Printable Superelastic Conductors with Extreme Stretchability and Robust Cycling Endurance Enabled by Liquid‐Metal Particles

Stretchable conductors are vital and indispensable components in soft electronic systems. The development for stretchable conductors has been highly motivated with different approaches established to address the dilemma in the conductivity and stretchability trade‐offs to some extent. Here, a new strategy to achieve superelastic conductors with high conductivity and stable electrical performance under stretching is reported. It is demonstrated that by electrically anchoring conductive fillers with eutectic gallium indium particles (EGaInPs), significant improvement in stretchability and durability can be achieved in stretchable conductors. Different from the strategy of modulating the chemical interactions between the conductive fillers and host polymers, the EGaInPs provide dynamic and robust electrical anchors between the conductive fillers. A superelastic conductor which can achieve a high stretchability with 1000% strain at initial conductivity of 8331 S cm^?1 and excellent cycling durability with about eight times resistance change (compared to the initial resistance at 0% strain before stretching) after reversibly stretching to 800% strain for 10 000 times is demonstrated. Applications of the superelastic conductor in an interactive soft touch device and a stretchable light‐emitting system are also demonstrated, featuring its promising applications in soft robotics or soft and interactive human–machine interfaces.     

Ultrastretchable Conductor Fabricated on Skin‐Like Hydrogel–Elastomer Hybrid Substrates for Skin Electronics

Printing technology can be used for manufacturing stretchable electrodes, which represent essential parts of wearable devices requiring relatively high degrees of stretchability and conductivity. In this work, a strategy for fabricating printable and highly stretchable conductors are proposed by transferring printed Ag ink onto stretchable substrates comprising Ecoflex elastomer and tough hydrogel layers using a water‐soluble tape. The elastic modulus of the produced hybrid film is close to that of the hydrogel layer, since the thickness of Ecoflex elastomer film coated on hydrogel is very thin (30 µm). Moreover, the fabricated conductor on hybrid film is stretched up to 1780% strain. The described transfer method is simpler than other techniques utilizing elastomer stamps or sacrificial layers and enables application of printable electronics to the substrates with low elastic moduli (such as hydrogels). The integration of printed electronics with skin‐like low‐modulus substrates can be applied to make wearable devices more comfortable for human skin.     

Kirigami‐Inspired Nanoconfined Polymer Conducting Nanosheets with 2000% Stretchability

Stretchable conductors are essential components of wearable electronics. However, such materials typically sacrifice their electronic conductivity to achieve mechanical stretchability and elasticity. Here, the nanoconfinement and air/water interfacial assembly is explored to grow freestanding mechanical endurance conducting polymer nanosheets that can be stretched up to 2000% with simultaneously high electrical conductivity, inspired by kirigami. Such stretchable conductors show remarkable electronic and mechanical reversibility and reproducibility under more than 1000 cycle durability tests with 2000% deformability, which can be accurately predicted using finite element modeling. The conductivity of nanoconfined freestanding conductor nanosheets increases by three orders of magnitude from 2.2 × 10^?3 to 4.002 S cm^?1 is shown, due to the charge‐transfer complex formation between polymer chain and halogen, while the electrical conductance of the stretchable kirigami nanosheets can be maintained over the entire strain regime. The nanoconfined polymer nanosheets can also act as a sensor capable of sensing the pressure with high durability and real‐time monitoring.     

Stretchable Transparent Conductive Films from Long Carbon Nanotube Metals

Flexible transparent conductors are an enabling component for large‐area flexible displays, wearable electronics, and implantable medical sensors that can wrap around and move with the body. However, conventional conductive materials decay quickly under tensile strain, posing a significant hurdle for functional flexible devices. Here, we show that high electrical conductivity, mechanical stretchability, and optical transparency can be simultaneously attained by compositing long metallic double‐walled carbon nanotubes with a polydimethylsiloxane substrate. When stretched to 100% tensile strain, thin films incorporating these long nanotubes (≈3.2 µm on average) achieve a record high conductivity of 3316 S cm^?1 at 100% tensile strain and 85% optical transmittance, which is 194 times higher than that of short nanotube controls (≈0.8 µm on average). Moreover, the high conductivity can withstand more than 1000 repeated stretch‐release cycles (switching between 100% and 0% strain) with a retention approaching 96%, whereas the short nanotube controls exhibit only 10%. Mechanistic studies reveal that long tubes can bridge the microscale gaps generated during stretching, thereby maintaining high electrical conductivity. When mounted on human joints, this elastic transparent conductor can accommodate large motions to provide stable, high current output. These results point to transparent conductors capable of attaining high electrical conductivity and optical transmittance under mechanical strain to allow large shape changes that may take place in the operation and use of flexible electronics.     

