Heterogeneous integration of rigid, soft, and liquid materials for self-healable, recyclable, and reconfigurable wearable electronics

Diels-Alder Network Blends as Self-Healing Encapsulants for Liquid Metal-Based Stretchable Electronics

Two dynamic covalent networks based on the Diels-Alder reaction were blended to exploit the properties of the dissimilar polymer backbones. Furan-functionalized polyether amines based on poly(propylene oxide) (PPO) FD4000 and polydimethylsiloxane (PDMS) FS5000 were mixed in a common solvent and reversibly cross-linked with the same bismaleimide DPBM. The morphology of the phase-separated blends is primarily controlled by the concentration of backbones. Increasing the PDMS content of the blends results in a dilute droplet morphology at 25 wt %, with a growing size and concentration of droplets and the formation of two separate PPO- and PDMS-rich layers at 50 wt %. Further increasing the PDMS content to 75 wt % leads to larger droplets and a thicker layer of the secondary phase. The hydrophobic PDMS phase creates a barrier against water, while the more hydrophilic PPO phase enhances the resistance against oxygen diffusion. Lowering the maleimide-to-furan stoichiometric ratio resulted in a decrease in cross-link density and thus more flexible and stretchable encapsulants. Changes in the stoichiometric ratio also affected the phase morphology due to resulting changes in phase separation and network formation kinetics. Lowering the stoichiometric ratio also resulted in enhanced self-healing properties of 96% at room temperature as a consequence of the increased chain mobility in the blended networks. The self-healing blends were used to encapsulate liquid metal circuits to create stretchable strain sensors with a linear electro-mechanical response without much drift or hysteresis, which could be efficiently recovered by 90% after the damage-healing cycles.

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

Digital Light 3D Printing of Double Thermoplastics with Customizable Mechanical Properties and Versatile Reprocessability

Digital light processing (DLP) is a 3D printing technology offering high resolution and speed. Printable materials are commonly based on multifunctional monomers, resulting in the formation of thermosets that usually cannot be reprocessed or recycled. Some efforts are made in DLP 3D printing of thermoplastic materials. However, these materials exhibit limited and poor mechanical properties. Here, a new strategy is presented for DLP 3D printing of thermoplastics based on a sequential construction of two linear polymers with contrasting (stiff and flexible) mechanical properties. The inks consist of two vinyl monomers, which lead to the stiff linear polymer, and α-lipoic acid, which forms the flexible linear polymer via thermal ring-opening polymerization in a second step. By varying the ratio of stiff and flexible linear polymers, the mechanical properties can be tuned with Young's modulus ranging from 1.1 GPa to 0.7 MPa, while the strain at break increased from 4% to 574%. Furthermore, these printed thermoplastics allow for a variety of reprocessability pathways including self-healing, solvent casting, reprinting, and closed-loop recycling of the flexible polymer, contributing to the development of a sustainable materials economy. Last, the potential of the new material in applications ranging from soft robotics to electronics is demonstrated.

Microconfined Assembly of High-Resolution and Mechanically Robust EGaIn Liquid Metal Stretchable Electrodes for Wearable Electronic Systems

Stretchable electrodes based on liquid metals (LM) are widely used in human-machine interfacing, wearable bioelectronics, and other emerging technologies. However, realizing the high-precision patterning and mechanical stability remains challenging due to the poor wettability of LM. Herein, a method is reported to fabricate LM-based multilayer solid-liquid electrodes (m-SLE) utilizing electrohydrodynamic (EHD) printed confinement template. In these electrodes, LM self-assembled onto these high-resolution templates, assisted by selective wetting on the electrodeposited Cu layer. This study shows that a m-SLE composed of PDMS/Ag/Cu/EGaIn exhibits line width of ≈20 µm, stretchability of ≈100%, mechanical stability ≈10 000 times (stretch/relaxation cycles), and recyclability. The multi-layer structure of m-SLE enables the adjustability of strain sensing, in which the strain-sensitive Ag part can be used for non-distributed detection in human health monitoring and the strain-insensitive EGaIn part can be used as interconnects. In addition, this study demonstrates that near field communication (NFC) devices and multilayer displays integrated by m-SLEs exhibit stable wireless signal transmission capability and stretchability, suggesting its applicability in creating highly-integrated, large-scale commercial, and recyclable wearable electronics.

Surfactant Self-Assembly Enhances Tribopositivity of Stretchable Ionic Conductors for Wearable Energy Harvesting and Motion Sensing

Boosting stretchability and electric output is critical for high-performance wearable triboelectric nanogenerators (TENG). Herein, for the first time, a new approach for tuning the composition of surface functional groups through surfactant self-assembly to improve the tribopositivity, where the assembly increases the transferred charge density and the relative permittivity of water polyurethane (WPU). Incorporating bis(trifluoromethanesulfonyl)imide (TFSI-) and alkali metal ions into a mixture of WPU and the surfactant forms a stretchable film that simultaneously functions as positive tribolayer and electrode, preventing the conventional detachment of tribolayer and electrode in long term usage. Further, the conductivity of the crosslinked film reaches 3.3 × 10-3 mS cm-1 while the elongation at break reaches 362%. Moreover, the surfactant self-assembly impedes the adverse impact of the fluorine-containing groups on tribopositivity. Consequently, the charge density reaches 155 µC m-2, being the highest recorded for WPU and stretchable ionic conductor based TENG. This work introduces a novel approach for boosting the output charge density while avoiding the adverse effect of ionic salts in solid conductors through a universal surfactant self-assembly strategy, which can be extended to other materials. Further, the device is used to monitor and harvest the kinetic energy of human body motion.

Biomimetic Materials for Skin Tissue Regeneration and Electronic Skin

Biomimetic materials have become a promising alternative in the field of tissue engineering and regenerative medicine to address critical challenges in wound healing and skin regeneration. Skin-mimetic materials have enormous potential to improve wound healing outcomes and enable innovative diagnostic and sensor applications. Human skin, with its complex structure and diverse functions, serves as an excellent model for designing biomaterials. Creating effective wound coverings requires mimicking the unique extracellular matrix composition, mechanical properties, and biochemical cues. Additionally, integrating electronic functionality into these materials presents exciting possibilities for real-time monitoring, diagnostics, and personalized healthcare. This review examines biomimetic skin materials and their role in regenerative wound healing, as well as their integration with electronic skin technologies. It discusses recent advances, challenges, and future directions in this rapidly evolving field.

