Skin-Inspired Electronics Enabled by Supramolecular Polymeric Materials

Open AccessCCS ChemistryMINI REVIEW1 Oct 2019Skin-Inspired Electronics Enabled by Supramolecular Polymeric Materials Kai Liu, Yuanwen Jiang, Zhenan Bao and Xuzhou Yan Kai Liu School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China) Google Scholar More articles by this author , Yuanwen Jiang Department of Chemical Engineering, Stanford University, Stanford, CA 94305 (United States) Google Scholar More articles by this author , Zhenan Bao *Corresponding authors: E-mail Address: email protected, E-mail Address: email protected Department of Chemical Engineering, Stanford University, Stanford, CA 94305 (United States) Google Scholar More articles by this author and Xuzhou Yan *Corresponding authors: E-mail Address: email protected, E-mail Address: email protected School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 (China) Google Scholar More articles by this author https://doi.org/10.31635/ccschem.019.20190048 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Next-generation electronics that intimately interact with the human body would play crucial roles in future health monitors and early disease diagnosis. Skin-inspired electronics have been rapidly growing in the past decade to emulate the remarkable sensory and responsive nature of the human skin tissue. Toward the goal of combining excellent electrical performance and exceptional mechanical compliance, one key step is to identify basic building blocks that are mechanically adaptive and electronically active. Among the available material candidates, supramolecular polymeric materials (SPMs) and their composites are emerging as promising candidates because of their intrinsic skinlike mechanical properties such as stretchability, flexibility, self-healing, and toughness, as well as their tunable electrical properties achieved through chemical synthesis or physical blending. In this mini review article, we critically summarize recent progress in skin-inspired electronics, enabled by SPMs with a focus on the design of electronically active SPMs and their applications as principle components of electronic skins. Finally, we conclude by highlighting on challenges and opportunities of SPMs toward a new-generation skin-inspired electronics. Download figure Download PowerPoint Introduction Skin is able to sense stimuli such as pressure, temperature, and pain through numerous sensor types, including mechanoreceptors, thermoreceptors, and nociceptors, all of which cooperate to play an essential role as the interface between brain and external ambient environment.1 To further improve quality of life and restore these natural sensing functionalities for people with skin damage and amputations, various materials and prosthetic devices have been developed to mimic the sensory capability of the human skin.2–5 Electronics inspired by the properties and functions of human skins termed “skin-inspired electronics” which need to simultaneously possess excellent electrical properties and exceptional mechanical compliance.6–8 While human skins are soft, stretchable, tough, and self-healable, traditional silicon electronics are rigid, brittle, and cannot self-repair. Therefore, it is challenging for these rigid, hard materials to integrate intimately with soft human skins as wearable bioelectronic devices.9,10 In recent years, strain engineering has been developed as an efficient tool to induce extrinsic stretchability to rigid electronic materials.11–14 However, the fabrication process remains complicated, and its relatively large size hampers their practical applications in wearable electronics. By contrast, mechanically adaptive electronic materials could integrate decent electronic performance with intrinsic stretchability, making them outstanding candidates for new-generation skin-inspired electronic materials. It is expected that future electronics would seamlessly connect with human skins and fulfill or even replace their functions. Supramolecular polymeric materials (SPMs) are a kind of adaptive materials whose components are bridged by dynamic bonds (e.g., reversible covalent bonds and noncovalent bonds). In the meantime, their structures and functions are also tightly regulated by these dynamic bonds.15–19 To present a broad concept of SPMs, we have considered both supramolecular polymers and polymeric systems with supramolecular interactions in this review. The dynamic bonds in SPMs could be easily broken to promote energy dissipation and thus endow SPMs with mechanical tolerance, while the bonds could be reconstructed autonomously or by the assistance of external stimuli such as solvent and temperature to achieve self-healing.20,21 Due to the reversible nature of the bonds, SPMs also have other interesting properties including intrinsic stretchability, shape memory, environmental adaptability, degradability, and recyclability.22–25 Combined with molecular chemistry, supramolecular chemistry could effectively tune the properties and functions of SPMs by judicious selection of dynamic bonds and building blocks.26 In addition, when electronic components are incorporated via