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.     

Mechano-Optical Sensors Fabricated with Multilayered Liquid Crystal Elastomers Exhibiting Tunable Deformation Recovery

Advances in tuning the mechanoresponsive behavior of liquid crystal elastomers have facilitated the development of next-generation applications such as reconfigurable photonic/electronic materials, energy-harvesting devices, and flexible sensors. However, the molecular-level control of mechanical responses remains difficult, with limited tunability achieved for recovery processes after stimulus removal. Herein, a design concept is proposed for facilely tuning the recovery of both the macroscopic deformation and molecular orientation change of liquid crystal elastomers using layered materials that exhibit the desired mechanoresponsive behavior. Changing the layering materials (a polydimethylsiloxane film with elastic response to a polymethylpenten film with plastic response) alters the relaxation time from <1 s to >6 months. To demonstrate this concept, highly sensitive, stretchable mechano-optical sensors with fast and ultraslow recovery times are developed that enable an applied strain to be quantitatively detected in real time or memorized with high spatial resolution, even with a conventional camera. This material design concept for arbitrarily controlling the recovery response can provide a platform for stimuli-responsive applications.     

Thermally Controlled,Patterned Graphene Transfer Printing for Transparent and Wearable Electronic/Optoelectronic System

Graphene has been highlighted as a platform material in transparent electronics and optoelectronics, including flexible and stretchable ones, due to its unique properties such as optical transparency, mechanical softness, ultrathin thickness, and high carrier mobility. Despite huge research efforts for graphene‐based electronic/optoelectronic devices, there are remaining challenges in terms of their seamless integration, such as the high‐quality contact formation, precise alignment of micrometer‐scale patterns, and control of interfacial‐adhesion/local‐resistance. Here, a thermally controlled transfer printing technique that allows multiple patterned‐graphene transfers at desired locations is presented. Using the thermal‐expansion mismatch between the viscoelastic sacrificial layer and the elastic stamp, a “heating and cooling” process precisely positions patterned graphene layers on various substrates, including graphene prepatterns, hydrophilic surfaces, and superhydrophobic surfaces, with high transfer yields. A detailed theoretical analysis of underlying physics/mechanics of this approach is also described. The proposed transfer printing successfully integrates graphene‐based stretchable sensors, actuators, light‐emitting diodes, and other electronics in one platform, paving the way toward transparent and wearable multifunctional electronic systems.     

Healable Transparent Electronic Devices

With the advent of the digital era, healable electronic devices are being developed to alleviate the propagation of breakdown in electronics due to the mechanical damage caused by bending, accidental cutting or scratching. Meanwhile, flexible transparent electronics, exhibiting high transmittance and robust flexibility, are drawing enormous research efforts due to their potential applications in various integrated wearable electronics. However, the breakdown of flexible transparent electronics seriously limits their reliability and lifetime. Therefore, transparent healable electronics are desired to tackle these problems, yet most of the healable electronics are not transparent nowadays. The combination of high performance, healability, and transparency into electronics is often mutually exclusive. Herein, after a brief introduction of self‐healing materials, healable electronics, and flexible transparent electronics, the recent progress in the healable electronic devices without transparency is reviewed in detail. Then, healable transparent electronic devices with high transparency, robust portability, and reliable flexibility are summarized. They are drawing great attention owing to their potential application in optical devices as well as smart wearable and integrated optoelectronic devices. Following that, the critical challenges and prospects are highlighted for the development of healable transparent electronic devices.     

