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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1606339 (1 of 16)

Healable Transparent Electronic Devices

Crystal Shaojuan Luo, Pengbo Wan,* Hui Yang, Sayyed Asim Ali Shah, and Xiaodong Chen*

With the advent of the digital era, healable electronic devices are being developed to alleviate the propagation of breakdown in electronics due to the mechanical damage caused by bending, accidental cutting or scratching. Meanwhile, flexible transparent electronics, exhibiting high transmittance and robust flexibility, are drawing enormous research efforts due to their potential applications in various integrated wearable electronics. However, the breakdown of flexible transparent electronics seriously limits their reli-ability and lifetime. Therefore, transparent healable electronics are desired to tackle these problems, yet most of the healable electronics are not trans-parent nowadays. The combination of high performance, healability, and transparency into electronics is often mutually exclusive. Herein, after a brief introduction of self-healing materials, healable electronics, and flexible transparent electronics, the recent progress in the healable electronic devices without transparency is reviewed in detail. Then, healable transparent elec-tronic devices with high transparency, robust portability, and reliable flexibility are summarized. They are drawing great attention owing to their potential application in optical devices as well as smart wearable and integrated opto-electronic devices. Following that, the critical challenges and prospects are highlighted for the development of healable transparent electronic devices.

DOI: 10.1002/adfm.201606339

electronic waste, maintenance cost, and raw materials consumption.[1–7] Self-healing is known to be a remarkable intrinsic ability of living organisms to repair damage, to increase their chance of survival, and to act as protective barrier to extend their lifetime. Inspired from these self-healing features of living organisms, self-healable materials are developed to not only repair the unexpected internal or external damages but also recover the functions and enhance the reliability of the materials. Consequently, the research focusing on mechanical and structural restoration and function recovery of self-healing materials have gained immense popularity in recent years.[8–18] Recently, numerous anti-corrosive,[19,20] anti-fouling,[21] superhydrophobic,[22–25] and electrically conductive materials[26] that possess self-healing properties are already successfully prepared; they are promising candidates for the fabrication of diverse functional electronic devices with strong robustness and long lifetime.[27]

Towards the fabrication of various functional healable devices, self-healable materials with easily achieved healability are indispensable components, which can assure the configu-ration integrality and original function recoverability of the devices after damage.[28–32] These healable materials can be fab-ricated via extrinsic self-healing through releasing embedded healing agent[33–37] or the intrinsic self-healing through recom-bining the inherent reversible dynamic covalent bonds[26,38–42] and reconstructing the noncovalent bonds[43–48] between the cracking interfaces. Driven by the recent achievements, there are exciting applications of self-healing materials employed in electrical conductors,[40,49,50] supercapacitors,[27] electronic skin,[46] and lithium ion batteries[51] with enhanced lifetime and durability.

Flexible transparent electronics, featured with reliable wear-ability, high transmittance, and robust flexibility, are attracting enormous research efforts on account of their potential applica-tions in diverse integrated wearable electronics. However, the breakdown of flexible, transparent electronics due to mechan-ical damage seriously restricts their reliability and lifetime. Therefore, transparent electronics incorporated with robust healability are desirably and urgently developed to avoid these problems.[8–18] Despite the fact that self-healing materials can recover both structure and function of the devices from damage, it is rare that these devices are visually transparent.

Healable Devices

Dr. C. S. J. Luo, Prof. P. B. Wan, Dr. S. A. A. ShahCenter of Advanced Elastomer MaterialsState Key Laboratory of Organic-Inorganic CompositesBeijing University of Chemical TechnologyBeijing 100029, P.R. ChinaE-mail: [email protected]. H. Yang, Prof. X. D. ChenSchool of Materials Science and EngineeringNanyang Technological University50 Nanyang Avenue, Singapore 639798, SingaporeE-mail: [email protected]. C. S. J. LuoShenzhen Key Laboratory of Two-Dimensional Materials and DevicesShenzhen UniversityShenzhen 518060, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/adfm.201606339.

1. Introduction

As the rapid development of smart, portable, and flexible elec-tronics in this modern era, the failure of electronics due to the mechanical damage caused by bending, accidental cutting, or scratching, even though caused by usage over time, has presented a huge problem. It is accompanied by enormous

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Thus, the highlights of healable transparent electronic devices (HTEDs) with high transparency, convenient portability, and reliable functionality are drawing great attention because of their potential applications in integrated optoelectronic devices, e.g., solar cell, photodetector, touch screen, smart window, dis-play, thin film transistor, and portable phone. Therefore, it is significant to develop HTEDs possessing desired function, optical transparency, and self-healable properties, as shown in Figure 1.