chemical design or physical blending, the resulting SPMs or their composites would simultaneously possess both high electric performance and mechanically adaptive properties.6 Hence, SPMs are emerging as prominent candidates for skin-inspired electronics. The fundamental principle of SPMs design for skin-inspired electronics is to incorporate suitable dynamic bonds into targeted polymeric materials, where molecular chemistry and supramolecular chemistry function simultaneously to achieve excellent electrical performance and exceptional mechanical compliance.21,26,27 For these SPMs, two characteristics are of particular interest, viz, self-healing and energy dissipation. The former helps to extend the device lifetime caused by materials failure over time, and the latter endows materials with mechanical toughness and adaptability. As such, SPMs could serve as chaperones for other electrically active components in the generation of skin-inspired electronics with unprecedented self-healability and mechanical conformability.27–31 During the last decade, we have witnessed significant progress of skin-inspired electronics enabled by SPMs and their composites. For example, various electronic devices with intrinsic stretchability and self-healing have been successfully fabricated using SPMs without sacrificing their electrical performance.27–31 Meanwhile, supramolecular polymers could selectively sort out semiconducting single-walled carbon nanotubes (SWNTs) from low-cost raw SWNT mixtures, and thus, their potentials could be exploited fully in the field of stretchable electronics.32,33 Moreover, as the basic and essential elements of skin-inspired electronics, organic field-effect transistors (OFETs) with intrinsically stretchable and self-healable properties could also be fabricated via delicate integration of supramolecular polymeric electrodes, semiconductors, and dielectrics in one device.34 More importantly, SPMs have enabled the development of multifunctional electronic skin systems with stretchability and self-recoverability.28,34,35 Despite these achieved notable progress, enormous challenges remain regarding the practical applications of skin-inspired electronics. To this end, we believe SPMs would play an increasingly important role in the fabrication of skin-inspired electronics with mechanically adaptive properties and tailorable electrical performance. Herein, we highlight some important advances in skin-inspired electronics enabled by SPMs. Specifically, key electronic components (e.g., conductors, dielectrics, and semiconductors), semiconducting SWNTs sorting, and integrated electronic (E)-skin systems associated with SPMs will be discussed, wherein stretchability, anti-tearing, and self-healing capabilities, biodegradability, and recyclability contributed by dynamic covalent and noncovalent chemistry will be emphasized. Finally, this review will conclude with an outlook on the challenges and opportunities in this burgeoning field. Supramolecular Polymeric Conductors Taking advantage of the intrinsically stretchable and self-healing properties of SPMs, conductors could possess ideal skinlike properties, the majority of which are composites prepared by combination of supramolecular polymeric matrixes and conducting fillers. In this section, we present recent advances in the exploration of supramolecular polymeric conductors. Conductors that showcase both mechanical and electrical self-healing properties are essential for E-skin applications. In 2012, a self-healing conductor, composed of a urea-based H-bonding supramolecular polymer and nanostructured nickel microparticles (μNi) was developed, which showed an unusual combination of flexibility, stretchability, high conductivity (40 S cm−1), and self-healing properties at ambient conditions (Figure 1a).28 Due to the reversible and fast H-bonding interactions on the cut surface, the rate of electrical healing (∼90% efficiency) was as short as 15 s, and the mechanical properties could be completely recovered in ∼10 min when the material sustained damage. Furthermore, the self-healing ability of E-skins, given their applied pressure and flexion sensing properties, were delicately demonstrated on the platform of this conducting supramolecular polymeric composite (see below). Figure 1 | Supramolecular polymeric conductors generated via hydrogen bonds. (a) Proposed interaction of oligomer chains with μNi particles (left). Repeated electrical healing for three cuts at the same severed location (middle). Demonstration of the healing behavior for a conductive composite with a LED in series with a self-healing electrical conductor (right). Reproduced with permission from ref 28. Copyright 2012, Nature Publishing Group. (b) Molecular design of PDMS–MPUx–IU1−x with high toughness, stretchability, and self-healing property (left). Optical images of reconfigurable assembly of electronic devices with modules and connectors (top-right). Optical images of stretchable modular wearable electronic device composed of five modules and four connectors on human arm and a three-dimensional (3D) assembled modular electronic device using 3D connectors (scale bar: 2 