Polymer Chemistries Underpinning Materials for Skin-Inspired

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Polymer Chemistries Underpinning Materials for Skin-Inspired Electronics Helen Tran,† Vivian R. Feig,‡ Kathy Liu,§ Yu Zheng,§ and Zhenan Bao*,† Department of Chemical Engineering, ‡Department of Material Science and Engineering, and §Department of Chemistry, Stanford University, Stanford, California 94305, United States

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ABSTRACT: Polymers play a multifaceted role in driving the progress of research in skin-inspired electronicsan emerging technology with promising implications in human health, the environment, and information infrastructure. Polymers may function as the substrate, encapsulant, adhesive, matrix, or active material. A vast chemical design space to molecularly engineer polymer structures allows physical properties and functionalities to be readily tuned. Diverse opportunities exist to exploit the recent synthetic advances and methodologies in polymer chemistry to advance skin-inspired electronics. This Perspective highlights chemistries underpinning polymeric materials currently employed in skin-inspired electronics and provides an outlook on chemistries that are potentially exciting to utilize. The aim of this Perspective is to concurrently expose device engineers to the chemical diversity of polymers and enthuse chemists to evaluate their synthetic expertise for potential skin-inspired electronics. Ultimately, the collective collaborations between chemists and engineers are critical to further advance the field of electronic skin.

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INTRODUCTION Skin-inspired electronics are poised to enhance our environmental perceptions and interactions, opening new markets in personalized patient care, environmental monitoring, consumer products, and networked information (e.g., Internet of Things).1,2 Tangible potential products include wearable electronics, prosthetic electronic skin, biodegradable implantable electronics, and disposable sensors. The natural functionalities of biogenic skin serve as the framework for engineering skin-inspired electronics: tactile sensing, resilient performance under dynamic conditions, and the ability to heal and regenerate. Such electronics are predicated on being ultrathin, flexible, lightweight, and seamlessly conformable while functioning reliably. Imparting functionality without compromising performance is a persistent challenge, which has led to deep interest in the synthesis of new materials and correspondingly engineering their processing parameters to fabricate the next generation of electronic devices.3−7 From fundamental transistors to sensing elements to energy harvesters as a power supply, device components traditionally fabricated on rigid substrates must be redesigned and reengineered to operate unobtrusively on deformable substrates.8 This has led to the convergence of chemistry, electrical engineering, materials science, mechanical engineering, and even biomedical engineering to design skin-inspired electronics. Successful outcomes from these collaborative efforts will yield electronic devices for applications inconceivable with the rigid systems that presently exist. To date, polymers have closely enabled incorporation of skin-inspired properties in electronics, such as stretchability, self-healing, and biodegradability (Figure 1).9 Polymers play an important but occasionally overlooked role in most design © XXXX American Chemical Society

Figure 1. Illustration of five-set Venn diagram showing permutations of various functionalities for skin-inspired electronics enabled by polymer chemistry: self-healing, stretchable/elastic, conformal, transient, and adhesive.

strategies and materials development for skin-inspired electronics. For example, in strain engineering, rigid inorganic Received: February 28, 2019 Revised: April 26, 2019

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Macromolecules materials are rendered extrinsically stretchable through serpentine-like interconnects.10 Importantly, stretchable and elastic substrates, such as urethane- or silicon-based elastomers, are necessitated. Similarly, in the formulation of composite materials with carbon nanotubes, silver nanowires, or graphene as conductive fillers, the selected matrix is typically a polymer and may additionally feature self-healing or degradable moieties.11−14 This Perspective focuses on the chemistries underpinning polymeric materials currently employed in skininspired electronics, highlighting work that illustrates fundamental chemistry concepts. Moreover, each section includes a reflection on how recent synthetic advances and methodologies in polymer chemistry can be potentially exploited for skin-inspired electronics. This Perspective concludes with an outlook on desirable functionalities yet to be amply explored, such as conformability, adhesion, and response to stimuli (Figure 1). In the development of materials for skin-inspired electronics, it is important to take into consideration where the functionality should be introduced to the device: the substrate, encapsulant, or active matrix. For example, while degradability is necessitated for all components in developing a transient device, adhesive properties are only useful on the interface (substrate or encapsulant). Moreover, incorporating such skininspired functionalities to electron-conducting polymers will disrupt the backbone conjugation, which may lead to suppressed electronic performance.15 While this Perspective focuses on exploring chemistries in designing materials for skin-inspired electronics, understanding where to impactfully incorporate them must be taken into consideration.