In order to achieve a HTED capable of the high transparency and superior healability, the choice and design of functional healable materials and optical transparent structures are fore-most in importance. A superior HTED should be embedded with following significant features: (i) good transparency and processability with light weight, (ii) operation at ambient envi-ronment, (iii) easy self-healing without deterioration, (iv) high and stable working performance, and (v) low cost and be envi-ronmentally friendly. Although sustained progress has been achieved in developing self-healing materials, the fabrication of HTED remains a huge challenge. Creating electronic devices with high performance, reliable healability, and excellent trans-parency is not so trivial, since the combination of these proper-ties into electronics is often mutually exclusive. More efforts are needed to develop transparent materials with ultrahigh heal-ability performance in electronics and to solve the intractable scientific and technical issues in electronic devices.[11,40,52–58]

Herein, after a brief introduction of the self-healable mate-rials, healable electronics, and flexible transparent electronics, we reviewed the recent progress of self-healing electronic devices without transparency including electronic skin, super-capacitor, and lithium-ion battery in details. Then we reviewed and highlighted the progress in healable transparent electronic devices with high transparency, robust portability, and reliable flexibility from the healable transparent conductors to devices (gas sensor, capacitive touch screen and artificial muscle). Fur-thermore, the critical challenges and prospects are emphasized for developing HTEDs to fulfill the future demands of high per-formance and integrated portability optoelectronic devices with multiple functions.

2. Healable Electronic Devices

Conductive polymers and their composites inherit excellent manufacturability and flexibility, and also possess favorable conductivity that is used widely in electronic devices, for instance, solar cells, sensors, displays, actuators and energy storage devices.[59] The circuit board of electronic devices would be ruined by a tiny mechanical damage of the electronic con-nector or component. Due to the high integration density of circuit boards, the replacement of the damaged circuit board is complicated and uneconomical, yet such a problem may be resolved by implementation of electrically conductive paint materials with self-healing ability. These revolutionary self-healing materials can recover their mechanics and functions after unexpected damage. Among the strategies that have been proposed for the preparation of self-healing conductive mate-rials, the mechanism of extrinsic self-healing through releasing the embedded healing agent has been widely investigated.[33,34]

Moore and co-workers reported a microcapsule system which is able to restore the conductivity of mechanically dam-aged electronic devices.[33] Solutions of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) in diverse solvents are blended into poly(urea-formaldehyde) core-shell microcap-sules independently. Rupture of a blend of TTF-containing microcapsules and TCNQ-containing microcapsules leads to the formation of the TTF-TCNQ crystalline salt, which shows conductivity up to ca. 400–500 S cm-1, making a partial res-toration of conductivity. Apart from in situ formation of the conductive repairing agent, they made the microcapsules with the encapsulation of liquid conductive species, Ga–In liquid metal alloy,[34] and silver particles dissolved in hexyl acetate with polymer binder layer.[1] Upon crack intrusion and the rup-ture of the microcapsules, the encapsulated liquid conductive species were released to the damaged area and restore electrical

Pengbo Wan received his B.S. degree (2006) at Wuhan University. Then, he was recommended to join Prof. Xi Zhang’s group as a Ph.D. candidate at Tsinghua University and received his Ph.D. degree in 2011. During his Ph.D. studies, he worked as an exchange Ph.D. student at University of Leuven, Belgium. After working as

a research fellow at Nanyang Technological University in Singapore, he joined Beijing University of Chemical Technology as an Associate Professor in 2013. His research interests include organic/inorganic hybrid nanomate-rials for sensors and energy conversion and storage devices, stimuli-responsive supramolecular assembly, and nanoelectronics.

Prof. Xiaodong Chen received his Ph.D. degree in biochemistry from University of Muenster (Germany) in 2006. After working as a postdoctoral fellow at Northwestern University (USA), he started his inde-pendent research career as a Singapore National Research Foundation Fellow and Nanyang Assistant Professor

at Nanyang Technological University in 2009. He was promoted to Associate Professor with tenure in September of 2013 and Professor in September of 2016. His research interests include interactive materials and devices, pro-grammable materials for energy conversion, and integrated nano-biointerface.

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conductivity. Due to the depletion of conductive healing agents, it is impossible to apply multiple healings in the same spot after a single damage, although this method is of autonomous self-healing nature. Thus healable materials based on revers-ible chemical bonding are capable of multiple self-healing or mending, which are alternatives to the microcapsule-based self-healing materials.

Williams and colleagues reported a novel class of organo-metallic polymer with conductivity at the order of 10-3 S/cm, which contained an N-heterocyclic carbine and a transition metal, can be used as self-healing conductor. The conductivity of the polymer was recoverable upon heating at 200 °C.[26] Burnworth and colleagues presented that a metallo-supramo-lecular polymer could concomitant reversibly decrease the poly-mers’ molecular mass and viscosity by selectively converting ultraviolet light stimuli to localized heat, leading a quick and efficient defect healing.[11] Liebler and co-workers developed a spontaneous healable thermoplastic elastomer material based on supramolecular interactions.[39,43,44] Zhang and co-workers reported a non-cytotoxic healable supramolecular elastomer synthesized using small-molecular biological acids (sebacic acid and citric acid) as raw materials through hydrogen-bonding interactions. The supramolecular elastomer exhibited spon-taneous self-healing behavior with the self-healing efficiency of 98.6% at room temperature without external assistance, ascribing to the efficient self-healing ability of the hydrogen bonding in the materials.[60] These smart supramolecular mate-rials systems exhibit extremely useful effect in raising structural

safety, materials lifetime, and environmental sustainability, as well as enabling electronic applications.