Printable elastic silver nanowire-based conductor for washable electronic textiles

Printable elastic conductors promote the wide application of consumable electronic textiles (e-textiles) for pervasive healthcare monitoring and wearable computation. To assure a clean appearance, the e-textiles require a washing process to clean up the dirt after daily use. Thus, it is crucial to develop low-cost printable elastic conductors with strong adhesion to the textiles. Here, we report a composite elastic conductor based on Ag nanowires (NWs) and polyurethane elastomer. The composite could be dispersed into ink and easily printed onto textiles. One-step print could form robust conductive coatings without sealing on the textiles. Interestingly, the regional concentration of Ag NWs within the polyurethane matrix was observed during phase inversion, endowing the elastic conductor with a low percolation threshold of 0.12 vol.% and high conductivity of 3,668 S·cm⁻¹. Thanks to the high adhesion of the elastic conductors, the resulted e-textiles could withstand repeated stretching, folding, and machine washing (20 times) without obvious performance decay, which reveals its potential application in consumable e-textiles.

     

Stretchable,Large‐area Organic Electronics

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

Ag@Ni Core–Shell Nanowire Network for Robust Transparent Electrodes Against Oxidation and Sulfurization

Silver nanowire (Ag NW) based transparent electrodes are inherently unstable to moist and chemically reactive environment. A remarkable stability improvement of the Ag NW network film against oxidizing and sulfurizing environment by local electrodeposition of Ni along Ag NWs is reported. The optical transmittance and electrical resistance of the Ni deposited Ag NW network film can be easily controlled by adjusting the morphology and thickness of the Ni shell layer. The electrical conductivity of the Ag NW network film is increased by the Ni coating via welding between Ag NWs as well as additional conductive area for the electron transport by electrodeposited Ni layer. Moreover, the chemical resistance of Ag NWs against oxidation and sulfurization can be dramatically enhanced by the Ni shell layer electrodeposited along the Ag NWs, which provides the physical barrier against chemical reaction and diffusion as well as the cathodic protection from galvanic corrosion.     

Surface‐Embedded Stretchable Electrodes by Direct Printing and their Uses to Fabricate Ultrathin Vibration Sensors and Circuits for 3D Structures

Printing is one of the easy and quick ways to make a stretchable wearable electronics. Conventional printing methods deposit conductive materials “on” or “inside” a rubber substrate. The conductors made by such printing methods cannot be used as device electrodes because of the large surface topology, poor stretchability, or weak adhesion between the substrate and the conducting material. Here, a method is presented by which conductive materials are printed in the way of being surface‐embedded in the rubber substrate; hence, the conductors can be widely used as device electrodes and circuits. The printing process involves a direct printing of a metal precursor solution in a block‐copolymer rubber substrate and chemical reduction of the precursor into metal nanoparticles. The electrical conductivity and sensitivity to the mechanical deformation can be controlled by adjusting the number of printing operations. The fabrication of highly sensitive vibration sensors is thus presented, which can detect weak pulses and sound waves. In addition, this work takes advantage of the viscoelasticity of the composite conductor to fabricate highly conductive stretchable circuits for complicated 3D structures. The printed electrodes are also used to fabricate a stretchable electrochemiluminescence display.     

Washable,stretchable, and reusable core–shell metal nanowire network-based electronics on a breathable polymer nanomesh substrate

Polymer nanofiber-based porous structures, referred to as “breathable devices,” have been recently developed to minimize user discomfort. Although these devices enable conformal integration to the skin with gas permeability, their performance and durability are significantly lower than those of conventional film-based devices. In this study, an ultradurable embedded Ag–Au core–shell nanowire network (AANN) on a nanomesh substrate is fabricated using the intense pulsed light irradiation and electroplating (IPL-EP) process. The AANN is designed to achieve breathability and durability without sacrificing device performance. It can be used in breathable nanomesh electronics and exhibits a low sheet resistance (1.4 Ω sq⁻¹), cycle stability (above 20,000 cycles), stability in chemicals (water-based solutions and highly corrosive H₂O₂ solution), washability (20 washings), and reusability. Additionally, it is used in reusable conductive electronic textiles, and its applications as a reusable strain sensor for motion detection and wearable heater for thermal therapy are demonstrated. Furthermore, the AANN-based conductive thread exhibits excellent electrical performance (0.3 Ω cm⁻¹) with durability and maintains its electrical characteristics after 50 wash cycles. The proposed process can enable large-scale fabrication of highly durable breathable electronics, electronic textiles, and other biomedical devices.     