Figure 2. (A) Illustration of polymer chains stretched, which leads to irreversible change. (B) Illustration of polymer chains with crosslinking units stretched, which is reversible. (C) Chemical structure of SEBS, a triblock copolymer of crystalline polystyrene and amorphous poly(ethylene−butylene). The crystalline domains of polystyrene arising from phase segregation function as physical cross-links to impart elasticity.

polystyrene regions are “hard”, and the amorphous regions of poly(ethylene−butylene) are “soft”. Because of the triblock architecture, phase segregation leads to polystyrene-rich regions, which effectively serve as physical cross-links and poly(ethylene−butylene)-rich regions, which accommodate strain. Processing, relative “hard” and “soft” composition, and temperature influence the degree of stretchability and elasticity. Because of the noncovalent nature of the crosslinks, SEBS and many urethane-based polymers are classified as viscoelastic, where time-dependent viscous and elastic characteristics are exhibited. Cross-linking concomitantly leads to increased solvent resistance, which may simplify device fabrication schemes. Currently, intrinsically stretchable electronic devices often must be built by using transfer processes because direct spincoating of materials dissolved in organic solvents damages underlying polymer layers. However, layer-by-layer transfer processes are difficult to reproduce and are not amenable to large-scale fabrication. Our group recently reported crosslinking of a dielectric to fabricate highly dense arrays of transistors and microelectronics for electrical stimulation.17,18 A cross-linked dielectric (SEBS) enables the semiconductor layer to be directly spin-coated on top to improve the dielectric−semiconductor interface, which is critically important in transistors (Figure 3A).17 We developed a cross-linking additive with two perfluorophenyl azide groups; upon irradiation with 254 nm wavelength light, these groups react with aliphatic hydrogens, which are common in most dielectrics and semiconductors (Figure 3B−D). Alternatively, the dielectric polymer structure itself may be designed to include cross-linkable units. For example, our group used a fluorinated polymer with terminal acrylates, which cross-link in the presence of a photoinitiator and 365 nm wavelength light (Figure 3E).19 These examples highlight the importance of

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STRETCHABILITY AND ELASTICITY A recognizable feature of biogenic skin is its ability to consistently adapt to dynamic motions and growth. An early goal of skin-inspired electronics is to impart such resilient capabilities. Developments in the design of devices that accommodate operation under dynamic conditions have evolved from focusing on flexibility toward materials and devices that can stretch repeatedly to large strains. Maintaining performance under deformation can be achieved through strain engineering, substrate structuring, and development of intrinsically stretchable materials.5,6 While inorganic materials can be made stretchable with geometric modifications like kirigami, polymers are the easiest to make intrinsically stretchable and therefore are highly versatile and easy to incorporate into a device. Many approaches rely on the unique polymer chain structure to achieve stretchability (Figure 2A). Upon strain, polymer chains within the amorphous regions become extended in response to tensile deformation. For most applications, some degree of elasticity is desired to maintain device fidelity and to avoid delamination from the underlying site of attachment when it returns to its original state. Elasticity is introduced by the presence of cross-linking sites to allow the polymer chains to return to their original state after the tensile stress is removed (Figure 2B). Polymer chains may be molecularly designed to include inherent cross-links or used in conjunction with reactive additives that generate cross-linking sites. Cross-linking sites may be ionic, covalent, or physical (e.g., hydrogen bonding, lock-and-key, van der Waals, π−π stacking, or other interactions). For example, poly(styrene−ethylene−butylene−styrene) (SEBS) is a commonly used triblock copolymer for electronic skin applications (Figure 2C).16 The crystalline B

