German Edition: DOI: 10.1002/ange.201902425Energy StorageInternational Edition: DOI: 10.1002/anie.201902425

The Rise of Fiber ElectronicsXiaojie Xu+, Songlin Xie+, Ye Zhang+, and Huisheng Peng*

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Keywords:energy harvesting ·energy storage ·fiber electronics ·sensing and lighting

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1. Introduction

With the increase in interdisciplinary integration andtechnological convergence, many new application fields thatmay revolutionize modern society and shape life in the future,such as implantable medical devices,[1, 2] soft robots,[3, 4]

wearable devices[5, 6] and electronic fabrics,[7, 8] have appeared.Unfortunately, bulky electronic devices based on the current-ly available technologies cannot effectively meet these newapplication requirements, including flexibility and stabilityrequirements.[9,10] For instance, the rigidity of widely exploredbulky devices has largely limited their conformity to irregularand soft surfaces.[11] Accordingly, the configurations ofelectronic devices have recently evolved from rigid three-dimensional bulk materials to flexible two-dimensional thinfilms and finally to ultra-flexible and even stretchable one-dimensional fibers.

The one-dimensional configuration offers fiber devicesunique properties. With diameters ranging from tens tohundreds of micrometers, they can efficiently accommodatecomplex deformations such as bending in any direction,twisting, and stretching.[12] On the one hand, compared withthree-dimensional bulk and two-dimensional thin film devi-ces, miniaturized fiber devices using soft fiber electrodesmatch well with biological tissues in terms of bendingstiffness, and the fiber shape also favors deep penetrationinto various tissues with negligible tissue damage and avoidscomplex implantation procedures. These properties help formstable tissue-electronics interfaces, which allow fiber devicesto be implanted into the human body to detect signals fromorgans such as the brain and treat various diseases.[13–15] Onthe other hand, fiber devices can be woven into electronicfabrics by mature textile technology. The resulting soft,breathable, and comfortable electronic fabrics can be blendedinto daily clothes and directly contact the skin over a largearea, effectively satisfying the need for portable and wearabledevices.[16, 17]

Therefore, a variety of fabrication techniques, such as wet-spinning,[18] dry-spinning,[19] and thermal drawing,[20] havebeen developed to fabricate fiber devices. These devices withvarious functions, such as energy harvesting, energy storage,sensing and lighting, are thus widely investigated. For energyharvesting, devices that convert renewable energy, includingsolar energy, mechanical energy, thermal energy, and hydroenergy, into electricity have been frequently investigated. For

energy storage, supercapacitors andbatteries have been extensively stud-ied. Sensing devices, including bothin vitro and in vivo sensors, have beenexplored. For lighting, the fiber devicesused include polymer light-emittingelectrochemical cells (PLECs), poly-mer light-emitting diodes, and inor-ganic electroluminescent devices. Oth-er functions of fiber devices, includingas transistors, have also attracted in-creasing interest. In just a decade,studies on fiber devices have led tosystematic development of methodsfor synthesizing electrodes and active

materials, for designing electrode microstructures and devicearchitectures, for optimizing interfaces between electrodesand active layers, for improving properties, and for integratingfunctions for practical applications. The above-mentionedstudies have given birth to a new direction or field, which wename here fiber electronics.

Based on the fundamental design principles, fiber elec-tronics can be mainly categorized into two groups: inside fiberand onto fiber, where the former focuses on integratingmultimaterials or devices inside fibers, and the latter is toconstruct a device on fiber electrode. This review article willshed light on the design principles of fiber electrodes anddevice configurations, the potential applications of fiberelectronic devices, and the remaining challenges and futureresearch directions.

2. Fiber Electrodes

Electrodes are key to advancements in traditional elec-tronics and are even more important for fiber electronics. Onthe one hand, we should pay more attention to controlling thecomposition and microstructure of the fiber electrode be-cause it is difficult to deposit thin and continuous active layersonto its curved surface. On the other hand, the fiber electrodeshould simultaneously possess many properties, that is, itshould be flexible, strong, and highly electrically conducting.In contrast to the bulk and film electrodes, which are largelystabilized on substrates during fabrication, fiber electrodes,which also serve as substrates, need to be flexible and strongto bear various deformations. Since charges transport alongthe fiber electrode with much longer pathways than those inbulk and film electrodes, higher electrical conductivities arerequired to guarantee stable performances of continuous

As a new direction in applied chemistry, fiber electronics allow deviceconfiguration to evolve from three to two dimensions and then to onedimension. The reduction in dimension brings unique properties, suchas ultraflexibility, tissue adaptability, and weavability, enabling theiruse in a variety of applications, particularly in various emerging fieldsrelated to implantable devices and wearable systems. The differenttypes of fiber electrode materials are summarized based on the one-dimensional configuration and their distinctive interfaces, variousdevices, and promising applications. The remaining challenges andfuture directions are finally highlighted.