Hard Magnetic Graphene Nanocomposite for Multimodal, Reconfigurable Soft Electronics

Soft electronics provide effective means for continuous monitoring of a diverse set of biophysical and biochemical signals from the human body. However, the sensitivities, functions, spatial distributions, and many other features of such sensors remain fixed after deployment and cannot be adjusted on demand. Here, laser-induced porous graphene is exploited as the sensing material, and dope it with permanent magnetic particles to create hard magnetic graphene nanocomposite (HMGN) that can self-assemble onto a flexible carrying substrate through magnetic force, in a reversible and reconfigurable manner. A set of soft electronics in HMGN exhibits enhanced performances in the measurements of electrophysiological signals, temperature, and concentrations of metabolites. All these flexible HMGN sensors can adhere to a carrying substrate at any position and in any spatial arrangement, to allow for wearable sensing with customizable sensitivity, modality, and spatial coverage. The HMGN represents a promising material for constructing soft electronics that can be reconfigured for various applications.

Ionic Polyimine-Based Composite Membrane with Inductive and Complexation Synergistic Effects for Sensitive and On-Site Fluorescent Detection of Volatile Iodine

Along with the development of nuclear power, concerns about radioactive emissions and the potential for nuclear leakage have been widely raised, particularly of harmful iodine isotopes. However, as a significant component of nuclear air waste, the enrichment and detection of air-dispersed gaseous iodine remain a challenge. In this work, it is focused on developing an attraction-immobilization-detection strategy-based fluorescence method for the on-site detection of volatile iodine, by employing a photoluminescent ionic polyimine network-polyvinylpyrrolidone (IPIN-PVP) composite membrane. This strategy synergizes ion-induced dipole interactions from IPIN and complexation effects from PVP, allowing effective iodine enrichment and immobilization. As a result, the optimized IPIN-PVP membrane exhibits rapid response times of 5 s and a low detection limit of 4.087 × 10-8 m for gaseous iodine. It also introduces a portable handheld detection device that utilizes the composite membrane, offering a practical solution for real-time on-site detection of volatile iodine. This innovation enhances nuclear safety measures and disaster management by providing rapid and reliable iodine detection capabilities.

Lightweight and Strong Cellulosic Triboelectric Materials Enabled by Cell Wall Nanoengineering

As intelligent technology surges forward, wearable electronics have emerged as versatile tools for monitoring health and sensing our surroundings. Among these advancements, porous triboelectric materials have garnered significant attention for their lightness. However, these materials face the challenge of improving structural stability to further enhance the sensing accuracy of triboelectric sensors. In this study, a lightweight and strong porous cellulosic triboelectric material is designed by cell wall nanoengineering. By tailoring of the cell wall structure, the material shows a high mechanical strength of 51.8 MPa. The self-powered sensor constructed by this material has a high sensitivity of 33.61 kPa-1, a fast response time of 36 ms, and excellent pressure detection durability. Notably, the sensor still enables a high sensing performance after the porous cellulosic triboelectric material exposure to 200 °C and achieves real-time feedback of human motion, thereby demonstrating great potential in the field of wearable electronic devices.

Self-healing polymers through hydrogen-bond cross-linking: synthesis and electronic applications

Recently, polymers capable of repeatedly self-healing physical damage and restoring mechanical properties have attracted extensive attention. Among the various supramolecular chemistry, hydrogen-bonding (H-bonding) featuring reversibility, directionality and high per-volume concentration has become one of the most attractive directions for the development of self-healing polymers (SHPs). Herein, we review the recent advances in the design of high-performance SHPs based on different H-bonding types, for example, H-bonding motifs and excessive H-bonding. In particular, the effects of the structural design of SHPs on their mechanical performance and healing efficiency are discussed in detail. Moreover, we also summarize how to employ H-bonding-based SHPs for the preparation of self-healable electronic devices, focusing on promising topics, including energy harvesting devices, energy storage devices, and flexible sensing devices. Finally, the current challenges and possible strategies for the development of H-bonding-based SHPs and their smart electronic applications are highlighted.

A biomimetic closed-loop recyclable, long-term durable, extreme-condition resistant, flame-retardant nanocoating synthesized by reversible flocculation assembly

Flame-retardant coatings have attracted increasing attention in mitigating the fire threat of flammable polymer materials. Their durable application inevitably provides high resistance to various complex environments, however, discarded stable materials will turn into another man-made waste disaster. The paradigm shift toward a sustainable future is to combine durability and recyclability of coatings. Herein, we demonstrate a biomimetic coating that reversibly captures active flame-retardant nanomaterials by flocculation assembly using anionic polyacrylamide covering the polyurethane foam surface. Strong hydrogen bonding and microstructural interlocking provide the coating with high durability under complex harsh conditions (underwater, chemical exposure, hydrothermal aging, long-term external extrusion, etc.). Meanwhile, the disassembly/reorganization of the coating can be easily repeated in response to pH stimulation with a recycling rate of 97%. The experiments and theoretical calculations reveal the mechanism of the reversible flocculation assembly. This biomimetic strategy of responsive flocculation assembly opens the way for functional coatings with integrated durability and recyclability.