2.1. Healable Electronic Skin

An ideal biomimetic electronic sensor should possess mechan-ical sensing and repeatable self-healing capability to achieve the requirements of electronic skin, which can be used in emerging fields, for instance, soft robotics and biomimetic prostheses; that is, self-healing electrode and tactile sensor are simultane-ously required.[61–63] However, it is still a challenging task to combine all these properties together. The lack of high electrical conductivity in self-healing materials becomes the most serious obstacle. Motivated by this, Bao and co-workers designed a supramolecular organic polymer composite with mechanical sensing and electrical self-healing properties by embedding nickel nanostructured microparticles (µNi).[46] The composite exhibited a bulk conductivity as high as 40 S cm-1, which was four orders of magnitude higher than the self-healing organo-metallic polymer reported by Williams.[26] During a mechanical damage event, a large quantity of weak hydrogen bonds was provided by the supramolecular polymeric hydrogen-bonding network. Moreover, the low glass transition temperature (Tg) of polymer enabled the chains to rearrange and diffuse on the fractured interfaces, thereby allowing ambient temperature self-healing.[31] Finally, through thermal crosslinking with urea, the µNi were dispersed into the polymer network via mixing process, as displayed in Figure 2.

The concentration of µNi was discreetly controlled at about 15 vol%, which is on the margin of the percolation threshold. The resistance of the composite could be reduced while the µNi particles were approached more closely to one another. This result was driven by the compressive stresses when the composite material was bended. Similarly, a tactile sensor could be constructed by sandwiching the piezoresistive composite between two layers of the self-healing conductive composite (Figure 3), showing the exponentially increased conductivity depending on stress. It is pressure- and flexion-sensitive and appropriate for electronic skin applications since the resist-ance of composite changes inversely with the applied flexion and tactile forces. The initial conductivity of the rupture could be restored with ≈90% efficiency after 15 s and the mechan-ical properties could be entirely restored after healing for ≈10 min. These results suggested that self-healing conductive and piezoresistive materials can mimic natural skin’s tactile and self-healing capability, while the applications of electronic skin systems could be potentially expanded.

2.2. Healable Supercapacitor

Recently, the increasing demand for high-performance energy storage devices is inescapable as fast growth of wearable elec-tronics and electric vehicles. Nevertheless, the worldwide energy dependency mostly on fossil fuels may lead to an energy crisis in the future. Therefore, renewable energy obtained from the sun, wind, waterpower or other renewable sources has been dramatically developed in recent years. The harvest

Figure 1. The healable transparent electronic devices with desirable electronic functions, robust optical transparency and reliable self-healing capability. (a-b) Reproduced with permission.[45] Copyright 2012, WILEY-VCH Verlag GmbH. (c) Reproduced with permission.[57] Copyright 2017, WILEY-VCH Verlag GmbH. (d) Reproduced with permission.[56] Copyright 2014, American Chemical Society. (e-f) Reproduced with permission.[58] Copyright 2015, WILEY-VCH Verlag GmbH.

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system must be equipped with a stable energy storage device to generate continual energy for practical usage since that sun-light and wind is unstable.[64,65] Supercapacitor is a promising class of energy storage devices, which is attracting enormous attention because of their fast charge/discharge rates, high power/energy densities, and long cycle lifetimes.[66–69] Until

now, the main direction of the current research on supercapac-itors is the fabrication of novel electrode materials with higher capacitance and unique configurations with greater compat-ibility. The electrode materials of these supercapacitors would become susceptible in bending or charge/discharge processes during practical applications, and the polymer-based flexible

Figure 2. Preparation of the healable composite. a) Proposed interaction among oligomer chains and µNi particles. The primary hydrogen bonds were formed. b) SEM of the oligomer-coated on the nano-corrugated µNi particle surface. Scale bar, 1 µm. c) Left: a randomly branched oligomer network in chloroform. Middle: a homogeneous suspension of µNi particles in the oligomer network. Right: optical image of the self-healing composite film with the flexibility. Scale bar, 1 mm. Reproduced with permission.[46] Copyright 2012, Macmillan Publishers Limited.

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substrates may possibly be mechanically cracked upon defor-mation over time or accidental cutting. These failures would cause the breakdown of the electronic devices. Chen and co-workers reported a mechanically and electrically healable supercapacitor to remedy the striking damage in the mechan-ical sustainability.[27]

The electrodes of supercapacitor were fabricated by uniformly casting acid-treated single-walled carbon nanotube (SWCNT) films with a large number of carboxylic and hydroxyl groups onto self-healing substrates. The self-healing substrates were composed of a supramolecular network with well-dispersed flower-like TiO2 nanospheres. Both the chains and cross-links with a large quantity of hydrogen bonds in supramolecular networks were obtained, which can guarantee the self-healing by bringing together cracked interfaces through dynamical association of hydrogen bonds.[39,46] Furthermore, the shape of supramolecular networks could be maintained by hydrogen bonds from the interaction between TiO2 nanospheres and supramoleculars (Figure 4a). Meanwhile, both the electro-lyte and separator of the polyvinyl pyrrolidone (PVP)-H2SO4 gel could be self-healable. The separated SWCNT layer in the ruptured supercapacitors was contacted again by the lateral movement of the self-healing layer, resulting in the enabled res-toration of the configuration and conductivity (Figure 4b). The as-prepared flexible all-solid-state supercapacitors demonstrated superior electrochemical performance with reliable self-healing

capability in the restoration (85.7% in specific capacitance), even after the 5th cutting. Therefore, the energy storage devices with expanded lifetime may provide the stepping stone for the successful preparation of next-generation healable electronics in the future.