[*] Dr. X. Xu,[+] S. Xie,[+] Dr. Y. Zhang,[+] Prof. H. PengState Key Laboratory of Molecular Engineering of Polymers andDepartment of Macromolecular Science, Fudan UniversityShanghai 200438 (China)E-mail: penghs@fudan.edu.cn

[+] These authors contributed equally to this work.

The ORCID identification number(s) for the author(s) of this articlecan be found under:https://doi.org/10.1002/anie.201902425.

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fiber devices for applicablelengths in meters or evenkilometers. Generally,polymer, metal and carbonnanomaterials are investi-gated for fiber electrodes.

Owing to the ease offunctional group controland surface modification,a spectrum of conductingpolymers, such as polyani-line,[21] polypyrrole,[22] andpoly(3,4-ethylene dioxy-thiophene) (PEDOT),[23]

have been explored. Theresulting fibers are flexiblethough typically have rela-tively low conductivities(< 102 S cm�1).[24] A secondphase with much higherconductivity, for example,carbon nanotubes(CNTs),[25] is thus added toincrease the conductivitiesto 103 Scm�1, which stillneed to be further en-hanced.

Compared to organicmaterials, metal wires suchas titanium[26] are recog-nized for having muchhigher conductivities of> 105 Scm�1. The thin met-al wires applicable to fiberdevices are also flexible and strong. However, owing to theirhigh densities, they are heavy when applied in a large scale,and are thus not ideal for portable and wearable applications.Furthermore, the low specific surface areas of metal wireslimit the loading capacity of active materials. To solve theabove problems, designing porous structures for metal wiresseems promising but challenging.

Advances in carbon materials provide a promising plat-form for the development of fiber electrodes. Carbon fibers

emerged first, as they are already available at market. Themain obstacle to the use of carbon fibers lies in their relativelylow conductivities,[27] namely,< 102 Scm�1. Further enhancingthe conductivity by refining the crystal structure of carbonseems difficult because this modification causes the fibers tobecome rigid. Furthermore, nanostructured carbon materialssuch as CNTs[28] and graphene[29] were successfully made intocontinuous fibers with excellent mechanical and electricalproperties. Nanostructured carbon fibers can be made fromtheir aqueous dispersions,[30] which resembles the solution-spinning process of liquid crystalline polymers, or dry-spinning processes,[19] which are inspired by the ancienttechnology of cotton spinning.

Nanostructured carbon fibers such as CNT fibers havea hierarchical structure (Figure 1a),[31] in which nanometer-sized building blocks (Figure 1b) are assembled into bundles(Figure 1c) and the bundles further assemble into macro-scopically continuous fibers (Figure 1d). The hierarchicalassembly provides excellent mechanical and electrical proper-ties. CNT fibers are flexible with a low bending stiffnesses (D)according to the equation D = (p � d3 � E)/64,[32] where d andE correspond to the fiber diameter and Young�s modulus,respectively. In contrast to those of individual CNTs (Young�smodulus of ca. 1.8 TPa[33]), the nanostructure and hierarchi-cally aligned architecture of CNT fibers greatly reduce their

Huisheng Peng is Changjiang Chair Profes-sor at the Department of MacromolecularScience and Laboratory of Advanced Mate-rials at Fudan University. He and co-workershave invented a new family of fiber-shapedenergy harvesting devices including perov-skite solar cells and fluidic generators, fiber-shaped energy storage devices includinglithium-ion batteries, lithium-sulfur batteries,and metal-air batteries, fiber-shaped light-emitting devices, and fiber-shaped sensors,thus starting a new direction in fiberelectronics. He is now interested in theapplication of fiber electronics to solvebiomedical problems.

Figure 1. Assembly strategies to prepare nanostructured fiber electrodes with desirable mechanical andelectrical properties from CNTs. a) A fiber electrode with a hierarchical assembly. b)–d) Transmission andscanning electron microscopy images of CNTs, bundles and fibers, respectively. e) Comparison of thesimulated stress distribution of CNT fibers and carbon fibers under the same bending condition. f) Effectiveinteractions of aligned CNTs.[35] Reproduced with permission from American Chemical Society from Ref. [35].g) Comparison of the specific strength and stiffness between CNT fibers and other strong fibers.[36] Adaptedand reproduced with permission from The American Association for the Advancement of Science fromRef. [36]. h) A three-dimensional electron hopping model.[37] i) Fitting conductivity (s) and temperature (T) atone, two, and three dimensions (left to right).[37] Reproduced with permission from American ChemicalSociety from Ref. [37].