Facile Synthesis of Highly Stretchable, Tough, and Photodegradable Hydrogels

Recently, highly stretchable and tough hydrogels that are photodegradable on-demand have been reported. Unfortunately, the preparation procedure is complex due to the hydrophobic nature of the photocrosslinkers. Herein, a simple method is reported to prepare photodegradable double-network (DN) hydrogels that exhibit high stretchability, toughness, and biocompatibility. Hydrophilic ortho-nitrobenzyl (ONB) crosslinkers incorporating different poly(ethylene glycol) (PEG) backbones (600, 1000, and 2000 g mol-1 ) are synthesized. These photodegradable DN hydrogels are prepared by the irreversible crosslinking of chains by using such ONB crosslinkers, and the reversible ionic crosslinking between sodium alginate and divalent cations (Ca2+ ). Remarkable mechanical properties are obtained by combining ionic and covalent crosslinking and their synergistic effect, and by reducing the length of the PEG backbone. The rapid on-demand degradation of these hydrogels is also demonstrated by using cytocompatible light wavelength (λ = 365 nm) that degrades the photosensitive ONB units. The authors have successfully used these hydrogels as skin-worn sensors for monitoring human respiration and physical activities. A combination of excellent mechanical properties, facile fabrication, and on-demand degradation holds promise for their application as the next generation of substrates or active sensors eco-friendly for bioelectronics, biosensors, wearable computing, and stretchable electronics.

Biomimetic Flexible Sensors and Their Applications in Human Health Detection

Bionic flexible sensors are a new type of biosensor with high sensitivity, selectivity, stability, and reliability to achieve detection in complex natural and physiological environments. They provide efficient, energy-saving and convenient applications in medical monitoring and diagnosis, environmental monitoring, and detection and identification. Combining sensor devices with flexible substrates to imitate flexible structures in living organisms, thus enabling the detection of various physiological signals, has become a hot topic of interest. In the field of human health detection, the application of bionic flexible sensors is flourishing and will evolve into patient-centric diagnosis and treatment in the future of healthcare. In this review, we provide an up-to-date overview of bionic flexible devices for human health detection applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we evaluate the working mechanisms of different classes of bionic flexible sensors, describing the selection and fabrication of bionic flexible materials and their excellent electrochemical properties; then, we introduce some interesting applications for monitoring physical, electrophysiological, chemical, and biological signals according to more segmented health fields (e.g., medical diagnosis, rehabilitation assistance, and sports monitoring). We conclude with a summary of the advantages of current results and the challenges and possible future developments.

Blood-Glucose-Powered Metabolic Fuel Cell for Self-Sufficient Bioelectronics

Currently available bioelectronic devices consume too much power to be continuously operated on rechargeable batteries, and are often powered wirelessly, with attendant issues regarding reliability, convenience, and mobility. Thus, the availability of a robust, self-sufficient, implantable electrical power generator that works under physiological conditions would be transformative for many applications, from driving bioelectronic implants and prostheses to programing cellular behavior and patients' metabolism. Here, capitalizing on a new copper-containing, conductively tuned 3D carbon nanotube composite, an implantable blood-glucose-powered metabolic fuel cell is designed that continuously monitors blood-glucose levels, converts excess glucose into electrical power during hyperglycemia, and produces sufficient energy (0.7 mW cm^-2 , 0.9 V, 50 mm glucose) to drive opto- and electro-genetic regulation of vesicular insulin release from engineered beta cells. It is shown that this integration of blood-glucose monitoring with elimination of excessive blood glucose by combined electro-metabolic conversion and insulin-release-mediated cellular consumption enables the metabolic fuel cell to restore blood-glucose homeostasis in an automatic, self-sufficient, and closed-loop manner in an experimental model of type-1 diabetes.

Endowing Polythioester Vitrimer with Intrinsic Crystallinity and Chemical Recyclability

Technologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C.

Self-Healing, Reconfigurable, Thermal-Switching, Transformative Electronics for Health Monitoring

Soft, deformable electronic devices provide the means to monitor physiological information and health conditions for disease diagnostics. However, their practical utility is limited due to the lack of intrinsical thermal switching for mechanically transformative adaptability and self-healing capability against mechanical damages. Here, the design concepts, materials and physics, manufacturing approaches, and application opportunities of self-healing, reconfigurable, thermal-switching device platforms based on hyperbranched polymers and biphasic liquid metal are reported. The former provides excellent self-healing performance and unique tunable stiffness and adhesion regulated by temperature for the on-skin switch, whereas the latter results in liquid metal circuits with extreme stretchability (>900%) and high conductivity (3.40 × 104  S cm-1 ), as well as simple recycling capability. Triggered by the increased temperature from the skin surface, a multifunctional device platform can conveniently conform and strongly adhere to the hierarchically textured skin surface for non-invasive, continuous, comfortable health monitoring. Additionally, the self-healing and adhesive characteristics allow multiple multifunctional circuit components to assemble and completely wrap on 3D curvilinear surfaces. Together, the design, manufacturing, and proof-of-concept demonstration of the self-healing, transformative, and self-assembled electronics open up new opportunities for robust soft deformable devices, smart robotics, prosthetics, and Internet-of-Things, and human-machine interfaces on irregular surfaces.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Graded Mxene-Doped Liquid Metal as Adhesion Interface Aiming for Conductivity Enhancement of Hybrid Rigid-Soft Interconnection

Hybrid rigid-soft electronic system combines the biocompatibility of stretchable electronics and the computing capacity of silicon-based chips, which has a chance to realize a comprehensive stretchable electronic system with perception, control, and algorithm in near future. However, a reliable rigid-soft interconnection interface is urgently required to ensure both the conductivity and stretchability under a large strain. To settle this demand, this paper proposes a graded Mxene-doped liquid metal (LM) method to achieve a stable solid-liquid composite interconnect (SLCI) between the rigid chip and stretchable interconnect lines. To overcome the surface tension of LM, a high-conductive Mxene is doped for the balance between adhesion and liquidity of LM. And the high-concentration doping could prevent the contact failure with chip pins, while the low-concentration doping tends to maintain the stretchability. Based on this dosage-graded interface structure, the solid light-emitting diode (LED) and other devices integrated into the stretchable hybrid electronic system could achieve an excellent conductivity insensitive to the exerted tensile strain. In addition, the hybrid electronic system is demonstrated for skin-mounted and tire-mounted temperature-test applications under the tensile strain up to 100%. This Mxene-doped LM method aims to obtain a robust interface between rigid components and flexible interconnects by attenuating the inherent Young's modulus mismatch between rigid and flexible systems and makes it a promising candidate for effective interconnection between solid electronics and soft electronics.

