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.     

Flexible and Stretchable Smart Display: Materials,Fabrication, Device Design,and System Integration

Recent technological advances in nanomaterials have driven the development of high‐performance light‐emitting devices with flexible and stretchable form factors. Deformability in such devices is mainly achieved by replacing the rigid materials in the device components with flexible nanomaterials and their assemblies (e.g., carbon nanotubes, silver nanowires, graphene, and quantum dots) or with intrinsically soft materials and their composites (e.g., polymers and elastomers). Downscaling the dimensions of the functional materials to the nanometer range dramatically decreases their flexural rigidity, and production of polymer/elastomer composites with functional nanomaterials provides light‐emitting devices with flexibility and stretchability. Furthermore, monolithic integration of these light‐emitting devices with deformable sensors furnishes the resulting display with various smart functions such as force/capacitive touch‐based data input, personalized health monitoring, and interactive human–machine interfacing. These ultrathin, lightweight, and deformable smart optoelectronic devices have attracted widespread interest from materials scientists and device engineers. Here, a comprehensive review of recent progress concerning these flexible and stretchable smart displays is presented with a focus on materials development, fabrication techniques, and device designs. Brief overviews of an integrated system of advanced smart displays and cutting‐edge wearable sensors are also presented, and, to conclude, a discussion of the future research outlook is given.     

Emerging Design Strategies Toward Developing Next-Generation Implantable Batteries and Supercapacitors

In recent years, the development of implantable bioelectronics has garnered significant attention. With the continuous advancement of IoT and information technology, implantable bioelectronics can be utilized more effectively for health monitoring to enhance treatment outcomes, reduce healthcare costs, and improve quality of life. Implantable energy storage devices have been widely studied as critical components for energy supply. Conventional power sources are bulky, inflexible, and potentially contain materials that are dangerous to the body. Meanwhile, human tissues are soft, flexible, dynamic, and closed, which puts new requirements on energy storage devices to improve the safety, stability, and matching of implantable batteries or supercapacitors. Herein, recent advances in state-of-the-art nonconventional power options for implantable electronics, specifically biocompatible, miniaturized, stretchable/deformable, biodegradable/bioresorbable, edible, and injectable energy storage devices, are reviewed in this paper. The material strategy and architectural design of the next-generation implantable energy storage device are discussed, including the selection principle of electrolytes, the all-in-one structure design strategy, and the way to realize self-charging. Finally, the challenges and prospects of emerging design strategies toward developing next-generation implantable batteries and supercapacitors for the future are put forward.     

Emerging Soft Conductors for Bioelectronic Interfaces

Bidirectional interfacing between electrodes and biological systems has enabled diagnostics and therapeutics in modern medicine; however, the inherent dissimilarity between the soft, ion‐rich, dynamic biological tissues and the rigid, dry, static electronic systems hinders the establishment of effective and reliable bioelectronic interfaces. In the past decade, the scope of flexible/stretchable electronics has been broadened into bioelectronics owing to the need of implementation of various biocompatible soft conductors. Herein, the basic requirements for the construction of both epidermal and implantable bioelectronic interfaces utilizing soft materials are discussed, the most recent progress in the development of soft conductors, which are customized to interface with skin and other tissues, are summarized. An outlook into the remaining obstacles is provided and possible strategies to facilitate technological advances in bioelectronics are also outlined.     

Stretchable,Skin‐Mountable,and Wearable Strain Sensors and Their Potential Applications: A Review

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.     

Stretchable and Transparent Biointerface Using Cell‐Sheet–Graphene Hybrid for Electrophysiology and Therapy of Skeletal Muscle

Implantable electronic devices for recording electrophysiological signals and for stimulating muscles and nerves have been widely used throughout clinical medicine. Mechanical mismatch between conventional rigid biomedical devices and soft curvilinear tissues, however, has frequently resulted in a low signal to noise ratio and/or mechanical fatigue and scarring. Multifunctionality ranging from various sensing modalities to therapeutic functions is another important goal for implantable biomedical devices. Here, a stretchable and transparent medical device using a cell‐sheet–graphene hybrid is reported, which can be implanted to form a high quality biotic/abiotic interface. The hybrid is composed of a sheet of C2C12 myoblasts on buckled, mesh‐patterned graphene electrodes. The graphene electrodes monitor and actuate the C2C12 myoblasts in vitro, serving as a smart cell culture substrate that controls their aligned proliferation and differentiation. This stretchable and transparent cell‐sheet–graphene hybrid can be transplanted onto the target muscle tissue, to record electromyographical signals, and stimulate implanted sites electrically and/or optically in vivo. Additional cellular therapeutic effect of the cell‐sheet–graphene hybrid is obtained by integrated myobalst cell sheets. Any immune responses within implanted muscle tissues are not observed. This multifunctional device provides many new opportunities in the emerging field of soft bioelectronics.     

