI water. The electrode did

not swell; thus, active material did not crack nor deconstruct, Figure 5.S.1.

Figure 5.S.1 Cathode (group 1) fabricated on PVAc substrate. The substrate does not swell

in water; thus, the active area is intact.

99

CHAPTER 6. A SOFT TRANSIENT ALL-ORGANIC SENSOR

Yuanfen Chen, Reihaneh Jamshidi, Reza Montazami

Abstract

All-organic intrinsically soft transient electronics which could accommodate stress

and strain through their molecular structure and morphology have great potential application

in biomedical areas. The key and challenge is to apply functional polymers. In this study, an

all-organic transient electrode with conjugated polymer E-jet printed onto a water-soluble

substrate is studied and presented. The electrode’s electronic properties dependence on

dynamic load shows fast time response between 0.05 Hz -3 Hz, and a reversible stretching

threshold of 3% strain. Transiency study shows that substrate will dissolve completely in

water, while conjugated polymer electrode remain intact. The substrate-less electrode forms

perfect contact with skin and was applied as an ultra-thin epidermal strain sensor. The all-

organic electrode presented in this study can be applied as flexible sensor as a whole, or as

ultra-thin temporary epidermal sensor with substrate-less electrode.

6.1 Introduction

Transient electronics are designed to operate over a pre-defined period, then undergo

self-deconstruction and vanish when transiency is triggered. These devices, especially soft

transient electronics, have a wide range of potential applications in biomedical research (1-5)

and environmental monitoring. (6, 7) In recent years, researchers have developed a variety of

soft transient electronic devices ranging from electronic components, (8-13) to integrated

systems. (14, 15) Typically, soft transient electronics are fabricated by deposition of an active

inorganic functional layer on a soft organic substrate. These inorganic functional materials,

100

mostly metallic nano/micro-materials, form rigid and brittle layers; therefore, in many cases

the topology of the functional layer is designed such that it accommodates strain. (16, 17)

Such designs fall into two main categories. The first is to intentionally pre-strained the

substrate or/and the printed structure before regular use, thus surface waves and buckles

generated in the first cycle would accommodate subsequent cycles of stretching. (18, 19) The

second is to laterally or topographically pattern the thin-films such that the global tensile

strains are converted to local bending strains. (20-23)

An alternative approach is to implement all-organic materials. The key concept, and

challenge, is to utilize functional polymers, which can accommodate strain through their

molecular structure and morphology, while (partially) maintaining their electrical properties.

(16) Application of intrinsically soft materials could simplify the fabrication, as well as

enhance mechanical compliance and robustness. (24, 25) In addition, the organic nature of

the all-polymer electronics contributes more advantages including oxide-free interfaces, (17)

and tunability by synthesis. (26, 27) Soft all-organic electronics that use intrinsically soft

functional materials have received increasing attention in recent years. Liang and colleagues

reported an elastomeric polymer light emitting device using a polyphenylenevinylene

derivative as the emissive. (28) Lipomi’s group investigated the effects of structural

parameters of a series of poly (3-alkylthiophenes) on their mechanical properties, (25) then

demonstrated a stretchable organic solar cell which could be conformally bonded to a

hemispherical surface. (24) Bao’s group studied electronic and morphological attributes of

poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) on stretchable

poly(dimethylsiloxane) (PDMS) substrates. (21) In another study, they reported a highly

stretchable and conductive polymer by adding a variety of enhancers to PEDOT:PSS. (29)

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Combing intrinsically soft organic functional material with transient technology to

achieve soft transient electronics will further expands potential applications of transient

electronics, especially in biomedical areas. (30, 31) For instance, soft “tissue-like” transient

epidermal sensor will form conformal contact with skin, and provide more reliable results.