2.3. Healable Lithium-Ion Battery

Lithium-ion batteries are another type of device that can meet most of the demands of energy storage and harvest system to store sustainable energy for practical applications.[70,71] How-ever, many rechargeable energy storage devices’ lifetimes are restricted by the predicament of mechanical fractures over the charge/discharge cycling process. Structural changes nor-mally occurred during the electrochemical reactions between the lithium battery materials, resulting in degradation and ultimate damage of the batteries over cycling. Recently, silicon is regarded as a potential anode material with the specific capacity theoretically ten times higher than traditional graphite anode, which is suffering from short cycling lifetime and quick capacity decay. It can be ascribed to volumetrical expansion for three times between lithium insertion (lithiation) and lithium extraction (delithiation) reactions, causing the cracking and pulverization of the electrodes and subsequently leading to the lack of efficient electrical contact and excessive generation

Figure 3. a) I–V curve for a light-emitting diode connected by the healable conductive composite wire. Inset: optical image of circuit (scale bar, 10 mm); b) Observation of the electrical healing process by resistance measurements at room temperature. c) Repeated electrical healing for 3 cuts at the same damaged location. d) Illustration of the healable process of a self-healing electrical conductor with a light-emitting diode bulb in series. 1, fresh conductor; 2, entirely damaged conductor (open circuit); 3, healing; 4, healed film being flexed to demonstrate its mechanical strength and flexibility 5 min later. Reproduced with permission.[46] Copyright 2012, Macmillan Publishers Limited.

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of solid-electrolyte interphase (SEI).[72–74] Mechanical fractures and damages were still observed even when high-modulus poly mer binders or metal alloys were incorporated to the silicon materials.

To overcome the challenge for rechargeable batteries, the incorporation of self-healing feature is desirable. Wang and co-workers[51] improved the cycling lifetime by coating a thin layer of self-healing polymer (SHP) with hydrogen bonding onto the silicon microparticles (SiMPs), resulting in the enabled healing of the mechanical damage and the increased lifetime of the electrodes, in comparison with the conventional polymer binders.

As shown in Figure 5a, the self-healing electrode is obtained by coating a thin layer of soft SHP onto the silicon electrode. The mechanical damage and cracks of the electrode could be repaired by the stretchable SHP, demonstrating more stable connections among the SiMPs in mechanics and electrical conductivity. The polymer binder was a conductive composite comprised of conductive carbon black nanoparticles (CB) and SHP (Figure 5b). The SHP is the self-healable supramo-lecular polymer network with randomly branched hydrogen-bonding. By the dynamic re-association of hydrogen bonds at the damaged interfaces, the spontaneous self-healing could be obtained (Figure 5c).[46] Finally, an excellent ten-times-longer cycle lifetime was attained, as compared to the previously reported anodes made from SiMPs and a high capacity up to ≈3000 mA h g-1 was still retained. The self-healing cracks of the coating with a relatively stable SEI formation and minimal side reactions during galvanostatic cycling can be spontaneously formed by the dynamic re-association of hydrogen bonds in the SHP. This self-healing electrode can provide promising oppor-tunities for substituting other electrode materials which are stuck with mechanical cracks during electrochemical process.

3. Healable Transparent Electronic Devices

In the past decades, the demands for integrated optoelectronic devices, such as touch screen, smart window, display, thin film transistor, and portable phone, are growing in leaps and bounds. Using a material with high visible light transparency as an essential component in the transparent electronic devices has attracted increasing academic attention. Like conventional electronics, the transparent electronics will also suffer deterio-ration by over-time usage and unexpected cutting or scratching, usually leading to accidental failure of electronic devices. Aimed at solving this problem, healable transparent electronics are proposed, which would reduce power and resource costs, even improve functional reliability. Until now, nearly all of the self-healing electronics are non-transparent. Thus it is of great importance to develop effective ways to fabricate healable, transparent, and conductive films-based electronics integrated with high transparency in both functional materials and self-healing substrates. On the basis of the above discussion, the pivotal step to execute the integration of a HTED is to develop healable transparent materials and design a hierarchical struc-ture of the devices.

3.1. Healable Transparent Conductors

3.1.1. Healable Transparent Conductor Based on Polyelectrolyte Multilayer Film

Layer-by-layer (LbL) assembly is a reliable technique for pre-paring nanocomposite films with well-tailored functions by alternate deposition of materials with complementary chemical interactions. Sun and co-workers showed that LbL-assembled

Figure 4. a) The fabrication for the flexible, healable supercapacitor. The self-healing substrate is composed of hierarchical TiO2 spheres (black spheres) and a supramolecular network (red wires) with hydrogen bonds. After coating CNT films on the self-healing substrates, the sandwiched supercapaci-tors were assembled. b) Images of as-prepared CNT films on self-healing substrates with the connected LED bulb in a circuit. (b1) the original; (b2) after damaged; (b3) after healing. c) Schematic illustration of electrically conductive self-healing capabilities for CNT films on healable substrates. Reproduced with permission.[27] Copyright 2014, WILEY-VCH Verlag GmbH.