Recent Progress in Solid Electrolytes for Energy Storage Devices

With the rapid advances in safe, flexible, and even stretchable electronic products, it is important to develop matching energy storage devices to more effectively power them. However, the use of conventional liquid electrolytes produces volatilization and leakage that are dangerous and requires strict packaging layers that are typically rigid. To this end, solid electrolytes that can overcome these problems have attracted increasing attention in recent decades. In this review article, three main types of solid electrolytes (i.e., inorganic, polymer, and composite electrolytes) are first described and compared in terms of their structures and properties. The advantages of solid electrolytes to make safe, flexible, stretchable, wearable, and self‐healing energy storage devices, including supercapacitors and batteries, are then discussed. The remaining challenges and possible directions are finally summarized to highlight future development in this field.     

Wearable and Implantable Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) are a promising technology to convert mechanical energy to electrical energy based on coupled triboelectrification and electrostatic induction. With the rapid development of functional materials and manufacturing techniques, wearable and implantable TENGs have evolved into playing important roles in clinic and daily life from in vitro to in vivo. These flexible and light membrane‐like devices have the potential to be a new power supply or sensor element, to meet the special requirements for portable electronics, promoting innovation in electronic devices. In this review, the recent advances in wearable and implantable TENGs as sustainable power sources or self‐powered sensors are reviewed. In addition, the remaining challenges and future possible improvements of wearable and implantable TENG‐based self‐powered systems are discussed.     

Bio‐Inspired Stretchable,Adhesive, and Conductive Structural Color Film for Visually Flexible Electronics

The rapid progress in flexible electronic devices has attracted immense interest in many applications, such as health monitoring devices, sensory skins, and implantable apparatus. Here, inspired by the adhesion features of mussels and the color shift mechanism of chameleons, a novel stretchable, adhesive, and conductive structural color film is presented for visually flexible electronics. The film is generated by adding a conductive carbon nanotubes polydopamine (PDA) filler into an elastic polyurethane (PU) inverse opal scaffold. Owing to the brilliant flexibility and inverse opal structure of the PU layer, the film shows stable stretchability and brilliant structural color. Besides, the catechol groups on PDA impart the film with high tissue adhesiveness and self‐healing capability. Notably, because of its responsiveness, the resultant film is endowed with color‐changing ability that responds to motions, which can function as dual‐signal soft human‐motion sensors for real‐time color‐sensing and electrical signal monitoring. These features make the bio‐inspired hydrogel‐based electronics highly potential in the flexible electronics field.     

The Era of Digital Health: A Review of Portable and Wearable Affinity Biosensors

Digital health facilitated by wearable/portable electronics and big data analytics holds great potential in empowering patients with real‐time diagnostics tools and information. The detection of a majority of biomarkers at trace levels in body fluids using mobile health (mHealth) devices requires bioaffinity sensors that rely on “bioreceptors” for specific recognition. Portable point‐of‐care testing (POCT) bioaffinity sensors have demonstrated their broad utility for diverse applications ranging from health monitoring to disease diagnosis and management. In addition, flexible and stretchable electronics‐enabled wearable platforms have emerged in the past decade as an interesting approach in the ambulatory collection of real‐time data. Herein, the technological advancements of mHealth bioaffinity sensors evolved from laboratory assays to portable POCT devices, and to wearable electronics, are synthesized. The involved recognition events in the mHealth affinity biosensors enabled by bioreceptors (e.g., antibodies, DNAs, aptamers, and molecularly imprinted polymers) are discussed along with their transduction mechanisms (e.g., electrochemical and optical) and system‐level integration technologies. Finally, an outlook of the field is provided and key technological bottlenecks to overcome identified, in order to achieve a new sensing paradigm in wearable bioaffinity platforms.     