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that are concurrently intrinsically stretchable and intrinsically electron-conducting introduces additional complexity. The possibility for semiconducting and conducting materials to have intrinsic stretchability is enabled by conjugated polymers, which conduct electricity through extended, delocalized πorbitals. There have been significant advances toward improving the stretchability of conjugated polymers using novel chemistries and material design strategies.20 In general, the thin film morphology that arises as a result of the polymer’s structure and its processing history dictates its functional properties: the amorphous regions of a film can impart stretchability, while interconnected semicrystalline and crystalline domains facilitate charge transport.21−24 Though there is usually a trade-off between stretchability and electronic performance, clever chemistry strategies can be used to improve mechanical and electronic properties simultaneously.6 Amorphous content imparts stretchability while semicrystalline and crystalline domains facilitate high mobilities and conductivities. Understanding polymer structures (e.g., backbone rigidity and cross-link conjugation) and thin film morphology are key design parameters. For example, we recently investigated the influence of various ionic liquids as both dopants and plasticizers for poly(3,4-ethylenedioxythiophene)−polystyrenesulfonate (PEDOT:PSS) to render the typically semicrystalline films stretchable while simultaneously enhancing conductivity (Figure 3F).25 The ionic liquid dopants facilitate the aggregation of PEDOT and plasticize the amorphous PSS regions, analogous to the “hard” and “soft” morphology of SEBS (Figures 2C and 3F).

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SELF-HEALING AND REGENERATION Remarkably, biogenic skin can autonomously repair itself and regenerate functionality in response to external damage. To similarly develop robust and damage-tolerant skin-inspired electronics, self-healing capabilities have been investigated.26−29 Intrinsically self-healing strategies are attractive to avoid the complex integration and compatibility considerations required for extrinsic healing strategies (e.g., addition of reactive capsules, vascular networks, or microorganisms).30,31 Autonomous recovery is accomplished by molecularly designing key chemical bond interactions or engineering the interplay between polymer chain diffusion and entanglement.32 Commonly utilized supramolecular interactions to induce self-healing within polymeric systems for skin-inspired electronics include hydrogen bonding, π−π stacking, host− guest interactions, and metal−ligand coordination (Figure 4A). Beyond these supramolecular examples, dynamic covalent bonds (e.g., reversible Diels−Alder reaction,33,34 disulfide exchange, and imine bonds) have been thoroughly investigated for alternative applications and may find use in skin-inspired electronics (Figure 4A).35 Also, ionomers exhibit unique aggregation behavior for ballistic puncture.36 In developing ionic materials as semiconductors and dielectrics, the influence of ionic species on charge transport must be considered for rational device design.37 Dynamic behaviors on multiple length scales are needed for autonomous self-healing to efficiently occur. On the molecular level, the design must include the presence of dynamic bonds and interactions. Moreover, the polymer chains must also be sufficiently dynamic. Lastly, on the macroscopic level, the interfaces must be adequately close for the dynamic reorganization to occur.

Figure 3. (A) A typical stretchable transistor configuration shows direct interface between the dielectric (SEBS-azide) and semiconductor (CONPHINE). (B) Chemical structure of flexible linker dual functionalized with perfluorophenyl azide. (C) General crosslinking of perfluorophenyl azide with aliphatic hydrogen. (D) Illustration of cross-linking polymer chain containing aliphatic hydrogens with a linker containing dual azide functionalities. (E) Chemical structure of a fluorinated elastomer featured acrylate groups at the end, which cross-link upon irradiation. (F) Illustration of PEDOT:PSS morphologies without (top) and with (bottom) ionic liquid, denoted as STEC. Images adapted with permission from refs 18 and 25. Copyright 2019 Nature Publishing Group and copyright 2017 American Association for the Advancement of Science, respectively.

considering device fabrication steps, in addition to functional properties, when designing new polymeric materials for skininspired devices. While the above considerations are generally true for all stretchable polymeric device components, designing materials C