(32, 33) Yet the application of intrinsically soft functional materials in soft transient

electronics remains scarce. (17)

In this work, we reported the application of conductive conjugated polymers for soft

transient organic electronics printed using the electrohydrodynamic jet (E-jet) printing

technique. PEDOT:PSS is applied as functional layer for its desired properties such as high

conductivity, (29, 34-36) chemical stability, (34, 37, 38) printability, (39-42) and non-

cytotoxicity. (43, 44) Polyethylene oxide (PEO) film is used as substrate for its

hydrophilicity, biocompatibility, and flexibility attributes. (44) E-jet printing is applied as the

fabrication technique because of its high resolution, (45, 46) low cost, (47), and drop-on-

demand printing (42, 48) characteristics. In this study, mechanical and electrical properties

along with transiency of an all-organic electrode are investigated. Proof-of-concept soft

transient strain/stress sensors are demonstrated and characterized. In particular, functionality

of a substrate-less electrode (with substrate layer completely dissolved in water) as ultrathin

epidermal sensor is demonstrated.

6.2 Materials and Methods

6.2.1 Materials

Polyethylene oxide (PEO) (Mw: 400,000 gmol-1), poly (3, 4-ethylenedioxythiophene)

polystyrene sulfonate (PEDOT: PSS) (3.0% - 4.0% wt. in H2O), were purchased from Sigma

Aldrich (St. Louis, MO, USA). Dimethyl sulfoxide (DMSO) was purchased from Fisher

102

Chemical (Lenexa, KS, USA). Conductive silver ink (Pelco, 187) was purchased from Ted

Pella (Redding, CA, USA).

5.2.2 Printing of PEDOT: PSS Conductive Patterns on PEO Substrate

1.0 g of PEO was added to 20 mL of Deionized (DI) water and stirred at 50 °C for 3

hours. The clear solution was then cooled down to room temperature, casted onto a plastic

mold, and dried at ambient conditions for 24 hours. Cured film was peeled off of the mold

and used as substrate for printing conductive patterns. The thickness of the substrate are

approximately 60 m.

An in-house made E-jet printing system (Figure 6.1a) was used for printing of

PEDOT:PSS conductive patterns (electrodes) on PEO substrate. As shown in Figure 1a, the

printer consists of two main parts: a steel nozzle and a moving stage. The nozzle has an inner

diameter of 210 m, with controllable standoff distance from the substrate. The standoff

distance was maintained 0.2 mm for all samples discussed in this report. The substrate was

placed on a stage that could move horizontally. The velocity of the stage was kept at 2.5 mm

s-1 during printing. A DC potential difference of 2.1 kV was applied between the nozzle and

the substrate. To prepare the printing ink, PEDOT:PSS was diluted into 0.6 wt.% -0.8 wt.%

in DI water, then 2.5 % volume ratio of DMSO was added to enhance the electron

conductivity of PETDOT:PSS. (29, 49) Ink was supplied into the nozzle through a 1 mL

syringe loaded onto a syringe pump (Kent Scientific Corporation) through Teflon tubing. The

flow rate was maintained at 2 L min-1. For each electrode, printing was repeated five runs to

obtain the suitable thickness and conductivity.

103

To study the morphology of the printed patterns, a scanning electron microscope

(SEM) (JCM-6000PLUS NeoScope Benchtop, JEOL) was used. Electrodes were freeze-

fractured in liquid nitrogen to prepare samples for cross-section imaging.

6.2.3 Mechanical Characterizations

Both static and dynamic mechanical properties of the all-organic electrodes were

characterized using a Dynamic Mechanical Analyzer, (DMA-1, Mettler Toledo). DMA was

loaded with tension clamps with a 10 mm sample opening. Static testing was performed on

force-controlled mode for a range of 0 – 6 N at a rate of 0.2 N min-1. Dynamic testing was

performed under displacement-controlled mode, at 3 Hz, 2 Hz, 1 Hz, 0.1 Hz, and 0.05 Hz

frequencies to examine response time, and a displacement range of 100 m – 400 m to test

the reversible stretching threshold. All mechanical characterizations were isothermal. Clamps

were insulated to prevent short circuit when electrical measurements were taken.

6.2.4 Electrical Characterizations

Electrical properties of printed electrodes were investigated under different

conditions, including under stress, during degradation, and when used as epidermal sensors.

Measurements were taken with a Potentiostat (VersaSTAT 4, Princeton Applied Research).

Bias potential of 1 V was applied during the experiments.