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Young�s moduli to about 10 GPa and 400 MPa, respective-ly.[34] Furthermore, the tensile stresses in CNT fibers areuniformly distributed along their length direction, whilesevere stress concentration occurs in non-nanostructuredcarbon fibers under bending (Figure 1 e). Regarding thehierarchically aligned structure, the CNT bundles shareexcellent mechanical properties with individual CNTs, andthe effective interactions among neighboring CNT bundlesfurther extend the outstanding mechanical properties to themacroscopic fiber (Figure 1 f).[35] As a result, state-of-the-artCNT fibers outperform other fiber materials in both specificstiffness and specific strength (Figure 1g).[36] Similarly, theyshow high conductivities of 104 S cm�1 and display a three-dimensional hopping conduction mechanism for efficientcharge transport[37] (Figure 1h,i). Thus, for the fiber electrodesin wearable devices, both electrical and mechanical propertiesneed to be balanced, for specific applications. Conductivitiesabove 104 Scm�1 are typically required for continuous fiberdevices, and matchable bending stiffnesses from neuron (ca.1.57 � 10�5 nnm2) to muscle (ca. 3.77 � 10�3 nnm) should becarefully designed (note that the size of cells was used as thediameter to calculate the bending stiffness).[38]

The fiber shape in the electrode offers unique advantages.Several primary fibers (Figure 2a) can be twisted into a multi-ply fiber (Figure 2b) and further over-twisted into a largerspring-like fiber (Figure 2 c). A variety of hierarchical gaps[39]

are formed, that is, nanometer-sized gaps among nanomate-rial building blocks such as CNTs (Figure 2d), micrometer-sized gaps among primary fibers (Figure 2e), and largermicrometer-sized gaps among the screw threads (Figure 2 f).The hierarchical gaps favor the diffusion of active materials orelectrolytes that infiltrate through the micrometer-sized andnanometer-sized gaps which results in both rapid transportand high loading (Figure 2g). As a demonstration, thedesigned red color rapidly moved along the length direction

of a spring-like fiber when one of its ends contacteda fluorescently labelled liquid (Figure 2h).

3. Configurations of Fiber Electronics

Compared with planar electronic devices, which typicallyhave a stacking configuration, one-dimensional fiber electro-des provide richer and unique configurations that can begenerally classified into three main categories: coaxial, twist-ing, and interlaced (Figure 3 a).

For the coaxial configuration, one fiber electrode serves asthe first core electrode, and the active layers or gel electrolyteand the second electrode are sequentially deposited onto it.For photovoltaic or lighting devices, the key is to producecontinuous active and transparent electrode layers on thecurved fiber surface.[40, 41] The active layer typically needs tobe very thin for effective charge separation and transport, andthe outer electrode layer should be transparent to guaranteesufficient light absorption or high light emission. For energy-storage or sensing fiber devices, the electrochemically activeor sensing materials can be much thicker, for example, tens tohundreds of micrometers,[42–44] allowing them to be easily andeffectively prepared with a continuous fabrication process.

For almost all kinds of fiber devices, the sheath electrodecan be changed to another fiber electrode, thereby formingthe twisting configuration. One fiber electrode is first coatedor incorporated with active materials and then wound withthe second fiber electrode. For photovoltaic or lightingdevices, the second fiber electrode should be flexible enoughto guarantee close contact with the active layer for effectivecharge separation and transport at the interface and shouldprevent the active layer from breaking when subjected todeformations during use. To this end, nanostructured carbonfibers can suitably meet the above requirements.[45] For

energy-storage and sensingfiber devices, there are nospecific requirements forthe intra-fiber contact, butthe fibers should have highspecific surface areas toachieve high loadings ofactive materials.[10, 46]

Fiber devices are oftenwoven into electronic tex-tiles for large-scale applica-tions. For both coaxial andtwisting fiber devices, twoelectrodes are connected atone end for use after beingwoven into a textile.[16, 47] Apossible challenge lies inthat once a short part ofa long fiber device fails, thewhole electronic textilebreaks down. One meansof overcoming this problemis weaving many fiber devi-ces with the same length

Figure 2. The interface design of fiber electrodes. a)–c) Assembly of primary fibers to form large fibers andfinally larger spring-like fibers. d)–f) Scanning electron microscopy images of the different sizes of gapsformed in (a–c). g) The infiltration of the dispersion into the spring-like fiber through the large and smallgaps. h) Fluorescence micrograph showing rapid transport of the labeled dispersion along the spring-likefiber. Reproduced with permission from Springer Nature from Ref. [39].

