Soft Core/Shell Packages for Stretchable Electronics

This paper presents materials and core/shell architectures that provide optimized mechanical properties in packages for stretchable electronic systems. Detailed experimental and theoretical studies quantitatively connect the geometries and elastic properties of the constituent materials to the overall mechanical responses of the integrated systems, with a focus on interfacial stresses, effective modulus, and maximum extent of elongation. Specific results include core/shell designs that lead to peak values of the shear and normal stresses on the skin that remain less than 10 kPa even for applied strains of up to 20%, thereby inducing minimal somatosensory perception of the device on the human skin. Additional, strain‐limiting mesh structures embedded in the shell improve mechanical robustness by protecting the active components from strains that would otherwise exceed the fracture point. Demonstrations in precommercial stretchable electronic systems illustrate the utility of these concepts.     

Stretchable Twisted‐Pair Transmission Lines for Microwave Frequency Wearable Electronics

Stretchable electrical interconnects based on serpentines combined with elastic materials are utilized in various classes of wearable electronics. However, such interconnects are primarily for direct current or low‐frequency signals and incompatible with microwave electronics that enable wireless communication. In this paper, design and fabrication procedures are described for stretchable transmission line capable of delivering microwave signals. The stretchable transmission line has twisted‐pair design integrated into thin‐film serpentine microstructure to minimize electromagnetic interference, such that the line's performance is minimally affected by the environment in close proximity, allowing its use in thin‐film bioelectronics, such as the epidermal electronic system. Detailed analysis, simulations, and experimental results show that the stretchable transmission line has negligible changes in performance when stretched and is operable on skin through suppressed radiated emission achieved with the twisted‐pair geometry. Furthermore, stretchable microwave low‐pass filter and band‐stop filter are demonstrated using the twisted‐pair structure to show the feasibility of the transmission lines as stretchable passive components. These concepts form the basic elements used in the design of stretchable microwave components, circuits, and subsystems performing important radio frequency functionalities, which can apply to many types of stretchable bioelectronics for radio transmitters and receivers.     

Shape-Engineerable Silk Fibroin Papers for Ideal Substrate Alternatives of Plastic Electronics

Plastic-based electronics fill the gaps in conventional rigid silicon-based devices toward the applications in soft interfaces. However, people in the future should also consider their potential environmental impact if tons of non-degradable plastics are applied. Silk fibroin is a superior substrate alternative for the development of “green” electronics; whereas, the brittleness of silk films is still a major limitation impeding their practical use. Different from the widely reported polyphasic composite approaches, here a trace-ion-assisted plasticization strategy is developed, and shape-engineerable pure silk fibroin paper (PSFP) is prepared for the first time, which can be engraved and crumpled like a sheet of paper in the dry state. The PSFPs exhibit higher tensile fracture energy (14.4 ± 4 kJ m⁻²) than any typical plastic-electronic-substrates as far as it is known. The intrinsic brittleness of pure silk films is overcome, and the PSFP can be easily engineered to form periodic meshes, electronic prototypes, and kirigami-based devices, which are beyond the reported regenerated silk films or silk composite films. Moreover, the scrape coating method employed here is simple, highly repeatable, and suitable for scaled production of low-cost PSFP continuously. Collectively, the PSFP is generalizable to various shapes and devices, represents an ideal substrate alternative to plastic electronics.     

Flexible Electronics: Stretchable Electrodes and Their Future

Flexible electronics, as an emerging and exciting research field, have brought great interest to the issue of how to make flexible electronic materials that offer both durability and high performance at strained states. With the advent of on‐body wearable and implantable electronics, as well as increasing demands for human‐friendly intelligent soft robots, enormous effort is being expended on highly flexible functional materials, especially stretchable electrodes, by both the academic and industrial communities. Among different deformation modes, stretchability is the most demanding and challenging. This review focuses on the latest advances in stretchable transparent electrodes based on a new design strategy known as kirigami (the art of paper cutting) and investigates the recent progress on novel applications, including skin‐like electronics, implantable biodegradable devices, and bioinspired soft robotics. By comparing the optoelectrical and mechanical properties of different electrode materials, some of the most important outcomes with comments on their merits and demerits are raised. Key design considerations in terms of geometries, substrates, and adhesion are also discussed, offering insights into the universal strategies for engineering stretchable electrodes regardless of the material. It is suggested that highly stretchable and biocompatible electrodes will greatly boost the development of next‐generation intelligent life‐like electronics.     

