Plasticizing Silk Protein for On‐Skin Stretchable Electrodes

Soft and stretchable electronic devices are important in wearable and implantable applications because of the high skin conformability. Due to the natural biocompatibility and biodegradability, silk protein is one of the ideal platforms for wearable electronic devices. However, the realization of skin‐conformable electronic devices based on silk has been limited by the mechanical mismatch with skin, and the difficulty in integrating stretchable electronics. Here, silk protein is used as the substrate for soft and stretchable on‐skin electronics. The original high Young's modulus (5–12 GPa) and low stretchability (<20%) are tuned into 0.1–2 MPa and > 400%, respectively. This plasticization is realized by the addition of CaCl₂ and ambient hydration, whose mechanism is further investigated by molecular dynamics simulations. Moreover, highly stretchable (>100%) electrodes are obtained by the thin‐film metallization and the formation of wrinkled structures after ambient hydration. Finally, the plasticized silk electrodes, with the high electrical performance and skin conformability, achieve on‐skin electrophysiological recording comparable to that by commercial gel electrodes. The proposed skin‐conformable electronics based on biomaterials will pave the way for the harmonized integration of electronics into human.     

Biofriendly,Stretchable, and Reusable Hydrogel Electronics as Wearable Force Sensors

The ever‐growing overlap between stretchable electronic devices and wearable healthcare applications is igniting the discovery of novel biocompatible and skin‐like materials for human‐friendly stretchable electronics fabrication. Amongst all potential candidates, hydrogels with excellent biocompatibility and mechanical features close to human tissues are constituting a promising troop for realizing healthcare‐oriented electronic functionalities. In this work, based on biocompatible and stretchable hydrogels, a simple paradigm to prototype stretchable electronics with an embedded three‐dimensional (3D) helical conductive layout is proposed. Thanks to the 3D helical structure, the hydrogel electronics present satisfactory mechanical and electrical robustness under stretch. In addition, reusability of stretchable electronics is realized with the proposed scenario benefiting from the swelling property of hydrogel. Although losing water would induce structure shrinkage of the hydrogel network and further undermine the function of hydrogel in various applications, the worn‐out hydrogel electronics can be reused by simply casting it in water. Through such a rehydration procedure, the dehydrated hydrogel can absorb water from the surrounding and then the hydrogel electronics can achieve resilience in mechanical stretchability and electronic functionality. Also, the ability to reflect pressure and strain changes has revealed the hydrogel electronics to be promising for advanced wearable sensing applications.     

On‐Skin Triboelectric Nanogenerator and Self‐Powered Sensor with Ultrathin Thickness and High Stretchability

Researchers have devoted a lot of efforts on pursuing light weight and high flexibility for the wearable electronics, which also requires the related energy harvesting devices to have ultrathin thickness and high stretchability. Hence, an elastic triboelectric nanogenerator (TENG) is proposed that can serve as the second skin on human body. The total thickness of this TENG is about 102 µm and the device can work durably under a strain of 100%. The carbon grease is painted on the surface of elastomer film to work as stretchable electrode and thus the fine geometry control of the electrode can be achieved. This elastic TENG can even work on the human fingers without disturbing body movement. The open‐circuit voltage and short‐circuit current from the device with a contact area of 9 cm² can reach 115 V and 3 µA, respectively. Two kinds of self‐powered sensor systems with optimized identification strategies are also designed to demonstrate the application possibility of this elastic TENG. The superior characteristics of ultrathin thickness, high stretchability, and fine geometry control of this TENG can promote many potential applications in the field of wearable self‐powered sensory system, electronics skin, artificial muscles, and soft robotics.     