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.     

Rewritable,Moldable, and Flexible Sticker‐Type Organic Memory on Arbitrary Substrates

Fabrication of functional devices on arbitrary non‐conventional substrates has significant advantages for broadening devices applications and the development of soft electronic systems such as flexible, stretchable, wearable, and epidermal electronic modules. Information storage device is one of crucial electronic elements in modern digital circuitries. Herein, a re‐writable, transferable, and flexible sticker‐type organic memory on universal substrates is demonstrated through a facile and cost‐effective one‐step strategy. The organic memory sticker based on the graphene electrode grown by chemical vapor deposition consists of a blending composite of polymer (poly (methyl methacrylate) (PMMA):poly (3‐hexylthiophene) (P3HT) in chlorobenzene (CB) fabricated by mature solution processes and facilities. By combining with the mechanical elastic of organic material and graphene electrode, the sticker‐type organic memory can be easily tagged on non‐planar or flexible substrates after etching away the supporting metal. Particularly, the new attachable sticker‐type memory processes a unique feature of re‐programmable capability. It is believed that the universal substrate selectivity of the sticker‐type organic memory with re‐writable characteristic revealed here may greatly enlarge information storage devices in immense areas and advance the future functional soft circuitries.     

Recent Progress in Flexible and Stretchable Organic Solar Cells

Flexible and stretchable organic solar cells (OSCs) have attracted enormous attention due to their potential applications in wearable and portable devices. To achieve flexibility and stretchability, many efforts have been made with regard to mechanically robust electrodes, interface layers, and photoactive semiconductors. This has greatly improved the performance of the devices. State‐of‐the‐art flexible and stretchable OSCs have achieved a power conversion efficiency of 15.21% (16.55% for tandem flexible devices) and 13%, respectively. Here, the recent progress of flexible and stretchable OSCs in terms of their components and processing methods are summarized and discussed. The future challenges and perspectives for flexible and stretchable OSCs are also presented.     

Skin‐Inspired Antibacterial Conductive Hydrogels for Epidermal Sensors and Diabetic Foot Wound Dressings

Recently, artificial intelligence research has driven the development of stretchable and flexible electronic systems. Conductive hydrogels are a class of soft electronic materials that have emerging applications in wearable and implantable biomedical devices. However, current conductive hydrogels possess fundamental limitations in terms of their antibacterial performance and a mechanical mismatch with human tissues, which severely limits their applications in biological interfaces. Here, inspired by animal skin, a conductive hydrogel is fabricated from a supramolecular assembly of polydopamine decorated silver nanoparticles (PDA@Ag NPs), polyaniline, and polyvinyl alcohol, namely PDA@Ag NPs/CPHs. The resultant hydrogel has many desirable features, such as tunable mechanical and electrochemical properties, eye‐catching processability, good self‐healing ability as well as repeatable adhesiveness. Remarkably, PDA@Ag NPs/CPHs exhibit broad antibacterial activity against Gram‐negative and Gram‐positive bacteria. The potential application of this versatile hydrogel is demonstrated by monitoring large‐scale movements of the human body in real time. In addition, PDA@Ag NPs/CPHs have a significant therapeutic effect on diabetic foot wounds by promoting angiogenesis, accelerating collagen deposition, inhibiting bacterial growth, and controlling wound infection. To the best of the authors' knowledge, this is the first time that conductive hydrogels with antibacterial ability are developed for use as epidermal sensors and diabetic foot wound dressing.     

Novel Electronics for Flexible and Neuromorphic Computing

Emerging classes of flexible electronic systems that can be attached to a wide range of surfaces from wearable clothes to internal organs have driven significant advances in communication protocols (e.g., Internet of Things, augmented reality) and clinical research, shifting today's personal computing paradigm. The field of “system on plastic” is on the verge of an innovative breakthrough toward a hypercognitive society by being fused with current neuromorphic applications in the spotlight, which can offer intelligent services such as personalized feedback therapy and autonomous driving. The novel concept of electronics for flexible and neuromorphic computing requires an important research leap in micro‐/nanoelectronics on plastics, system‐level integration techniques (interconnection and packaging), and synaptic devices. Here, representative advances and developments in the area of flexible and neuromorphic technologies are reviewed with regard to device configurations, materials, fabrication processes, and their potential research fields.     