Reinventing Butyl Rubber for Stretchable Electronics

The development of stretchable electronic devices that are soft and conformable has relied heavily on a single material—polydimethylsiloxane—as the elastomeric substrate. Although polydimethylsiloxane has a number of advantageous characteristics, its high gas permeability is detrimental to stretchable devices that use materials sensitive to oxygen and water vapor, such as organic semiconductors and oxidizable metals. Failing to protect these materials from atmosphere‐induced decomposition leads to premature device failure; therefore, it is imperative to develop elastomers with gas barrier properties that enable stretchable electronics with practical lifetimes. Here, butyl rubber—a material with an intrinsically low gas permeability traditionally used in the innerliners of tires to maintain air pressure—is reinvented for stretchable electronics. This new material is smooth and optically transparent, possesses the low gas permeability typical of butyl rubber, and vastly outperforms polydimethylsiloxane as an encapsulating barrier to prevent the atmospheric degradation of sensitive electronic materials and the premature failure of functioning organic devices. The merits of transparent butyl rubber presented here position this material as an important counterpart to polydimethylsiloxane that will enable future generation stretchable electronics.     

Highly Stretchable,Global, and Distributed Local Strain Sensing Line Using GaInSn Electrodes for Wearable Electronics

For identifying human or finger movement, it is necessary to sense subtle movements at multiple points, including the local strain and global deformation simultaneously; however, this has not yet been realized. Therefore, a highly stretchable, global, and distributed local strain sensing electrode made of GaInSn and polydimethylsiloxane is developed for wearable devices. To investigate the electrical properties of multiple sections of the GaInSn electrode when stretching, tensile, cyclic, and three‐point‐bending tests are performed. The results demonstrate that the electrode can withstand a strain up to 50% and has little hysteresis without any delay. Moreover, the distributed local strain and global strain can be simultaneously measured using just a single electrode line. Finally, a prototype of a data glove as an application of the strain sensing line is manufactured, and it is demonstrated that the folding state of fingers could be identified. The proposed technology may allow the creation of a lightweight master hand manipulator or 3D data entry device.     

Load‐Controlled Roll Transfer of Oxide Transistors for Stretchable Electronics

A stretchable and transparent In‐Ga‐Zn‐O (IGZO) thin film transistors with high electrical performance and scalability is demonstrated. A load‐controlled roll transfer method is realized for fully automated and scalable transfer of the IGZO TFTs from a rigid substrate to a nonconventional elastomeric substrate. The IGZO TFTs exhibit high electrical performance under stretching and cyclic tests, demonstrating the potentiality of the load‐controlled roll transfer in stretchable electronics. The mechanics of the load‐controlled roll transfer is investigated and simulated, and it is shown that the strain level experienced by the active layers of the device can be controlled to well below their maximum fracture level during transfer.     