3D-Printed Silicone Substrates as Highly Deformable Electrodes for Stretchable Li-Ion Batteries

Stretchable energy storage devices receive a considerable attention at present due to their growing demand for powering wearable electronics. A vital component in stretchable energy storage devices is its electrode which should endure a large and repeated number of mechanical deformations during its prolonged use. It is crucial to develop a technology to fabricate highly deformable electrode in an easy and an economic manner. Here, the fabrication of stretchable electrode substrates using 3D-printing technology is reported. The ink for fabricating it contains a mixture of sacrificial sugar particles and polydimethylsiloxane resin which solidifies upon thermal curing. The printed stretchable substrate attains a porous structure after leaching the sugar particles in water. The resulting printed porous stretchable substrates are then utilized as electrodes for Li-ion batteries (LIBs) after loading them with electrode materials. The batteries with stretchable electrodes exhibit a decent electrochemical performance comparable to that of the conventional electrodes. The stretchable electrodes also exhibit a stable electrochemical performance under various mechanical deformations and even after several hundreds of stretch/release cycles. This work provides a feasible route for constructing LIBs with high stretchability and enhanced electrochemical performance thereby providing a platform for realizing stretchable batteries for next generation wearable electronics.     

An All‐Stretchable‐Component Sodium‐Ion Full Battery

Stretchable energy‐storage devices receive considerable attention due to their promising applications in future wearable technologies. However, they currently suffer from many problems, including low utility of active materials, limited multidirectional stretchability, and poor stability under stretched conditions. In addition, most proposed designs use one or more rigid components that fail to meet the stretchability requirement for the entire device. Here, an all‐stretchable‐component sodium‐ion full battery based on graphene‐modified poly(dimethylsiloxane) sponge electrodes and an elastic gel membrane is developed for the first time. The battery exhibits reasonable electrochemical performance and robust mechanical deformability; its electrochemical characteristics can be well‐maintained under many different stretched conditions and after hundreds of stretching–release cycles. This novel design integrating all stretchable components provides a pathway toward the next generation of wearable energy devices in modern electronics.     

A Polypyrrole Elastomer Based on Confined Polymerization in a Host Polymer Network for Highly Stretchable Temperature and Strain Sensors

For the purpose of stretchable electronics, broad interests have been paid to elastic conductors by which high tensile strain over 100% can be readily achieved. Here, a scalable‐processing, dyeing‐like strategy for highly stretchable polypyrrole elastomer (1450% in strain) is conceived without particular topological design. This approach effectively improves the mechanical properties of the classic insoluble polypyrrole by confined polymerization within an elastic polymer network. In terms of the easy processing, it is technically possible to prepare stretchable electronics with arbitrary shape and size for wearable electronics with low cost. The mechanism of interpenetrated networks coexisting with microphase separation is comprehensively illustrated at molecular scale. The as‐fabricated polypyrrole elastomers are utilized as temperature or strain sensors for automatic fishing and region‐distinct dual signal sensing. Further integration of multiple sensors offers immediate alarm for old people falling at home, which thereby proves its promising potential in practical applications.     

Stretchable,Large‐area Organic Electronics

Stretchability will significantly expand the application scope of electronics, particularly large‐area electronics—displays, sensors, and actuators. If arbitrary surfaces and movable parts could be covered with stretchable electronics, which is impossible with conventional electronics, new classes of applications are expected to emerge. A large hurdle is manufacturing electrical wiring with high conductivity, high stretchability, and large‐area compatibility. This Review describes stretchable, large‐area electronics based on organic field‐effect transistors for applications to sensors and displays. First, novel net‐shaped organic transistors are employed to realize stretchable, large‐area sensor networks that detect distributions of pressure and temperature simultaneously. The whole system is functional even when it is stretched by 25%. In order to further improve stretchability, printable elastic conductors are developed by dispersing single‐walled carbon nanotubes (SWNTs) as dopants uniformly in rubbers. Further, we describe integration of printable elastic conductors with organic transistors to construct a rubber‐like stretchable active matrix for large‐area sensor and display applications. Finally, we will discuss the future prospects of stretchable, large‐area electronics with delineating a picture of the next‐generation human/machine interfaces from the aspect of materials science and electronic engineering.     

