This journal is©The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 8765--8773 | 8765

Cite this: J.Mater. Chem. C, 2017,

5, 8765

Light-reversible hierarchical patterns by dynamicphoto-dimerization induced wrinkles†

Honghao Hou, Fudong Li, Zhilong Su, Jie Yin and Xuesong Jiang *

Hierarchical patterns are playing an increasing role for various fields due to their integrated functions.

Herein we created a novel library of multi-scale complex patterns where hierarchical wrinkles can be

dynamically generated and eliminated by in situ photo-control. Atom Force Microscopy (AFM) results

and UV-vis kinetics manifest that the change of surface modulus induced by dynamic photo-

dimerization of the anthracene-containing polymer (PAN) top-layer plays a crucial role in triggering the

morphology switch of the resulting wrinkled surface with self-healing, tunable adhesion and wettability

properties. Based on the temporal and spatial characteristics of dynamic photo-dimerization, the ordered

and hierarchical patterns can be obtained through adapting selective exposure and photolithography.

Furthermore, a series of hierarchical patterns in which the smaller-scale wrinkles can be prescriptively

generated on the assigned micro-pillar arrays, making up sets of Braille characters, was demonstrated

for Braille text refreshable typography through photo-reversible formation and erasure. This novel and

effective approach for photoreversible hierarchical wrinkle patterns offers great promise for smart devices

and surfaces with dynamic tunable morphology and surface properties on demand in response to light

stimuli without altering the bulk properties.

1. Introduction

Micro/nano-patterned surfaces are of fundamental importanceto materials science,1,2 physics,3–5 chemistry6,7 and biology,2,8,9

bearing significant applications both in natural and man-madeevents, such as biological functions,9–12 interface engineering,13–15

and micro-nano manufacturing.11,16,17 The structural hierarchyfurther advances the topographical patterns to possess variousintriguing and prominent properties due to the synergic effectsof hierarchical structures across multiple scales.18–23 Despitethe great promise of the hierarchical surface patterns, they stillencounter some challenges. Firstly, large amounts of variousmaterials and key components are consumed and multistepand/or complicated fabrication processes are usually involvedduring the generation of hierarchical patterns.24,25 Secondly, andmore importantly than the burdensome preparation procedures,it is still a challenge to produce dynamic hierarchical surfacepatterns since the morphologies of unstable patterns are prepenselydefined or fixed by the fabrication systems. Zhao’s group21

harnessed localized ridges to develop hierarchical patterns withdynamic ability based on precise manual intervention in the

release and stretch of pre-strain. Crosby and his co-workers26

demonstrated a solvent-responsive micro-lens array patternfabricated by photomask-assistant ultraviolet/ozone (UVO) oxida-tion treatment and wrinkling instability for reversible opticaldisplay taking advantage of well control for wetting and dewettingof externally imposed solvents. Even shape memory polymershave been adapted for dynamic pattern systems over limitedshape memory cycles.27–31 However, these works still cannotbreak through the limitations of discrete, special-purposemanual behaviours or requiring the alteration of the material’sintrinsic properties, thus it is still unrealizable to dynamicallytune the morphologies of surface patterns in situ. The subtlecontrol subject to contrived regulation of the surface patternwith discrete size and fixed morphology significantly hinders theimprovement of hierarchical patterns and its resulting surfaceproperties. To develop a novel and effective method to create asmart surface with reversible hierarchical patterns and desirabletunable properties is a hot task. Seki and Lu’s groups demon-strated light-erasable wrinkles on a PDMS substrate, in which thetop azobenzene-containing polymer layer can be softened andrelease compressive stress by the trans/cis photo-isomerization ofazobenzene moieties upon light irradiation.32,33 Very recently,our group reported a novel and robust strategy for fabrication ofa dynamic wrinkle pattern with highly reversible morphologyand tunable multi-functional properties in situ via temperaturecontrol of the dynamic Diels–Alder (D–A) chemistry.34 A wrinkle,as a typical form of surface mechanical instability, occurs once

School of Chemistry & Chemical Engineering, State Key Laboratory for Metal

Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240,

People’s Republic of China. E-mail: ponygle@sjtu.edu.cn

† Electronic supplementary information (ESI) available: Materials, synthesis andcharacterization of materials, Fig. S1–S7. See DOI: 10.1039/c7tc02569f

Received 10th June 2017,Accepted 25th July 2017

DOI: 10.1039/c7tc02569f

rsc.li/materials-c

Journal ofMaterials Chemistry C

PAPER

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article OnlineView Journal | View Issue

8766 | J. Mater. Chem. C, 2017, 5, 8765--8773 This journal is©The Royal Society of Chemistry 2017

the compressive stress s0 in the top layer exceeds the criticalstress sc as a result of minimizing the total energy stored in thefilm. The characteristic wavelength (l) and amplitude (A) ofwrinkle patterns are determined by the intrinsic properties ofthe films, film thickness (t) and applied strain s0, as depicted ineqn (1) and (2):

l ¼ 2pt�Ef

3 �Es

� �13

(1)