A Textile-Based Temperature-Tolerant Stretchable Supercapacitor for Wearable Electronics

Among the extensive development of wearable electronics, which can be implanted onto bodies or embedded in clothes, textile-based devices have gained significant attention. For daily basis applications, wearable energy storage devices are required to be stable under harsh environmental conditions and different deformational conditions. In this study, a textile-based stretchable supercapacitor with high electrochemical performance, mechanical stability, and temperature tolerance over a wide temperature range is reported. It exhibits high areal capacitances of 28.0, 30.4, and 30.6 mF cm⁻² at −30, 25, and 80 °C, respectively, while the capacitance remains stable over three repeated cycles of cooling and heating from −30 to 80 °C. The supercapacitor is stable under stretching up to 50% and 1000 repetitive cycles of stretching. A temperature sensor and an liquid-crystal display are simultaneously driven at temperatures between −20 and 80 °C by the supercapacitors. The supercapacitors are woven into a nylon glove power a micro-light-emitting diode stably regardless of the bending of the index finger. Furthermore, the encapsulated supercapacitors retain the capacitance during being immersed in water for a few days. This study demonstrates the potential application of the fabricated supercapacitor as a wearable energy storage device that works under extreme temperature variations, high humidity, and body movements.     

Calcium‐Modified Silk as a Biocompatible and Strong Adhesive for Epidermal Electronics

With the increasing interest and demand for epidermal electronics, a strong interface between a sensor and a biological surface is essential, yet achieving such interface is still a challenge. Here, a calcium (Ca)‐modified biocompatible silk fibroin as a strong adhesive for epidermal electronics is proposed and the physical principles behind its interfacial and adhesive properties are reported. A strong adhesive characteristic (>800 N m^?1) is observed because of the increase in both viscoelastic property and mechanical interlocking through the incorporation of Ca ions. Furthermore, additional key characteristics of the Ca‐modified silk: reusability, stretchability, biocompatibility, and conductivity, are reported. These characteristics enable a wide range of applications as demonstrated in four epidermal electronic systems: capacitive touch sensor, resistive strain sensor, hydrogel‐based drug delivery, and electrocardiogram monitoring sensor. As a reusable, biocompatible, conductive, and strong adhesive with water‐degradability, the Ca‐modified silk adhesive is a promising candidate for the next‐generation adhesive for epidermal biomedical sensors.     

A Skin‐Inspired Substrate with Spaghetti‐Like Multi‐Nanofiber Network of Stiff and Elastic Components for Stretchable Electronics

Mimicking the skin's non‐linear self‐limiting mechanical characteristics is of great interest. Skin is soft at low strain but becomes stiff at high strain and thereby can protect human tissues and organs from high mechanical loads. Herein, the design of a skin‐inspired substrate is reported based on a spaghetti‐like multi‐nanofiber network (SMNN) of elastic polyurethane (PU) nanofibers (NFs) sandwiched between stiff poly(vinyldenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) NFs layers embedded in polydimethylsiloxane elastomer. The elastic moduli of the stretchable skin‐inspired substrate can be tuned in a range that matches well with the mechanical properties of skins by adjusting the loading ratios of the two NFs. Confocal imaging under stretching indicates that PU NFs help maintain the stretchability while adding stiff P(VDF‐TrFE) NFs to control the self‐limiting characteristics. Interestingly, the Au layer on the substrate indicates a negligible change in the resistance under cyclic (up to 7000 cycles at 35% strain) and dynamic stretching (up to 35% strain), which indicates the effective absorption of stress by the SMNN. A stretchable chemoresistive gas sensor on the skin‐inspired substrate also demonstrates a reasonable stability in NO₂ sensing response under strain up to 30%. The skin‐inspired substrate with SMNN provides a step toward ultrathin stretchable electronics.     

A Generic Soft Encapsulation Strategy for Stretchable Electronics

Recent progress in stretchable forms of inorganic electronic systems has established a route to new classes of devices, with particularly unique capabilities in functional biointerfaces, because of their mechanical and geometrical compatibility with human tissues and organs. A reliable approach to physically and chemically protect the electronic components and interconnects is indispensable for practical applications. Although recent reports describe various options in soft, solid encapsulation, the development of approaches that do not significantly reduce the stretchability remains an area of continued focus. Herein, a generic, soft encapsulation strategy is reported, which is applicable to a wide range of stretchable interconnect designs, including those based on two‐dimensional (2D) serpentine configurations, 2D fractal‐inspired patterns, and 3D helical configurations. This strategy forms the encapsulation while the system is in a prestrained state, in contrast to the traditional approach that involves the strain‐free configuration. A systematic comparison reveals that substantial enhancements (e.g., ≈6.0 times for 2D serpentine, ≈4.0 times for 2D fractal, and ≈2.6 times for 3D helical) in the stretchability can be achieved through use of the proposed strategy. Demonstrated applications in highly stretchable light‐emitting diodes systems that can be mounted onto complex curvilinear surfaces illustrate the general capabilities in functional device systems.     