Stretchable Organometal‐Halide‐Perovskite Quantum‐Dot Light‐Emitting Diodes

Stretchable light‐emitting diodes (LEDs) and electroluminescent capacitors have been reported to potentially bring new opportunities to wearable electronics; however, these devices lack in efficiency and/or stretchability. Here, a stretchable organometal‐halide‐perovskite quantum‐dot LED with both high efficiency and mechanical compliancy is demonstrated. The hybrid device employs an ultrathin (<3 µm) LED structure conformed on a surface‐wrinkled elastomer substrate. Its luminescent efficiency is up to 9.2 cd A^?1, which is 70% higher than a control diode fabricated on the rigid indium tin oxide/glass substrate. Mechanical deformations up to 50% tensile strain do not induce significant loss of the electroluminescent property. The device can survive 1000 stretch–release cycles of 20% tensile strain with small fluctuations in electroluminescent performance.     

Inkjet printing of single-walled carbon nanotube/RuO<Subscript>2</Subscript> nanowire supercapacitors on cloth fabrics and flexible substrates

Single-walled carbon nanotube (SWNT) thin film electrodes have been printed on flexible substrates and cloth fabrics by using SWNT inks and an off-the-shelf inkjet printer, with features of controlled pattern geometry (0.4–6 cm²), location, controllable thickness (20–200 nm), and tunable electrical conductivity. The as-printed SWNT films were then sandwiched together with a piece of printable polymer electrolyte to form flexible and wearable supercapacitors, which displayed good capacitive behavior even after 1,000 charge/discharge cycles. Furthermore, a simple and efficient route to produce ruthenium oxide (RuO₂) nanowire/SWNT hybrid films has been developed, and it was found that the knee frequency of the hybrid thin film electrodes can reach 1,500 Hz, which is much higher than the knee frequency of the bare SWNT electrodes (˜158 Hz). In addition, with the integration of RuO₂ nanowires, the performance of the printed SWNT supercapacitor was significantly improved in terms of its specific capacitance of 138 F/g, power density of 96 kW/kg, and energy density of 18.8 Wh/kg. The results indicate the potential of printable energy storage devices and their significant promise for application in wearable energy storage devices.      

Skin‐Like Stretchable Fuel Cell Based on Gold‐Nanowire‐Impregnated Porous Polymer Scaffolds

Skin‐like energy devices can be conformally attached to the human body, which are highly desirable to power soft wearable electronics in the future. Here, a skin‐like stretchable fuel cell based on ultrathin gold nanowires (AuNWs) and polymerized high internal phase emulsions (polyHIPEs) scaffolds is demonstrated. The polyHIPEs can offer a high porosity of 80% yet with an overall thickness comparable to human skin. Upon impregnation with electronic inks containing ultrathin (2 nm in diameter) and ultrahigh aspect‐ratio (>10 000) gold nanowires, skin‐like strain‐insensitive stretchable electrodes are successfully fabricated. With such designed strain‐insensitive electrodes, a stretchable fuel cell is fabricated by using AuNWs@polyHIPEs, platinum (Pt)‐modified AuNWs@polyHIPEs, and ethanol as the anode, cathode, and fuel, respectively. The resulting epidermal fuel cell can be patterned and transferred onto skin as “tattoos” yet can offer a high power density of 280 µW cm^?2 and a high durability (>90% performance retention under stretching, compression, and twisting). The results presented here demonstrate that this skin‐thin, porous, yet stretchable electrode is essentially multifunctional, simultaneously serving as a current collector, an electrocatalyst, and a fuel host, indicating potential applications to power future soft wearable 2.0 electronics for remote healthcare and soft robotics.     