A ¼ te0ec� 1

� �12; ec ¼ �

1

4

3 �Es

�Ef

� �23

(2)

where the subscripts f and s refer to the top-layer film andsubstrate, respectively, t is the thickness of the top-layer skinfilm, and �Ef and �Es are the plane-strain modulus for the top-layerfilm and substrate, respectively. The intrinsic nature of the dynamiccovalent cross-linking network endows a wide tunable range ofsurface moduli and highly-reversible erasure and re-generation ofwrinkle patterns, and also guarantees excellent long-term stability.However, the thermo-responsive time of the D–A reaction (4 hours)is too long so limits applications requiring a rapid response. Inaddition, the lack of a uniform pattern in the hierarchy and offspatiotemporal control bounds the advance of the resultingproperties. Light,32,33,35–37 as a non-contact and on–off switchablestimulus, provides an expedient and efficient approach to spatiallyand temporally tune the surface features and inherently possessesthe unique advantages of ultra-rapid response, controllable oper-ability and region-selectivity for the resulting photo-controllablepattern over other stimuli, such as solvents,14,26 temperature,34

electrical field,38 magnetic field39 and mechanical force.21,22,40,41

Herein, we present a facile and effective strategy to fabricatelight-controllable hierarchical patterns with highly reversiblemorphology and rapid response time by reversible photo-dimerization-induced dynamic wrinkle (Fig. 1). Thanks to the

advantages of photo-orthogonality, we demonstrated a rich varietyof light reversible hierarchical pattern with various multi-scaletopography features based on the homogeneous materials. Asidefrom the nature of the dynamic reaction enabling the reversiblewrinkle pattern to possess self-healing and tunable adhesionproperties, this light-responsive patterned surface was used forthe novel application of ‘‘Braille typography’’, in which we canrepeatedly print and erase Braille texts on demand by utilizingthe reversible wrinkle patterns. We think the pattern generationby light-controlled dynamic chemistry paves the way for con-venient, rapid, large-area, yet reversible tunability techniques toorient and locate smart surfaces in desired micro-structuredtopography, which thus has been proposed in various techno-logical fields.

2. Results and discussion

Fig. 1 illustrates the total strategy for fabricating the light-reversible hierarchical surface pattern. A polydimethylsiloxane(PDMS, Dow Corning Sylgard 184) elastomer sheet with thicknessof 400 mm, pre-attached on the glass slide substrate, was spin-coated with a thin layer of photosensitive anthracene-containingcopolymer PAN (Fig. 1a, Fig. S1 and S2 and Table S1 in ESI†).The designs of both the micropattern and the coating func-tional polymer incorporated our considerations for a variety ofphysical and chemical features. The anthracene-based polymer(PAN) was chosen as the top-layer material because of its highreversibility and rapid response speed for photo-dimerization.Moreover, the constitutions and glass transition temperaturesof functional polymers are accommodated by copolymerizationbetween the rigid anthracene-based monomer and soft n-butylacrylate (BA) to obtain anthracene-containing copolymers withdesirable surface moduli suitable for preparing the wrinklepattern. As a typical bilayer buckling system, the characteristic

Fig. 1 (a–e) Strategy for the light reversible hierarchical wrinkle pattern induced by photo-dimerization of anthracene-containing polymer and(f) chemical structures of anthracene-containing functional copolymer PAN and schematically illustrating the dynamic photo-dimerization reaction.(a 2 b) Surface wrinkle pattern of PAN coated bilayers is generated after undergoing (a) 365 nm UV light exposure and then an external stimuli of heatingto 70 1C or (b) erased by 254 nm UV light exposure or heating at 150 1C. (a 2 c) Reversible prescribed generation or erasure of the hierarchical wrinklepattern through (c) 365 nm light selective exposure utilizing photomasks and then an external stimuli of heating to 70 1C or (a) 254 nm UV light exposureor heating at 150 1C. (d 2 e) Dynamic generation and erasure of multi-scale hierarchical wrinkle pattern via 365 or 254 nm UV light selective exposurecombining the photo-lithography.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

This journal is©The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 8765--8773 | 8767

size of the wrinkle pattern can be easily tuned by changing thethickness of the top-layer film from 40 nm to 230 nm aftercross-linking by photo-dimerization of AN via irradiation with365 nm UV light (Fig. S3, ESI†).

The coated bilayers were exposed to 365 nm UV light to becross-linked with a high surface modulus and then heated tothe desired temperature. Upon cooling to room temperature,the wrinkle patterns are generated as a result of releasing thelocalized compressive stress to minimize the total energy of thesystem owing to the considerable mismatch in the modulusand thermal expansion ratios of the bilayers (Fig. 1b). This canalso be verified from a series of resulting wrinkles with higheramplitude or aspect ratios at various heating temperatures(Fig. S4, ESI†). According to the results of the in situ AFMcharacterization, the wrinkle wavelength (l) does not changesignificantly while the wrinkle amplitude (A) increases linearlywith the heating temperature. This result confirms the linearbuckling theory that heating to a higher temperature gives thecross-linked top-layer a bigger thermal stimulus for creating ahigher applied strain s, while the higher s can lead to the largeramplitude A of the wrinkle, as depicted in eqn (2), but there is noobvious difference from the wavelength l according to eqn (1).

Following exposure to 254 nm UV light or baking above150 1C, the wrinkled surface became smooth and the wrinkles wereerased because the de-crosslinking of the top-layer led to the releaseof compression strain between bilayers. The cross-linking andde-crosslinking led to the change of the surface modulus, resultingin generation and erasing of the wrinkle pattern owing to accumu-lation and release of the compression strain. As a result, the bilayerfilm undergoing repeated irradiation from different wavelengthsof UV light shows highly reversible generation and erasure of thewrinkle pattern (Fig. 1a and b), and such a simple low-costfabrication can generate a variety of light reversible wrinklepatterns induced by dynamic photo-dimerization.