Experimental and Theoretical Studies of Serpentine Microstructures Bonded To Prestrained Elastomers for Stretchable Electronics

Stretchable electronic devices that exploit inorganic materials are attractive due to their combination of high performance with mechanical deformability, particularly for applications in biomedical devices that require intimate integration with human body. Several mechanics and materials schemes have been devised for this type of technology, many of which exploit deformable interconnects. When such interconnects are fully bonded to the substrate and/or encapsulated in a solid material, useful but modest levels of deformation (<30–40%) are possible, with reversible and repeatable mechanics. Here, the use of prestrain in the substrate is introduced, together with interconnects in narrow, serpentine shapes, to yield significantly enhanced (more than two times) stretchability, to more than 100%. Fracture and cyclic fatigue testing on structures formed with and without prestrain quantitatively demonstrate the possible enhancements. Finite element analyses (FEA) illustrates the effects of various material and geometric parameters. A drastic decrease in the elastic stretchability is observed with increasing metal thickness, due to changes in the buckling mode, that is, from local wrinkling at small thicknesses to absence of such wrinkling at large thicknesses, as revealed by experiment. An analytic model quantitatively predicts the wavelength of this wrinkling, and explains the thickness dependence of the buckling behaviors.     

A Highly Stretchable Fiber‐Based Triboelectric Nanogenerator for Self‐Powered Wearable Electronics

The development of flexible and stretchable electronics has attracted intensive attention for their promising applications in next‐generation wearable functional devices. However, these stretchable devices that are made in a conventional planar format have largely hindered their development, especially in highly stretchable conditions. Herein, a novel type of highly stretchable, fiber‐based triboelectric nanogenerator (fiber‐like TENG) for power generation is developed. Owing to the advanced structural designs, including the fiber‐convolving fiber and the stretchable electrodes on elastic silicone rubber fiber, the fiber‐like TENG can be operated at stretching mode with high strains up to 70% and is demonstrated for a broad range of applications such as powering a commercial capacitor, LCD screen, digital watch/calculator, and self‐powered acceleration sensor. This work verifies the promising potential of a novel fiber‐based structure for both power generation and self‐powered sensing.     

Design of Strain‐Limiting Substrate Materials for Stretchable and Flexible Electronics

Recently developed classes of electronics for biomedical applications exploit substrates that offer low elastic modulus and high stretchability, to allow intimate, mechanically biocompatible integration with soft biological tissues. A challenge is that such substrates do not generally offer protection of the electronics from high peak strains that can occur upon large‐scale deformation, thereby creating a potential for device failure. The results presented here establish a simple route to compliant substrates with strain‐limiting mechanics based on approaches that complement those of recently described alternatives. Here, a thin film or mesh of a high modulus material transferred onto a prestrained compliant substrate transforms into wrinkled geometry upon release of the prestrain. The structure formed by this process offers a low elastic modulus at small strain due to the small effective stiffness of the wrinkled film or mesh; it has a high tangent modulus (e.g., >1000 times the elastic modulus) at large strain, as the wrinkles disappear and the film/mesh returns to a flat geometry. This bilinear stress–strain behavior has an extremely sharp transition point, defined by the magnitude of the prestrain. A theoretical model yields analytical expressions for the elastic and tangent moduli and the transition strain of the bilinear stress–strain relation, with quantitative correspondence to finite element analysis and experiments.