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polyelectrolyte multilayer (PEM) films can autonomically restore damage cracks in several tens of micrometers deep and wide from the PEM films’ flowability and inter-diffusion through carrying the cracking surfaces into contact in the presence of water.[45,75] The healable PEM film is composed of a LbL-assembled branched poly(ethylenimine) (bPEI) and poly(acrylic acid)-hyaluronic acid (PAA-HA) mixture. The poly(vinylpyrrolidone) (PVPON)-decorated Ag nanowires (AgNWs) solution was drop-cast onto the healable (bPEI/PAA-HA)∗50 film to produce a healable, transparent, and conduc-tive AgNWs/(bPEI/PAA–HA)∗50 film. The conductivity of the AgNWs/(bPEI/PAA–HA) film is dependent on the thick-ness of AgNWs layer. A transparent conductive film with a sheet resistance at 7 Ω □-1 and a visible light transmission at ≈56% is obtained through adjusting the quantity of coated AgNWs on (bPEI/PAA–HA)∗30 film with an average transmis-sion at ≈90%. After the cutting-healing cycle, the incorporated LED bulb in a circuit with connected healable AgNWs/(bPEI/PAA–HA) film, was lit again (Figure 6a). It demonstrated the healability of the film to recover its conductivity after damage by the lateral flowability of the underlying (bPEI/PAA–HA) film to bring the separated AgNWs areas back into contact (Figure 6b–e). In addition, the analog signals could be trans-mitted from the AgNWs/(bPEI/PAA-HA)∗50 film to a liquid crystal display (LCD) monitor for a high-quality display simul-taneously.[45] It is believed that the water-triggered healable method developed in this research for the design of healable, transparent, and conductive films, will offer a new technique to fabricate diverse healable films.

3.1.2. Healable Semitransparent Composite Conductor

The water-triggered, healable PEM films have been well estab-lished by Sun and co-workers for healable electrodes and conductors.[45,75] Although this PEM film can repair the elec-trical conductivity and structure by dropping deionized water onto the damaged interfaces, the water usage in such compli-cated electronic systems might lead to circuit shorting, water leakage, and even the lack of water for leading to the loss of healability. Conductive composite materials composed of supramolecular polymeric hydrogen-bonding network with embedded nickel nanostructured microparticles or carbon black have been well designed.[46,51] Although these composites can be healed in both structural and electrical cracks effectively, none of such healable, flexible, and conductive film materials are optically transparent, which are important components for flexible transparent optoelectronic devices.

Healable conductors were generally fabricated by dispersing conductive materials into healable polymers or thermoplastic elastomers. Healable polymer matrix based on reversible Diels-Alder (D-A) reaction, has been studied for multiple transparent healings.[40,56,76–79] Pei and co-workers selected the monomer 1,1′-(methylene di-4,1-phenylene)bismaleimide (MDPB) with two maleimide groups, and the monomer with four furan groups (FGEEDR) for cross-copolymerization via a reversible Diels-Alder cycloaddition reaction to gain the healable polymer network, as demonstrated in Figure 7a–b.[40] Due to the revers-ible D-A reaction, the generated mechanical crack could be healed by D-A cycloaddition reaction between furan groups and

Figure 5. a) Scheme 1: the silicon electrode with the electrode failure due to damage in particles and traditional polymer binder, resulting in electrical contact loss. Scheme 2: the self-healing electrode maintains electrical contact in the broken particles and no cracks in the polymer binder are observed. b) Chemical structure of SHP. c) Illustration of the healability of the conductive composite with the connection of LED to the battery-powered circuit. Reproduced with permission.[51] Copyright 2013, Macmillan Publishers Limited.

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maleimide groups again upon heating to restore the broken bonds (Figure 7c–d).

The AgNWs percolation network was inlaid to the polymer’s surface layer to obtain a healable AgNW–polymer composite conductor, which was semitransparent, flexible, and highly con-ductive (Figure 8a). A crack on the surface of conductive film could be healed under heating at 110 °C, accompanying the reformation of the copolymer substrate and the reconnection of the AgNWs network. When heated at 110 °C, the reversible D-A reaction had sufficient energy to take place, the damaged D-A bonds were reformed to release the compression stress on the film, resulting in recombination of the two cut parts and the reconnection of the AgNWs network. It only takes 5 min to recover 97% of the surface conductivity. The repeatable healing could be carried out at the same location over multiple cycles (Figure 8b). For potential applications, the better transparency and the room-temperature healing of the composite film con-ductors are more preferred, which can provide the new vistas for fabricating healable, transparent and smart optoelectronic devices.

3.2. Healable Transparent Electronic Sensor

With the rapid development of modern society, as well as the increased world-wide air pollution and personalized physi-ological monitoring significance of exhaled gas, the electronic gas sensor is becoming a rising device of critical importance. Recently, healable transparent electronics with the combi-nation of high visible light transparency in both functional materials and self-healing materials have attracted increasing attention owing to their potential applications in integrated

optoelectronics. Nevertheless, healable transparent gas sensor devices with the potential integration into healable transparent optoelectronic devices for timely chemical gases sensing, have rarely been displayed. Therefore, an electronic gas sensor device with repeatable healablility, optical transparency, and high performance is still imminently desirable.