A series of control experiments was conducted to verifythe strategy for the reversible wrinkle patterns (Fig. S5, ESI†).Without 365 nm UV exposure, no obvious wrinkling pattern onthe surface of the coated PDMS sheet before and after heatingwas observed in the optical images (step A), suggesting that theself-wrinkling pattern was not caused by the coating of PAN andalso verifying that the UV cross-linking reaction is necessaryfor the generation of the wrinkling pattern. The initially softPAN is not rigid enough to constrain the volume expansion andshrinkage of the bulk material, thus cannot generate enoughcompressive stress to trigger the formation of the pattern. Incontrast, the same wrinkled results of step B and C demonstratethe procedures of 365 nm UV cross-linking and heating arenecessary for the generation of the wrinkle pattern. Only the changeof the surface modulus resulted from the photo-dimerization canaffect the generation and morphology of wrinkles (step D and E)because the photodimerization cross-linking increases the surfacemodulus, thus decreasing the critical strain ec below the appliedstrain that arises from the considerable mismatch of thermalexpansion between the bilayers according to eqn (2).

A library of surface labyrinth wrinkle patterns with variousfeature sizes was prepared by simply varying the exposure timeof 365 nm UV light. As shown in Fig. 2, in situ atomic forcemicroscopy (AFM) reveals that the characteristic amplitude (A)and wavelength (l) of wrinkles and the corresponding surfaceYoung’s modulus (E, determined by AFM force curves showed inFig. S6, ESI†) both obviously increased as a function of 365 UVexposure time. With approximately 60% conversion of photo-dimers after 10 min UV illumination, the sharp change of thetop-layer’s E from the initial 0.45 GPa to 3.92 GPa dominatedthe topography generation from the original flat surface to thewrinkle pattern with characteristic A of 1240 nm. Meanwhile,the kinetics of the anthracene photo-dimerization reaction weretraced via the evolution of the ultraviolet visible (UV-vis) spectra.

Fig. 2 (a–d) 3D AFM images showing the evolution of the wrinkle morphologies on the PAN-1 coated PDMS sheet with different 365 nm light exposuretime: (a) 6 min, (b) 10 min, (c) 14 min, and (d) 18 min, respectively. (e) Characteristic amplitude (A, black square, left vertical axis) and wavelength (l, redcircle, right vertical axis) of wrinkle pattern. (f) Conversion of reversible photo-dimerization (black triangle, left vertical axis) and surface Young’s modulus(blue pentagram, right vertical axis) of the wrinkling PAN coated bilayers as a function of 365 nm light exposure time. The top-layer thickness of PAN-1 isfixed at approximately 130 nm. The intensity of 365 nm UV light and 254 nm light is approximately 20 mW cm�2 and 3.5 mW cm�2, respectively.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

8768 | J. Mater. Chem. C, 2017, 5, 8765--8773 This journal is©The Royal Society of Chemistry 2017

The evolution of the UV absorption characteristic quartet peakat near 370 nm, typical for the anthracene ring, was chosen forconversion calculations of the photocyclodimers’ formationand breaking (Fig. S7, ESI†). A significant decrease of thecharacteristic absorption peak of anthracene at 320–400 nmwas observed (Fig. S7a, ESI†). As depicted in Fig. 2f, the gradualincrease in the conversion of photo-dimerization combinedwith surface moduli simultaneously results in a rapid increaseof the wrinkle amplitude, reaching a maximal value as thephoto-dimerization reaction proceeds. Thus, the wrinkle ampli-tude A can be predicted and tailored from the surface modulus�Ef via the simple light control of the photo-dimerization

according to the qualitative relationship ln A2 þ t2� �

� 2

3ln �Ef ,

as revealed by eqn (2). A deviation from the ideal relationship inthe last photopolymerization period, especially from UV expo-sure time of 6–10 min, may be ascribed to these followingreasons. The conversion of photodimerization determined byUV-vis transmittance spectra cannot be involved in the sidereaction of oxidization by molecular oxygen. The amplitudemaybe continues to increase slightly, although the conversionwill be close to a maximum value. The surface modulus deter-mined by AFM (which usually works for the uppermost severalnm thickness layer) cannot be completely responsible for thetotal gradient cross-linked top-layer resulting from the funnellingeffect in photopolymerization. When the top-layer reaches acertain cross-linking degree under 365 nm UV exposure, thesurface modulus of the thin layer sensitive to the AFM probetip almost arrived at a maximum value after 10 minutes. Further-more, both the varying cross-linking degrees, glass transmissiontemperatures, and Poisson’s ratio of the dynamic polymernetwork have an effect on the ideal quantitative relationshipbetween surface modulus E and characteristic amplitude andwavelength according to eqn (1) and (2). However, this devia-tion in the last photopolymerization period does not affect therelationship revealed from these data because the photodimer-ization degree was close to its maximum value after 10 min.

Since the key to controlling the wrinkle pattern is the photo-dimerization reaction, the wrinkle can be dynamically tuned bysimple regulation of reversible photo-dimerization. Upon heatingat an appropriate temperature (such as 150 1C in our experiment)or 254 nm UV light illumination for the dedimerization, thewrinkling pattern vanishes as the photo-dimerization with areverse de-cross-linking process (Fig. S7b and c, ESI†) restoresthe initial status of the surface materials and the modulus in thetop layer, which is not rigid enough to generate enough com-pressive stress to trigger the formation of the wrinkle pattern.