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The selection of the specific self-healing moiety may strongly influence the molecular packing of the polymer chains and consequently the macroscopic mechanical properties (Figure 4A,B).38 Thioureas and ureas are both hydrogen-bonding moieties (Figure 4A). However, thioureas experience equilibrium switching between the cis and trans configuration whereas ureas remain in the trans configuration. The reported poly(ether−thioureas) are amorphous because the thiourea moieties hydrogen bond in a zigzag array (Figure 4B), yielding a strong but self-healable material. The reported poly(ether− urea) is semicrystalline because the urea moieties hydrogen bond in a linear array (Figure 4B), yielding a brittle material. In the design of future self-healing materials, therefore, it is important to not only consider the incorporation of a specific moiety but also understand how the moiety affects the morphology and resulting mechanical properties. For self-healing to occur, the crack interfaces must be sufficiently close or external pressure is needed to reunite the crack interfaces (Figure 4C). The reunification allows for chemical reorganization or polymer interdiffusion between the two interfaces of the crack. Shape memory polymers offer opportunities to autonomously reunite crack interfaces by entropy-driven recovery of the original structure.39−42 We envision shape memory polymers to be incorporated as the substrate in skin-inspired electronics as an auxiliary feature to facilitate self-healing whenever fractures are too large. Shape memory polymers may be programmed to switch between temporary and permanent shapes as well as return to their original shape.43 This programmed response is achieved by controlling the molecular structure, morphology, and processing to create a nonequilibrium configuration, which can be entropically recovered. For example, as inspired by the natural response of leaves, Urban and co-workers designed a selfhealing urethane-based polymer with a spiropyran mechanophore as a visual indicator (Figure 4D,E).39 Based on our discussion on elasticity, the segments derived from 1,4butanediol (BDO) function as the “hard” physical cross-links (Figures 2C and 4E). We envision shape memory polymers to be incorporated in skin-inspired electronics to facilitate selfhealing whenever fractures are too large.

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TRANSIENCE AND BIODEGRADABILITY Harmonizing with healing, biogenic skin routinely sheds the outer epidermis layer as new cells are regenerated. Skininspired electronics draw inspiration from this natural cycle of degradation and its potential for recyclability. In contrast to conventional microelectronics engineered to last indefinitely, transient (or biodegradable) electronics physically disintegrate in a programmable manner.44−46 By the design of electronics to be transient, nontoxic byproducts and the potential for recyclability may alleviate the adverse human impact on ecology and yield disintegrable implantable devices.47 Because transient devices do not require a secondary surgical retrieval, we foresee advancements in developing passive or self-powered devices or developing biodegradable energy sources (i.e., degradable batteries) to exploit the transient functionality. Moreover, as most implantable biodegradable devices will be placed intimately with dynamic soft tissue, it will be invaluable to concurrently engineer transience and stretchability. Beyond chemical bond design, it is important to consider the processing steps entailed in device fabrication for proper selection of biodegradable materials. We recently reported the fabrication of biodegradable pressure/strain sensors for

Figure 4. (A) Chemical structures of various moieties employed for self-healing. (B) Chemical structures of thiourea, poly(ether− thiourea), urea, and poly(ether−urea). Poly(ester−thiourea) is amorphous whereas poly(ester−urea) is semicrystalline due to (C) differences in hydrogen bonding. (D) Shape memory polymers may reunite crack interfaces after damage to improve self-healing. (E) Chemical structure of a shape memory polymer. The “soft” block is derived from 1,4-butanediol (BDO), and the “hard” block contains polycaprolactone (PCL) and a spiropyran (SP) mechanophore. (F) Photograph of Delosperma cooperi with a kink that formed after selfhealing at the injury point, as indicated by the arrow. Images adapted with permission from refs 38−40. Copyright 2018 American Association for the Advancement of Science and copyright 2018 and 2015 Elsevier Inc., respectively. D

