Liquid Metal Based Island‐Bridge Architectures for All Printed Stretchable Electrochemical Devices

The adoption of epidermal electronics into everyday life requires new design and fabrication paradigms, transitioning away from traditional rigid, bulky electronics towards soft devices that adapt with high intimacy to the human body. Here, a new strategy is reported for fabricating achieving highly stretchable “island‐bridge” (IB) electrochemical devices based on thick‐film printing process involving merging the deterministic IB architecture with stress‐enduring composite silver (Ag) inks based on eutectic gallium‐indium particles (EGaInPs) as dynamic electrical anchors within the inside the percolated network. The fabrication of free‐standing soft Ag‐EGaInPs‐based serpentine “bridges” enables the printed microstructures to maintain mechanical and electrical properties under an extreme (≈800%) strain. Coupling these highly stretchable “bridges” with rigid multifunctional “island” electrodes allows the realization of electrochemical devices that can sustain high mechanical deformation while displaying an extremely attractive and stable electrochemical performance. The advantages and practical utility of the new printed Ag‐liquid metal‐based island‐bridge designs are discussed and illustrated using a wearable biofuel cell. Such new scalable and tunable fabrication strategy will allow to incorporate a wide range of materials into a single device towards a wide range of applications in wearable electronics.     

Stretchable Electrode Based on Laterally Combed Carbon Nanotubes for Wearable Energy Harvesting and Storage Devices

Carbon nanotubes (CNTs) are a promising material for use as a flexible electrode in wearable energy devices due to their electrical conductivity, soft mechanical properties, electrochemical activity, and large surface area. However, their electrical resistance is higher than that of metals, and deformations such as stretching can lead to deterioration of electrical performances. To address these issues, here a novel stretchable electrode based on laterally combed CNT networks is presented. The increased percolation between combed CNTs provides a high electrical conductivity even under mechanical deformations. Additional nickel electroplating and serpentine electrode designs increase conductivity and deformability further. The resulting stretchable electrode exhibits an excellent sheet resistance, which is comparable to conventional metal film electrodes. The resistance change is minimal even when stretched by ≈100%. Such high conductivity and deformability in addition to intrinsic electrochemically active property of CNTs enable high performance stretchable energy harvesting (wireless charging coil and triboelectric generator) and storage (lithium ion battery and supercapacitor) devices. Monolithic integration of these devices forms a wearable energy supply system, successfully demonstrating its potential as a novel soft power supply module for wearable electronics.     

Anhydride‐Assisted Spontaneous Room Temperature Sintering of Printed Bioresorbable Electronics

Bioresorbable electronic devices are promising replacements for conventional build‐to‐last electronics in implantable biomedical systems and consumer electronics. However, bioresorbable devices are typically achieved by complex complementary metal oxide semiconductor fabrication processes that minimize exposure to humidity. Emerging printable techniques for bioresorbable electronics demand further improvement in electrical conductivity and mechanical robustness. This paper presents a room‐temperature spontaneous sintering method of bioresorbable inks that contain zinc nanoparticles and anhydride. The entire process can be conducted in atmosphere environment under 90% humidity within 300 min. It has minimum requirement for external heating and special ambient conditions, allowing humidity to trigger the surface chemistry of zinc nanoparticles and spontaneous welding between neighboring nanoparticles. The resulting bioresorbable patterns are highly conductive (σ = 72 400 S m^?1) and mechanically robust (>1500 bending cycles) to enable practical applications. A radio circuit achieved through the above method can operate stably over 14 days in air and disappear in water for less than 30 min. The spontaneous room‐temperature sintering represents a rapid and energy‐efficient approach to achieve high‐performance bioresorbable electronics with improved mechanical robustness and electrical performance, leading to broader impacts in the areas of healthcare, information security, and consumer electronics.     