In-situ forming dynamic covalently crosslinked nanofibers with one-pot closed-loop recyclability

Polymeric nanofibers are attractive nanomaterials owing to their high surface-area-to-volume ratio and superior flexibility. However, a difficult choice between durability and recyclability continues to hamper efforts to design new polymeric nanofibers. Herein, we integrate the concept of covalent adaptable networks (CANs) to produce a class of nanofibers ⎯ referred to dynamic covalently crosslinked nanofibers (DCCNFs) via electrospinning systems with viscosity modulation and in-situ crosslinking. The developed DCCNFs possess homogeneous morphology, flexibility, mechanical robustness, and creep resistance, as well as good thermal and solvent stability. Moreover, to solve the inevitable issues of performance degradation and crack of nanofibrous membranes, DCCNF membranes can be one-pot closed-loop recycled or welded through thermal-reversible Diels-Alder reaction. This study may unlock strategies to fabricate the next generation nanofibers with recyclable features and consistently high performance via dynamic covalent chemistry for intelligent and sustainable applications.

Highly Damping and Self-Healable Ionic Elastomer from Dynamic Phase Separation of Sticky Fluorinated Polymers

Shock-induced low-frequency vibration damage is extremely harmful to bionic soft robots and machines that may incur the malfunction of fragile electronic elements. However, current skin-like self-healable ionic elastomers as the artificial sensing and protecting layer still lack the ability to dampen vibrations, due to their almost opposite design for molecular frictions to material's elasticity. Inspired by the two-phase structure of adipose tissue (the natural damping skin layer), here, a highly damping ionic elastomer with energy-dissipating nanophases embedded in an elastic matrix is introduced, which is formed by polymerization-induced dynamic phase separation of sticky fluorinated copolymers in the presence of lithium salts. Such a supramolecular design decouples the elastic and damping functions into two distinct phases, and thus reconciles a few intriguing properties including ionic conductivity, high stretchability, softness, strain-stiffening, elastic recovery, room-temperature self-healability, recyclability, and most importantly, record-high damping capacity at the human motion frequency range (loss factor tan δ > 1 at 0.1-50 Hz). This study opens the door for the artificial syntheses of high-performance damping ionic skins with robust sensing and protective applications in soft electronics and robotics.

Strong and Tough Supramolecular Covalent Adaptable Networks with Room-Temperature Closed-Loop Recyclability

Development of closed-loop chemically recyclable plastics (CCRPs) that can be widely used in daily life can be a fundamental solution to the global plastic waste crisis. Hence, it is of great significance to develop easy-to-recycle CCRPs that possess superior or comparable material properties to the commodity plastics. Here, a novel dual crosslinked CCRP, namely, supramolecular covalent adaptable networks (supra-CANs), is reported, which not only displays mechanical properties higher than the strong and tough commodity polycarbonate, but also exhibits excellent solvent resistance as thermosets. The supra-CANs are constructed by introducing reversible noncovalent crosslinks into the dynamic covalent polymer networks, resulting in highly stiff and strong thermosets that also exhibit thermoplastic-like ductile and tough behaviors as well as reprocessability and rehealability. In great contrast, the analogs that do not have noncovalent crosslinks (CANs) show elastomeric properties with significantly decreased mechanical strength. Importantly, the developed supra-CANs and CANs can be converted back into the initial monomers in high yields and purity at room temperature, even with additives, which enables the sustainable polymer-monomer-polymer circulation. This work provides new design principles for high-performance chemically recyclable polymers as sustainable substitutes for the conventional plastics.

Reconfigurable Touch Panel Based on a Conductive Thixotropic Supramolecular Hydrogel

Touch panels based on ionic conductive hydrogels perform excellent flexibility and biocompatibility, becoming promising candidates for the next-generation human-machine interface. However, these ionic hydrogels are usually composed of cross-linked polymeric networks that are difficult to be recycled or reconfigured, resulting in environmental issues. Herein, we designed a lithium ion-triggered gelation strategy to provide a conductive molecular hydrogel with thixotropy, which can be mechanically recycled or reconfigured at room temperature. In this hydrogel, lithium ions function as ionic bridges to construct supramolecular nanoassemblies and charge carriers to impart ionic conductivity. With polymer additives, the mechanical accommodability of the hydrogel was improved to meet the requirements of the daily use of touch panels. When this molecular hydrogel was fabricated into a surface capacitive touch panel, real-time sensing and reliable touch locating abilities were achieved. Remarkably, this touch panel can be reconfigured into 1D, 2D, and 3D device structures by a simple stirring-remolding method under ambient conditions. This work brings new insight into enriching the functionalities of hydrogel-based ionotronics with a supramolecular approach.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Dynamic Sustainable Polyimide Film Combining Hardness with Softness via a "Mimosa-Like" Bionic Strategy

Dielectric polyimides (PIs) are ubiquitous as insulation in electrical power systems and electronic devices. Generally, dynamic polyimide is required to solve irreversible failure processes of electrical or mechanical damage, for example, under high temperature, pressure, and field strength. The challenge lies in the design of the molecular structure of rigid polyimide to achieve dynamic reversibility. Herein, a low-molecular-weight polyimide gene unit is designed to crosslink with polyimide ligase to prepare the smart film. Interestingly, due to the variability of gene unit and ligase combinations, the polyimide films combining hardness with softness are designed into three forms via a "Mimosa-like" bionic strategy to adapt to different application scenarios. Meanwhile, the films have good degradation efficiency, excellent recyclability, and can be self-healable, which makes them reuse. Clearly, the films can be used in the preparation of ultrafast sensors with a response time ≈0.15 s and the application of corona-resistant films with 100% recovery. Furthermore, the construction of polyimide and carbon-fiber-reinforced composites (CFRCs) has been verified to apply to the worse environment. Nicely, the composites have the property of multiple cycles and the non-destructive recycle rate of carbon fiber (CF) is as high as 100%. The design idea of preparing high-strength dynamic polyimide by crosslinking simple polyimide gene unit with ligase could provide a good foundation and a clear case for the sustainable development of electrical and electronic polyimides, from the perspective of Mimosa bionics.