6.2.5. Degradation

Transiency of the all-organic electrodes in DI water was studied using an ordinary

charged coupled device (CCD) camera and a digital microscope (AM7013MZT4, 5MP,

Dino-Lite Premier). To better monitor electrical properties of the electrodes during

degradation, electrodes were degraded on a glass slide with a DI water reservoir. Epidermal

sensors were degraded in a larger container filled with DI water.

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6.3 Results and Discussion

6.3.1 Printed All-Organic Electrodes

The all-organic electrodes are fabricated by printing PEDOT:PSS ink onto a PEO

substrate using a customized E-jet printer (Figure 6.1a). The width of the electrodes ranges

from 30 m to 220 m, depending on the stand-off distance d, flow rate Q, and the applied

potential V. To obtain continuous electrode pattern, d, Q, and V were set to be 0.2 mm, 2 L

min-1, and 2.1 kV, respectively. A typical electrode array is shown in Figure 6.1b, with

length of 17.5 mm, width of 180 m, and average thickness t of 10 m. The resistance R of

each electrode is approximately 20 k. Deduced from Equation 6.1, the conductivity of the

printed PEDOT:PSS electrode is calculated to be 486 S m-1. Conductivity of the PEDOT:PSS

electrode could be increased by post-treatment of the film, such as annealing the film at

elevated temperature, or washing the residual of PSS chains with water/methanol. (36, 50)

The printed electrodes were not post-treated in this study, for the reason that PEO substrate is

not physically/ chemically stable in water/methanol and high temperature environment.

Presented in Figure 6.1c, SEM image of an all-organic electrode (top-view) shows

that the printed electrode consists of three bands, a granular structure band in the center, and

two transition bands on the sides. The granular structure band is thought to be ink jetting

area, of which each grain is formed by an ink drop. One evidence is that grains size of

approximately 50 m is smaller than the needle diameter, which agrees with the E-jet

printing characteristic. (48, 51, 52) In addition, the grain shapes agree with the mechanism

that ink drop would deform from round shape to elliptic shape towards moving direction of

substrate. The transition region is anticipated to be the result of wetting/drying dynamics of

printing process and interactions between the aqueous ink and water-soluble PEO substrate

105

during printing process. Figure 6.1d shows that cross-section of printed electrode has an arc-

shape, which could be explained by the drying dynamics.

σ =1

ρ=

𝑙

𝑅 ∙ 𝐴=

𝑙

𝑅 ∙ 𝑤∙ 𝑡 Equation 6.1

Figure 6.1 a) Schematic of E-Jet printer; b) Image of all- polymer electrodes taken with

CCD camera; c) Image of electrode taken with SEM; d) Image of cross section area of

electrode taken with SEM.

6.3.2 Strain - Electric Resistance Correlations

Flexible electronics will be subject to tension and compression in its regular use. In

particular, epidermal sensors will experience static or periodic stress exerted by the skin,

depending on specific applications. It is important to establish correlations among the applied

stress, strain, and electrical resistance of flexible electronics. In this study, electrical

resistance of all-organic electrodes is characterized under static and dynamic Stress.

106

6.3.2.1 Static Stress

Resistance of an electrode similar to that presented in Figure 6.1b was examined in

response to strain due to a force of 0 – 6 N increasing at a step size of 0.2 N min-1. Load

condition at each step was treated as static. As presented in Figure 6.2a, resistance was

measured and monitored as a function of PEO substrate strain. An inset graph is added to

better show the increasing trend for strain below 5%. Resistance remains approximately

unchanged for up to 2% strain; and increases gradually within 6.5% strain; then increases

with a sharp slope thereafter. In the meanwhile, as shown in the stress-strain curve in Figure

6.2b, PEO substrate remains linearly elastic within 2% strain, then turns into plastic, and

yields at about 6% strain. Figure 6.2c and Figure 6.2d show images of the electrode after

7% strain. At this strain, cracks initiated in the PEO substrate, and micro-cracks in the

PEDOT:PSS electrode. Considering the fact that PEDOT:PSS electrodes consist of nano-

scale regions of conducting PEDOT-rich areas surrounded by insulating PSS-rich areas, (53-

55) the electrical property of the organic electrode in response to static load is explained as

the following. Within elastic region of the substrate (ε < 2%), strain inside the printed

electrode is accommodated by PSS chain rearrangement within the PEDOT-rich regions,

resistance remains almost unchanged. As the strain increases further (2% < ε < 6%),

disconnections between PEDOT-rich regions increase gradually when the PEO substrate

undergoes plastic deformation, thus resistance increases gradually. When substrate starts to

yield (ε ≈ 6.5%), cracks formed on the substrate initiate formation of micro-cracks within the

PEDOT:PSS electrode; which, in turn, results in a dramatic increase in resistance.