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organized in parallel together[42] so that the electronic textilescould work even if one or a few fiber devices fail; anothermethod consists of arranging the cathode and anode fibers aswarp and weft yarns during weaving. In other words, the fiberelectrodes are assembled into devices during the weavingprocess.[48] For large-scale production, the resulting interlacedconfiguration is compatible with the well-developed textiletechnology. For the coaxial and twisting configurations in thetextile, one anode contacts just one cathode. In contrast, in theinterlaced configuration, one anode contacts several catho-des, and vice versa. The charges separate and transportthrough the contact points between two kinds of fiberelectrodes.

The charge transport pathways differ among the threeconfigurations (Figure 3b), which is especially important forstructural design in fiber optoelectronics. The interlacedconfiguration is more favorable for light-harvesting and light-emitting devices. To harvest solar energy, for instance, thecounter electrode collects photocarriers along not only thecircumferential direction but also the neighboring longitudi-nal area. Therefore, the diameter and density of the counterelectrode are critical for power conversion efficiency. Thiseffect is similar to that in the interdigitated metal contacts incommercial planar photovoltaics, where the width and spac-ing of the interdigitated contacts are carefully designed toreach a balance between light absorption and charge collec-

tion.[49, 50] However, it is stillimpossible to determine theoptimal diameter and den-sity of the counter electrodein photovoltaic textiles.Moreover, there are muchfewer studies on the inter-laced configuration than thecoaxial and twisting config-urations.

The other configura-tions, such as hybrid fiberswith multimaterials co-de-posited inside polymer fi-bers by thermal drawingtechnique, are particularlyattractive to multifunctionaldevices, such as photodetec-tors and LEDs for opticalcommunication.[51]

Fiber devices sharepromising properties or ad-vantages compared withbulk and planar counter-parts based on their uniqueconfiguration. Taking thecoaxial configuration as anexample (Figure 3c), to har-vest sunlight, a fiber devicemay absorb light from 3608with its cylindrical shape,and it also offers a highlystable power supply that is

not affected by the angle of the incident light.[40] Therefore,for practical applications, the resulting photovoltaic textilecan output stable electricity to effectively power electronicdevices under deformations such as bending and twisting. Forenergy storage with O2 in air as the reactant, a 3608 solid–liquid–air interface enables enhanced ion transfer in the fiberdevice.[43] Of course, a fiber device can emit light at 3608,which is important for applications in biomedical fields suchas optogenetic stimulation of peripheral nerve bundles.[52] Itcan illuminate the entire nerve bundle in all angles, suggestingits potential as a powerful tool for biological research anddiagnosis. The fiber device may also receive stimuli, partic-ularly biological signals from the environment, at 3608 withboth high sensitivity and stability.[15] Upon implantation, thefiber sensor can accurately and stably detect chemical signalsof the target tissue.

4. Fiber Devices and Applications

Fiber devices are mainly classified into four categoriesbased on function, that is, energy harvesting, energy storage,sensing, and lighting devices, of which the recent progress andchallenges will be discussed below.

Figure 3. The three primary configurations of fiber-shaped electronic devices with unique charge transportpathways and properties. a) Coaxial (left), twisting (middle), and interlaced (right) configurations. b) Chargetransport pathways in the three configurations. c) Unique properties endowed by the fiber shape in energy-harvesting (for example, harvesting sunlight at 3608), energy-storage (for example, a metal–air battery withoxygen diffusion at 3608), sensing (receiving stimuli at 3608), and lighting (emitting light at 3608) devices,exemplified by the coaxial configuration.

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4.1. Energy Harvesting

Energy-harvesting fiber devices are expected to be woveninto fabrics and blended into daily clothes to convert energyfrom the environment and human body into electricity andfunction as the power supply for next-generation wearabledevices. Therefore, compared to the other types of fiberdevices, they face more stringent requirements, such as highpower conversion efficiency (PCE), safety, flexibility, andstretchability.