Closed-loop chemical recycling of cross-linked polymeric materials based on reversible amidation chemistry

Closed-loop chemical recycling provides a solution to the end-of-use problem of synthetic polymers. However, it remains a major challenge to design dynamic bonds, capable of effective bonding and reversible cleaving, for preparing chemically recyclable cross-linked polymers. Herein, we report a dynamic maleic acid tertiary amide bond based upon reversible amidation reaction between maleic anhydrides and secondary amines. This dynamic bond allows for the construction of polymer networks with tailorable and robust mechanical properties, covering strong elastomers with a tensile strength of 22.3 MPa and rigid plastics with a yield strength of 38.3 MPa. Impressively, these robust polymeric materials can be completely depolymerized in an acidic aqueous solution at ambient temperature, leading to efficient monomer recovery with >94% separation yields. Meanwhile, the recovered monomers can be used to remanufacture cross-linked polymeric materials without losing their original mechanical performance. This work unveils a general approach to design polymer networks with tunable mechanical performance and closed-loop recyclability, which will open a new avenue for sustainable polymeric materials.

Fabrication and Functionality Integration Technologies for Small-Scale Soft Robots

Small-scale soft robots are attracting increasing interest for visible and potential applications owing to their safety and tolerance resulting from their intrinsic soft bodies or compliant structures. However, it is not sufficient that the soft bodies merely provide support or system protection. More importantly, to meet the increasing demands of controllable operation and real-time feedback in unstructured/complicated scenarios, these robots are required to perform simplex and multimodal functionalities for sensing, communicating, and interacting with external environments during large or dynamic deformation with the risk of mismatch or delamination. Challenges are encountered during fabrication and integration, including the selection and fabrication of composite/materials and structures, integration of active/passive functional modules with robust interfaces, particularly with highly deformable soft/stretchable bodies. Here, methods and strategies of fabricating structural soft bodies and integrating them with functional modules for developing small-scale soft robots are investigated. Utilizing templating, 3D printing, transfer printing, and swelling, small-scale soft robots can be endowed with several perceptual capabilities corresponding to diverse stimulus, such as light, heat, magnetism, and force. The integration of sensing and functionalities effectively enhances the agility, adaptability, and universality of soft robots when applied in various fields, including smart manufacturing, medical surgery, biomimetics, and other interdisciplinary sciences.

Liquid metal droplet motion transferred from an alkaline solution by a robot arm

The excellent motion performance of gallium-based liquid metals (LMs) upon the application of a modest electric field has provided a new opportunity for the development of autonomous soft robots. However, the locomotion of LMs often appears in an alkaline solution, which hampers the application under other different conditions. In this work, a novel robot arm is designed to transfer the motion of the LM from an alkaline solution in a synchronous drive mode. The liquid metal droplet (LMD) at the bottom of the robot arm is actuated using a DC voltage to provide the driving force for the system. By introducing an end effector at the center of the robot arm, the synchronous motion of the system is replicated and can be applied to different situations. The theoretical understanding of continuous electrowetting (CEW) at the LM interface is explained, and then the motion performance of the robot arm against the function of the applied voltage and driving direction is investigated. Moreover, several applications using this robot arm, such as pattern drawing, cargo transportation, and drug concentration detection, are demonstrated. The presented robot arm has the potential to observably expand the application fields of the LM.

Conformally Adhesive, Large-Area, Solidlike, yet Transient Liquid Metal Thin Films and Patterns via Gelatin-Regulated Droplet Deposition and Sintering

Adhesion and spreading of liquid metals (LMs) on substrates are essential steps for the generation of flexible electronics and thermal management devices. However, the controlled deposition is limited by the high surface tension and peculiar wetting and adhesion behavior of LMs. Herein, we introduce gelatin-regulated LM droplet deposition and sintering (GLMDDS), for the upscalable production of conformally adhesive, solidlike, yet transient LM thin films and patterns on diverse substrates. This method involves four steps: homogeneous deposition of LM microdroplets, gelation of the LM-gelatin solution, toughening of the gelatin hydrogel by solvent displacement, and peeling-induced sintering of LM microdroplets. The LM thin film exhibits a three-layer structure, comprising an LM microdroplet-embedded tough organohydrogel adhesion layer, a continuous LM layer, and an oxide skin. The composite exhibits high stretchability and mechanical robustness, conformal adhesion to various substrates, high conductivity (4.35 × 10⁵ S·m^-1), and transience (86% LM recycled). Large-scale deposition (i.e., 5.6 dm²) and the potential for patterns on diverse substrates demonstrate its upscalability and broad suitability. Finally, the LM thin films and patterns are applied for flexible and wearable devices, i.e., pressure sensors, heaters, human motion tracking devices, and thermal management devices, illustrating the broad applicability of this strategy.

Research Progresses in Microstructure Designs of Flexible Pressure Sensors

Flexible electronic technology is one of the research hotspots, and numerous wearable devices have been widely used in our daily life. As an important part of wearable devices, flexible sensors can effectively detect various stimuli related to specific environments or biological species, having a very bright development prospect. Therefore, there has been lots of studies devoted to developing high-performance flexible pressure sensors. In addition to developing a variety of materials with excellent performances, the microstructure designs of materials can also effectively improve the performances of sensors, which has brought new ideas to scientists and attracted their attention increasingly. This paper will summarize the flexible pressure sensors based on material microstructure designs in recent years. The paper will mainly discuss the processing methods and characteristics of various sensors with different microstructures, and compare the advantages, disadvantages, and application scenarios of them. At the same time, the main application fields of flexible pressure sensors based on microstructure designs will be listed, and their future development and challenges will be discussed.

Emerging Liquid Metal Biomaterials: From Design to Application

Liquid metals (LMs) as emerging biomaterials possess unique advantages including their favorable biosafety, high fluidity, and excellent electrical and thermal conductivities, thus providing a unique platform for a wide range of biomedical applications ranging from drug delivery, tumor therapy, and bioimaging to biosensors. The structural design and functionalization of LMs endow them with enhanced functions such as enhanced targeting ability and stimuli responsiveness, enabling them to achieve better and even multifunctional synergistic therapeutic effects. Herein, the advantages of LMs in biomedicine are presented. The design of LM-based biomaterials with different scales ranging from micro-/nanoscale to macroscale and various components is explored in-depth to promote the understanding of structure-property relationships, guiding their performance optimization and applications. Furthermore, the related advanced progress in the development of LM-based biomaterials in biomedicine is summarized. Current challenges and prospects of LMs in the biomedical field are also discussed.