Silver Nanoflower Decorated Graphene Oxide Sponges for Highly Sensitive Variable Stiffness Stress Sensors

Soft conductive materials should enable large deformation while keeping high electrical conductivity and elasticity. The graphene oxide (GO)‐based sponge is a potential candidate to endow large deformation. However, it typically exhibits low conductivity and elasticity. Here, the highly conductive and elastic sponge composed of GO, flower‐shaped silver nanoparticles (AgNFs), and polyimide (GO‐AgNF‐PI sponge) are demonstrated. The average pore size and porosity are 114 µm and 94.7%, respectively. Ag NFs have thin petals (8–20 nm) protruding out of the surface of a spherical bud (300–350 nm) significantly enhancing the specific surface area (2.83 m² g^?1). The electrical conductivity (0.306 S m^?1 at 0% strain) of the GO‐AgNF‐PI sponge is increased by more than an order of magnitude with the addition of Ag NFs. A nearly perfect elasticity is obtained over a wide compressive strain range (0–90%). The strain‐dependent, nonlinear variation of Young's modulus of the sponge provides a unique opportunity as a variable stiffness stress sensor that operates over a wide stress range (0–10 kPa) with a high maximum sensitivity (0.572 kPa^?1). It allows grasping of a soft rose and a hard bottle, with the minimal object deformation, when attached on the finger of a robot gripper.     

One step synthesis of silver nanowires used in preparation of conductive silver paste

For mass preparation of conductive silver paste, a convenient approach to synthesize silver nanowires (Ag NWs) is presented. Monodisperse Ag NWs with ca. 40 nm in diameter and over 10 μm in length were successfully obtained in high yield only by heating a mixture of AgNO₃, poly(vinyl pyrrolidone), ethylene glycol and HCl prepared previously. This method not only simplifies the conventional polyol process but also extremely improves its repeatability. More important, the patterns fabricated with the Ag NWs naturally have a high conductivity, up to ca. 13 % of bulk silver, due to the cross-linked network structure formed by accumulation of nanowires. The conductivity even can be improved to 41 % of bulk silver conductivity through a chemical sintering at room temperature, which makes us believe that the Ag NWs could be broadly applied in preparation of conductive paste.     

Fabrication of Highly Stretchable Conductors via Morphological Control of Carbon Nanotube Network

Stretchable conductors, which can keep their excellent electrical conductivity while highly stretched, have been investigated extensively due to their wide range of applications in flexible and stretchable electronics, wearable displays, etc.; however, their preparation is often complicated and expensive. Herein, an efficient method to prepare high performance stretchable conductors through morphological control of conductive networks formed with carbon nanotubes (CNTs) in an elastomer matrix is reported. It is observed that an interface‐mediated method could be used to align randomly oriented filler during stretching and to induce buckling of CNTs during relaxation. Further morphological studies indicate the possible formation of a wavy CNT structure induced by cyclic pre‐straining. Subsequent thermal annealing is observed to collapse the oriented network and improve the local contacts between conductive networks. Through such a simple procedure, a conductivity of nearly 1000 S m^?1 and a stretchability of 200% can be achieved for composites containing 20 wt% CNTs. CNTs are observed to buckle over a large area in polymer bulk, and the combination of pre‐straining and thermal annealing modifies the conductive network in the elastomer matrix. As a general method, this could be used for easy fabrication of high‐performance stretchable conductors for arbitrary‐shaped objects on a large scale.     

Highly Stable and Stretchable Conductive Films through Thermal‐Radiation‐Assisted Metal Encapsulation

Stretchable conductors are the basic units of advanced flexible electronic devices, such as skin‐like sensors, stretchable batteries and soft actuators. Current fabrication strategies are mainly focused on the stretchability of the conductor with less emphasis on the huge mismatch of the conductive material and polymeric substrate, which results in stability issues during long‐term use. Thermal‐radiation‐assisted metal encapsulation is reported to construct an interlocking layer between polydimethylsiloxane (PDMS) and gold by employing a semipolymerized PDMS substrate to encapsulate the gold clusters/atoms during thermal deposition. The stability of the stretchable conductor is significantly enhanced based on the interlocking effect of metal and polymer, with high interfacial adhesion (>2 MPa) and cyclic stability (>10 000 cycles). Also, the conductor exhibits superior properties such as high stretchability (>130%) and large active surface area (>5:1 effective surface area/geometrical area). It is noted that this method can be easily used to fabricate such a stretchable conductor in a wafer‐scale format through a one‐step process. As a proof of concept, both long‐term implantation in an animal model to monitor intramuscular electric signals and on human skin for detection of biosignals are demonstrated. This design approach brings about a new perspective on the exploration of stretchable conductors for biomedical applications.     