A variety of energy sources, including sunlight, mechan-ical movement, heat and water, have been converted intoelectricity in fiber devices. Photovoltaics started from dye-sensitized solar cells in a coaxial configuration with very lowPCEs.[53] Thus, the main efforts made were to enhance thePCE by developing fiber counter electrodes from metalwires[54] to nanostructured carbon fibers[45] and finally tonanostructured composite fibers[55] with higher specific sur-face areas and improved interface contacts. Accordingly,PCEs were increased from 2.78 % to 5.64 % and finally to8.45%. A further improvement in the available activeelectrochemical surface through hydrophilic modification ofthe counter electrode produced the highest PCE of 10 % todate.[56] The microstructure of TiO2 on the photoanode is alsocritical, and perpendicularly aligned nanotubes outperformedTiO2 nanoparticles owing to the shorter transport pathways ofcharges.[57]

In the above cases, liquid electrolytes are typicallyrequired; however, effective encapsulation to prevent theleakage of liquid electrolytes remains a challenge and may bea safety concern for wearable applications. Gel or solidelectrolytes were proposed but have much lower PCEs thanliquid electrolytes,[58] so many efforts were made to fabricateall-solid-state polymer solar cells in the fiber shape. Despitethe growing interest in increasing PCEs of planar polymersolar cells,[40, 59] the maximum PCE of the fiber-shapedpolymer solar cell remains relatively low (3.27%),[40] possiblybecause it is difficult to produce the desired thin andcontinuous photoactive layer (typically 100–400 nm)[60] ofsemiconducting polymers on a curved fiber surface. With therapid advancement in perovskite solar cells, promisingexamples of the realization of highly efficient all-solid-statefiber-shaped photovoltaic devices with a PCE of 9.49% haveappeared in just the past several years.[61] The large perovskitecrystals formed were the key to the high performance andmay be optimized for even higher PCEs in the future.

Mechanical energy, which is ubiquitous and closelyrelated to body movements, can be harvested by two kindsof devices, namely, piezoelectric and triboelectric nanogener-ators. A typical fiber piezoelectric nanogenerator consists ofcore fiber and sheath electrodes with piezoelectric materialssandwiched in between.[62] As the piezoelectric effect arisesfrom the breaking of the central symmetry of the crystalstructures in materials under an external force,[63] theselection of piezoelectric materials and the design of devicearchitectures are two key factors in improving its perfor-mance. ZnO and polyvinylidene fluoride are recognized astwo promising candidates, as they have good piezoelectric

constants and can be easily grown/coated on curved fiberelectrodes.[64, 65]

Regarding the device architecture, the main efforts madewere to increase the deformation and polarization of thepiezoelectric layers under deformation.[64]

Triboelectric nanogenerators, which derive from thecombination of triboelectrification and electrostatic induc-tion,[66] typically consist of two materials with differentelectron affinities, allowing them to generate an electricalpotential at the interface when contact friction occurs. Thesurface charge density and morphology of the frictionmaterials are the key factors determining the performance.[64]

The fiber triboelectric nanogenerator shows the same struc-ture as the fiber piezoelectric nanogenerator, with activematerials sandwiched between the core fiber and sheathelectrodes.[67] Nanoscale roughness is generally introduced tothe tribo-surfaces to improve the output power density. Forexample, with the introduction of different nanopatterns onthe polydimethylsiloxane layer, the power density followedthe trend of film < line < cube < pyramid.[68] The design ofnanostructured or microstructured active layers may repre-sent an important strategy for achieving high power outputs inthe future.

The human body is a permanent heat sink, so theconversion of body heat to electricity in various textilesthrough the use of fiber generators is appealing. Typically,there are two ways to make wearable thermoelectric (TE)fabric: printing TE materials onto textiles[69] or embeddingTE materials inside fibers.[70] Besides, the energy stored inaqueous solutions such as sweat and blood with various forms,including flowing ionic water, stationary water and mois-ture,[71] can also be harvested and converted into electricity.For instance, the mechanical energy of flowing water can beconverted into electricity with a high PCE of 23.3%.[72]

Although there are several successful examples, the develop-ment of both thermoelectric and water-to-electricity gener-ators is at an early stage. Efforts should be made to enhancethe PCEs by synthesizing new active materials and designingnovel assembly techniques.

4.2. Energy Storage

As the current output of energy-harvesting fibers islargely dependent on the environment, energy-storage fibersare designed to work as a power source or to be integratedwith energy-harvesting fibers to provide a stable and efficientpower supply for wearable electronics.

Fiber-shaped capacitor appeared first, possibly because itwas relatively easier to fabricate them.[73] Dielectric materials,such as poly(vinylidene fluoride), separated two electrodesmade of conducting polymers. These materials were thermallydrawn into capacitor fibers.[74] To improve the energy-storagecapacity, supercapacitors were then developed. For instance,two identical CNT fibers whose surfaces were coated witha gel electrolyte were twisted together to produce a fiber-shaped electric double-layer capacitor with an energy densityof 0.6 Whkg�1.[30] After modifications to the CNT fibers, suchas the introduction of nitrogen-doped reduced graphene

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oxide sheets that may interconnect the neighboring CNTs toprovide larger specific surface areas and higher electricalconductivities, an improved energy density of 6.3 mWhcm�3

was achieved.[75] Other active materials capable of providingpseudocapacitance, such as conducting polymers[76] and metaloxides,[77] have also been incorporated into the fiber electrodeto further enhance their energy density.