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Figure 5. (A) Illustration for a fully biodegradable wireless pressure sensor, featuring a magnesium antenna and microstructured pyramids. (B) Illustration for a fully biodegradable dual pressure and strain sensor, which is composed of multiple layers of biodegradable materials. (C) Chemical structures of PLLA, PGS, and POMaC. (D) Chemical structure of acylhydrazone linkage and its reversible reaction. (E) Chemical structure of the o-nitrobenzyl group with the labile bond highlighted. (F) The [2 + 2] photoaddition of cinnamoyl to truxylic acid is reversible. The two forms (two monomers or dimer) are switched with distinct wavelengths of light. Images adapted with permission from refs 48 and 49. Copyright 2018 and 2019 Nature Publishing Group, respectively.

is simple, the ability to chemically tune mechanical properties and degradation behavior through molecular structure design has made the ester moiety ubiquitous in biodegradable systems, both synthetically and in nature. Within the field of polymer chemistry, there is rich knowledge of labile bonds which may potentially drive nextgeneration transient designs for electron skin applications.52 In addition to the common moieties susceptible to hydrolysis and oxidation previously reviewed,46 interesting labile linkages include the acylhydrazones, aminals, hemiacetals, and vicinal tricarbonates (Figure 5D).52 Moreover, transience though external stimuli will enable on-demand degradation, which would be valuable as fuses in response to undesired high heat or as indicators in response to harmful irradiation. A common photoresponsive linker is cleavage with the o-nitrobenzyl group (Figure 5E)53 and reversible [2 + 2] photodimerization (e.g., cinnamoyl to truxylic acid, Figure 5F).54,55 Also, molecular design parameters from triggerable depolymerizable and selfimmolative polymers may be interesting.56,57

orthopedic tissue rehabilitation and arterial pulse sensors for wireless blood monitoring (Figure 5A,B).48,49 The main biodegradable polymers employed in both studies were selected for their processing parameters. Poly(octamethylene maleate (anhydride) citrate) (POMaC) is an elastomer cured by light,50 poly(glycerol sebacate) (PGS) is an elastomer cured by heat,51 and poly(L-lactic acid) (PLLA) is a thermoplastic. The microstructured pyramids are integral to achieving sensitive pressure sensing and must be fully elastic to avoid deformation after multiple cycles of compression. Moreover, minimal shrinkage upon curing is desired. On the basis of these criteria, PGS was selected. PLLA was selected for the insulation layer between the magnesium antenna because it provides rigid support and can be directly spin-coated to form a thin film. Thermal curing, as needed for PGS, may result in deformation of the underlying delicate patterns and yield nonuniform films. POMaC was ideal for the soft encapsulation layers for its quick 3 min photocuring facilitated fabrication. Similar to our discussion of hydrogen bond packing of thioureas versus ureas (Figure 4A), the molecular structure influences the tensile properties and degradation behavior of common moieties susceptible to hydrolysis and oxidation.46 For instance, there are various types of polymers based on lactic acid, arising from the inherent chirality of lactic acid: PLLA, poly(D-lactic acid) (PDLA), and poly(DL-lactic acid) (PDLLA). PLLA and PDLA are synthesized with different optical stereoisomers but exhibit similar crystalline characteristics. In contrast, PDLLA is composed of both stereoisomers and hence is relatively amorphous, which leads to faster degradation times. Similarly, the degree of crystallinity is influenced by the neighboring atoms which contribute to or disrupt local molecular order, as observed in polycaprolactone and derivatives with branching. Even though the ester moiety

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ENVISAGED FUNCTIONALITIES Skin-inspired electronic devices must be able to make stable, conformal contact with biological tissues to achieve improved signal integrity and long-term usability. Two forthcoming challenges that must be tackled are (1) developing devices that can conform to unusual surfaces and (2) adhesion of stretchable devices under wet conditions. The most common strategy to improve adhesion and conformability is by making devices ultrathin, which increases the magnitude of van der Waals forces between devices and skin.58 This general approach is generally sufficient to adhere skin-inspired electronics to relatively dry skin. Moving toward applications within the body or on more complex curvilinear surfaces, it is E