We developed a healable, transparent, and flexible chemical gas sensor device that was assembled from functional multi-walled carbon nanotubes (FMWCNTs) network-coated healable PEM film.[58,81] The healability of FMWCNT network layer was achieved by the lateral movement of the underlying LbL-assembled PEM films to bring the damaged FMWCNTs areas back into contact. The conductivity and transmittance of the FMWCNTs-coated PEM films could be balanced by varying the amount of the dispersed FMWCNTs. This FMWCNTs-coated PEM film showed a specific response to NH3 with better repeat-ability, in comparison with other volatile organic compounds (Figure 9). Besides, the device exhibited good flexibility, reliable transparency, and robust water-triggered healability of sensing performance (Figure 10). It is foreseeable that this direction of research can guide the development of other healable, trans-parent, and flexible optoelectronic devices with decreased raw material consumption and maintenance costs, extended device lifetime, and tunable functionality.

3.3. Healable Transparent Capacitive Touch Screen

In the modern digital era, capacitive touch screen with intui-tive user interface, excellent sensitivity, reliable multi-touch capability and working stability, have become indispensable components of most mobile devices such as smart phones,

Figure 6. a) Healing process after a cut on a healable AgNWs/(bPEI/PAA–HA)∗50 film with connected LED bulb: (a1) the original film; (a2) the film after cut; (a3-a5) healing of the conductivity after the dropped deionized water on the cut; (a6) water is removed after the completed healing process. SEM images of AgNWs/(bPEI/PAA–HA)∗50 film with a cut before (b) and after (c,d) healing. e) Schematic representation of water-enabled healing process. Reproduced with permission.[45] Copyright 2012, WILEY-VCH Verlag GmbH.

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pads, kindles, and laptops. However, cracks or damages occurred on the commercially available ITO/glass or ITO/polyethylene terephthalate (PET)-based touch screens because of the brittle feature of ITO ceramic in accidental falling or scratching. To replace the ITO-based touch screens, qualified material with robust transparency and healable mechanical and electrical function would extend the reliability and lifetime of the touch screens. Moreover, it would be healed in the solid state and in dry environment for avoiding circuit shortage in the electronic devices. Thus, healable polymer composite films based on the reversible Diels-Alder (D-A) cycloaddition reaction are particularly suitable in virtue of being fully solid state and capable of multiple self-healing by heating. Pei and co-workers reported the fabrication of highly transparent, conductive, and healable composite electrodes by embedding Ag NWs per-colation network into the surface of a polymer substrate con-taining the healable D-A cycloaddition copolymers for healable capacitive touch screen.[56] The as-prepared Ag NWs embedded healable copolymer layer could hold Ag NWs together along substrate surface of reversible D-A polymers, which helps to obtain a better conductivity after healing (Figure 11). The heal-able composite film electrodes with a transmittance of 80% at 550 nm and a sheet resistance of 18 Ω □-1 were obtained,

which is comparable to the commercial ITO/PET film with a sheet resistance of 35 Ω □.-1 After heating at 100 °C for 6 min, the damaged on the conductive surface would heal and the sheet resistance could be recovered to 21 Ω □-1.

A healable capacitive touch screen sensor was prepared by packing two conductive composite film electrodes arranged in 8 rows and 8 columns, resulting in an array of 8 × 8 capacitive sensing points (Figure 12). Figure 12c showed a smiley face from the touch screen. To mimic physical damage during prac-tical application, a cut damage was made along the red dash line. Consequently, the LED matrix in the left half part turned off when touching the related surfaces, while the right half of smiley face could still be lit. After healing by using a hair dryer for heating at 80 °C for 30 s, the entire smiley face could be redrawn. Moreover, this cutting-healing could be reversible for up to 4 times, demonstrating the potential applications on various healable electronic display screens.

3.4. Healable Transparent Artificial Muscle

Among the functional biomaterials, animal muscle is strong, stretchable, and could be self-healed when wounded,

Figure 7. a) Synthesis of a healable copolymer network. b) Synthesis of FGEEDR. c) Optical image of the copolymer film (light yellow). d) Optical micrographs demonstrating the healing progress of a copolymer film with heating for different time at 110 °C.[40] Copyright 2013, WILEY-VCH Verlag GmbH.

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continuously fascinating the scientific world.[82] Inspired from the self-healing animal muscles, the various self-healing artifi-cial muscles were developed from the synthetic polymers.[83,84] Bao and co-authors reported a self-healing artificial muscle manufactured from the poly(dimethylsiloxane) polymer com-posites crosslinked with coordination complexes of 2,6-pyri-dinedicarboxamide ligands and Fe(III) centers, demonstrating high stretchability and autonomous self-healing ability.[84] The dynamic bonds between iron and ligands facilitated the revers-ible unfolding and refolding of the polymer chains, and thus the high stretchability of the material with self-healing capa-bility. A high dielectric strength was restored by the recovery of mechanical damage from the healable elastomer-based dielec-tric elastomer actuator with nontransparent dielectric layers, implying the promising applications in artificial muscle. How-ever, the development of healable artificial muscle with simulta-neous transparency, mechanical stretchability, and self-healing features are still being pursued.