Binary Synergistic Sensitivity Strengthening of Bioinspired Hierarchical Architectures based on Fragmentized Reduced Graphene Oxide Sponge and Silver Nanoparticles for Strain Sensors and Beyond

Recently, stretchable electronics have been highly desirable in the Internet of Things and electronic skins. Herein, an innovative and cost‐efficient strategy is demonstrated to fabricate highly sensitive, stretchable, and conductive strain‐sensing platforms inspired by the geometries of a spiders slit organ and a lobsters shell. The electrically conductive composites are fabricated via embedding the 3D percolation networks of fragmentized graphene sponges (FGS) in poly(styrene‐block‐butadiene‐block‐styrene) (SBS) matrix, followed by an iterative process of silver precursor absorption and reduction. The slit‐ and scale‐like structures and hybrid conductive blocks of FGS and Ag nanoparticles (NPs) provide the obtained FGS–Ag‐NP‐embedded composites with superior electrical conductivity of 1521 S cm^?1, high break elongation of 680%, a wide sensing range of up to 120% strain, high sensitivity of ≈10⁷ at a strain of 120%, fast response time of ≈20 ms, as well as excellent reliability and stability of 2000 cycles. This huge stretchability and sensitivity is attributed to the combination of high stretchability of SBS and the binary synergistic effects of designed FGS architectures and Ag NPs. Moreover, the FGS/SBS/Ag composites can be employed as wearable sensors to detect the modes of finger motions successfully, and patterned conductive interconnects for flexible arrays of light‐emitting diodes.     

Highly Conductive,Stretchable, and Cell‐Adhesive Hydrogel by Nanoclay Doping

Electrically conductive materials that mimic physical and biological properties of tissues are urgently required for seamless brain–machine interfaces. Here, a multinetwork hydrogel combining electrical conductivity of 26 S m^?1, stretchability of 800%, and tissue‐like elastic modulus of 15 kPa with mimicry of the extracellular matrix is reported. Engineering this unique set of properties is enabled by a novel in‐scaffold polymerization approach. Colloidal hydrogels of the nanoclay Laponite are employed as supports for the assembly of secondary polymer networks. Laponite dramatically increases the conductivity of in‐scaffold polymerized poly(ethylene‐3,4‐diethoxy thiophene) in the absence of other dopants, while preserving excellent stretchability. The scaffold is coated with a layer containing adhesive peptide and polysaccharide dextran sulfate supporting the attachment, proliferation, and neuronal differentiation of human induced pluripotent stem cells directly on the surface of conductive hydrogels. Due to its compatibility with simple extrusion printing, this material promises to enable tissue‐mimetic neurostimulating electrodes.     

Fully Integrated Design of a Stretchable Solid‐State Lithium‐Ion Full Battery

A solid‐state lithium‐ion battery, in which all components (current collector, anode and cathode, electrolyte, and packaging) are stretchable, is introduced, giving rise to a battery design with mechanical properties that are compliant with flexible electronic devices and elastic wearable systems. By depositing Ag microflakes as a conductive layer on a stretchable carbon–polymer composite, a current collector with a low sheet resistance of ≈2.7 Ω □^?1 at 100% strain is obtained. Stretchable electrodes are fabricated by integrating active materials with the elastic current collector. A polyacrylamide–“water‐in‐salt” electrolyte is developed, offering high ionic conductivity of 10^?3 to 10^?2 S cm^?1 at room temperature and outstanding stretchability up to ≈300% of its original length. Finally, all these components are assembled into a solid‐state lithium‐ion full cell in thin‐film configuration. Thanks to the deformable individual components, the full cell functions when stretched, bent, or even twisted. For example, after stretching the battery to 50%, a reversible capacity of 28 mAh g^?1 and an average energy density of 20 Wh kg^?1 can still be obtained after 50 cycles at 120 mA g^?1, confirming the functionality of the battery under extreme mechanical stress.     

Material Design and Fabrication Strategies for Stretchable Metallic Nanocomposites

Stretchable conductive nanocomposites fabricated by integrating metallic nanomaterials with elastomers have become a vital component of human‐friendly electronics, such as wearable and implantable devices, due to their unconventional electrical and mechanical characteristics. Understanding the detailed material design and fabrication strategies to improve the conductivity and stretchability of the nanocomposites is therefore important. This Review discusses the recent technological advances toward high performance stretchable metallic nanocomposites. First, the effect of the filler material design on the conductivity is briefly discussed, followed by various nanocomposite fabrication techniques to achieve high conductivity. Methods for maintaining the initial conductivity over a long period of time are also summarized. Then, strategies on controlled percolation of nanomaterials are highlighted, followed by a discussion regarding the effects of the morphology of the nanocomposite and postfabricated 3D structures on achieving high stretchability. Finally, representative examples of applications of such nanocomposites in biointegrated electronics are provided. A brief outlook concludes this Review.     