Advances on Emerging Materials for Flexible Supercapacitors: Current Trends and Beyond

The progressive size reduction of electronic components is experiencing bottlenecks in shrinking charge storage devices like batteries and supercapacitors, limiting their development into wearable and flexible zero‐pollution technologies. The inherent long cycle life, rapid charge–discharge patterns, and power density of supercapacitors rank them superior over other energy storage devices. In the modern market of zero‐pollution energy devices, currently the lightweight formula and shape adaptability are trending to meet the current requirement of wearables. Carbon nanomaterials have the potential to meet this demand, as they are the core of active electrode materials for supercapacitors and texturally tailored to demonstrate flexible and stretchable properties. With this perspective, the latest progress in novel materials from conventional carbons to recently developed and emerging nanomaterials toward lightweight stretchable active compounds for flexi‐wearable supercapacitors is presented. In addition, the limitations and challenges in realizing wearable energy storage systems and integrating the future of nanomaterials for efficient wearable technology are provided. Moreover, future perspectives on economically viable materials for wearables are also discussed, which could motivate researchers to pursue fabrication of cheap and efficient flexible nanomaterials for energy storage and pave the way for enabling a wide‐range of material‐based applications.     

Flexible Breathable Nanomesh Electronic Devices for On‐Demand Therapy

Flexible electronics are drawing tremendous interest for various applications in wearable healthcare biomonitoring, on‐demand therapy, and human–machine interactions. However, conventional plastic substrates with uncomfortableness, mechanical mismatches, and impermeability have limited the application of flexible on‐skin electronic devices for healthcare biomonitoring and on‐demand therapy. Herein, flexible breathable electronic devices with the capabilities of real‐time temperature sensing and timely on‐demand anti‐infection therapy at wound sites are presented. These devices are assembled from a crosslinked electrospun moxifloxacin hydrochloride (MOX)‐loaded thermoresponsive polymer nanomesh film with a conductive pattern. The conductive polymer nanomesh film demonstrates excellent flexibility, reliable breathability, and robust environmental stability. Furthermore, the assembled temperature sensor displays a linear relationship between the electrical resistance and temperature, potentially enabling real‐time biomonitoring of tissue temperature at the wound site. Smart artificial electronic skins (E‐skins) are assembled from the thermoresponsive polymer nanomesh film for spatial touching sensing mapping of temperature changes. Furthermore, the flexible temperature sensor is coupled with a wireless transmitter for real‐time wireless temperature monitoring. Notably, the thermoresponsive polymer nanomesh film can also be assembled as a highly efficient flexible heater to trigger the on‐demand release of antibiotics loaded in the fibers to eliminate bacterial colonization in the wound site once infection has occurred.     

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.     

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.     

On‐Body Bioelectronics: Wearable Biofuel Cells for Bioenergy Harvesting and Self‐Powered Biosensing

The growing power demands of wearable electronic devices have stimulated the development of on‐body energy‐harvesting strategies. This article reviews the recent progress on rapidly emerging wearable biofuel cells (BFCs), along with related challenges and prospects. Advanced on‐body BFCs in various wearable platforms, e.g., textiles, patches, temporary tattoo, or contact lenses, enable attractive advantages for bioenergy harnessing and self‐powered biosensing. These noninvasive BFCs open up unique opportunities for utilizing bioenergy or monitoring biomarkers present in biofluids, e.g., sweat, saliva, interstitial fluid, and tears, toward new biomedical, fitness, or defense applications. However, the realization of effective wearable BFC requires high‐quality enzyme‐electronic interface with efficient enzymatic and electrochemical processes and mechanical flexibility. Understanding the kinetics and mechanisms involved in the electron transfer process, as well as enzyme immobilization techniques, is essential for efficient and stable bioenergy harvesting under diverse mechanical strains and changing operational conditions expected in different biofluids and in a variety of outdoor activities. These key challenges of wearable BFCs are discussed along with potential solutions and future prospects. Understanding these obstacles and opportunities is crucial for transforming traditional bench‐top BFCs to effective and successful wearable BFCs.