Indeed, the as-prepared wrinkled surface that is subjected tomultiple cycles of alternate UV exposure or heating processesexhibits highly reversible morphology as well as a transition ofphoto-dimerization conversion. As depicted in Fig. 3, the wrinkledsurface with characteristic A of 530 nm undergo a dramaticdecrease to approximately 90 nm after 254 light illuminationfor 10 min, and then become smooth for the further exposureof 20 min, accompanied by a sharp augment in the surfaceYoung’s modulus and photo-dimerization conversion monitored

by UV-vis spectra. The wrinkled surface of PAN erased by254 nm light restored to the initial wrinkle morphology, how-ever, once subsequently exposed under 365 nm light for 10 min.Furthermore, because the high temperature also can depoly-merize the anthracene photo-dimers, we then baked thewrinkled sample at 150 1C, the amplitude of the wrinklesdecreased from A = 365 nm (40 min) to A = 180 nm (90 min),with a large decline of the photo-dimerization degree to a verylow value simultaneously. Interestingly, the heating erasure wasaccomplished after 180 min of heating at 150 1C, whose timeis 6 times than the erasure time via 254 nm light exposure.This result manifests the unique advantage of light-responsivewrinkle pattern. It is remarkable that the patterned film canundergo a thermo/light dual reversible morphology from awrinkle pattern to a plane film for more than eight cycles.Because the surface modulus of PAN top-layer is regulatedstrictly by the reversible photo-dimerization, the mechanicalmismatch in modulus between bilayers caused by the photo-dimerization reaction degraded to the equilibrium with theprocess of depolymerization and the wrinkle was relaxed. Thegeneration and disappearance of wrinkles are determined bymodulus mismatch between bilayers as result of the reversiblephoto-dimerization.

Our strategy of using photochemistry to produce reversiblewrinkles enables spatioselective control and ordered alignmentof the spontaneous pattern (Fig. 1c–e). As a demonstration,by adapting the photolithography and selective exposure toour reversible wrinkle pattern system, a library of hierarchical

Fig. 3 (a–g) 3D AFM images and (h) the evolution of their correspondingamplitude (A) and conversion of photo-dimerization demonstrating thereversibly tunable morphology of the wrinkled PAN coated bilayer withsequential cycles of alternate 365 nm UV light exposure for the photo-dimerization and subsequently heating at a higher temperature of 150 1Cor 254 nm UV light illumination for the depolymerization. The arrowsindicate the developmental axis of the tunable surface morphology. Thewhite, green and pink regions represent 365 nm UV light exposure forthe photo-dimerization, 254 nm UV light illumination and heating at ahigher temperature of 150 1C for the depolymerization, respectively. Thetop-layer thickness of PAN-1 is fixed at approximately 130 nm. The intensityof 365 nm UV light and 254 nm light is approximately 20 mW cm�2 and3.5 mW cm�2, respectively. All the scale bar is 60 mm.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

This journal is©The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 8765--8773 | 8769

patterns with various feature topographies was created (Fig. 4).Firstly, selective illumination of the PAN coated bilayer is imple-mented through photomasks with different exposed/unexposedregions. In the case of 365 nm UV light illumination, the exposedregions that undergo the photo-dimerization became cross-linkedwith high modulus, while the masked areas remain unchangedand low-modulus, thereby creating well-defined boundaries forwrinkle confinement. The low modulus regions allow the highmodulus regions to relieve in-plane stresses, aligning the wrinklesperpendicular to the boundary of the mask. Subsequently, bymeans of the simple selective 254 nm UV exposure, the wrinkledsamples can be fully erased in exposed regions while kept primarystate in unexposed regions. Surprisingly, the initially disorderedwrinkles dynamically evolved into highly-ordered hierarchicalpattern with feature geographies or even globally wrinkle-freesurface through selective erasure and formation in a stepwiseand cyclic manner via a succession of alternative irradiation(Fig. 4a–d). This approach provides a rapid method for gener-ating complex wrinkle pattern with excellent pattern fidelityand also verifies the feasibly of our strategy for obtaining lightreversible hierarchical pattern. Since photodimer of AN doesnot emit fluorescence, this complex pattern can offer multiplefluorescence information besides morphology feature (Fig. 4b),which may find application for anti-counterfeiting11 or fluores-cent ink.42 Furthermore, thanks to the advantages of anthracene

photochemistry, it not only allows wrinkle formation, butalso enables the bulk polymer to be moulded into desiredgeometries. By simply adapting the photolithography to ourreversible wrinkle pattern system, we can form a series of multi-scale complex pattern where smaller-scale reversible wrinklesgrow atop larger-scale microstructure arrays such as squares,straight bars or pillars. This light-control of photodimerization-induced dynamic wrinkle on a micropattern array with tunablemorphology by alternative irradiation results in the novel light-reversible hierarchical pattern that combines micro/nano-scalewrinkles and larger-scale micropattern arrays (Fig. 4e and f).As expected, the as-prepared hierarchical patterned surfacegoing through multiple cycles of alternate UV light irradiationat different wavelengths exhibits highly reversible wrinklemorphology. Thereafter, such a facile and low-cost fabricationcan remarkably generate a variety of light reversible hierarchicalpattern with various feature topographies by photo-dimerizationinduced wrinkle. It is noted that this novel approach to attainthe reversible hierarchical pattern is particularly advantageousbecause the hierarchical patterns combining a new mode ofmicro/nano-scale wrinkles and larger-scale micropattern arraysare based on the identical materials and exhibit homogeneouschemical property. Also no additional chemical materials ortreatments are required nor are extra physical defects introducedinto this photo-reversible pattern system.