cm) (bottom-right). Reproduced with permission from refs 37 and 38. Copyright 2018, 2019 Wiley-VCH. (c) Chemical structures of SPMs (top). Cartoon representation of the proposed mechanism for highly stretchable SPMs (bottom-left). Photographs of an SPM-2 test specimen before and after stretching to 17,000% (middle). Stretchability comparison of the notched SPM- and PDMS-supported electrodes (Inset is the mechanical simulation by FEM to show the strain magnification effect induced by the notch.) (middle-right). Optical images to illustrate the stretching state of SPM-2-based stretchable thin-film gold electrode with notch (bottom-right). Adapted with permission from ref 26. Copyright 2018, American Chemical Society. Download figure Download PowerPoint Although the shortcoming of significant viscoelasticity for H-bonding-based supramolecular materials has been addressed using the graphene oxide as a macro-cross-linker,36 moisture and water sensitivity for these materials remain outstanding. Toward this goal, we reported a series of poly(dimethylsiloxane) (PDMS)-based supramolecular polymers with strong 4,4′-methylenebis(phenyl urea) (MPU) and weak isophorone bisurea (IU) dynamic bonding units embedded in the polymeric backbones, termed PDMS–MPUx–IU1−x (Figure 1b).37 Among them, PDMS–MPU0.4–IU0.6 exhibited the best performance, such as notch-insensitive stretchability (1200%), high fracture toughness (∼12,000 J·m−2), and autonomous self-healability, even in artificial sweat. In this supramolecular polymeric network, the strong MPU hydrogen bonds enabled robustness and elasticity, whereas the weak IU ones were responsible for energy dissipation, both of which synergistically contributed to unique mechanical behaviors through breakage of H-bonding and step-by-step reconstruction. Moreover, a film electrode consisting of PDMS–MPU0.4–IU0.6 as an encapsulation and supporting layer, and the liquid metal alloy named Eutectic Gallium–Indium, was fabricated as self-assembled monolayer of the conductive filler, which exhibited high stretchability (500%) with stable (100 cycles) and reversible low resistance. This electrode was also demonstrated to be self-healable under water and even in artificial sweat at room temperature. Based on this electrode, a skinlike capacitive strain sensor with high toughness and robustness against damage was further developed. Furthermore, we developed a skinlike modular electronic system with stretchability and self-healability using this tough SPM, in which the electronic components could be electrically interconnected and digitally interact with each other (Figure 1b).38 In terms of the self-healing ability of the connected conductive lines, users could readily change the electronic system into their desired configurations via a simple Lego assembly process. Thin-film gold electrodes deposited on PDMS substrates are subjected typically to a few drawbacks, including sensitivity to fracture/notch, poor interfacial adhesion, and lack of self-healing capability. To solve these problems, we designed and synthesized a series of SPMs (SPMs 1– 3), composed of soft polymeric chains (polytetramethylene glycol and tetraethylene glycol) and reversible quadruple H-bonding cross-linkers, ranging from 0–30 mol% (Figure 1c).26 The soft chains formed soft domains of the SPMs, while the strong quadruple H-bonding units contributed to the desired mechanical properties, thus, generating flexible, stretchable, yet tough elastomers. SPM- 2 is highly stretchable (up to 17,000% strain), tough (with a record fracture energy ∼30,000 J·m−2), and self-healing, which are all good features for subsequent thin-film gold-electrode fabrication. By directly depositing a gold coating on the SPM- 2 substrate, the resultant electrode maintained its conductivity and possessed high stretchability (∼400%), self-healing, notch/fracture insensitivity, and improved interfacial adhesion between the gold film and the substrate. Finally, we successfully employed this electrode to measure electromyography signals both on human skin (in vitro) and as implants within live rats (in vivo). Aside from pure H-bonding, metal-coordination interactions could also serve as driving forces in the preparation of SPMs for fabrication of conductors. For instance, we developed a supramolecular polymeric network with the micro-phase-separated structure using cooperative metal-coordinated bonds (β-diketone–europium interaction) and hydrogen bonds (Figure 2a). As such, the resultant polymer possessed high stress at break (∼1.8 MPa), high fracture strain (∼900%), along with excellent elasticity.39 Impressively, this dual-dynamic cross-linking system could achieve 98% self-healing efficiency after 48 h at room temperature without external stimuli. Importantly, such SPM could play dual functions of the dielectric layer and the conductive layer (silver flakes as the filler, conductivity 46.5 S cm−1) in the fabrication of a stretchable and self-healable touchpad, based on integrated capacitive sensors (Figure 2a). Figure 2 | Supramolecular polymeric conductors via metal-coordination interactions and dipole–dipole interactions. (a) Molecular structure and schematic