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On the other hand, covalent bonding interactions are less sensitive to water and therefore are promising to adhere a broad array of skin-inspired materials to tissues. One common class of adhesives is based on catechol chemistries, which can form multiple types of bonds with a variety of surfaces (Figure 6).65 In the catechol state, these moieties can form hydrogen

envisaged that improved adhesion, especially in wet environments, and three-dimensional conformability will be necessary. These cooperative attributes will ensure devices remain in the same position to monitor or stimulate precisely the same targeted position. Because these functionalities are emerging fields of interest, this section of the Perspective highlights potential chemical strategies for adhesion and conformability for skin-inspired electronics. Conformability. Many conformal electronic skin devices are effectively two-dimensional. Such device footprints are appropriately small, cover relatively smooth surfaces, or have curvature only in one direction. Areas selected for demonstrations typically include the back of a hand, the forehead, and portions of the arm. However, next-generation skin-inspired electronics may target applications such as sensing over the entire tip of a finger and whole-area organ mapping. Notably, a combination of optical segmentation techniques and 3D printing has been utilized to fabricate a conformal electronic device for spatiotemporal cardiac measurements and stimulation over part of a heart.59 Ultrathin electronics and optoelectronics were placed on the 3D model and then coated with a thin layer of a silicone-based elastomer for transfer. However, this device is still effectively wrapping over a planar region with few curvilinear shapes. Although this method serves as an important report for covering both the anterior and posterior surface of an organ, more direct and scalable approaches are desired and may be accomplished with polymer chemistry. For example, it is conceivable to fabricate a fully stretchable electronic device on a substrate that can then conform over an object with mild heating. This idea functions similarly to shrink wrap, which is enabled by shape memory polymers. In shrink wrap, amorphous polymer chains are extended at temperatures above their glass transition temperature and cooled to create a temporary ordered state. Upon heating, entropy drives the return of the polymer chains from their aligned to amorphous state, which corresponds to shrinkage. Conformal shrinking and wrapping may also function as an extrinsic adhesive, since the device can remain securely positioned. Radical ideas are embraced in designing conformal electronics on unusual shapes and irregular features. Adhesion. The chemical and mechanical properties of biological tissues enable a variety of bonding interactions with synthetic materials. Covalent bonds can be formed using common amine reactions due to the prevalence of −NH2 groups in proteins.60 The surfaces of many tissues are also rich in carboxylic acid groups, which can form hydrogen bonds or electrostatic interactions with charged materials like polyelectrolytes.61 Finally, the soft and deformable nature of tissues allows adhesion by using short-range interactions, like van der Waals forces. Besides chemical bonding, molecules can physically bridge the interface and adhere via mechanical interlocking.60 An early strategy to improve the adhesiveness of polymeric materials was inspired by the foot hairs of geckos, which adhere to solid surfaces because both van der Waals and capillary forces are comparably strong at their length scale.62,63 One major drawback is that the capillary forces involved are significantly weakened by excess water.64 This is problematic for many biointerfacing applications of skin-inspired electronics, since many of the materials used for these devices are hydrogels and since many of the target health-related applications inevitably expose devices to physiological fluids like sweat.

Figure 6. Schematic diagram of plausible adhesion and cross-linking mechanisms of catechol and o-quinone polymers. The catecholic polymer may undergo surface bonding or metal complexation. The oquinone polymer may undergo amine-based reactions or cross-linking via radical formation. Images adapted with permission from ref 65. Copyright 2013 John Wiley & Sons, Inc.