Highly Stretchable Bilayer Lattice Structures That Elongate via In‐Plane Deformation

Many emerging technologies such as wearable batteries and electronics require stretchable functional structures made from intrinsically less deformable materials. The stretch capability of most demonstrated stretchable structures often relies on either initially out‐of‐plane configurations or the out‐of‐plane deflection of planar patterns. Such nonplanar features may dramatically increase the surface roughness, cause poor adhesion and adverse effects on subsequent multilayer processing, thereby posing a great challenge for flexible devices that require smooth surfaces (e.g., transparent electrodes in which flat‐surface‐enabled high optical transmittance is preferred). Inspired by the lamellar layouts of collagenous tissues, this work demonstrates a planar bilayer lattice structure, which can elongate substantially via only in‐plane motion and thus maintain a smooth surfaces. The constructed bilayer lattice exhibits a large stretchability up to 360%, far beyond the inherent deformability of the brittle constituent material and comparable to that of state‐of‐the‐art stretchable structures for flexible electronics. A stretchable conductor employing the bilayer lattice designs can remain electrically conductive at a strain of 300%, demonstrating the functionality and potential applications of the bilayer lattice structure. This design opens a new avenue for the development of stretchable structures that demand smooth surfaces.     

Liquid Metal based Stretchable Room Temperature Soldering Sticker Patch for Stretchable Electronics Integration (Adv. Funct. Mater. 36/2023)

Researchers are eagerly developing various stretchable conductors to fabricate devices for next-generation electronics. Most of the major problems in stretchable electronics happen at the connection between rigid and soft parts and the development of reliable soldering material is a major hurdle in stretchable electronics. Though there are attempts to devise new soldering processes for integrating chips and stretchable conductors, they still possess limitations such as mechanical stability, mass production, sophisticated processes, and restricted candidates for conductors and substrates. Here, this study presents a room-temperature universal stretchable sticker-like soldering process that can stretchably solder multiple spots at once and directly fabricates a stretchable device in an in situ manner while a target conductor is installed on one's body. The solder developed in this research possesses high conductivity with a unique freestanding feature enabling the process. It can be elongated when directly positioned between a rigid chip and a rigid conductor, demonstrating its extraordinary stretchability. It is expected that this simple but unique stretchable soldering technique utilizing the invented solder will allow the integration of functional stretchable conductors with highly advanced rigid chips for next-generation stretchable electronics.     

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.     

Design Criteria for Horseshoe and Spiral-Based Interconnects for Highly Stretchable Electronic Devices

Stretchable electronics can be used for numerous advanced applications such as soft and wearable actuators, sensors, bio-implantable devices, and surgical tools because of their ability to conform to curvilinear surfaces, including human skin. The efficacy of these devices depends on the development of stretchable geometries such as interconnection-based configurations and the associated mechanics that helps to achieve optimum configurations. This work presents the essential mechanics of silicon (Si) island-interconnection structures, which include horseshoe and spiral interconnections, without reducing the areal efficiency. In particular, this study demonstrates the range of the geometrical parameters where they have a high stretchability and cyclic life. The numerical results predict the areas that are prone to breaking followed by experimental validation. The figure-of-merit for these configurations is achieved by mapping the fracture-free zones for in-plane and out-of-plane stretching with essential implications in stretchable and wearable system design. Furthermore, this work demonstrates the mechanical response for a range of materials (i.e., copper, gold, aluminum, silver, and graphene) that experience the plastic deformations in contrast to conventionally used Si-based devices that represent the extended usage for advanced stretchable electronic devices. The detailed mechanics of these configurations provides comprehensive guidelines to manufacture wearable and stretchable electronic devices.     

Bioinspired Self-healing Soft Electronics

Inspired by nature, various self-healing materials that can recover their physical properties after external damage have been developed. Recently, self-healing materials have been widely used in electronic devices for improving durability and protecting the devices from failure during operation. Moreover, self-healing materials can integrate many other intriguing properties of biological systems, such as stretchability, mechanical toughness, adhesion, and structural coloration, providing additional fascinating experiences. All of these inspirations have attracted extensive research on bioinspired self-healing soft electronics. This review presents a detailed discussion on bioinspired self-healing soft electronics. Firstly, two main healing mechanisms are introduced. Then, four categories of self-healing materials in soft electronics, including insulators, semiconductors, electronic conductors, and ionic conductors, are reviewed, and their functions, working principles, and applications are summarized. Finally, human-inspired self-healing materials and animal-inspired self-healing materials as well as their applications, such as organic field-effect transistors (OFETs), pressure sensors, strain sensors, chemical sensors, triboelectric nanogenerators (TENGs), and soft actuators, are introduced. This cutting-edge and promising field is believed to stimulate more excellent cross-discipline works in material science, flexible electronics, and novel sensors, accelerating the development of applications in human motion monitoring, environmental sensing, information transmission, etc.     