For increased energy densities, fiber-shaped lithium-ionbatteries in which two fiber electrodes were twisted togetherand served as skeletons supporting active materials for boththe cathode and anode were proposed.[78, 79] For nanostruc-tured carbon fiber electrodes,[80] active materials with highloadings up to 90 % and an energy density of 17.7 mWhcm�3

or 27 Whkg�1 were incorporated. Later, metal–air batteries,such as lithium–air[44] and zinc–air batteries,[81] were devel-oped to further improve the energy density. These batterieswere typically made in a coaxial configuration with a metalwire as the inner anode, polymer gel as the electrolyte andcarbon-based film as the outer cathode. Obviously, the use ofmetal electrodes may lead to serious safety problems andreduce device flexibility. Accordingly, alternate anodes suchas lithiated silicon/CNT hybrid fibers[43] were used to replacethe metal wire to produce lithium-ion air batteries with anenergy density of 512 Whkg�1.

Fiber-shaped energy-storage devices can tolerate variousdeformations, such as bending, twisting and tying; forexample, the storage properties remained almost unchangedafter bending for tens of thousands of cycles.[43] They are alsomade stretchable by incorporating a spring-like structure,such as by over-twisting several aligned CNT fibers togetherinto coiled loops[82] (Figure 2c). Compared with their planarcounterparts that need elastomeric substrates, stretchablefiber batteries do not require substrates. With the decrease ofthe volume and weight, the energy densities dramaticallyincreased. The flexible energy-storage devices were woveninto soft textiles to power wearable electronics.[39] Aqueousfiber-shaped energy-storage devices can also efficiently powermedical devices needing low electricity to function in vivo anduse body fluids as the electrolyte.[83] To this end, they may beinjected into the body in a minimally invasive way withreduced pain and cost, advantageous over conventionalsurgery.

4.3. Sensing

Sensing fibers are expected to provide real-time monitor-ing of physiological activities and are promising for mobileelectronic engineering and public health. Typically, they canbe divided into two main categories, that is, in vivo and invitro detection. For in vivo detection, sensing fibers efficientlypenetrated into various tissues without complex surgeries orobvious damage to the film sensors. Decades ago, metalwires[84] were developed to detect electricity signals ofneurons and monitor brain activities. They enabled importantdiscoveries in neuroscience, ranging from the discovery ofgrid cells to the mapping and stimulation of the motor cortex.However, rigid metal electrodes often lead to acute foreignbody reactions and cause damage to surrounding tissues

during use, including neuronal death and glial scarring aroundthe probe.[38]

Low-modulus elastomers showing reduced mechanicalmismatches with biological tissue were thus developed toobtain more stable and effective interfaces. For example,a polymer-based elastomeric composite with a Young�smodulus five orders of magnitude lower than that of itstungsten counterparts can be extruded. Improved neuronattachment and viability were observed during an electro-physiology experiment.[85] Another effective method is todesign of nanostructured or microstructured fiber electrodesto achieve better electronic–tissue interfaces. For example,the bond between the probe and tissue could be greatlyenhanced by forming nanometer-sized curves and micro-meter-sized needles on a silicon fiber.[86]

Multifunctional and high-density recording[87, 88] repre-sents another trend of fiber sensors in vivo. With theadvancement of optical fibers and genetic engineering, opticalfunction was introduced to fiber probes and has boosted theiroptogenetic capabilities.[89] By using a hybrid optical fiber, wemay simultaneously record signals and selectively turnspecific neurons on or off via light. To integrate morefunctions, thermal drawing had been widely explored. Itallowed multimaterials or devices in one fiber with thevarious shapes, positions, structures and interfaces to be welldefined. The as-fabricated fiber devices realized multiplefunctions, for example, dense recording, physiological mon-itoring, stimulation, and drug delivery.[90–92] Additionally,because of the many different encapsulation materialsapplicable, a wide range of elasticities can be obtained; forexample, up to 500 % elastic deformation has been demon-strated by the utilization of thermoplastic elastomers.[93]