Printable Fabrication of Nanocoral‐Structured Electrodes for High‐Performance Flexible and Planar Supercapacitor with Artistic Design

Planar supercapacitors with high flexibility, desirable operation safety, and high performance are considered as attractive candidates to serve as energy‐storage devices for portable and wearable electronics. Here, a scalable and printable technique is adopted to construct novel and unique hierarchical nanocoral structures as the interdigitated electrodes on flexible substrates. The as‐fabricated flexible all‐solid‐state planar supercapacitors with nanocoral structures achieve areal capacitance up to 52.9 mF cm^?2, which is 2.5 times that of devices without nanocoral structures, and this figure‐of‐merit is among the highest in the literature for the same category of devices. More interestingly, due to utilization of the inkjet‐printing technique, excellent versatility on electrode‐pattern artistic design is achieved. Particularly, working supercapacitors with artistically designed patterns are demonstrated. Meanwhile, the high scalability of such a printable method is also demonstrated by fabrication of large‐sized artistic supercapacitors serving as energy‐storage devices in a wearable self‐powered system as a proof of concept.     

Printable Skin‐Driven Mechanoluminescence Devices via Nanodoped Matrix Modification

Mechanically driven light generation is an exciting and under‐exploited phenomenon with a variety of possible practical applications. However, the current driving mode of mechanoluminescence (ML) devices needs strong stimuli. Here, a flexible sensitive ML device via nanodopant elasticity modulus modification is introduced. Rigid ZnS:M²⁺(Mn/Cu)@Al₂O₃ microparticles are dispersed into soft poly(dimethylsiloxane) (PDMS) film and printed out to form flexible devices. For various flexible and sensitive scenes, SiO₂ nanoparticles are adopted to adjust the elasticity modulus of the PDMS matrix. The doped nanoparticles can concentrate stress to ZnS:M²⁺(Mn/Cu)@Al₂O₃ microparticles and achieve intense ML under weak stimuli of the moving skin. The printed nano‐/microparticle‐doped matrix film can achieve skin‐driven ML, which can be adopted to present fetching augmented animations expressions. The printable ML film, amenable to large areas, low‐cost manufacturing, and mechanical softness will be versatile on stress visualization, luminescent sensors, and open definitely new functional skin with novel augmented animations expressions, the photonic skin.     

Stretchable Organic Semiconductor Devices

Stretchable electronics are essential for the development of intensely packed collapsible and portable electronics, wearable electronics, epidermal and bioimplanted electronics, 3D surface compliable devices, bionics, prosthesis, and robotics. However, most stretchable devices are currently based on inorganic electronics, whose high cost of fabrication and limited processing area make it difficult to produce inexpensive, large‐area devices. Therefore, organic stretchable electronics are highly attractive due to many advantages over their inorganic counterparts, such as their light weight, flexibility, low cost and large‐area solution‐processing, the reproducible semiconductor resources, and the easy tuning of their properties via molecular tailoring. Among them, stretchable organic semiconductor devices have become a hot and fast‐growing research field, in which great advances have been made in recent years. These fantastic advances are summarized here, focusing on stretchable organic field‐effect transistors, light‐emitting devices, solar cells, and memory devices.     

EGaIn‐Assisted Room‐Temperature Sintering of Silver Nanoparticles for Stretchable,Inkjet‐Printed,Thin‐Film Electronics

Coating inkjet‐printed traces of silver nanoparticle (AgNP) ink with a thin layer of eutectic gallium indium (EGaIn) increases the electrical conductivity by six‐orders of magnitude and significantly improves tolerance to tensile strain. This enhancement is achieved through a room‐temperature “sintering” process in which the liquid‐phase EGaIn alloy binds the AgNP particles (≈100 nm diameter) to form a continuous conductive trace. Ultrathin and hydrographically transferrable electronics are produced by printing traces with a composition of AgNP‐Ga‐In on a 5 µm‐thick temporary tattoo paper. The printed circuit is flexible enough to remain functional when deformed and can support strains above 80% with modest electromechanical coupling (gauge factor ≈1). These mechanically robust thin‐film circuits are well suited for transfer to highly curved and nondevelopable 3D surfaces as well as skin and other soft deformable substrates. In contrast to other stretchable tattoo‐like electronics, the low‐cost processing steps introduced here eliminate the need for cleanroom fabrication and instead requires only a commercial desktop printer. Most significantly, it enables functionalities like “electronic tattoos” and 3D hydrographic transfer that have not been previously reported with EGaIn or EGaIn‐based biphasic electronics.     

