Tuning the Rigidity of Silk Fibroin for the Transfer of Highly Stretchable Electronics

The transfer of stretchable electrodes or devices from one substrate to another thin elastomer is challenging as the elastic stamp often yields a huge strain beyond the stretchability limit of the electrodes at the debonded interface. This will not happen if the stamp is rigid. However, a rigid material cannot be used as the substrate for stretchable electrodes. Herein, silk fibroin with tunable rigidity (Young's modulus can be changed from 134 kPa to 1.84 GPa by controlling the relative humidity) is used to transfer highly stretchable metal networks as highly conformable epidermal electrodes. The silk fibroin stamp is tuned to be rigid during peeling, and then be soft and highly stretchable as a substrate when laminated on moisturized human skin. In addition, the epidermal electrodes exhibit no skin irritation or inflammation after attaching for over 10 d. The high compliance results in a lower interface impedance and lower noises of the electrode in measuring electromyographic signals, compared with commercial Ag‐AgCl gel electrodes. The strategy of tuning the rigidity at different stages of transfer is a general method that can be extended to the transfer of other stretchable electrodes and devices for epidermal electronics, human machine interfaces, and soft robotics.     

Stretchable Energy‐Harvesting Tactile Interactive Interface with Liquid‐Metal‐Nanoparticle‐Based Electrodes

Energy‐harvesting electronic skin (E‐skin) is highly promising for sustainable and self‐powered interactive systems, wearable human health monitors, and intelligent robotics. Flexible/stretchable electrodes and robust energy‐harvesting components are critical in constructing soft, wearable, and energy‐autonomous E‐skin systems. A stretchable energy‐harvesting tactile interactive interface is demonstrated using liquid metal nanoparticles (LM‐NPs)‐based electrodes. This stretchable energy‐harvesting tactile interface relies on triboelectric nanogenerator composed of a galinstan LM‐NP‐based stretchable electrode and patterned elastic polymer friction and encapsulation layer. It provides stable and high open‐circuit voltage (268 V), short‐circuit current (12.06 µA), and transferred charges (103.59 nC), which are sufficient to drive commercial portable electronics. As a self‐powered tactile sensor, it presents satisfactory and repeatable sensitivity of 2.52 V·kPa^?1 and is capable of working as a touch interactive keyboard. The demonstrated stretchable and robust energy‐harvesting E‐skin using LM‐NP‐based electrodes is of great significance in sustainable human–machine interactive system, intelligent robotic skin, security tactile switches, etc.     

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 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.     

Block Copolymer-Based Supramolecular Ionogels for Accurate On-Skin Motion Monitoring

Interest in wearable and stretchable on-skin motion sensors has grown rapidly in recent years. To expand their applicability, the sensing element must accurately detect external stimuli; however, weak adhesiveness of the sensor to a target object has been a major challenge in developing such practical and versatile devices. In this study, freestanding, stretchable, and self-adhesive ionogel conductors are demonstrated which are composed of an associating polymer network and ionic liquid that enable conformal contact between the sensor and skin even during dynamic movement. The network of ionogel is formed by noncovalent association of two diblock copolymers, where phase-separated micellar clusters are interconnected via hydrogen bonds between corona blocks. The resulting ionogels exhibit superior adhesive characteristics, including a very high lift-off force of 93.3 N m⁻¹, as well as excellent elasticity (strain at break ≈ 720%), toughness ( ≈ 2479 kJ m⁻³), thermal stability ( ≈ 150  ° C), and high ionic conductivity ( ≈ 17.8 mS cm⁻¹ at 150  ° C). These adhesive ionogels are successfully applied to stretchable on-skin strain sensors as sensing elements. The resulting devices accurately monitor the movement of body parts such as the wrist, finger, ankle, and neck while maintaining intimate contact with the skin, which was not previously possible with conventional non-adhesive ionogels.     