For in vitro detection, optical fibers also initially enhancedthe development of physical sensors starting from acousto-optic interactions,[94] and a number of fiber-optic sensors forboth typical[95] and multiplex[96] detection have been widelyexplored. Fiber sensors were further assembled into textiles tomonitor a variety of physiological signals, such as musclemotion, heart rate, vocal cord vibration, body temperature,and biomarkers in sweat. Those mechanical strain sensorshave been most widely explored. Under a slightly varyingforce, such as a pulse beat, a change in electrical resistance,[97]

capacitance,[22] or the piezoelectric[98] effect on the fibersensor may occur, and physical signals are thus transformedinto electrical signals for output. Apart from physical signals,different kinds of sensing fibers were woven into textiles tosimultaneously detect concentrations of chemical signals suchas glucose; Na+, K+, and Ca2+ ions; and pH in sweat.[42]

4.4. Lighting

For application in vivo, the advancements in lightingfibers promote the development of implantable devices,especially in optogenetics or bio-optical treatments. To beused in vitro, lighting fibers can be woven into wearable fabricdisplays, which may revolutionize display technology.

Organic light-emitting diodes represent one of the mostwidely explored kinds of light-emitting devices and were first

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made into a fiber shape with lengths of millimeters by thermalvacuum deposition.[99] Later, with a solution-based dip-coat-ing process, organic active materials were coated on the fiberelectrode, followed by evaporation of a metal contact on oneside of the cylindrical fiber as the outer electrode, leaving theother side as a light-emitting area.[41, 100] Similar to the fiber-shaped organic solar cell, it is challenging to obtain thin yetcontinuous organic active layers on curved surfaces; thus,further development in compatible new fiber electrodes andactive materials along with more effective coating methods isrequired.

In contrast to the active layers in organic light-emittingdiodes, which have a multi-layered structure, the polymerelectroluminescent layer in PLECs will form an in situ p-i-n junction for charge injection from electrodes,[101] relievingthe device from the requirements of exquisite band alignmentand surface roughness and thus making it more feasible forthem to be constructed on a fiber electrode. As a proof ofconcept, the fiber-shaped PLEC in a coaxial configurationwith an electroluminescent polymer layer sandwiched be-tween the cathode and a transparent anode was made.[52] Thedevelopment of fiber-shaped PLEC has just started and needsmore systematic studies to realize long lifetimes and contin-uous production.

For the above organic materials, the resulting thin activelayers, typically at tens to hundreds of nanometers, arevulnerable to friction, twisting, or squeezing force during use,and the organic luminescent materials are sensitive to oxygenand humidity. Therefore, fiber-shaped lighting devices shouldbe carefully sealed for high stability. Alternatively, fiber-shaped lighting devices with inorganic active materials such asZnS-based phosphors are also widely studied.[102] A typicalZnS-based lighting fiber with an outer electroluminescentlayer and two inner parallel hydrogel electrodes was preparedvia an extrusion process.[103] As each inorganic luminescentparticle may act as an independent light-emitting centerunder an alternating electric field, the mechanical robustnessof the fiber was installed by the polymer matrix. The fiber wasalso flexible and stretchable. Inorganic material-based light-ing fibers need relatively high operating voltages and haveshort lifetimes, so additional efforts should be made toexplore novel phosphors to lower operating voltages.

4.5. With Other Functions

Fiber devices with the other functions, for example,transistors as key components in electronic systems, havealso appeared.[104] For a typical fiber transistor, the source anddrain contact patterned on one fiber electrode were con-nected by active materials, and the second fiber electrode wasinterlaced with the channel between the source and draincontact, serving as the gate.[105] After modification of theabove architecture, further effort was subsequently made toconstruct preliminary logic circuits by assembling individualfibers coated with active transistor components into a fabricwith the designated junction realized by an electrolyte.[7]

However, developing fiber transistors into sophisticated logic

circuits remains challenging, despite the promising prospec-tive of adopting this approach for micro-electronics.

4.6. Integration of Fiber Devices

Fiber devices may achieve complex integrations that aredifficult or even impossible for their planar counterparts. Onthe one hand, several of the same type of devices, such assupercapacitors, can be connected in series along the sharingfiber electrode to output high voltages in a manner thatmimics the electric eel,[106] or they may be connected inparallel to produce high electrical currents.[46, 47] On the otherhand, different types of devices, such as energy-harvestingand energy-storage devices, may be integrated in series toharvest and store energy at the same time.[107] Furthermore, itis also possible to design a core-sheath structure with anenergy-harvesting sheath and energy-storage core.[108] Gen-erally, supercapacitors show a high power density but a lowenergy density, while the opposite is true for lithium-ionbatteries. Interestingly, three fiber electrodes incorporatedwith battery-active and capacitor-active materials were twist-ed together to produce both high energy and power densitiesin one device.[109]