depiction of a dual-dynamic cross-linking polymer based on curcumin coordination bonds and hydrogen bonds (top and left). Demonstration of the healing process of a capacitive touch pad device composed of silver electrode and the polymer (bottom-right). Reproduced with permission from ref 39. Copyright 2018 Wiley-VCH. (b) Healing process of underwater self-healable material (top-left). Chemical structures of fluorinated polymer and plasticizer, DFT-optimized structure of two p-PVDF-HFP monomers interacting via van der Waals and dipole–dipole forces and scheme of dipole–dipole interaction (bottom-left). Optical microscopy image of damaged p-PVDF-HFP-15 wt % DBP after different healing times at 60 °C in air (scale bar, 500 μm) as well as typical stress–strain curves of original and healed samples for different healing times at room temperature and at 60 °C underwater (top-right). Demonstration of the stretchability and healing process for an underwater self-healable electrical conductor with an LED in series (bottom-right). Reproduced with permission from ref 40. Copyright 2018 Wiley-VCH. Download figure Download PowerPoint Self-healing and stretchable electronic skins which could work under harsh aqueous conditions (including seawater, highly acidic and basic media) are needed urgently for some underwater and biological applications. However, most previously reported self-healing SPMs did not work effectively in these aqueous environments due to the poor underwater stability of the dynamic bonds (such as hydrogen bond, metal–ligand coordination, and ionic interaction). To tackle this issue, Wang and co-workers40 utilized a novel dynamic dipole–dipole interactions of C–F bonds between highly polar and hydrophobic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the driving force for underwater self-healing study (Figure 2b). By mixing soft and amorphous p-PVDF-HFP (40% HFP content) with 15 wt % of dibutyl phthalate (DBP) as the plasticizer generated a transparent polymer complex with high stretchability (up to 1000%), underwater stability (several months), and autonomous self-healability under several water conditions. This new type of elastomer could be combined further with silver flakes to make underwater self-healable and stretchable supramolecular polymeric conductors. In general, chain mobility is the prerequisite of a self-healing polymer because it could contribute to mutual penetration between broken polymer/polymer interfaces. Realization of self-healing in stiff polymers which have reduced motilities of polymer chains is relatively difficult. To address this issue, Li et al.41 utilized dynamic covalent boroxine bonds as cross-links to prepare PDMS-based supramolecular polymeric network, which was synchronously strong (with tensile Young’s module up to 182 ± 15.8 MPa and compressive Young’s module up to 142 ± 9.8 MPa) and stiff (with elongation and compressibility less than 10% at 10 MPa stress) (Figure 3a). To our surprise, such rigid material was still able to self-heal with the aid of water. Given the moisture-sensitive and reversible covalent bonds, the material was exploited further as reusable and healable adhesives (adhesion strength 7.5 MPa after curing 24 h at 70 °C). Finally, a healable semitransparent conductor (sheet resistances of 12 Ω sq−1) with enhanced adhesive was developed by directly coating silver nanowires (Ag NWs) on this polymer matrix. Unifying simultaneous stiffness and water-enabled healing ability in a dynamic SPM is quite rare. Such combination might provide more possibilities for their potential applications. Figure 3 | Supramolecular polymeric conductors via dynamic covalent bonds. (a) The structure and dynamic process of PDMS-Boroxine and the mechanism for the water-enabled healing process. Reproduced with permission from ref 41. Copyright 2016 Wiley-VCH. (b) Schematic illustration of rehealability and full recyclability of the E-skin (left). Polymerization of the polyimine (top-right). The malleable E-skin can be conformally mounted onto a human arm and can be rehealed by applying a small amount of rehealing agent and heat pressing (middle-right). The E-skin can be fully recycled using the recycling solution, yielding the solution with dissolved oligomers/monomers and AgNPs at the bottom. The solution and AgNPs can be reused to make a new E-skin (bottom-right). Reproduced with permission from ref 42. Copyright 2018 AAAS. Download figure Download PowerPoint In addition to underwater stability, a good chemical and thermal stability represent another requisite for real applications of E-skins. Meanwhile, considering the recyclability of E-skins is also essential for the reduction of electronic waste, thereby, having an environmental impact on the eco-friendly manufacturing process. To combine these benefits, Zou et al.42 developed a silver nanoparticles-doped polyimine-based thermoset conductor for E-skin, consisting of tactile sensing system in which temperature, flow, and humidity sensors were delicately put together (Figure 3b). As the robust thermoset matrix was prepared via dynamic and reversible imine bonds, in addition to its robustness, the newly developed