107

Figure 6.2 Relative resistance changed as a function of static strain; b) Stress- stain curve of

the PEO substrate; c) and d) SEM image of electrode after stretched for 7%, cracks and

micro-crack are marked by yellow arrows.

6.3.2.2 Dynamic Load

Electrical resistance dependence on dynamic load was investigated at different

frequencies and strains to evaluate the time response and reversible stretching threshold of

the all-organic electrodes. Frequency - resistance correlations are presented in Figure 6.3a.

Stacking plots show the normalized resistance fluctuations of the electrode for 200 m

displacement (ε = 2%) at frequencies of 3 Hz, 2 Hz,1 Hz, 0.1 Hz, and 0.05 Hz, respectively.

Correlation between frequency and resistance at this frequency range is not found. Overall

behaviors of relative resistance fluctuations are almost identical for the five different

108

frequencies. The time response performances validate the potential applications of the

presented all-organic electrode as an epidermal sensor, from monitoring pulse and respiration

activity (frequencies from 1 Hz – 3 Hz); to monitoring finger and arm movements (usually

lower than 1 Hz).

Figure 6.3 a) Resistance fluctuations of a polymer electrode for 200 m displacement at 1

Hz, 0.1 Hz, and 0.05 Hz frequency, respectively; b) Resistance fluctuations at 1 Hz for 100,

200 and 300 m, respectively. c) Flexible electrode bend around a glass tube of 1cm

diameter. d) Resistance fluctuations at 1 Hz for 400 m.

Resistance fluctuations of electrodes at 1 Hz frequency for different (100, 200 and

300 m) displacements (corresponding to ε = 1%, 2% and 3%, respectively) are presented in

Figure 6.3b. Correlation between strain and resistance is obvious. As strain increases, the

109

relative resistance fluctuations increase. Presented in Figure 6.3d is the relative resistance

fluctuation at 1 Hz for 4% strain. It could be seen that strain-resistance correlation is not

reversible for 4%. In each cycle, resistance fluctuations shift towards higher values compared

to the preceding cycle. The irreversibility might be contributed by the plastic deformation of

the PEO substrate. Figure 6.3c shows an array of electrodes wrapped around a glass vial to

demonstrate the flexibility. The electrodes are detached and rewrapped several times, and no

change in the electrode performance was observed.

6.3.3 Transient Property

PEO substrate is water-soluble, while printed PEDOT:PSS electrodes are not. We

demonstrated in previous studies, that insoluble active electrodes might undergo

disintegration by swelling-induced stress of the substrate. (12, 56-59) Here, transient

properties of the all-organic electrodes in water are studied and presented in Figure 6.4a. As

showed in the image series, PEO substrate swells and dissolves in water in less than 20

minutes., while PEDOT:PSS electrode does not dissolve or disintegrate. The swelling

induced stress at the interface of PEO layer and PEDOT:PSS layer is not high enough to

fracture the PEDOT:PSS substrate, most probably because the modulus of the swelling PEO

substrate is lower than that of PEDOT:PSS. As found in our previous analysis of interfacial

stress between dissimilar materials, the interfacial swelling induced stress is highly related to

the ratio of elastic modulus between the swelling substrate and printed layer. (56, 57) The

substrate-less PEDOT:PSS electrode was then transferred onto a Aluminums foil to take

SEM image. As presented in Figure 6.4b, no micro crack is observed in the PEDOT:PSS

electrode.

110

Figure 6.4 a) Sequential images of transiency of all polymer electrode in water, scale bars

represent 10 mm; b) SEM image of electrode with substrate dissolved, then transferred onto

an Aluminum foil; c) Resistance of the electrode as a function of time in water, inset image is

the electrode on a glass slide after all the substrate are completely dissolved.