In situ multimodal transparent electrophysiological hydrogel for in vivo miniature two-photon neuroimaging and electrocorticogram analysis

Hydrogels are widely used in nerve tissue repair and show good histocompatibility. There remain, however, challenges with hydrogels for applications related to neural signal recording, which requires a tissue-like biomechanical property, high optical transmission, and low impedance. Here, we describe a transparent hydrogel that is highly biocompatible and has a low Young's modulus (0.15 MPa). Additionally, it functions well as an implantable electrode, as it conformably adheres to brain tissue, results in minimal inflammation and has a low impedance of 150 Ω at 1 kHz. Its high transmittance, corresponding to 93.35% at a wavelength of 300 nm to 1100 nm, supports its application in two-photon imaging. Consistent with these properties, this flexible multimodal transparent electrophysiological hydrogel (MTEHy) electrode was able to record neuronal Ca²⁺ activity using miniature two-photon microscopy. It also used to monitor electrocorticogram (ECoG) activity in real time in freely moving mice. Moreover, its compatibility with magnetic resonance imaging (MRI), indicates that MTEHy is a new tool for studying activity in the cerebral cortex. STATEMENT OF SIGNIFICANCE: : Future brain science research requires better-performing implantable electrodes to detect neuronal signaling in the brain. In this study, we developed a new hydrogel material, MTEHy-3, that shows high biocompatibility, high optical transmittance (93.35%) and a low Young's modulus (0.15 MPa). Using as high-biocompatible metal-free hydrogel electrode, MTEHy-3 can be implanted for a long time to study the cerebral cortex, and synchronously record the Ca²⁺ signaling activity of individual neurons and monitor electrocorticogram activity through ionic conduction in freely moving mice. At the same time, non-metallic MTEHy-3 is also suitable for magnetic resonance imaging. Thus MTEHy-3 provides one in situ multimodal tool to detect neuronal signaling with both high spatial resolution and high temporal resolution in the brain.

Optically Triggering and Monitoring Single-Cell-Level Metabolism Using Ormosil-Decorated Ultrathin Fibers

The integration of biological components and artificial devices requires a bio-machine interface that can simultaneously trigger and monitor the activities in biosystems. Herein, we use an organically modified silicate (ormosil) composite coating containing a light-responsive nanocapsule and a fluorescent bioprobe for reactive oxygen species (ROS) to decorate ultrathin optical fibers, namely, ormosil-decorated ultrathin fibers (OD-UFs), and demonstrate that these OD-UFs can optically trigger and monitor the intracellular metabolism activities in living cells. The sizes and shapes of UF tips were finely controlled to match the dimension and mechanical properties of living cells. The increased elasticity of the ormosil coating of OD-UFs reduces possible mechanical damage during the cell membrane penetration. The light-responsive nanocapsule was physically absorbed on the surface of the ormosil coating and could release a stimulant to trigger the metabolism activities in cells upon the guided laser through OD-UFs. The fluorescent bioprobe was covalently linked with the ormosil matrix for monitoring the intracellular ROS generation, which was verified by the in vitro experiments on the microdroplets of a hydrogen peroxide solution. Finally, we found that the living cells could maintain most of their viability after being inserted with OD-UFs, and the intracellular metabolism activities were successfully triggered and monitored at the single-cell level. The OD-UF provides a new platform for the investigation of intracellular behaviors for drug stimulations and represents a new proof of concept for a bio-machine interface based on the optical and chemical activities of organic functional molecules.

Transient Heat Conduction in the Orthotropic Model with Rectangular Heat Source

Epidermal electronic systems (EESs) are a representative achievement for utilizing the full advantages of ultra-thin, stretchable and conformal attachment of flexible electronics, and are extremely suitable for integration with human physiological systems, especially in medical hyperthermia. The stretchable heater with stable electrical characteristics and a uniform temperature field is an irreplaceable core component. The inorganic stretchable heater has the advantage of maintaining stable electrical characteristics under tensile deformation. However, the space between the patterned electrodes that provides tensile properties causes uneven distribution of the temperature field. Aiming at improving the temperature distribution uniformity of stretchable thermotherapy electrodes, an orthotropic heat transfer substrate for stretchable heaters is proposed in this paper. An analytical model for transient heat conduction of stretchable rectangular heaters based on orthotropic transfer characteristics is established, which is validated by finite element analysis (FEA). The homogenization effect of orthotropic heat transfer characteristics on temperature distribution and its evolutionary relationship with time are investigated based on this model. This study will provide beneficial help for the temperature distribution homogenization design of stretchable heaters and the exploration of its transient heat transfer mechanism.

Mechanical Gradients Enable Highly Stretchable Electronics Based on Nanofiber Substrates

Stretchable electronics play a pivotal role in the age of information and intelligence. Integrated circuit components are an integral part of high-performance and multifunctional stretchable electronic devices. Therefore, it is an ideal design concept for stretchable electronic devices to not only ensure the reliability of the connection between rigid inorganic electronic components and stretchable circuits but also maintain the stretchability of the device. In this work, we constructed a mechanical gradient strategy to fabricate high-performance stretchable electronic devices. Briefly, polyvinyl alcohol glue is used to fix integrated circuits on stretchable circuits, which are fabricated by printing liquid metal on a thermoplastic polyurethane nanofiber membrane. The strategy of integrated circuits (rigid)-polyvinyl alcohol glue (high elastic modulus)-thermoplastic polyurethane nanofiber membrane (low elastic modulus)-liquid metal (liquid) realizes the strain gradient during the stretching process of the device, thus ensuring the stability and reliability. Moreover, we explored the mechanism through experiments and finite element analysis. The flexible electronic devices fabricated by this scheme are not only ultra-stretchable (900%) but also have good stability and comfort. As proof, the application in stretchable sensors, human-computer interaction devices, and displays was realized.