Highly Conductive and Transparent PEDOT:PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes

Highly conductive and transparent poly‐(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) films, incorporating a fluorosurfactant as an additive, have been prepared for stretchable and transparent electrodes. The fluorosurfactant‐treated PEDOT:PSS films show a 35% improvement in sheet resistance (R_s) compared to untreated films. In addition, the fluorosurfactant renders PEDOT:PSS solutions amenable for deposition on hydrophobic surfaces, including pre‐deposited, annealed films of PEDOT:PSS (enabling the deposition of thick, highly conductive, multilayer films) and stretchable poly(dimethylsiloxane) (PDMS) substrates (enabling stretchable electronics). Four‐layer PEDOT:PSS films have an R_s of 46 Ω per square with 82% transmittance (at 550 nm). These films, deposited on a pre‐strained PDMS substrate and buckled, are shown to be reversibly stretchable, with no change to R_s, during the course of over 5000 cycles of 0 to 10% strain. Using the multilayer PEDOT:PSS films as anodes, indium tin oxide (ITO)‐free organic photovoltaics are prepared and shown to have power conversion efficiencies comparable to that of devices with ITO as the anode. These results show that these highly conductive PEDOT:PSS films can not only be used as transparent electrodes in novel devices (where ITO cannot be used), such as stretchable OPVs, but also have the potential to replace ITO in conventional devices.     

Super‐ and Ultrathin Organic Field‐Effect Transistors: from Flexibility to Super‐ and Ultraflexibility

Physically flexible electronics offer a wide range of benefits, including the development of next‐generation consumer electronics and healthcare products. The advancement of physical flexibility, typically achieved by the reduction of the total device thickness, including substrates and encapsulation layers, shows great promise for skin‐laminated electronics. Organic electronics—devices relying on carbon‐based materials—offer many advantages over their inorganic counterparts, including the following: significantly lower fabrication temperatures resulting in alternative fabrication techniques, including inkjet and roll‐to‐roll printing, enabling low‐cost and large‐area fabrication; biocompatibility; and spectacular physical flexibility. This article presents a review, spanning the last two decades, of organic field‐effect transistors with the total thickness of just a few microns as well as devices demonstrated in this decade with a total thickness of few hundred of nanometers. A handful of demonstrations of other organic electronic thin film devices are also presented.     

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.     

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.     

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.     

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.     

A Sensing and Stretchable Polymer-Dispersed Liquid Crystal Device Based on Spiderweb-Inspired Silver Nanowires-Micromesh Transparent Electrode

Polymer-dispersed liquid crystal (PDLC) devices are truly promising optical modulators for information display, smart window as well as intelligent photoelectronic applications due to their fast switching, large optical modulation as well as cost-effectiveness. However, realizing highly soft PDLC devices with sensing function remains a grand challenge because of the intrinsic brittleness of traditional transparent conductive electrodes. Here, inspired by spiderweb configuration, a novel type of silver nanowires (AgNWs) micromesh-based stretchable transparent conductive electrodes (STCEs) is developed to support the realization of soft PDLC device. Benefiting from the embedding design of AgNWs micromesh in polydimethylsiloxane (PDMS), the STCEs can maintain excellent electrical conductivity and transparency even in various extreme conditions such as bending, folding, twisting, stretching as well as multiple chemical corrosion. Further, STCEs with the embedded AgNWs micromesh endow the assembled PDLC device with excellent photoelectrical properties including rapid switching speed (<1 s), large optical modulation (69% at 600 nm), as well as robust mechanical stability (bending over 1000 cycles and stretching to 40%). Moreover, the device displays the pressure sensing function with high sensitivity in response to pressure stimulus. It is conceivable that AgNWs micromesh transparent electrodes will shape the next generation of related soft smart electronics.     