Recently, Wang and co-workers developed a healable trans-parent artificial muscle based on a transparent, stretchable ionic conductor with autonomous healability by employing the dynamic bonds of ion–dipole interactions between polar, stretchable polymers and the mobile, high ionic-strength salt.[57] The poly(vinylidene fluoride-co-hexafluoropropylene)

(PVDF-co-HFP) with higher HFP component (45%, amorphous and highly polar) was chosen as the polar polymer, resulting in a lower crystallinity and a higher dipole moment (5.507 Debye) (Figure 13). Meanwhile, the imidazolium salts were selected as the ionic liquids due to the high ionic strength and excel-lent electrochemical stability. After the addition of ionic liquids into the polymer network, the reversible strong ion–dipole interactions (22.4 kcal mol-1) were formed between polymer chains and ionic liquids for enabling autonomous self-healing capability in the polymer network. A transparent, stretchable material film (0.01 mm thick) was obtained with the average transmittance at 92%. Then, the self-healing ionic conductors were employed as components of dielectric elastomer actua-tors (DEAs). The schematic DEAs based on a layer of dielectric elastomer sandwiched between two stretchable conductors was displayed in Figure 14a. By applying high voltage, the electro-static stress was exerted from the conductors on the dielectric elastomer, leading to compress in thickness and expand in area. After applying 5 kV (Figure 14b, top-right) on the pristine device, the viscoelastic expansion of the active area of the device was observed. The scissors were used to cut self-healing ionic conductor layer and the cut surfaces were put in contact with each other (Figure 14b, bottom-left). 24 h later, the cut healed (Figure 14b, bottom-center). By actuating the healed DEA with

Figure 8. a) The fabricated, healable AgNWs-copolymer conductor. b) i) A lit LED connected in series with a composite conductor. ii) Cut the composite conductor to break the circuit. iii) The LED was lit again after healing upon heating. Reproduced with permission.[40] Copyright 2013, WILEY-VCH Verlag GmbH.

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Figure 9. The healable transparent sensor of FMWCNT network-coated PEM films: a) the efficient sensing; b) cut the sensor, c) water-triggered healing, and d) after healing. Reproduced with permission.[58,81] Copyright 2015, WILEY-VCH Verlag GmbH.

Figure 10. a) The sensing performance of FMWCNTs network-coated healable PEM film, in the presence of different concentrations of NH3. b) Sensing selectivity of FMWCNTs network-coated PEM film to various gases (100 ppm). c) Sensing performances in bended and extended states to 25 ppm NH3. d) The sensing sensitivity to 25 ppm NH3 after healing. Reproduced with permission.[58,81] Copyright 2015, WILEY-VCH Verlag GmbH.

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5 kV, it performed the same as the pristine state with a similar area expansion and without noticeable separation in the cut region (Figure 14b, bottom-right), demonstrating the probable improvement of the lifetime of healable transparent artificial muscles.

4. Conclusions and Perspectives

Various healable electronic devices with or without transparency have recently been developed from the different self-healing materials.[86–88] On the one hand, the composite approach could be employed to prepare the self-healing composite film, which consisted of a supramolecular organic polymer composite with embedded conductive metal, metal oxide, or carbon-based materials. Though these self-healing composites fabricated by the composite approach could be healed effectively, none of these film materials are visually transparent for integra-tion into the flexible transparent optoelectronics. Moreover, many self-healing conductors have been well studied, but they all have low optical transparency and instable healability after multiple self-healing. On the other hand, the healable trans-parent composite conductors with better stability and adhesion were prepared by embedding the transparent AgNWs or CNTs percolation network onto the surface layer of self-healing poly-mers. For instance, with the addition of functional MWCNTs network onto a healable PEM film, a healable, transparent and flexible chemical gas sensor can be assembled. In addition, the healable transparent artificial muscle could be obtained by the composite approach of employing the dynamic bonds of ion-dipole interactions between polar polymers and the high ionic-strength salt. This recent achievement provides a new

direction for the fabrication transparent biomimetic electronic sensors.

Healable transparent electronic devices show great promise for their potential applications in integrated optoelectronic devices, including touch screens, smart windows, displays, thin film transistors, and gas sensors. Various alternatives are investigated for developing healable transparent electronics, but most of them possess many weaknesses such as low con-ductivity, low performance, low transparency, and instable healability for multiple self-healing. In order to achieve reliable healability and excellent transparency of the HTEDs, the following main issues need to be addressed: (i) the healability, transparency, and processability of the hierarchically nano-structured functional materials and their compatibility on the substrates; (ii) the configuration and distribution of nanostruc-tured materials on substrates; (iii) the trade-off between trans-parency and performance of the devices.

When preparing HTEDs, a dilemma is normally encoun-tered from the fact that the film transparency decays with the thickness, whereas the relatively electronic performance and healable capability increase with the thickness.[5,81] Thus, the combination of flexibility, transparency, and ultrahigh healable and electronic performance for HTEDs remains a consider-able challenge. The composites consisted of supramolecular polymers with embedded conductive particles could serve as healable substrate for potential applications in electronic skins, supercapacitors and lithium ion batteries.[27,46,51] Nevertheless, these healable devices are not transparent. LbL assembly of functional materials with complementary chemical interactions and polymers based on reversible D-A reaction were employed to prepare the healable transparent substrates. Though the LbL-assembled PEM films displayed a high transmittance, the

Figure 11. a) Transmittance spectra of copolymer P(FR-BME) film (FR: furan oligomer, BME: 1,8-bis(maleimido)-1-ethylpropane), two Ag NWs-embedded composite film electrodes with different resistances, and a commercial ITO/PET film. b) Optical images of P(FR-BME) polymer film and conductive composite films. c) Transient resistance of composite film electrodes with 18 Ω during healing process. d) Resistances of the composite film electrodes after multiple-healing cycles at the same location. Reproduced with permission.[56] Copyright 2014, American Chemical Society.