Fig. 4 (a–d) Demonstration of the light reversible hierarchical patterns obtained when the wrinkled PAN/PDMS bilayer was selectively or blanketexposed to 365 or 254 nm UV light through the photomask. (a) The as-prepared wrinkled PDMS bilayers generated by 365 UV light exposure for 10 minand an external stimuli of heating to 70 1C was (b) selectively erased by illumination under 254 nm UV light overlaying the copper grids (mesh size:100 mm) for 30 min, (c) subsequently erased to a globally wrinkle-free plane by heating at 150 1C for 3 h, and (d) selectively generated under 365 nm UVlight overlaying the copper grids (mesh size: 100 mm) for 10 min, (a) then fully generated to restore via 365 nm UV light blanket exposure for 10 min. Theinsets at the upper right are the 2D fast Fourier transform (FFT) images from the corresponding photographs, and the inset of (b) is the corresponding3D laser confocal image and fluorescent photograph. (e and f) Demonstration of light reversible multiscale hierarchical wrinkle pattern upon alternatingillustration of (e) 365 nm or (f) 254 nm UV light or 150 1C. The top-layer thickness of PAN-1 is fixed at 130 nm. The intensity of the 365 nm UV light and254 nm light is approximately 20 mW cm�2 and 3.5 mW cm�2, respectively.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

8770 | J. Mater. Chem. C, 2017, 5, 8765--8773 This journal is©The Royal Society of Chemistry 2017

Given the nature and versatility of dynamic photochemistryand the combined reversibility and thermal/photo-responsivenessof our wrinkle pattern system, surface patterned coatings fabri-cated in this dynamical manner should find numerous applica-tions. We only demonstrate several samples based on the natureof dynamic reaction and the high-designability of this opensystem. For example, the highly reversible nature of the dynamicreaction enables the wrinkle pattern to possess the talent of self-healing and tunable adhesion and wettability properties as well(Fig. 5 and 6). The adhesive/hydrophobic and non-adhesive/hydrophilic states were reversibly cycled for more than 7 timesvia the control of the photo-dimerization by switching the alter-native heating or light control (Fig. 5). Also the damaged wrinklepattern can be erased and restored by an in situ thermal operationwith the proceeding of the dynamical crosslinking chemistry (Fig. 6).We think these advantages of generalizability and open archi-tecture framework make it possible to become a general methodfor reversible wrinkled surface to tune the surface properties andexpend the applications in different practical conditions.

Moreover, the combination of nano-micro-hierarchy, dynamictunability, and outstanding designability promises extraordinaryfunctions for the new surface hierarchical patterns. Here, we

highlight initial applications to demonstrate the unique aspectsof this top-constrained, light reversible hierarchical pattern byphoto-dimerization induced dynamic wrinkle system to serve asrecyclable typography for Braille publications. Braille is a tactilewriting system used by people who are blind or visually impairedto assist them in completing common reading and writing tasks.Braille characters are small rectangular blocks called cells thatcontain six tiny palpable bumps called raised dots arranged in twolateral rows, each having three dots. The number and arrange-ment of these dots distinguish one character from another,providing 64 possible transcription codes (sets of characterdesignations) of printed writing systems to represent an alphabetletter, number, punctuation mark, or even an entire word.Advances with Braille are meaningful because it helps its usersaccess the civilized world. Despite the Braille system’s universalreach, some disadvantages impede lots of visually impairedpeople from being able to read Braille publications. One ofthe big problems is the absence of refreshable printings forponderous Braille texts.43–45 Generally, it is written with embossedpaper where Braille raised dots pillar arrays are produced bythermal or photo-cured materials, resulting in the disposabilityand inconvenience of these cumbersome books.

As demonstrated in Fig. 7, the as-prepared micro-pillar arraypatterned surface atop smaller-scale wrinkles through the photo-lithography was subjected to 254 nm UV light selective exposureoverlaying the stencil masks, leading to the formation of hierarchicalpatterns in which the smaller-scale wrinkles generate on assignedwrinkled micro-pillar arrays. The resulting exposed/wrinkled pillarscan be easily identified from the unexposed plane regions becausethe exposed parts have a rough tactile sense in contrast to the flatparts owing to the presence of wrinkle microstructures, thus thishierarchical pattern whose prescribed micro-pillars are wrinkled canmake up a set of character designations for Braille character ‘‘S’’(Fig. 7a). Subsequently, the wrinkles in different micro-pillars can be

Fig. 5 Demonstration of reversible (a) wettability and (b) adhesion properties ofthe dynamic photo-dimerization induced reversible wrinkle pattern. (a) Changesin water contact angle (black line, left vertical axis) and surface free energy (blueline, right vertical axis) of the reversible wrinkling PAN coated PDMS sheet withsequential, cyclic control of the ongoing photo-dimerization under 365 nmUV exposure and subsequent depolymerization reaction under 254 nm UVexposure. The black squares and blue circles represent water contact angle andsurface free energy, respectively. (b) Evolution of the adhesion force measuredon the reversible wrinkled surface on the reversible photo-dimerization reactiontime with sequential, cyclic control of the ongoing photo-dimerizationunder 365 nm UV exposure (white regions) and subsequent depolymerizationreaction under 254 nm UV exposure (grey regions). The thickness of the toplayer is approximately 75 nm. The intensity of the 365 and 254 nm UV light isapproximately 20 mW cm�2 and 3.5 mW cm�2, respectively.