Stretchable ITO-Free Organic Solar Cells with Intrinsic Anti-Reflection Substrate for High-Efficiency Outdoor and Indoor Energy Harvesting

Flexible photovoltaic devices are promising candidates for triggering the Internet of Things (IoT). However, the power conversion efficiencies (PCEs) of flexible organic photovoltaic (OPV) devices with high conductivity poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes on plastic are lagging behind the rigid devices due to the low transmittance of polyethylene terephthalate (PET)/PEDOT:PSS. Moreover, the poor stretchability of the commonly used plastic substrates largely hinders the practical application of wearable devices. Herein, a novel stretchable indium tin oxide (ITO)-free OPV device with a surface-texturing polydimethylsiloxane (PDMS) substrate for outdoor strong- and indoor dim-light energy harvesting is reported. The high diffuse transmittance and haze effect of the substrate enable stretchable ITO-free devices, yielding a high PCE of 15.3% under 1 sun illumination. More excitingly, the stretchable device based on textured PDMS/PEDOT:PSS maintains a comparable PCE of 20.5% (20.8% for the rigid device) under indoor light illumination. Notably, the stretchable device is much more insensitive to the light direction, maintaining 38.5% of the initial PCE at an extremely small incident angle of 10° (16.3% for glass/ITO-based counterpart). The texturing stretchable substrate provides a new direction for achieving high performance and enhanced light utilization for the stretchable light-harvesting device, suitable for indoor and outdoor applications.     

Tacky Elastomers to Enable Tear‐Resistant and Autonomous Self‐Healing Semiconductor Composites

Mechanical failure of π‐conjugated polymer thin films is unavoidable under cyclic loading conditions, due to intrinsic defects and poor resistance to crack propagation. Here, the first tear‐resistant and room‐temperature self‐healable semiconducting composite is presented, consisting of conjugated polymers and butyl rubber elastomers. This new composite displays both a record‐low elastic modulus (<1 MPa) and ultrahigh deformability with fracture strain above 800%. More importantly, failure behavior is not sensitive to precut notches under deformation. Autonomous self‐healing at room temperature, both mechanical and electronic, is demonstrated through the physical contact of two separate films. The composite film also shows device stability in the ambient environment over 5 months due to much‐improved barrier property to both oxygen and water. Butyl rubber is broadly applicable to various p‐type and n‐type semiconducting polymers for fabricating self‐healable electronics to provide new resilient electronics that mimic the tear resistance and healable property of human skin.     

Gallium-Based Liquid–Solid Biphasic Conductors for Soft Electronics

Soft and stretchable electronics have diverse applications in the fields of compliant bioelectronics, textile-integrated wearables, novel forms of mechanical sensors, electronics skins, and soft robotics. In recent years, multiple material architectures have been proposed for highly deformable circuits that can undergo large tensile strains without losing electronic functionality. Among them, gallium-based liquid metals benefit from fluidic deformability, high electrical conductivity, and self-healing property. However, their deposition and patterning is challenging. Biphasic material architectures are recently proposed as a method to address this problem, by combining advantages of solid-phase materials and composites, with liquid deformability and self-healing of liquid phase conductors, thus moving toward scalable fabrication of reliable stretchable circuits. This article reviews recent biphasic conductor architectures that combine gallium-based liquid-phase conductors, with solid-phase particles and polymers, and their application in fabrication of soft electronic systems. In particular, various material combinations for the solid and liquid phases in the biphasic conductor, as well as methods used to print and pattern biphasic conductive compounds, are discussed. Finally, some applications that benefit from biphasic architectures are reviewed.     

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.