Highly Stretchable,Compliant, Polymeric Microelectrode Arrays for In Vivo Electrophysiological Interfacing

Polymeric microelectrode arrays (MEAs) are emerging as a new generation of biointegrated microelectrodes to transduce original electrochemical signals in living tissues to external electrical circuits, and vice versa. So far, the challenge of stretchable polymeric MEAs lies in the competition between high stretchability and good electrode–substrate adhesion. The larger the stretchability, the easier the delamination of electrodes from the substrate due to the mismatch in their Young's modulus. In this work, polypyrrole (PPy) electrode materials are designed, with PPy nanowires integrated on the high conductive PPy electrode arrays. By utilizing this electrode material, for the first time, stretchable polymeric MEAs are fabricated with both high stretchability (≈100%) and good electrode–substrate adhesion (1.9 MPa). In addition, low Young's modulus (450 kPa), excellent recycling stability (10 000 cycles of stretch), and high conductivity of the MEAs are also achieved. As a proof of concept, the as‐prepared polymeric MEAs are successfully used for conformally recording the electrocorticograph signals from rats in normal and epileptic states, respectively. Further, these polymeric MEAs are also successful in stimulating the ischiadic nerve of the rat. This strategy provides a new perspective to the highly stretchable and mechanically stable polymeric MEAs, which are vital for compliant neural electrodes.     

Bath Electrospinning of Continuous and Scalable Multifunctional MXene‐Infiltrated Nanoyarns

Electroactive yarns that are stretchable are desired for many electronic textile applications, including energy storage, soft robotics, and sensing. However, using current methods to produce these yarns, achieving high loadings of electroactive materials and simultaneously demonstrating stretchability is a critical challenge. Here, a one‐step bath electrospinning technique is developed to effectively capture Ti₃C₂T_x MXene flakes throughout continuous nylon and polyurethane (PU) nanofiber yarns (nanoyarns). With up to ≈90 wt% MXene loading, the resulting MXene/nylon nanoyarns demonstrate high electrical conductivity (up to 1195 S cm^?1). By varying the flake size and MXene concentration, nanoyarns achieve stretchability of up to 43% (MXene/nylon) and 263% (MXene/PU). MXene/nylon nanoyarn electrodes offer high specific capacitance in saturated LiClO₄ electrolyte (440 F cm^?3 at 5 mV s^?1), with a wide voltage window of 1.25 V and high rate capability (72% between 5 and 500 mV s^?1). As strain sensors, MXene/PU yarns demonstrate a wide sensing range (60% under cyclic stretching), high sensitivity (gauge factor of ≈17 in the range of 20–50% strain), and low drift. Utilizing the stretchability of polymer nanofibers and the electrical and electrochemical properties of MXene, MXene‐based nanoyarns demonstrate potential in a wide range of applications, including stretchable electronics and body movement monitoring.     

An Optically Flat Conductive Outcoupler Using Core/Shell Ag/ZnO Nanochurros

Transparent conductive electrodes (TCEs) featuring a smooth surface are indispensable for preserving pristine electrical characteristics in optoelectronic and transparent electronic devices. For high‐efficiency organic light emitting diodes (OLEDs), a high outcoupling efficiency, which is crucial, is only achieved by incorporating a wavelength‐scale undulating surface into a TCE layer, but this inevitably degrades device performance. Here, an optically flat, high‐conductivity TCE composed of core/shell Ag/ZnO nanochurros (NCs) is reported embedded within a resin film on a polyethylene terephthalate substrate, simultaneously serving as an efficient outcoupler and a flexible substrate. The ZnO NCs are epitaxially grown on the {100} planes of a pentagonal Ag core and the length of ZnO shells is precisely controlled by the exposure time of Xe lamp. Unlike Ag nanowires films, the Ag/ZnO NCs films markedly boost the optical tunneling of light. Green‐emitting OLEDs (2.78 × 3.5 mm²) fabricated with the Ag/ZnO TCE exhibit an 86% higher power efficiency at 1000 cd m^?2 than ones with an Sn‐doped indium oxide TCE. A full‐vectorial electromagnetic simulation suggests the suppression of plasmonic absorption losses within their Ag cores. These results provide a feasibility of multifunctional TCEs with synthetically controlled core/shell nanomaterials toward the development of high‐efficiency LED and solar cell devices.