Fig. 6 Demonstration of the self-healing properties of the dynamicphoto-dimerization induced reversible wrinkle pattern. Schematic illustra-tion, 3D AFM images and their corresponding cross-sections of thewrinkling PAN coated PDMS sheet (a) fabricated under 365 UV exposurefor 10 min and then slightly scratched by a spindly probe, (b) subsequentlyheated at 150 1C for the depolymerization for 20 min to erase thedamaged pattern to a wrinkle-free plane, (c) followed by 365 nm UVexposure for 10 min to restore the wrinkle pattern.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

This journal is©The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 8765--8773 | 8771

fully erased by 254 nm selective exposure overlaying various stencilmasks (Braille ‘‘J’’, Fig. 7b), and the initial wrinkle-free micro-pillarscan be fully erased by 254 nm selective exposure overlaying variousstencil masks (Braille ‘‘J’’, Fig. 7b), and the initial wrinkle-free micro-pillars can also generate a wrinkle pattern by 365 nm UV lightexposure (Braille ‘‘T’’ and ‘‘U’’, Fig. 7c and d). Thus, multi-scalehierarchical patterns representing various Braille character informa-tion can be conveniently created by programmed wrinkle generationand erasure through simple photo-control. Here, the first 365 nm UVlight selective exposure to generate the wrinkle pattern can beregarded as the process of ‘‘writing/printing’’ information, and thesecond 254 nm UV light blanket exposure represents the step oferasing the information. The alternating 365 or 254 nm UV selectiveexposure to generate or eliminate the wrinkles as we program is theprocess of ‘‘modification and recycling’’. Thus, taking advantageof the optically controllable reversibility of the wrinkles combinedwith the reversible photodimerization-induced wrinkling/selectiveexposure programmable procedure, we can repeatedly write anderase Braille texts in a novel manner of ‘‘typography’’. We think thatthis simple and robust approach of Braille typography technologyfacilitates the printing of Braille texts and also allows blind people toaccess the texts conveniently and economically.

3. Experimental3.1 Preparation and erasure of wrinkled patterns

A toluene solution of PAN was firstly spin-coated on a precastPDMS sheet on a glass substrate and then exposed under365 nm UV light for the desired time for the photo-dimerizationcrosslinking to take place, forming a relatively hard top-layer

film on the PDMS sheet. Then the coated PDMS sheet was cooledto room temperature rapidly after heating to 70 1C to generate thewrinkle pattern. As for erasure of the wrinkle pattern, the wrinkledPDMS bilayer undergoes heating to B150 1C or illumination under254 nm UV light for the desired duration for de-cross-linking ofthe top-layer to make the surface wrinkle-free.

3.2 The photolithography process

The PAN coated bilayers were selectively exposed under 365 nmUV light through photomasks to undergo the photo-dimerization,forming a partially cross-linked top-layer film on the PDMS sheet.After development in the THF solution and heating to removethe unexposed areas and residue solvents, the desired patternedsurface was obtained.

3.3 Characterization1H NMR spectra were obtained using a Varian Mercury Plus400 MHz instrument with chloroform-d (CDCl3) or dimethyl-sulfoxide-d6 (DMSO-d6) as the solvents and tetramethylsilane(TMS) as the internal standard at room temperature. TheFT-IR spectrum was obtained using a Spectrum 100 Fourier-transformation infrared absorption spectrometer (FT-IR, NicoletIS10), which was recorded from 4000 to 400 cm�1 with a 4 cm�1

resolution over 32 scans. UV-vis absorption spectra were obtainedvia a UV-2550 spectrophotometer (Shimadzu, Japan). The conver-sion of photo-dimerization was calculated from the ratio of UVabsorbance at one moment of exposure time to the absorbanceprior to polymerization. Average molecular weights were determinedby means of gel permeation chromatography (GPC, LC-20A,Shimadzu, Japan), using tetrahydrofuran as an eluent at a flow

Fig. 7 (a–d) Optical images demonstrating the application for ‘‘Braille typography’’ fabricated by light reversible hierarchical patterns in which the smaller-scale wrinkles are generated on assigned micro-pillar arrays by multiple cycles of selective or blanket exposure under 365 or 254 nm UV light. The noteabove depicts the Braille character represented by the six-raised dots array in the corresponding picture. (e and f) SEM images showing (e) the exposed/wrinkled pillar and (f) the unexposed plane pillar. The inset is a local magnified laser confocal micrograph of the wrinkled pillars. The diameter (1.5 mm) andheight (approximately 400 mm) of pillars are fixed to the same as the Chinese standard size for printed Braille characters. The top-layer thickness of PAN-2 isfixed at approximately 230 nm. The intensity of the 365 nm UV light and 254 nm light is approximately 20 mW cm�2 and 3.5 mW cm�2, respectively.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