Optically Transparent and Mechanically Robust Ionic Hydrogel Electrodes for Bright Electroluminescent Devices Achieving High Stretchability Over 1400%

To realize wearable displays and interactive soft robots, significant research efforts are focused on developing highly deformable alternating-current electroluminescent (ACEL) devices. Although soft emission layers are well developed, designing stretchable, conductive, and transparent soft electrodes remains challenging. In this study, ionic hydrogels are prepared comprising a double network (DN) of poly(N-hydroxyethylacrylamide-co-acrylamide)/crosslinked chitosan swollen in aqueous lithium bis(trifluoromethanesulfonyl) imide. Owing to the finely tuned DN structure of the polymeric crosslinker and transparent electrolyte, the developed ionic hydrogels exhibit remarkable stretchability (1400%), excellent optical transmittance (>99%), and high conductivity (1.95 × 10⁻² Sm⁻¹). Based on the high performance of the ionic hydrogels, ACEL devices are fabricated with an emission layer containing phosphor microparticles and demonstrate stable, high luminance under extreme deformation, and ultra-high elongation. The excellent transparency of the ionic hydrogel further enables the fabrication of novel soft ACEL devices with tandem structures by stacking several emission and electrode layers, in which each emission layer is independently controlled with a switch circuit.     

A Highly-Adhesive and Self-Healing Elastomer for Bio-Interfacial Electrode

Stretchable electrodes are playing important roles in the measurement of bio-electrical signals especially in wearable electronic devices. These electrodes usually adopt commercial elastomers such as polydimethylsiloxane or polystyrene-ethylene-butylene-styrene as substrates, which result in poor stability and reliability due to weak interfacial adhesion between electrodes and human skin. Here, dopamine is introduced into the hydrogen bonding based elastomer as pendent groups. The elastomer shows both mechanical strength and adhesion strength at the same time. It exhibits high stress at break (1.9 MPa) and high fracture strain (5100%). Significantly, it exhibits a high adhesive strength (≈62 kPa) and underwater adhesive strength (≈16 kPa) with epithelial tissue. Thus, a stretchable bio-interfacial electrode is fabricated by spray-coating silver nanowires on the elastic substrate, which is stretchable, self-healable, and highly adhesive and suitable for electromyogram measurement.     

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.     

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.     

Ultralow Voltage High-Performance Bioartificial Muscles Based on Ionically Crosslinked Polypyrrole-Coated Functional Carboxylated Bacterial Cellulose for Soft Robots

The development of ultralow voltage high-performance bioartificial muscles with large bending strain, fast response time, and excellent actuation durability is highly desirable for promising applications such as soft robotics, active biomedical devices, flexible haptic displays, and wearable electronics. Herein, a novel high-performance low-priced bioartificial muscle based on functional carboxylated bacterial cellulose (FCBC) and polypyrrole (PPy) nanoparticles is reported, exhibiting a large bending strain of 0.93%, long actuated bending durability (96% retention for 5 h) under an ultralow harmonic input of 0.5 V, broad frequency bandwidth up to 10 Hz, fast response time (≈4 s) in DC responses, high energy density (6.81 KJ m⁻³), and high power density (5.11 KW m⁻³), all of which mainly stem from its high surface area and porosity, large specific capacitance, tuned mechanical properties, and strong ionic interactions of cations and anions in ionic liquid with FCBC and PPy nanoparticles. More importantly, bioinspired applications such as the grapple robot, bionic medical stent, bionic flower, and wings-vibrating have been realized. These successful demonstrations offer a viable means for developing high-performance bioartificial muscles for next-generation soft bioelectronics including bioinspired robotics, biomedical microdevices, and wearable electronics.     

Self-Healing,Robust, and Stretchable Electrode by Direct Printing on Dynamic Polyurea Surface at Slightly Elevated Temperature

Printed electronics on elastomer substrates have found wide applications in wearable devices and soft robotics. For everyday usage, additional requirements exist for the robustness of the printed flexible electrodes, such as the ability to resist scratching and damage. Therefore, highly robust electrodes with self-healing, and good mechanical strength and stretchability are highly required and challenging. In this paper, a cross-linking polyurea using polydimethylsiloxane as the soft segment and dynamic urea bonds is prepared and serves as a self-healing elastomer substrate for coating and printing of silver nanowires (AgNWs). Due to the dynamic exchangeable urea bond at 60 °C, the elastomer exhibits dynamic exchange of the cross-linking network while retaining the macroscopic shape. As a result, the AgNWs are partially embedded in the surface of the elastomer substrate when coated or printed at 60 °C, forming strong interfacial adhesion. As a result, the obtained stretchable electrode exhibits high mechanical strength and stretchability, the ability to resist scratching and sonication, and self-healing. This strategy can be applied to a variety of different conducting electrode materials including AgNWs, silver particles, and liquid metal, which provides a new way to prepare robust and self-healing printed electronics.     