A Liquid‐Metal–Elastomer Nanocomposite for Stretchable Dielectric Materials

Stretchable high‐dielectric‐constant materials are crucial for electronic applications in emerging domains such as wearable computing and soft robotics. While previous efforts have shown promising materials architectures in the form of dielectric nano‐/microinclusions embedded in stretchable matrices, the limited mechanical compliance of these materials significantly limits their practical application as soft energy‐harvesting/storage transducers and actuators. Here, a class of liquid metal (LM)–elastomer nanocomposites is presented with elastic and dielectric properties that make them uniquely suited for applications in soft‐matter engineering. In particular, the role of droplet size is examined and it is found that embedding an elastomer with a polydisperse distribution of nanoscale LM inclusions can enhance its electrical permittivity without significantly degrading its elastic compliance, stretchability, or dielectric breakdown strength. In contrast, elastomers embedded with microscale droplets exhibit similar improvements in permittivity but a dramatic reduction in breakdown strength. The unique enabling properties and practicality of LM–elastomer nanocomposites for use in soft machines and electronics is demonstrated through enhancements in performance of a dielectric elastomer actuator and energy‐harvesting transducer.     

A Flexible Stretchable Hydrogel Electrolyte for Healable All‐in‐One Configured Supercapacitors

The development of integrated high‐performance supercapacitors with all‐in‐one configuration, excellent flexibility and autonomously intrinsic self‐healability, and without the extra healable film layers, is still tremendously challenging. Compared to the sandwich‐like laminated structures of supercapacitors with augmented interfacial contact resistance, the flexible healable integrated supercapacitor with all‐in‐one structure could theoretically improve their interfacial contact resistance and energy densities, simplify the tedious device assembly process, prolong the lifetime, and avoid the displacement and delamination of multilayered configurations under deformations. Herein, a flexible healable all‐in‐one configured supercapacitor with excellent flexibility and reliable self‐healing ability by avoiding the extra healable film substrates and the postassembled sandwich‐like laminated structures is developed. The healable all‐in‐one configured supercapacitor is prepared from in situ polymerization and deposition of nanocomposites electrode materials onto the two‐sided faces of the self‐healing hydrogel electrolyte separator. The self‐healing hydrogel film is obtained from the physically crosslinked hydrogel with enormous hydrogen bonds, which can endow the healable capability through dynamic hydrogen bonding. The assembled all‐in‐one configured supercapacitor exhibits enhanced capacitive performance, good cycling stability, reliable self‐healing capability, and excellent flexibility. It holds broad prospects for obtaining various flexible healable all‐in‐one configured supercapacitors for working as portable energy storage devices in wearable electronics.     

Microtopography‐Guided Conductive Patterns of Liquid‐Driven Graphene Nanoplatelet Networks for Stretchable and Skin‐Conformal Sensor Array

Flexible thin‐film sensors have been developed for practical uses in invasive or noninvasive cost‐effective healthcare devices, which requires high sensitivity, stretchability, biocompatibility, skin/organ‐conformity, and often transparency. Graphene nanoplatelets can be spontaneously assembled into transparent and conductive ultrathin coatings on micropatterned surfaces or planar substrates via a convective Marangoni force in a highly controlled manner. Based on this versatile graphene assembled film preparation, a thin, stretchable and skin‐conformal sensor array (144 pixels) is fabricated having microtopography‐guided, graphene‐based, conductive patterns embedded without any complicated processes. The electrically controlled sensor array for mapping spatial distributions (144 pixels) shows high sensitivity (maximum gauge factor ≈1697), skin‐like stretchability (<48%), high cyclic stability or durability (over 10⁵ cycles), and the signal amplification (≈5.25 times) via structure‐assisted intimate‐contacts between the device and rough skin. Furthermore, given the thin‐film programmable architecture and mechanical deformability of the sensor, a human skin‐conformal sensor is demonstrated with a wireless transmitter for expeditious diagnosis of cardiovascular and cardiac illnesses, which is capable of monitoring various amplified pulse‐waveforms and evolved into a mechanical/thermal‐sensitive electric rubber‐balloon and an electronic blood‐vessel. The microtopography‐guided and self‐assembled conductive patterns offer highly promising methodology and tool for next‐generation biomedical devices and various flexible/stretchable (wearable) devices.     