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water-enabled healing process might cause circuit shorting in complicated electronic systems,[45,58,81] and the near-infrared (NIR) light-enabled healable PEM films showed relative poor transparency.[47] The most frequently employed healable poly-mers based on reversible D-A systems of the furan/maleimide pair,[40,56,77,89] would be healed with the assistance of heating, which may exceed the limitation of some plastic-based flexible devices.[40]

Due to the potential applications in flexible electronic devices, the conductivity of the healable transparent film elec-trodes should be a crucial parameter. By coating or embedding the conductive nanomaterials on the substrate surface, such as metal nanowires, nanotubes, graphene, conducting polymers, and hierarchical nanocomposites, the healable transparent film electrodes with impressive surface conductivity could be achieved. Moreover, the conductive nanomaterials should reform their percolation network when the damaged substrate is healed. In general, the high conductivity is accompanied with the relative low visible light transmittance of conductive nanomaterials-coated substrates. The trade-off between the transparency, the conductivity, and the performance should be considered carefully. Due to the complex configuration of the electronic devices, the transparency, healability and reli-ability of the composite films in a complicated device are still major impediments to realize real-world applications of HTEDs.

The as-prepared healable transparent conductors have been comprehensively reviewed for the fabrication of healable and intelligent optoelectronic devices. Most conductors were prepared by using Ag NWs as conductive materials. Indeed, the conductive optoelectronic devices fabricated by these healable transparent conductors might suffer oxidation and corrosion in the real applications when facing chemical reac-tions and various environmental changes, leading to the conse-quential decreased performance of the devices. In contrast, nanocomposites are desirable choices, which synergistically integrate the merits of individual unit. For instance, hierarchical nanocomposite networks of conducting polymers and carbon nanotubes or graphene would possess excellent conductivity, reliable flexibility, high transparency, and better compatibility with the substrate. They could be potentially employed in HTEDs for the construction of electrochromic supercapacitors, gas sensors and touch screens, etc.

Future efforts will likely be devoted to promoting the further exploration and broader applications of HTEDs. Exploiting heal-able transparent substrates with steady chemical and phy sical self-healing ability, low cost and environmental friendliness is of great importance. According to the as-reported transparent electronics, choosing appropriate conductive nanomaterials with uniform distribution on the substrates should be feasible for developing various HTEDs. In addition, HTEDs were gener-ally restricted to one kind of function. It is preferred to integrate

Figure 12. a) Optical image of the healable touch screen sensor fabricated by laminating two composite films on a LED display. b) Illustration of the entire capacitive touch screen system. c) A smile face obtained on the screen after the operation of the touch screen. d) The healing for the screen. A smile face drawn on the touch screen (left). After cutting, one-half of the smiley face obtained (middle). The whole smiley face could be reproduced (right) by healing at 80 °C for 30 s with a hair drier. Reproduced with permission.[56] Copyright 2014, American Chemical Society.

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diverse HTEDs into one device with various functions. It can thus simultaneously provide more than one performance at the same time, such as “back to back” HTED, which combines both gas sensor and piezoelectric supercapacitor. It can provide diverse gas concentrations and a complete air quality evaluation

using the self-powered supercapacitor in potential real-world applications. With continued research advances, it is believable that healable transparent electronic devices will likely become a common integrated component of intelligent optoelectronics and wearable devices in our everyday lives.

Figure 13. a) Scheme of self-healing process and structure of the polymer and the imidazolium salt. b) DFT calculations for the optimized structure of imidazolium salt interacting with PVDF-co-HFP monomer. Reproduced with permission.[57] Copyright 2017, WILEY-VCH Verlag GmbH.

Figure 14. a) Scheme of a DEA in the rest state and in the actuated state. b) The transparent, self-healing, highly stretchable ionic conductor was employed as the DEA stretchable conductor. Top: Pristine DEA in the rest state and in the actuated state. Bottom: After self-healing for 24 h on the cut layer of ionic conductor, the actuator’s performance was fully restored. Reproduced with permission.[57] Copyright 2017, WILEY-VCH Verlag GmbH.

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AcknowledgementsThis work was financially supported by National Natural Science Foundation of China, Beijing Natural Science Foundation (2152023), National Key Research and Development Project (2016YFC0801302), Natural Science Foundation of Guangdong Province (2016A030310048), China Postdoctoral Science Foundation and the Fundamental Research Funds for the Central Universities. Crystal S. Luo and P. B. Wan contributed equally to this work.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsflexible electronic devices, nanocomposites, self-healing, supramolecular materials, transparent conducting films

Received: December 1, 2016Revised: March 4, 2017

Published online: April 19, 2017

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