To further investigate the electric properties of the PEDOT:PSS electrode (post

substrate transiency), resistance fluctuations of the electrode in water were monitored and

recorded as a function of time. Presented in Figure 6.4c is the normalized resistance that

instantaneously increases to approximately 200% as the electrode is exposed to water. The

peak resistance occurs after approximately 100 seconds. of exposure, then drops and

stabilizes around 25% after approximately 700 seconds. The instantaneous increase in

resistance is attributed to the fast swelling of PEO substrate in water, which results in

application of swelling stress, and therefore strain, on the PEDOT:PSS electrode. As PEO

substrate swells further, the relative resistance increases gradually. Subsequently, the PEO

substrate starts to dissolve away and so the swelling stress disappears. Consequently,

PEDOT:PSS electrode starts to relax and recover the lost conductivity until it stabilizes at

111

25%. The conductivity is not fully because the electrode does not return to its original length,

as shown in the inset. Figure 6.4c shows that PEDOT:PSS electrode still retains most of its

electrical properties after the PEO substrate are completely dissolved in water. Based on this

observation, we examined PEDOT:PSS electrodes as epidermal strain sensors, after

substrates were fully dissolved.

6.3.4 Epidermal Strain Sensor

In the transiency study, it was found that the freestanding PEDOT:PSS electrode,

with PEO substrate completely dissolved, still retains most of its electrical properties. The

freestanding electrode thus has a great potential to be applied as an ultra-thin epidermal

sensor which forms perfect contact with curvilinear surface of the host tissue, skin in this

study. Researcher has reported that ultra-thin graphene-based sensors that attach to the skin

with Van der Waals force could exhibit higher sensitivity compared to thicker sensor. (32)

Considering PEDOT:PSS is noncytotoxic, (43, 44) its application as epidermal sensor should

not pose any risk to the host tissue. To apply the sensor on skin, it was attached to forearm

and opisthenar, then PEO substrate was dissolved using water (Figure 6.5a). Presented in

Figure 6.5b are digital and microscopy images of an electrode on the skin. Conformal

contact between the electrode and skin are evident from the magnified microscopy image.

Response of the electrode to cyclic hand movement is shown in Figure 6.5c. The

freestanding electrode did not detach, delaminate, chip, or crack while on skin during the

examination, and it could remain on the skin for hours. Van der Waals force is speculated to

be responsible for electrode-skin bonding.

112

Figure 6.5 a) Sequential images of transiency of all polymer electrode on a human forearm

skin, scale bars present 10 mm; b) Image of a pure electrode (after the substrate are

completely dissolved) on forearm skin taken with a CCD camera and digital microscope; c)

Resistance fluctuation of a pure electrode mounted on opisthenar to monitor the hand

movement.

6.4 Conclusion

An all-organic electrode with PEO membrane as the substrate and PEDOT: PSS as

the active electrode was fabricated with an E-jet printer. To evaluate the all-organic

electrode’s potential application as a flexible sensor, electrode’s electronic properties

dependence on static load and dynamic load was studied. Under static load, the resistance

increases gradually within 6.5% strain. With dynamic load, correlation between frequency

and resistance was not found for frequency ranging from 0.05 Hz to 3 Hz, revealing good

time response of the all-organic electrodes; while correlation existed between strain and

resistance, and the threshold for reversible stretching strain is 3%. The transiency of the all

organic electrode was also studied. The PEO substrate could dissolve completely in water,

113

while PEDPT: PSS electrode remain almost intact. It is concluded the swelling induced stress

from the PEO substrate is not enough to disintegrate the printed PEDOT:PSS electrode. The

ultra-thin substrate-less electrode could form perfect contact with skin, thus was applied as an

epidermal sensor to monitor the hand motion. The all-organic electrode presented in this

study could be applied as flexible sensor as a whole, or as ultra-thin temporary epidermal

sensor with substrate-less electrode after dissolving the substrate in water.