Conductive PEDOT:PSS on surface-functionalized chitosan biopolymers for stretchable skin-like electronics

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising alternative transparent electrode to replace conventional indium tix oxide (ITO) for flexible and stretchable electronics. For their applications in optoelectronic devices, realizing both high conductivity and transmittance for the films is of great necessity as a suitable high performance transparent electrode. Here, we demonstrate simultaneously enhanced electrical and optical properties of PEDOT:PSS films prepared on chitosan bio-substrates by using an organic surface modifier, 11-aminoundecanoic acid (11-AA). The sheet resistance of PEDOT:PSS films decreases from 1120.8 to 292.8 Ω/sq with an increase in a transmittance from 75.9 to 80.4% by 11-AA treatment on the chitosan films. The functional groups of 11-AA effectively enhance the adhesion property at the interface between the chitosan substrate and PEDOT:PSS by forming strong interfacial bondings and decrease insulating PSS from PEDOT:PSS films. The wearable heater devices and on-skin sensors based on the 11-AA-treated PEDOT:PSS on the chitosan bio-substrates are successfully fabricated, showing the excellent thermal and sensing performances. The 11-AA surface-modification approach for highly conductive PEDOT:PSS on chitosan bio-substrates presents a great potential for applications toward transparent, flexible and stretchable electronics.     

Highly efficient and mechanically robust stretchable polymer solar cells with random buckling

Stretchable polymer solar cells have shown great potential as stretchable power generators for applications in stretchable electronics, such as wearable electronics, electronic skins and stretchable displays. However, their mechanical stability and power conversion efficiency (PCE) thus are still far below the requirement for the practical applications. Here, we have developed highly efficient and stretchable polymer solar cells (PSCs) based on a random buckling process. The stretchable PSCs are fabricated by attaching the ultrathin PSC onto a pre-stretched elastomeric substrate and then releasing the prestrain to form random bucklings. Its PCE of 5.8% under 70% tensile strain is the largest to date among the reported PSCs. The stretchable PSCs exhibit small fluctuations in performance after 400 stretching-releasing cycles. This is an important step towards producing stretchable PSCs for commercial applications.     

Intrinsically Stretchable Organic Solar Cells beyond 10% Power Conversion Efficiency Enabled by Transfer Printing Method

Stretchable organic solar cells (OSCs) simultaneously possessing high-efficiency and robust mechanical properties are ideal power generators for the emerging wearable and portable electronics. Herein, after incorporating a low amount of trimethylsiloxy terminated polydimethylsiloxane (PDMS) additive, the intrinsic stretchability of PTB7-Th:IEICO-4F bulk heterojunction (BHJ) film is greatly improved from 5% to 20% strain without sacrificing the photovoltaic performance. The intimate multi-layers stacking of OSCs is also realized with the transfer printing method assisted by electrical adhesive “glue” D-Sorbitol. The resultant devices with 84% electrode transmittance exhibit a remarkable power conversion efficiency (PCE) of 10.1%, which is among the highest efficiency for intrinsically stretchable OSCs to date. The stretchable OSCs also demonstrate the ultra-flexibility, stretchability, and mechanical robustness, which keep the PCE almost unchanged at small bending radium of 2 mm for 300 times bending cycles and retain 86.7% PCE under tensile strain as large as 20% for the devices with 70% electrode transmittance. The results provide a universal method to fabricate highly efficient intrinsically stretchable OSCs.     

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.     

Conductive Hydrogel-Based Electrodes and Electrolytes for Stretchable and Self-Healable Supercapacitors

Stretchable self-healing supercapacitors (SCs) can operate under extreme deformation and restore their initial properties after damage with considerably improved durability and reliability, expanding their opportunities in numerous applications, including smart wearable electronics, bioinspired devices, human–machine interactions, etc. It is challenging, however, to achieve mechanical stretchability and self-healability in energy storage technologies, wherein the key issue lies in the exploitation of ideal electrode and electrolyte materials with exceptional mechanical stretchability and self-healing ability besides conductivity. Conductive hydrogels (CHs) possess unique hierarchical porous structure, high electrical/ionic conductivity, broadly tunable physical and chemical properties through molecular design and structure regulation, holding tremendous promise for stretchable self-healing SCs. Hence, this review is innovatively constructed with a focus on stretchable and self-healing CH based electrodes and electrolytes for SCs. First, the common synthetic approaches of CHs are introduced; then the stretching and self-healing strategies involved in CHs are systematically elaborated; followed by an explanation of the conductive mechanism of CHs; then focusing on CH-based electrodes and electrolytes for stretchable self-healing SCs; subsequently, application of stretchable and self-healing SCs in wearable electronics are discussed; finally, a conclusion is drawn along with views on the challenges and future research directions regarding the field of CHs for SCs.     