E-skin was rehealable, fully recyclable, and malleable. Specifically, the E-skin could be rehealed by applying a small amount of rehealing agent compounds 1, 2, 3, and silver nanoparticles (AgNPs) in ethanol at the cut area and by heat pressing (8.5 kPa at 80 °C). Further, the E-skin was 100% recyclable at room temperature after soaking it in recycling solution (compounds 2 and 3 in ethanol). Notably, the conductor could maintain its original mechanical and electrical properties after all the rehealing and recycling processes, thus, representing a significant leap in the development of E-skins. Supramolecular Polymeric Dielectrics of E-skins Stretchable and self-healable dielectrics are crucial to skinlike OFETs. Traditional stretchable dielectric materials such as PDMS and poly(styrene-block-(ethylene-co-butylene)-block-styrene) (SEBS) often exhibit low dielectric constants and lack both self-healability and high mechanical tolerance.9 One promising approach to tackle these problems simultaneously is the introduction of metal–ligand interactions into polymeric backbones, wherein the resulting supramolecular polymeric dielectrics could achieve high polarizability, and thus, show high dielectric and mechanical strength.30 In this section, we review recent reports on skinlike dielectrics. Strong metal–ligand interactions in self-healing polymeric materials usually require external stimuli to recover, whereas weak ones lack sufficient mechanical strength. To overcome these disadvantages, a PDMS-based supramolecular polymeric network cross-linked by 2,6-dicarboxylic amide pyridine (PDCA) chelating ligands and Fe(III) centers was developed, wherein the coordination geometry assumes three different interactions as follows: a strong pyridyl–iron one, and two weaker carboxamido–iron ones through both the nitrogen and oxygen atoms of the carboxamide groups (Figure 4).30 In these dual-strength metal-coordination bonds, the weaker ones could readily break and reconstruct, while the strong one could remain stable, enabling reversible unfolding and refolding of the chains, and therefore, supports high stretchability (maximum fracture strain up to 4500 ± 20%), toughness (fracture energy ∼2571 ± 20 J·m−2), and Specifically, the healing process could be out even under (such as and is not by and In addition, this SPM with a of between Fe(III) and PDMS a high dielectric ± and dielectric strength ± In combination with adaptive mechanical behaviors of the a device of artificial was which was able to in a broad (Figure area of in the elastomer was achieved after applying a high electric field on Figure | Supramolecular polymeric of the supramolecular polymeric elastomer and the and proposed mechanism for chain and tensile stretching (top). The stress–strain curves of the film healed based on the material at different conditions (bottom-left). to the of the healed for in a dielectric elastomer Photographs of the dielectric elastomer before and after the of a high (bottom-right). Reproduced with permission from ref Copyright 2016 Nature Publishing Group. Download figure Download PowerPoint The high of metal-coordination bonds could the dielectric when are integrated into Such is for dielectric materials to this a self-healing dielectric elastomer was reported with metal-coordination bonds as the cross-links in PDMS matrixes (Figure Due to the dynamic and reversible nature of the coordination, the dielectric elastomer showed self-healing ability at ambient conditions. Healing of ± ± and ± were for and and cross-linked PDMS in as dielectrics the dielectric and characteristics were It could be that the strong interactions between the metal and the small could in PDMS elastomer under Notably, stable characteristics and low after at 100% strain were achieved even in the fully stretchable transistors fabricated based on dielectrics. The outstanding electrical performance and skinlike mechanical properties of these metal–ligand cross-linked PDMS make them prominent candidates for applications in stretchable electronics. Figure | Supramolecular polymeric The of metal cross-linked PDMS and the proposed dynamic interactions metal the and the in the polymer systems under mechanical stress (top and Schematic of the device structure in a geometry on rigid (bottom-left). Self-healing test for polymer at ambient without curves of with and as dielectrics (bottom-right). Adapted from Copyright 2016 American Chemical Society. Download figure Download PowerPoint Supramolecular Polymeric polymers are often rigid and to maintain good electronic properties, the mechanical strength their practical applications in electronics to some various physical and chemical including strain energy dissipation via noncovalent bonds, and cross-linking with amorphous were developed to stretchability on the polymers while their electrical Among them, noncovalent or dynamic covalent chemistry an to rigid semiconducting polymers to be without sacrificing their electrical performance. This is because their dynamic bonds could be readily broken and under mechanical as energy dissipation and of their mechanical Herein, we address recent in supramolecular polymeric