A Fully Self-Healing and Highly Stretchable Liquid-Free Ionic Conductive Elastomer for Soft Ionotronics

Soft ionic conductors hold great potential for soft ionotronics, such as ionic skin, human–machine interface and soft luminescent device. However, most hydrogel and ionogel-based soft ionic conductors suffer from freezing, evaporation and liquid leakage problems, which limit their use in complex environments. Herein, a class of liquid-free ionic conductive elastomers (ICEs) is reported as an alternative soft ionic conductor in soft ionotronics. These liquid-free ICEs offer a combination of desirable properties, including extraordinary stretchability (up to 1913%), toughness (up to 1.08 MJ cm⁻³), Young's modulus (up to 0.67 MPa), rapid fully self-healing capability at room temperature, and good conductivity (up to 1.01 × 10⁻⁵ S cm⁻¹). The application of these ICEs is demonstrated by creating a wearable sensor that can detect and discriminate minimal deformations and human body movements, such as finger or elbow joint flexion, walking, running, etc. In addition, self-healing soft ionotronic devices are demonstrated to confront mechanical breakdown, such as an ionic skin and an alternating-current electroluminescent device that can reuse from damage. It is believed that these liquid-free ICEs hold great promises for applications in wearable devices and soft ionotronics.     

Supercooled Liquid Ga Stretchable Electronics

By controlling the properties of its medium, supercooled liquid Ga (SLGa) based stretchable remains stretchable at −22 °C, i.e., 52 °C below its thermodynamic melting point of Ga. Thus far, our oldest deposited SLGa circuit and film have remained liquids for 2 years at room temperature. The study investigates the crystallization of SLGa triggered by the surface energy of nucleation agents, temperature, circuit cross-section, and mechanical impact. Based on these parameters, a method is presented to integrate electronic components with SLGa circuits without compromising its supercooling effect. Further, the large stiffness variation induced by phase transition is demonstrated in different applications. For the desired stiffness variation, the crystallization rate can be controlled by varying the temperature and cross-section area. Finally, spray-printing an ink of microscale SLGa microscale particles can conformally pattern Ga on a rough surface, e.g., to fabricate a stretchable array of SLGa microelectrodes. A smart patch with stretchable SLGa electrode arrays records human electrocardiogram signals in cold water and does not stain the skin after use. Its low and stable impedance in water will enable novel applications in wearable electronics.     

Evaporation-Induced Closely-Packing of Core–Shell PDMS@Ag Microspheres Enabled Stretchable Conductor with Ultra-High Conductance

Stretchable conductors are indispensable building blocks for stretchable electronic devices that are used in next-generation wearable electronics, on-skin electronics, and soft robotics. Whereas, the ability to realize synergy high conductance and sufficient conductivity under high strain remains challenging. Herein, a stretchable conductor made from tightly assembled core–shell polydimethylsiloxane@silver microspheres (PDMS@Ag MPs) is elaborated. By judiciously using evaporation-induced capillary effect, 3D interconnected conductive paths consisting of closely packed conductive PDMS@Ag MPs are constructed inside the elastic matrix. The spatially selective distributed Ag-shell enables conductor metallic conductivity (67185 S cm⁻¹) at ultralow Ag fraction (19.5 wt.%), and well-maintained conductance over wide strain (820 S cm⁻¹ at 400%). Due to the suppressed Ag content, both the rapture strain and Young's modulus (613%, 0.79 MPa for CPSC4) of the conductor are largely retained. Besides, the synergy hierarchical surface topology and low surface energy endow conductors with high water-repellent properties. The fabricated conductors with remarkably high conductivity, well-retained conductance under large strain, and robust hydrophobicity are of great significance for advanced stretchable electronics.     