An Intrinsically Stretchable High‐Performance Polymer Semiconductor with Low Crystallinity

For wearable and implantable electronics applications, developing intrinsically stretchable polymer semiconductor is advantageous, especially in the manufacturing of large‐area and high‐density devices. A major challenge is to simultaneously achieve good electrical and mechanical properties for these semiconductor devices. While crystalline domains are generally needed to achieve high mobility, amorphous domains are necessary to impart stretchability. Recent progresses in the design of high‐performance donor–acceptor polymers that exhibit low degrees of energetic disorder, while having a high fraction of amorphous domains, appear promising for polymer semiconductors. Here, a low crystalline, i.e., near‐amorphous, indacenodithiophene‐co‐benzothiadiazole (IDTBT) polymer and a semicrystalline thieno[3,2‐b]thiophene‐diketopyrrolopyrrole (DPPTT) are compared, for mechanical properties and electrical performance under strain. It is observed that IDTBT is able to achieve both a high modulus and high fracture strain, and to preserve electrical functionality under high strain. Next, fully stretchable transistors are fabricated using the IDTBT polymer and observed mobility ≈0.6 cm² V^?1 s^?1 at 100% strain along stretching direction. In addition, the morphological evolution of the stretched IDTBT films is investigated by polarized UV–vis and grazing‐incidence X‐ray diffraction to elucidate the molecular origins of high ductility. In summary, the near‐amorphous IDTBT polymer signifies a promising direction regarding molecular design principles toward intrinsically stretchable high‐performance polymer semiconductor.     

3D Interconnected Conductive Graphite Nanoplatelet Welded Carbon Nanotube Networks for Stretchable Conductors

Stretchable conductors with stable electrical conductivity under harsh mechanical deformations are essential for developing next generation portable and flexible wearable electronics. To achieve both high stretchability and conductivity with electromechanical stability, highly stretchable conductors based on 3D interconnected conductive graphite nanoplatelet welded carbon nanotube (GNP-w-CNT) networks are fabricated by welding the junctions of CNTs using GNPs followed by infiltrating with poly(dimethylsiloxane) (PDMS). It is observed that GNPs can weld the adjacent CNTs to facilitate the formation of continuous conductive pathways and avoid interfacial slippage under repetitive stretching. The enhanced interfacial bonding enables the conductor both high electrical conductivity (>132 S m⁻¹) and high stretchability (>150% strain) while ensuring long-term stability (1000 stretching-releasing cycles under 60% tensile strain). To demonstrate the outstanding flexibility and electrical stability, a flexible and stretchable light-emitting diode circuit with stable performance during stretching, bending, twisting, and pressing conditions is further fabricated. The unique welding mechanism can be easily extended to other material systems to broaden the application of stretchable conductors to a myriad of new applications.     

Capacitance Characterization of Elastomeric Dielectrics for Applications in Intrinsically Stretchable Thin Film Transistors

Stretchable electronics exhibit unique mechanical properties to expand the applications areas of conventional electronics based on rigid wafers. Intrinsically stretchable thin film transistor is an essential component for functional stretchable electronics, which presents a great opportunity to develop mechanically compliant electronic materials. Certain elastomers have been recently adopted as the gate dielectrics, but their dielectric properties have not been thoroughly investigated for such applications. Here, a charging measurement technique with a resistor–capacitor circuit is proposed to quantify the capacitance of the dielectric layers based on elastomers. As compared with conventional methods, the technique serves as a universal approach to extract the capacitance of various elastomers under static conditions, irrespective of the charging mechanisms. This technique also offers a facile approach to reliably quantify the mobility of thin film transistors based on elastomeric dielectrics, paving the way to utilize this class of dielectrics in the development of intrinsically stretchable transistors.     