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CHAPTER 7. GENERAL CONCLUSIONS AND FUTURE WORKS

General Conclusions

The goal of this dissertation is to present some studies on transient materials, their

mechanical properties, their transiency mechanism, and their applications in transient

electronics. Exploratory efforts of swelling induced stress and fracture of active electrodes

are first presented. Subsequent research reported a transient lithium-ion battery based on

hybrid transiency as a potential transient onboard power unit for transient electronics. Lastly,

an all-organic transient strain sensor is presented for application as ultra-thin epidermal

sensors. The concluding remarks of each study are summarized as following.

In Chapters 2, 3 and 4, swelling-induced stress and fragmentation are studied as

major contributors to disintegration of transient layered-structures consisting of dissimilar

materials. Interfacial stress distribution is related to the thickness and real-time mechanical

properties of each layer. Fragmentation size are reported to be determined by initial defects

of the active electrode layer. It is concluded that the pre-existing characteristics of the layers

can be used as a means to determine and control the mechanics of fragmentation of the

layered structure as a whole, can be applied to design, tuning, and programming transiency

and disintegration of printed layered soft electronics. The stress analysis method presented in

these chapters could be extended to analyze interfacial stress distribution in a wide range of

layered structures with dynamic properties and help to better understand the physics of such

dynamic structures. In addition, study of swelling induced fracture provides another transient

technique for active components of transient electronics other than dissolution.

In chapter 5, transient Lithium-ion battery was fabricated and tested for the transient

property and the electrochemical performance to investigate the battery’s potential to be an

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onboard power source for transient electronics. With a fast disintegration time and high

output voltage, the transient Li-ion battery reported here is a promising candidate for on-

board transient energy storage element that can be applied in the non-biological field such as

environmental sensor.

In chapter 6, an all polymeric electrode with PEO membrane as the substrate and

PEDOT: PSS as the active electrode was fabricated by a customized E-jet printer. Electrode’s

electronic properties dependence on dynamic load shows good time response of the all-

organic electrodes in the range of 0.05 Hz -3 Hz. The all-organic electrode was found to be

partially transient with PEO substrate dissolved completely in water, and PEDPT: PSS

electrode remain intact. The ultrathin intact electrode could form conformal contact with

skin, thus was applied as an epidermal sensor to monitor the forearm motion. The all

polymeric electrode presented in this study could be applied as flexible sensor as a whole or

applied as ultrathin temporary tattoo with the freestanding electrode after dissolving the

substrate in water.

Future Works

In chapter 5, a transient lithium ion battery was demonstrated. The next step of is to

incorporate the advanced manufacturing techniques, such as E-jet printing reported in chapter

6, to achieve a fully printed transient battery. A more compact structure will assure good

electrode contact, increasing the cycling efficiency as well. Moreover, reducing the volume

will increase the power density.

Another interesting topic would be investigating application of self-power transient

environmental monitors. Soil nutrient detectors that could be degraded naturally into the soil

after a well-defined reliable operation time have extensive application in agriculture area.

During the stable operation period, self-power transient soil nutrient monitors could collect

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real-time data continuously and send to the data storage center nearby, proving helpful

information for farmers. A web of transient soil nutrient monitors could also provide

information of nutrient distribution and movements over a large area of farm land over a long

period of time. After the predefined working time, these transient environmental monitors

would degradable into the surround environment with eco-friendly end products, saving

expense in recollection of unwanted devices, and eliminating harmful wastes.

Another recommendation for future work is to advance the performance of the

ultrathin epidermal sensor reported in chapter 6. The PEDOT:PSS electrode are applied as

epidermal sensor after the substrate are dissolving in water, yet, the dissolution of substrate

takes approximately 40 minutes. The long dissolution time limits application of the PEDO:

PSS electrodes as epidermal sensor. Studies on the PEO substrate to reduce the dissolution

time or even achieve spray-on PEDOT: PSS tattoo as epidermal sensor are demanding.

Proposed strategy including modification on the polymer matrix of PEO substrate, such

modulating the porosity of the PEO substrate by 3D printing technique, or adding additive,

such as sugar, to expedite the dissolution of PEO in water. In addition, the printed electrode

needs to be improved to accommodate higher strain, either by better design printed pattern,

or modification of the printed ink, such as addition of enhancement.