8772 | J. Mater. Chem. C, 2017, 5, 8765--8773 This journal is©The Royal Society of Chemistry 2017

rate of 1.0 mL min�1 with a combination of two columns(Shodex, KF-802 and 804, 300 � 8 mm) and equipped with aRID-10A differential refractive index detector. The sampleconcentration was 2 mg mL�1 and 50 mL of the injection volume.The calibration curve was obtained using polystyrene standards(Shodex, Japan). Static contact angle measurements were recordedon a spinning drop interface tensiometer (SL200C, USA KINOIndustry) using the sessile drop method. The mean results ofwater contact angle and diiodomethane contact angle measuredon five different locations of the surface for each sample wereused to calculate the surface free energy by means of Owens’smethod. The fluorescent photograph was obtained through super-resolution multiphoton confocal microscope (TCS SP8 STED 3X,Leica, Germany). Observation of the surface morphology wasperformed by laser scanning confocal microscopy (LSCM, LEXTOLS4100, Olympus, Japan), laser profile micrometer (VF-7510,Keyence, Japan) and AFM (E-Sweep, SII, Japan). For topographymeasurements of tapping-mode AFM, silicon tips with a radius of10 nm, spring constant of 3 N m�1 and resonance frequency of78 Hz were chosen. Surface line profiles were analyzed withdigitized Nanonavi III (Seiko) offline software from the acquiredAFM images, and the values of amplitude and distance betweentwo peaks of each image were statistically calculated from20 typical wrinkles. The adhesion measurements were conductedin both contact and force modes under vacuum conditions usinga silicon tip with a contact area radius of 10 nm, spring constantof 3 N m�1 and resonance frequency of 60–80 Hz. Surface Young’smodulus (E) is calculated as the average value of the approach andthe retract trace of each force curve according to the Hertz modelusing the force–distance approach of AFM. Detailed informationon the measurement of Young’s modulus (E) by AFM can befound in our previous report.34

4. Conclusions

We demonstrated a novel and effective strategy for obtaining alight reversible hierarchical pattern, in which a homogeneousphotodimerization-induced reversible wrinkle pattern wasfabricated atop a larger-scale microstructure arrays pattern.Unique characteristics of the dynamic photochemical reaction,such as the rapid responsibility, high reversibility, on–off control-lable, non-contact convenient operability and light orthogonality,make this approach promising for practical applications. Besidesthe self-healing and tunable adhesion properties, the hierarchicalpatterns exhibit a new mode that combines micro/nano-scalewrinkles and larger-scale micropattern arrays, which enablethe novel application for ‘‘Braille typography’’ in which we canrepeatedly print and erase Braille texts and enable blind peopleto access texts conveniently and economically. This novel androbust strategy for light-reversible hierarchical patterns bydynamic photochemistry inherently possesses unique advan-tages of rapid response, controllable operability and accurateregion selectivity. It thus paves the way for convenient, rapid,large-area, yet reversible and tuneable techniques to orient andlocate smart materials in a desired multi-scale topography and

obtain the resulting surface properties for smart surfaces anddevices by dynamically tuning surface geometry in acute responseto light stimuli without altering the bulk properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Nature Science Foundation ofChina (21522403, 51373098), the National Basic Research Program(2013CB834506) and the Education Commission of ShanghaiMunicipal Government (15SG13) for their financial support.

Notes and references

1 Z. Nie and E. Kumacheva, Nat. Mater., 2008, 4, 277–290.2 S. Park, Y. J. Kang and S. Majd, Adv. Mater., 2015, 27,

7583–7619.3 P. Muller and J. Kierfeld, Phys. Rev. Lett., 2014, 112, 094303.4 H. Jiang, D. Y. Khang, J. Song, Y. Sun, Y. Huang and J. A.

Rogers, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 15607–15612.5 T. Xie, X. Xiao, J. Li and R. Wang, Adv. Mater., 2010, 22,

4390–4394.6 J. Leopoldes and P. Damman, Nat. Mater., 2006, 5, 957–961.7 K. Efimenko, M. Rackaitis, E. Manias, A. Vaziri, L. Mahadevan

and J. Genzer, Nat. Mater., 2005, 4, 293–297.8 C. Zhang, D. A. McAdams 2nd and J. C. Grunlan, Adv.

Mater., 2016, 28, 6292–6321.9 C. Rianna, L. Rossano, R. H. Kollarigowda, F. Formiggini,

S. Cavalli, M. Ventre and P. A. Netti, Adv. Funct. Mater., 2016,26, 7572–7580.

10 L. J. Kesong Liu, ACS Nano, 2011, 5, 6786–6790.11 H. J. Bae, S. Bae, C. Park, S. Han, J. Kim, L. N. Kim, K. Kim, S. H.