3R Electronics: Scalable Fabrication of Resilient, Repairable, and Recyclable Soft-Matter Electronics

E-waste is rapidly turning into another man-made disaster. It is proposed that a paradigm shift toward a more sustainable future can be made through soft-matter electronics that are resilient, repairable if damaged, and recyclable (3R), provided that they achieve the same level of maturity as industrial electronics. This includes high-resolution patterning, multilayer implementation, microchip integration, and automated fabrication. Herein, a novel architecture of materials and methods for microchip-integrated condensed soft-matter 3R electronics is demonstrated. The 3R function is enabled by a biphasic liquid metal-based composite, a block copolymer with nonpermanent physical crosslinks, and an electrochemical technique for material recycling. In addition, an autonomous laser-patterning method for scalable circuit patterning with an exceptional resolution of <30 µm in seconds is developed. The phase-shifting property of the BCPs is utilized for vapor-assisted "soldering" circuit repairing and recycling. The process is performed entirely at room temperature, thereby opening the door for a wide range of heat-sensitive and biodegradable polymers for the next generation of green electronics. The implementation and recycling of sophisticated skin-mounted patches with embedded sensors, electrodes, antennas, and microchips that build a digital fingerprint of the human electrophysiological signals is demonstrated by collecting mechanical, electrical, optical, and thermal data from the epidermis.

End-to-end design of wearable sensors

Wearable devices provide an alternative pathway to clinical diagnostics by exploiting various physical, chemical and biological sensors to mine physiological (biophysical and/or biochemical) information in real time (preferably, continuously) and in a non-invasive or minimally invasive manner. These sensors can be worn in the form of glasses, jewellery, face masks, wristwatches, fitness bands, tattoo-like devices, bandages or other patches, and textiles. Wearables such as smartwatches have already proved their capability for the early detection and monitoring of the progression and treatment of various diseases, such as COVID-19 and Parkinson disease, through biophysical signals. Next-generation wearable sensors that enable the multimodal and/or multiplexed measurement of physical parameters and biochemical markers in real time and continuously could be a transformative technology for diagnostics, allowing for high-resolution and time-resolved historical recording of the health status of an individual. In this Review, we examine the building blocks of such wearable sensors, including the substrate materials, sensing mechanisms, power modules and decision-making units, by reflecting on the recent developments in the materials, engineering and data science of these components. Finally, we synthesize current trends in the field to provide predictions for the future trajectory of wearable sensors.

An Analytic Orthotropic Heat Conduction Model for the Stretchable Network Heaters

Compared with other physiotherapy devices, epidermal electronic systems (EES) used in medical applications such as hyperthermia have obvious advantages of conformal attachment, lightness and high efficiency. The stretchable flexible electrode is an indispensable component. The structurally designed flexible inorganic stretchable electrode has the advantage of stable electrical properties under tensile deformation and has received enough attention. However, the space between the patterned electrodes introduced to ensure the tensile properties will inevitably lead to the uneven temperature distribution of the thermotherapy electrodes and degrade the effect of thermotherapy. It is of great practical value to study the temperature uniformity of the stretchable patterned electrode. In order to improve the uniformity of temperature distribution in the heat transfer system with stretchable electrodes, a temperature distribution manipulation strategy for orthotropic substrates is proposed in this paper. A theoretical model of the orthotropic heat transfer system based on the horseshoe-shaped mesh electrode is established. Combined with finite element analysis, the effect of the orthotropic substrate on the uniformity of temperature distribution in three types of heat source heat transfer systems is studied based on this model. The influence of the thermal conductivity ratio in different directions on the temperature distribution is studied parametrically, which will help to guide the design and fabrication of the stretchable electrode that can produce a uniform temperature distribution.

Wearable microfluidic patch with integrated capillary valves and pumps for sweat management and multiple biomarker analysis

Wearable sweat sensors are essential for providing insight into human physiological health. The currently developed microfluidic sweat sensors have demonstrated the function of collecting and storing sweat. However, they detect more average concentrations of substances based on time periods, which leads to the fact that in situ real-time measurement for multiple biomarkers remains a grand challenge. Here, we propose a wearable epidermal microfluidic patch with integrated microfluidic pumps and micro-valves for accelerated and continuous collection of the sweat, where the micro-pumps ensure the complete separation of old and new sweat for real-time detection of real concentration of biomarkers in sweat. The biomarker concentration at different time periods is detected by introducing a burst valve, which is used to assist in the analysis of the real-time detection. A quantitative relationship between the minimum burst pressure difference required for sequential collection and the size of the microchannel structure is established to overcome the effects of additional resistance at the gas-liquid interface. Additionally, the sensing modules, including sodium ion, chlorine ion, glucose, and pH level in sweat, are integrated into the patch to realize in situ, real-time detection of multiple biomarkers in the human sweat, decoding the correlation between changes in substance concentrations and physiological conditions. This work provides a unique and simplifying strategy for developing wearable sweat sensors for potential applications in health monitoring and disease diagnostics.

A Mechanically Interlocking Strategy Based on Conductive Microbridges for Stretchable Electronics

Stretchable electronics incorporating critical sensing, data transmission, display and powering functionalities, is crucial to emerging wearable healthcare applications. To date, methods to achieve stretchability of individual functional devices have been extensively investigated. However, integration strategies of these stretchable devices to achieve all-stretchable systems are still under exploration, in which the reliable stretchable interconnection is a key element. Here, solderless stretchable interconnections based on mechanically interlocking microbridges are developed to realize the assembly of individual stretchable devices onto soft patternable circuits toward multifunctional all-stretchable platforms. This stretchable interconnection can effectively bridge interlayer conductivity with tight adhesion through both conductive microbridges and selectively distributed adhesive polymer. Consequently, enhanced stretchability up to a strain of 35% (R/R₀  ≤ 5) is shown, compared with conventional solder-assisted connections which lose electrical conduction at a strain of less than 5% (R/R₀  ≈ 30). As a proof of concept, a self-powered all-stretchable data-acquisition platform is fabricated by surface mounting a stretchable strain sensor and a supercapacitor onto a soft circuit through solderless interconnections. This solderless interconnecting strategy for surface-mountable devices can be utilized as a valuable technology for the integration of stretchable devices to achieve all-soft multifunctional systems.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Stencil Printing of Liquid Metal upon Electrospun Nanofibers Enables High-Performance Flexible Electronics