Design of Engineered Elastomeric Substrate for Stretchable Active Devices and Sensors

In the field of flexible electronics, emerging applications require biocompatible and unobtrusive devices, which can withstand different modes of mechanical deformation and achieve low complexity in the fabrication process. Here, the fabrication of a mesa‐shaped elastomeric substrate, supporting thin‐film transistors (TFTs) and logic circuits (inverters), is reported. High‐relief structures are designed to minimize the strain experienced by the electronics, which are fabricated directly on the pillars' surface. In this design configuration, devices based on amorphous indium‐gallium‐zinc‐oxide can withstand different modes of deformation. Bending, stretching, and twisting experiments up to 6 mm radius, 20% uniaxial strain, and 180° global twisting, respectively, are performed to show stable electrical performance of the TFTs. Similarly, a fully integrated digital inverter is tested while stretched up to 20% elongation. As a proof of the versatility of mesa‐shaped geometry, a biocompatible and stretchable sensor for temperature mapping is also realized. Using pectin, which is a temperature‐sensitive material present in plant cells, the response of the sensor shows current modulation from 13 to 28 °C and functionality up to 15% strain. These results demonstrate the performance of highly flexible electronics for a broad variety of applications, including smart skin and health monitoring.     

High-Throughput Screening of Self-Healable Polysulfobetaine Hydrogels and their Applications in Flexible Electronics

Hydrogels are promising materials in the applications of wound adhesives, wearable electronics, tissue engineering, implantable electronics, etc. The properties of a hydrogel rely strongly on its composition. However, the optimization of hydrogel properties has been a big challenge as increasing numbers of components are added to enhance and synergize its mechanical, biomedical, electrical, and self-healable properties. Here in this work, it is shown that high-throughput screening can efficiently and systematically explore the effects of multiple components (at least eight) on the properties of polysulfobetaine hydrogels, as well as provide a useful database for diverse applications. The optimized polysulfobetaine hydrogels that exhibit outstanding self-healing and mechanical properties, have been obtained by high-throughput screening. By compositing with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), intrinsically self-healable and stretchable conductors are achieved. It is further demonstrated that a polysulfobetaine hydrogel-based electronic skin, which exhibits exceptionally fast self-healing capability of the whole device at ambient conditions. This work successfully extends high-throughput synthetic methodology to the field of hydrogel electronics, as well as demonstrates new directions of healable flexible electronic devices in terms of material development and device design.     

Stretchable,Transparent, and Thermally Stable Triboelectric Nanogenerators Based on Solvent‐Free Ion‐Conducting Elastomer Electrodes

The development of stretchable/soft electronics requires power sources that can match their stretchability. In this study, a highly stretchable, transparent, and environmentally stable triboelectric nanogenerator with ionic conductor electrodes (iTENG) is reported. The ion‐conducting elastomer (ICE) electrode, together with a dielectric elastomer electrification layer, allows the ICE‐iTENG to achieve a stretchability of 1036% and transmittance of 91.5%. Most importantly, the ICE is liquid solvent‐free and thermally stable up to 335 °C, avoiding the dehydration‐induced performance degradation of commonly used hydrogels. The ICE‐iTENG shows no decrease in electrical output even after storing at 100 °C for 15 h. Biomechanical motion energies are demonstrated to be harvested by the ICE‐iTENG for powering wearable electronics intermittently without extra power sources. An ICE‐iTENG‐based pressure sensor is also developed with sensitivity up to 2.87 kPa^?1. The stretchable ICE‐iTENG overcomes the strain‐induced performance degradation using percolated electrical conductors and liquid evaporation‐induced degradation using ion‐conducting hydrogels/ionogels, suggesting great promising applications in soft/stretchable electronics under a relatively wider temperature range.     