Auxetic Mechanical Metamaterials to Enhance Sensitivity of Stretchable Strain Sensors

Stretchable strain sensors play a pivotal role in wearable devices, soft robotics, and Internet‐of‐Things, yet these viable applications, which require subtle strain detection under various strain, are often limited by low sensitivity. This inadequate sensitivity stems from the Poisson effect in conventional strain sensors, where stretched elastomer substrates expand in the longitudinal direction but compress transversely. In stretchable strain sensors, expansion separates the active materials and contributes to the sensitivity, while Poisson compression squeezes active materials together, and thus intrinsically limits the sensitivity. Alternatively, auxetic mechanical metamaterials undergo 2D expansion in both directions, due to their negative structural Poisson's ratio. Herein, it is demonstrated that such auxetic metamaterials can be incorporated into stretchable strain sensors to significantly enhance the sensitivity. Compared to conventional sensors, the sensitivity is greatly elevated with a 24‐fold improvement. This sensitivity enhancement is due to the synergistic effect of reduced structural Poisson's ratio and strain concentration. Furthermore, microcracks are elongated as an underlying mechanism, verified by both experiments and numerical simulations. This strategy of employing auxetic metamaterials can be further applied to other stretchable strain sensors with different constituent materials. Moreover, it paves the way for utilizing mechanical metamaterials into a broader library of stretchable electronics.     

Energy Harvesters for Wearable and Stretchable Electronics: From Flexibility to Stretchability

The rapid advancements of wearable electronics have caused a paradigm shift in consumer electronics, and the emerging development of stretchable electronics opens a new spectrum of applications for electronic systems. Playing a critical role as the power sources for independent electronic systems, energy harvesters with high flexibility or stretchability have been the focus of research efforts over the past decade. A large number of the flexible energy harvesters developed can only operate at very low strain level (≈0.1%), and their limited flexibility impedes their application in wearable or stretchable electronics. Here, the development of highly flexible and stretchable (stretchability >15% strain) energy harvesters is reviewed with emphasis on strategies of materials synthesis, device fabrication, and integration schemes for enhanced flexibility and stretchability. Due to their particular potential applications in wearable and stretchable electronics, energy‐harvesting devices based on piezoelectricity, triboelectricity, thermoelectricity, and dielectric elastomers have been largely developed and the progress is summarized. The challenges and opportunities of assembly and integration of energy harvesters into stretchable systems are also discussed.     

Recent Advances in Transparent Electronics with Stretchable Forms

Advances in materials science and the desire for next‐generation electronics have driven the development of stretchable and transparent electronics in the past decade. Novel applications, such as smart contact lenses and wearable sensors, have been introduced with stretchable and transparent form factors, requiring a deeper and wider exploration of materials and fabrication processes. In this regard, many research efforts have been dedicated to the development of mechanically stretchable, optically transparent materials and devices. Recent advances in stretchable and transparent electronics are discussed herein, with special emphasis on the development of stretchable and transparent materials, including substrates and electrodes. Several representative examples of applications enabled by stretchable and transparent electronics are presented, including sensors, smart contact lenses, heaters, and neural interfaces. The current challenges and opportunities for each type of stretchable and transparent electronics are also discussed.