The integration of fiber devices into textiles is alsoeffective in realizing intended applications. Two kinds ofenergy-harvesting fibers, photovoltaic and piezoelectric fi-bers, have been effectively integrated into one fabric througha conventional weaving process for simultaneously harvestingsolar and mechanical energy in textiles.[110] Attempts havealso been made to integrate electronic fibers with differentfunctions into fabrics for novel applications, for example,thermally drawn light-emitting and light-receiving fibers havebeen woven into one piece of fabric to obtain a wearableoptical communication system that can monitor physiologicalstatus in real time.[111]

5. Outlook

In recognition of the growing importance of flexible, soft,wearable, and breathable electronic systems, electronic de-vices in a fiber shape are being developed at an unprece-dented speed. Generally, the active materials widely used inbulky and planar devices are directly transferred to fiberdevices.[40, 42, 52,80] It is rare to synthesize new materials tospecifically fit fiber electronics, while the curved surface ofthe fiber electrode does differ from the flat surface. There isalso a lack of deep understanding of the difference betweencurved surface and flat surface in terms of microstructure andfunctionality, and further design is needed to optimize theinterface for high performance. More importantly, althoughsome unique phenomena are observed for fiber electronics,systematic investigation of the underlying mechanism andrules, critical for guiding further development of fiberelectronics, is rare.

The fiber electronic devices can be used in many fields.For one example, scaling up the production of fiber batteriesis underway. These fiber batteries are then woven into flexible

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textiles to power wearableelectronic products and bio-medical facilities, which re-mains challenging for thecurrent bulky batteries. Foranother example, fiber sen-sors are found to play a crit-ical role in implantable bio-medical devices. The rela-tively low bending stiffnessand 3608 interacting inter-face offer the advantages ofdeep penetration and long-term non-destructive detec-tion in vulnerable organssuch as the brain, which ishard to achieve for the pla-nar electrodes.

From the applicationviewpoint, the develop-ments of general and effec-tive fabrication methods areurgently needed to bringfiber electronics out of thelab and into large-scale pro-duction. In particular, en-capsulating fiber devices remains challenging in terms of bothmaterials and technology. When traditional encapsulationmaterials and technologies are used, the stability of the fiberdevice is lower than that of its planar counterparts.[61]

Conventional potting process, which consists of immersingthe electronic parts in liquid resin in a confined mold andcuring, presents new challenges for one-dimensional cylin-drical structures in terms of consistent quality and scalability.Conformal coating also remains as one of the key challengesas microbeads may easily occur, especially during the large-scale production. Exploring encapsulation techniques com-patible with fiber devices is greatly desired. Possible solutionsmay include developing heat shrink tubes with effectivemoisture and oxygen barrier coating based on the discovery ofnew materials and preparation methods. Although thousandsof publications are available, it is still difficult to compareeven the same type of fiber devices, and effective evaluationstandards are urgently needed.

Fiber electronics may even revolutionize a number ofmultidisciplinary fields (Figure 4). For instance, next-gener-ation daily clothes may harvest sufficient energy from theenvironment and store it as electricity to power variouselectronic products effectively both in vitro and in vivo. Theycan also be used for communicating in order to free humanbeings from heavy and bulky electronic devices such as cellphones, for remotely turning on/off household electricalappliances, and for operating machines for production evenat home. These clothes can receive and process data throughfiber-shaped electronic circuits and textile-type displays andmake the internet of things convenient and efficient. Thinclothes based on fiber electronics can rapidly adjust to a broadrange of temperatures to serve as spacesuits for spaceexploration, where we have to be able to survive extremely

hot daytimes and cold nights. Similarly, fiber sensors can beinjected to simultaneously monitor health conditions, detectsevere diseases at an early stage and administer treatment.The roles that fiber electronics can play in our future lives areenormous.

Acknowledgements

This work was supported by MOST (2016YFA0203302),NSFC (21634003, 51573027, 51673043, 21604012, 21805044,21875042), STCSM (16JC1400702, 17QA1400400,18QA1400700, 18QA1400800, 19QA1400500), SHMEC(2017-01-07-00-07-E00062), and Yanchang Petroleum Group.The authors appreciated the helpful advice and languageediting from Lingyi Bi.

Conflict of interest

The authors declare no conflict of interest.

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Manuscript received: February 24, 2019Accepted manuscript online: April 15, 2019Version of record online: && &&, &&&&

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Minireviews

Energy Storage

X. Xu, S. Xie, Y. Zhang,H. Peng* &&&&—&&&&

The Rise of Fiber Electronics

Fiber electronics, a new multidisciplinarydirection in applied chemistry, is pre-sented. The unique properties of a newfamily of fiber-shaped electronic devicesare summarized. The focus is on fiberelectrode materials based on a one-dimensional configuration and their dis-tinctive interfaces, various devices, andpromising applications.

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