bonds and metal−ligand complexes. When oxidized to the quinone state, they can pair covalently with −NH2 groups via either Schiff base reactions or Michael addition or form hydrogen bonds.65,66 However, a challenge of working with catechol chemistries is lack of precise control over the oxidation state. While the highly reactive quinone structure of oxidized catechols can covalently bond with −NH2 groups, they can also rapidly form intermolecular bonds that drastically reduce adhesiveness. By intercalating dopamine monomers between clay nanosheets, Han et al. were able to controllably oxidize dopamine into polydopamine, while preventing overoxidation into the quinone structure.67 Highly tough adhesives have been created using a combination of tethering covalent bonds and energy dissipation mechanisms.68 For instance, nanoparticle solutions can bond a variety of diverse materials, including biological tissues, by relying on adsorption of macromolecules to the high density of available surface sites on the nanoparticles.69,70 Adhesion energies over 1000 J/m2 have been achieved by combining a dissipative matrix with an adhesive surface that uses electrostatic interactions, covalent bonds, and physical interpenetration to bond to the target substrate. The dissipative matrix bolsters the adhesion toughness due to the interpenetration of a covalently cross-linked polyacrylamide gel with a dissipative, ionically cross-linked alginate network.61 F

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Macromolecules Looking forward, imparting adhesion to skin-inspired electronics will occur at the interface of the device and its anticipated location, such as on the surface of skin or an internal organ. These adhesive chemistries may be directly incorporated into the substrate or used in the design of a separate adhesive layer. As some tissue adhesives are hydrogelbased and many reported sophisticated electronic skin devices have been fabricated on elastomers, understanding the bonding between hydrogels and elastomers could provide a universal route to bridge the advances developed for each skin-inspired functionality.71 While soft mechanics and ultrathin geometries have sufficiently offered adhesion to the epidermis, future applications in more hydrating environments, particularly bioelectronics interfacing internal organs, will necessitate adhesives. Stimuli-Responsive. Stimuli-responsive polymers are increasingly of interest for a range of applications, such as drug delivery, adaptive coatings, artificial muscle actuators, biosensors, and supports for tissue regeneration.72−74 In the context of skin-inspired electronics, stimuli-responsive polymers may function as an environment-dependent on/off switch for standalone wearable devices, passive colorimetric detection of analytes, or energy harvester.75 Common stimuli are pH, light, mechanical force, and temperature. Common responses are bond cleavage, structural conformation, or phase changes, such as the cis/trans photoisomerization of azobenzene and the lower critical solution temperature (LCST) phase transition of poly(N-isopropylacrylamide) (PNIPAM). Ultimately, these responses manifest in changes in the bulk physical properties of the polymeric system. For example, a polymer metal−organic cage (polyMOC) network can be photoswitched between two topological states, each with distinct self-healing abilities, shear modulus, defect tolerance, fatigue behavior, and other characteristics (Figure 7).76 These major macroscopic changes arise from the structural rearrangement of the terminal bis(pyridyl)dithienylethene (DTE) on poly(ethylene glycol) (Figure 7A). Another example of a stimuli-responsive material with macroscopic changes is a mechanoresponsive self-growing hydrogel inspired by muscle training (Figure 7B).77 Upon strain, radicals are produced, which initiate the polymerization of a newly formed network. Such chemistries may enable unconventional devices for skininspired electronics.

Figure 7. (A) The two photoswitchable topologies of a polymer metal−organic cage network exhibit different physical properties, such as self-healing and defect tolerance. The chemical structure of poly(ethylene glycol) with terminal bis(pyridyl)dithienylethene groups, which structurally rearrange with UV light or green light. The angle between the pyridine groups drastically changes between the two topologies. (B) Illustration of a mechanoresponsive hydrogel inspired by muscle training. Mechanical stress leads to covalent bond scission and radicals, which subsequently react with reactive species to form a new network. Images adapted with permission from refs 76 and 77. Copyright 2018 Nature Publishing Group and copyright 2019 American Association for the Advancement of Science, respectively.