Highly Stretchable or Transparent Conductor Fabrication by a Hierarchical Multiscale Hybrid Nanocomposite

As is frequently seen in sci‐fi movies, future electronics are expected to ultimately be in the form of wearable electronics. To realize wearable electronics, the electric components should be soft, flexible, and even stretchable to be human‐friendly. An important step is presented toward realization of wearable electronics by developing a hierarchical multiscale hybrid nanocomposite for highly flexible, stretchable, or transparent conductors. The hybrid nanocomposite combines the enhanced mechanical compliance, electrical conductivity, and optical transparency of small CNTs (d ≈ 1.2 nm) and the enhanced electrical conductivity of relatively bigger Ag nanowire (d ≈ 150 nm) backbone to provide efficient multiscale electron transport path with Ag nanowire current backbone collector and local CNT percolation network. The highly elastic hybrid nanocomposite conductors and highly transparent flexible conductors can be mounted on any non‐planar or soft surfaces to realize human‐friendly electronics interface for future wearable electronics.     

Cross‐Stacked Superaligned Carbon Nanotube Films for Transparent and Stretchable Conductors

Transparent and stretchable conductors are essential components in many stretchable electronics. However, it is still a challenge to make this kind of conductor easily and cost‐effectively. Here, a way to utilize cross‐stacked superaligned carbon nanotube films to make transparent and stretchable conductors is reported. The as‐produced cross‐stacked films are isotropic in electrical conductivity, but anisotropic in mechanical properties, because of their microscale cross structures. Along some directions, the films can sustain a high strain, of more than 35%, which is helpful for applications as stretchable conductors. These cross‐stacked films can be further made into composite films with polyvinyl alcohol by a dip‐coating method, and with polydimethylsiloxane by an embedding method. The former composite films have similar isotropic electrical and anisotropic mechanical properties to SACNT films, but much larger capability in terms of tensile load. The latter composite films possess quite highly stretchable and reversible electrical behaviors, which can be used in stretchable touch panels, solar cells, strain sensors, and implanted conductors.     

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.     

Reliable Patterning,Transfer Printing and Post‐Assembly of Multiscale Adhesion‐Free Metallic Structures for Nanogap Device Applications

Patterning of metallic nanogaps with ultrasmall gap size on arbitrary substrates is of great importance for various applications in nanoelectronics, nanoplasmonics, and flexible optoelectronics. Common lithographic approaches suffer from limited resolution in defining ultrasmall nanogaps and restrictive available substrates for flexible and stretchable devices. In this work, a process portfolio to overcome the above limitations is proposed, enabling the fabrication of multiscale metallic nanogaps with reduced gap size on specific substrates for functional devices. The portfolio combines the recently developed sketch and peel lithography strategy, nanotransfer printing, and post‐mechanical assembly. Among the portfolio, the sketch and peel lithography strategy provides the unique capability to rapidly and reliably define multiscale adhesion‐free metallic nanostructures and nanogaps, which significantly facilitates the subsequent transfer printing process. Nanoplasmonic and nanoelectronic devices with ultrasmall nanogaps that are inaccessible with existing patterning approaches are fabricated to demonstrate the applicability of this fabrication strategy. The portfolio could also have potential for a variety of other applications in flexible and stretchable optics, electronics, and optoelectronics.     

Stretchable Active Matrix Inorganic Light‐Emitting Diode Display Enabled by Overlay‐Aligned Roll‐Transfer Printing

An active matrix‐type stretchable display is realized by overlay‐aligned transfer of inorganic light‐emitting diode (LED) and single‐crystal Si thin film transistor (TFT) with roll processes. The roll‐based transfer enables integration of heterogeneous thin film devices on a rubber substrate while preserving excellent electrical and optical properties of these devices, comparable to their bulk properties. The electron mobility of the integrated Si‐TFT is over 700 cm² V^?1 s^?1, and this is attributed to the good interface between the Si channel and the thermally grown SiO₂ insulator. The light emission properties of the LED are of wafer quality. The resulting display stably operates under tensile strains up to 40%, over 200 cycles, demonstrating the potential of stretchable displays based on inorganic materials.     

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.