Flexible electronics as an emerging technology has demonstrated potential for applications in various fields. With the advent of the Internet of Things era, countless flexible electronic systems need to be developed and deployed. However, materials and fabrication technologies are the key factors restricting the development and commercialization of flexible electronics. Here we report a simple, fast, and green flexible electronics preparation technology. The stencil printing method is adopted to pattern liquid metal on the thermoplastic polyurethane membrane prepared by electrospinning. Besides, with layer-by-layer assembly, flexible circuits, resistors, capacitors, inductors, and their composite devices can be prepared parametrically. Furthermore, these devices have good stretchability, air permeability, and stability, while they are multilayered and reconfigurable. As proof, this strategy is used to fabricate flexible displays, flexible sensors, and flexible filters. Finally, flexible electronic devices are also recycled and reconfigured.

High-Efficiency Large-Area Printed Multilayer Liquid Metal Wires for Stretchable Biomedical Sensors with Recyclability

Stretchable conductors are essential for soft robots, wearable on-skin electronic technologies, and bioelectronics. The utilization of sophisticated stretchable conductors requires a new, simple, rapid, and large-scale printing process whose features include high stretchability, high precision, multilayers, and recyclability simultaneously for commercial wearable electronics. To address this need, an LM (liquid metal) wire was developed using a simple, rapid, and large-scale soft stamper-based printing process and employed to realize LM wire-based conductors and capacitors, which simultaneously offer high stretchability (>380%), high precision past 50 μm, and electromechanical response stability after stretching for up to an hour. Based on the excellent electromechanical responses, the LM wire-based capacitors, as strain sensors, attached to finger joints resulted in precise gesture detection. Meanwhile, a simple transparent wearable e-skin consisting of a 6 × 6 LM wire-based capacitor array without rigid parts successfully monitored a multi-point touch. At last, a portable noninvasive stretchable multilayer LM wire-based pulse sensor with recyclability is fabricated to monitor the patient's heartbeats. The experimental results reveal that the stretchable biomedical sensors have the potential to help patients to improve their life in healthcare, including replacement prosthetic devices, daily and sports activity tracking, continuous health monitoring, and others.

Recent progress in the fabrication of flexible materials for wearable sensors

Wearable sensors have been widely studied because of their small size, light weight, and potential for the noninvasive tracking and monitoring of human physiological information. Wearable flexible sensors generally consist of two parts: a flexible substrate in contact with the skin and a signal processing module. At present, wearable electronics cover many fields, such as machinery, physics, chemistry, materials science, and biomedicine. The design concept and selection of materials are very important to the function of a sensor. In this review, we summarize the latest developments in flexible materials for wearable sensors, including developments in flexible materials, electrode materials, and new flexible biodegradable materials, and describe the important role of innovation in material and sensor design in the development of wearable flexible sensors. Strategies and challenges related to the improvement of the performances of wearable flexible sensors, as well as the development prospects of wearable devices based on flexible materials, are also discussed.

Flexible and recyclable bio-based transient resistive memory enabled by self-healing polyimine membrane

The recyclable, self-healing and easily-degradable transient electronic technology has aroused tremendous attention in flexible electronic products. However, integrating the above advantages into one single flexible electronic device is still a huge challenge. Herein, we demonstrate a flexible and recyclable bio-based memory device using fish colloid as the resistive switching layer on a polyimine substrate, which affords reliable mechanical and electrical properties under repetitive conformal deformation operation. This flexible bio-based memory device presents potential analog behaviors including memory characteristics and excitatory current response, which undergoes incremental potentiation in conductance under successive electrical pulses. Moreover, this device is expected to greatly alleviate the environmental problems caused by electronic waste. It can be decomposed rapidly in water and well recycled, which is a promising candidate for transient memories and information security. We believe that this study can provide new possibilities to the field of high-performance transient electronics and flexible resistive memory devices.

Liquid Metal-Based Soft Electronics for Wearable Healthcare

Wearable healthcare devices have garnered substantial interest for the realization of personal health management by monitoring the physiological parameters of individuals. Attaining the integrity between the devices and the biological interfaces is one of the greatest challenges to achieving high-quality body information in dynamic conditions. Liquid metals, which exist in the liquid phase at room temperatures, are advanced intensively as conductors for deformable devices because of their excellent stretchability and self-healing ability. The unique surface chemistry of liquid metals allows the development of various sensors and devices in wearable form. Also, the biocompatibility of liquid metals, which is verified through numerous biomedical applications, holds immense potential in uses on the surface and inside of a living body. Here, the recent progress of liquid metal-based wearable electronic devices for healthcare with respect to the featured properties and the processing technologies is discussed. Representative examples of applications such as biosensors, neural interfaces, and a soft interconnection for devices are reviewed. The current challenges and prospects for further development are also discussed, and the future directions of advances in the latest research are explored.

Scalable Fabrication of Crystalline COF Membranes from Amorphous Polymeric Membranes

Covalent organic framework (COF) membranes hold potential for widespread applicability, but scalable fabrication is challenging. Here, we demonstrate the disorder-to-order transformation from amorphous polymeric membrane to crystalline COF membrane via monomer exchange. Solution processing is used to prepare amorphous membrane and the replacing monomer is selected based on the chemical and thermodynamical stability of the final framework. Reversible imine bonds allow the extraneous monomers to replace the pristine monomers within amorphous membrane, driving the transformation from disordered network to ordered framework. Incorporation of intramolecular hydrogen bonds enables the crystalline COF to imprint the amorphous membrane morphology. The COF membranes harvest proton conductivity up to 0.53 S cm^-1 at 80 °C. Our strategy bridges amorphous polymeric and crystalline COF membranes for large-scale fabrication of COF membranes and affords guidance on materials processing.