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CONCLUSION The past decade has witnessed substantial progress in polymer design, synthesis, and processing for skin-inspired electronics. Key knowledge in imparting stretchability, self-healing, biodegradability, adhesion, and conformability has been gained. This chemical and engineering knowledge has been utilized for fabricating skin-inspired electronics, such as a wireless biodegradable arterial pulse monitor and ultralow voltage in vivo neural stimulator. The next stage involves incorporating dual and multiple functionalities, inspired by skin, and developing standalone devices. We will need to understand how to orthogonally design functionalities through a combination of chemical, materials processing, and engineering strategies. Alternatively, we will need to understand how chemistries can be designed to symbiotically impart our desired functionalities. A key challenge will be designing electron-conducting polymers with these functionalities due to the added constraint of needing a conjugated backbone. This Perspective highlighted the significant role of polymers in skin-

inspired electronics and discussed insights into the underlying chemistries enabling skin-inspired functionalities. From the selection of a sulfur or oxygen atom to the tuning of polymer chain conformation, the vast chemical design space of polymers provides a rich and precise toolkit to design materials for skin-inspired electronics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Helen Tran: 0000-0002-4041-7340 Zhenan Bao: 0000-0002-0972-1715 Notes

The authors declare no competing financial interest. G

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Kathy Liu is currently an undergraduate student at Stanford University conducting research in Prof. Zhenan Bao’s group. Her research focuses on developing and characterizing stretchable, biodegradable electronics and conductive tissue-like materials.

Dr. Helen Tran is an Intelligence Community postdoctoral researcher in Prof. Zhenan Bao’s group at Stanford University. She received her B.S. in Chemistry from UC Berkeley in 2009, worked as a postbaccalaureate fellow at the Molecular Foundry at Berkeley National Laboratories from 2009−2011, and completed her Ph.D. at Columbia University in 2016 under the supervision of Prof. Luis Campos. Her current research focuses on understanding the structure−property−function relationships of polymers to develop high-performance, stretchable electronics that are biodegradable.

Yu Zheng is currently a Ph.D student in the Chemistry department at Stanford University under the supervision of Prof. Zhenan Bao. She received a B.A. in Chemistry from Nankai University in 2017. Her current research focuses on the design and characterization of polymer semiconductor materials for stretchable electronics.

Professor Zhenan Bao is a Professor of Chemical Engineering at Stanford University. Prior to joining Stanford in 2004, she was a Distinguished Member of Technical Staff in Bell Laboratories, Lucent Technologies from 1995−2004. She has over 450 refereed publications and over 60 US patents. She pioneered a number of design concepts for organic electronic materials. Her work has enabled flexible electronic circuits and displays. In her recent work, she has developed skin-inspired organic electronic materials, which resulted in unprecedented performance or functions in medical devices, energy storage, and environmental applications.

Vivian R. Feig is a Ph.D. candidate in the Materials Science and Engineering department at Stanford University, under the supervision of Prof. Zhenan Bao. She received a B.S. in Chemical Engineering from Columbia University in 2012 and then worked at ExxonMobil until 2015. Her current research focuses on developing soft, conductive materials for stable long-term bioelectronic interfaces. Her research is funded through a National Defense Science and Engineering Graduate Research Fellowship.

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ACKNOWLEDGMENTS H.T. was supported by an appointment to the Intelligence Community Postdoctoral Research Fellowship Program at Stanford University, administered by Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the Office of the Director of National Intelligence. V.R.F. was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate (NDSEG) Fellowship Program. The authors thank L. Beker for discussions and Y. Gu for providing his ChemDraw file for Figure 7. This work is partly based upon work supported by the Air Force Office of Scientific Research under award number FA9550-18-1-0143. H

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ABBREVIATIONS SEBS, styrene−ethylene−butylene−styrene; PDMS, polydimethylsiloxane.; PEDOT:PSS, poly(3,4-ethylenedioxythiophene)−polystyrenesulfonate; PCL, polycaprolactone; BDO, 1,4-butanediol; SP, spiropyran.; POMaC, poly(octamethylene maleate (anhydride) citrate); PGS, poly(glycerol sebacate); PLLA, poly(L-lactic acid).; LCST, lower critical solution temperature; PNIPAM, poly(N-isopropylacrylamide).; DTE, bis(pyridyl)dithienylethene; PolyMOC, polymer metal−organic cage.

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DOI: 10.1021/acs.macromol.9b00410 Macromolecules XXXX, XXX, XXX−XXX