Song, W. Park and S. Kwon, Adv. Mater., 2015, 27, 2083–2089.12 Q. Wang and X. Zhao, MRS Bull., 2016, 41, 115–122.13 Y. H. Kim, Y. M. Lee, J. Y. Lee, M. J. Ko and P. J. Yoo,

ACS Nano, 2012, 2, 1082–1093.14 L. Xue and Y. Han, Prog. Polym. Sci., 2011, 36, 269–293.15 J. Rodrıguez-Hernandez, Prog. Polym. Sci., 2015, 42, 1–41.16 Y. H. Lee, T. K. Lee, I. Song, H. Yu, J. Lee, H. Ko, S. K. Kwak

and J. H. Oh, Adv. Mater., 2016, 28, 4976–4982.17 A. Biswas, I. S. Bayer, A. S. Biris, T. Wang, E. Dervishi and

F. Faupel, Adv. Colloid Interface Sci., 2012, 170, 2–27.18 W. G. Bae, H. N. Kim, D. Kim, S. H. Park, H. E. Jeong and

K. Y. Suh, Adv. Mater., 2014, 26, 675–700.19 J. H. Lee, H. W. Ro, R. Huang, P. Lemaillet, T. A. Germer,

C. L. Soles and C. M. Stafford, Nano Lett., 2012, 12, 5995–5999.20 Y. Zhang, C. T. Lin and S. Yang, Small, 2010, 6, 768–775.21 C. Cao, H. F. Chan, J. Zang, K. W. Leong and X. Zhao, Adv.

Mater., 2014, 26, 1763–1770.22 W. K. Lee, J. Kang, K. S. Chen, C. J. Engel, W. B. Jung,

D. Rhee, M. C. Hersam and T. W. Odom, Nano Lett., 2016,16, 7121–7127.

Paper Journal of Materials Chemistry C

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online

This journal is©The Royal Society of Chemistry 2017 J. Mater. Chem. C, 2017, 5, 8765--8773 | 8773

23 I. E. Jacobs, J. Li, S. L. Burg, D. J. Bilsky, B. T. Rotondo,M. P. Augustine, P. Stroeve and A. J. Moule, ACS Nano, 2015,2, 1905–1912.

24 A. Munoz-Bonilla, M. Fernandez-Garcıa and J. Rodrıguez-Hernandez, Prog. Polym. Sci., 2014, 39, 510–554.

25 R. X. L. Y. Wen, Y. Mi and Y. Lei, Nat. Nanotechnol., 2017, 12,244–250.

26 H. S. Kim and A. J. Crosby, Adv. Mater., 2011, 23, 4188–4192.27 Z. Wang, C. Hansen, Q. Ge, S. H. Maruf, D. U. Ahn, H. J. Qi

and Y. Ding, Adv. Mater., 2011, 23, 3669–3673.28 C.-C. Fu, A. Grimes, M. Long, C. G. L. Ferri, B. D. Rich,

S. Ghosh, S. Ghosh, L. P. Lee, A. Gopinathan and M. Khine,Adv. Mater., 2009, 21, 4472–4476.

29 J. Li, Y. An, R. Huang, H. Jiang and T. Xie, ACS Appl. Mater.Interf., 2012, 4, 598–603.

30 J. Hu, Y. Zhu, H. Huang and J. Lu, Prog. Polym. Sci., 2012, 37,1720–1763.

31 T. Gong, K. Zhao, X. Liu, L. Lu, D. Liu and S. Zhou, Small,2016, 12, 5769–5778.

32 T. Takeshima, W.-y. Liao, Y. Nagashima, K. Beppu, M. Hara,S. Nagano and T. Seki, Macromolecules, 2015, 48, 6378–6384.

33 C. Zong, Y. Zhao, H. Ji, X. Han, J. Xie, J. Wang, Y. Cao,S. Jiang and C. Lu, Angew. Chem., 2016, 55, 3931–3935.

34 H. Hou, J. Yin and X. Jiang, Adv. Mater., 2016, 28, 9126–9132.

35 H. Petek and J. T. Yates, Chem. Rev., 2006, 10, 4113–4115.36 S. J. Ma, S. J. Mannino, N. J. Wagner and C. J. Kloxin,

ACS Macro Lett., 2013, 2, 474–477.37 M. Takahashi, T. Maeda, K. Uemura, J. Yao, Y. Tokuda,

T. Yoko, H. Kaji, A. Marcelli and P. Innocenzi, Adv. Mater.,2007, 19, 4343–4346.

38 N. Wu, L. F. Pease and W. B. Russel, Adv. Funct. Mater.,2006, 16, 1992–1999.

39 D. M. Drotlef, P. Blumler and A. del Campo, Adv. Mater.,2014, 26, 775–779.

40 E. Lee, M. Zhang, Y. Cho, Y. Cui, J. Van der Spiegel,N. Engheta and S. Yang, Adv. Mater., 2014, 26, 4127–4133.

41 P. Y. Chen, J. Sodhi, Y. Qiu, T. M. Valentin, R. S. Steinberg,Z. Wang, R. H. Hurt and I. Y. Wong, Adv. Mater., 2016, 28,3564–3571.

42 H. Wu, Y. Chen and Y. Liu, Adv. Mater., 2017, 29, DOI:10.1002/adma.201605271.

43 Y. Haga, W. Makishi, K. Iwami, K. Totsu, K. Nakamura andM. Esashi, Sens. Actuators, A, 2005, 119, 316–322.

44 Y. Kamotani, T. Bersano-Begey, N. Kato, Y.-C. Tung,D. Huh, J. W. Song and S. Takayama, Biomaterials, 2008,29, 2646–2655.

45 K. Ren, S. Liu, M. Lin, Y. Wang and Q. M. Zhang, Sens.Actuators, A, 2008, 143, 335–342.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

25

July

201

7. D

ownl

oade

d by

Sha

ngha

i Jia

oton

g U

nive

rsity

on

07/1

2/20

17 1

0:19

:44.

View Article Online