Ferromagnetic,Folded Electrode Composite as a Soft Interface to the Skin for Long‐Term Electrophysiological Recording

A class of ferromagnetic, folded, soft composite material for skin‐interfaced electrodes with releasable interfaces to stretchable, wireless electronic measurement systems is introduced. These electrodes establish intimate, adhesive contacts to the skin, in dimensionally stable formats compatible with multiple days of continuous operation, with several key advantages over conventional hydrogel‐based alternatives. The reported studies focus on aspects ranging from ferromagnetic and mechanical behavior of the materials systems, to electrical properties associated with their skin interface, to system‐level integration for advanced electrophysiological monitoring applications. The work combines experimental measurement and theoretical modeling to establish the key design considerations. These concepts have potential uses across a diverse set of skin‐integrated electronic technologies.     

Fully 3D-Printed,Stretchable, and Conformable Haptic Interfaces

Integrating rich cutaneous haptic feedback enhances realism and user immersion in virtual and augmented reality settings. One major challenge is providing accurately localized cutaneous stimuli on fingertips without interfering with the user's dexterity. This sub 200 µm thick, fully printed, stretchable Hydraulically Amplified Taxels (HAXELs) enable both static indentation and vibrating haptic stimuli, localized to a 2.5 mm diameter region. The HAXELs are directly bonded to the user's skin, are soft enough to conform to any body part, and can be fabricated in dense arrays with no crosstalk. All functional materials (elastomers, stretchable conductors, and sacrificial layers) are deposited by inkjet printing, which allows rapid prototyping of multi-material, polymer-based structures. The actuators consist of oil-filled stretchable pouches, whose shape is controlled by electrostatic zipping. The 5 mm wide actuators weigh <250 mg and generate cutaneous stimuli well above reported perception thresholds, from DC to 1 kHz. They operate well even when stretched to over 50%, allowing great freedom in placement. The 2 × 2 arrays are tested on the fingers of human volunteers: the actuated quadrant is correctly identified 86% of the time. Printing soft actuators allows tailoring dense and effective cutaneous haptics to the unique shape of each user.     

3D Printing of Interdigitated Dielectric Elastomer Actuators

Dielectric elastomer actuators (DEAs) are soft electromechanical devices that exhibit large energy densities and fast actuation rates. They are typically produced by planar methods and, thus, expand in‐plane when actuated. Here, reported is a method for fabricating 3D interdigitated DEAs that exhibit in‐plane contractile actuation modes. First, a conductive elastomer ink is created with the desired rheology needed for printing high‐fidelity, interdigitated electrodes. Upon curing, the electrodes are then encapsulated in a self‐healing dielectric matrix composed of a plasticized, chemically crosslinked polyurethane acrylate. 3D DEA devices are fabricated with tunable mechanical properties that exhibit breakdown fields of 25 V µm^?1 and actuation strains of up to 9%. As exemplars, printed are prestrain‐free rotational actuators and multi‐voxel DEAs with orthogonal actuation directions in large‐area, out‐of‐plane motifs.     

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.     

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.     

Strain-Tolerant,High-Detectivity,and Intrinsically Stretchable All-Polymer Photodiodes

A skin-like photodiode (PD) that is stretchable and skin-conformable is crucial to opening the next-generation wearable electronics for optical biometric monitoring, biomedical imaging, and others. To achieve reliable PD characteristics under large deformation, stretchable PDs with high detectivity and high mechanical stretchability must be developed. Herein, intrinsically stretchable polymer-based PDs (is-PPDs) comprising all-polymeric constituent layers are demonstrated. In particular, elastomeric photoactive layers consisting of an elastomer with p-/n-type semiconducting polymers and conducting polymer-based stretchable transparent electrodes with modulated work functions improve both the mechanical stability and the detectivity (D*) of is-PPDs. Accordingly, is-PPDs show excellent D* over 10¹³ Jones with a suppressed dark current density of 0.1 nA cm⁻² before and after 100% stretching. The proposed is-PPDs record high-quality and stable photoplethysmography signals at the wrist with outward extension.