The Manufacture of Unbreakable Bionics via Multifunctional and Self-Healing Silk-Graphene Hydrogels

Bio-Inspired Synthesis of Injectable, Self-Healing PAA-Zn-Silk Fibroin-MXene Hydrogel for Multifunctional Wearable Capacitive Strain Sensor

Conductive hydrogels have important application prospects in the field of wearable sensing, which can identify various biological signals for human motion monitoring. However, the preparation of flexible conductive hydrogels with high sensitivity and stability to achieve reliable signal recording remains a challenge. Herein, we prepared a conductive hydrogel by introducing conductive Ti3C2Tx MXene nanosheets into a dual network structure formed by Zn2+ crosslinked polyacrylic acid and silk fibroin for use as a wearable capacitive strain sensor. The prepared injectable hydrogel has a uniform porous structure and good flexibility, and the elongation at break can reach 1750%. A large number of ionic coordination bonds and hydrogen bond interactions make the hydrogel exhibit good structural stability and a fast self-healing property (30 s). In addition, the introduction of Ti3C2Tx MXene as a conductive medium in hydrogel improves the conductivity. Due to the high conductivity of 0.16 S/m, the capacitive strain sensor assembled from this hydrogel presents a high gauge factor of 1.78 over a wide strain range of 0-200%, a fast response time of 0.2 s, and good cycling stability. As a wearable sensor, the hydrogel can accurately monitor the activities of different joints in real-time. This work is expected to provide a new approach for wearable hydrogel electronic devices.

Advanced Liquid Metal-Based Hydrogels for Flexible Electronics

With the rapid development of flexible electronics in wearable devices, healthcare devices, and the Internet of Things (IoT), liquid metals (LMs)-based hydrogels have emerged as cutting-edge functional materials due to their high electrical conductivity, tunable mechanical properties, excellent biocompatibility, and unique self-healing properties. Through various physical or chemical methods, LMs can be integrated to form multifunctional LMs-based hydrogels, thus broadening the potential application fields. In this Review, the recent advancement in LMs-based hydrogels for flexible electronics is comprehensively and systematically reviewed from three aspects of synthesis methods, properties, and applications. For the first time, the existing innovative synthesis methods of LMs-based hydrogels are classified and summarized, including patterned LMs on/inside hydrogel substrates, LMs as conductive fillers in polymeric hydrogels, LMs as initiators in hydrogels, and LMs as cross-linkers with organic/inorganic materials. The synthesis mechanism is also stated in detail to highlight the multiple roles of LMs in adjusting the hydrogel properties. The versatile applications of LMs-based hydrogels in flexible electronics, including flexible sensors, wireless communications, electromagnetic interference (EMI) shielding, soft robot actuators, energy storage and conversion, etc., are separately described. Finally, the current challenges and future prospects of LMs-based hydrogels are proposed.

Self-Maintainable Electronic Materials with Skin-Like Characteristics Enabled by Graphene-PEDOT:PSS Fillers

Conventional devices lack the adaptability and responsiveness inherent in the design of nature. Therefore, they cannot autonomously maintain themselves in natural environments. This limitation is primarily because of using rigid and fragile material components for their construction, which hinders their ability to adapt and evolve in changing environments. Moreover, they often cannot self-repair after injuries or significant damage. Even devices with self-healing, soft, and responsive properties often fail to seamlessly integrate all these attributes into a single, scalable, and cohesive platform. In this study, a significant breakthrough is introduced by utilizing graphene-poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (graphene-PEDOT:PSS) fillers to transform a typically weak, insulating, and jelly-like material into a soft electronic material with properties akin to those of living organisms, such as skin tissue. The developed electronic materials exhibit a range of other capabilities attributed to the hierarchical organization originating from filler enhancement, which includes methods such as heat regulation, 3D printability, and multiplex sensing. The introduction of this new class of materials can facilitate the self-maintenance of life-like soft robots and bioelectronics that can be seamlessly integrated within dynamic environments, such as the human body, while demonstrating the ability to sense, respond, and adapt to challenging environments.

Intrinsically Conductive, Highly Compressible, Porous Hydrogel with Exceptional Sensitivity at Low Pressure

Conductive hydrogels have emerged as a promising material in the field of flexible sensing, holding great potential for advanced wearable devices and medical diagnostics, because of their unique conductivity, mechanical deformability, and tissue-like softness. However simultaneously achieving intrinsic conductivity, excellent compressibility and resilience remains a significant challenge. Herein, a novel macroporous, highly compressive, resilient, and intrinsically conductive hydrogel (MPGEL) based on a newly developed easy, eco-friendly, and zero-waste strategy is reported. The MPGEL is prepared using nitrogen as the inert gas and foaming agent, polymerizable Pluronic F127 as a surfactant and crosslinker, and ionic conductive lithium acrylate (LiAA) as the monomer. The resulting MPGEL exhibits highly compressibility and resilience with a low compressive modulus (3.75 kPa), yielding an exceptional compressive sensitivity of 31.67 kPa-1 at low pressure. Therefore, the MPGEL not only can monitor various human movements, but also can effectively detect human cardiac motion, and even precisely distinguish between central and peripheral arterial blood pressure waveforms. This highlights the immense potential of MPGEL for future medical diagnostic technologies and advanced wearable health-monitoring devices.

Anti-freezing conductive hydrogels with exceptional mechanical properties and stable sensing performance at -30 °C

Conductive hydrogels with stable sensing performance are highly required in soft electronic devices. However, these hydrogels tend to solidify and experience structural damage at sub-zero temperatures, leading to material breakdown and device malfunction. The main challenge lies in effectively designing the micro/nano-structure to enhance mechanical properties and stable strain sensing while preventing freezing in hydrogels. Here, we present a rapid strategy for developing a MXene bridging double-network structure-based strain sensor using polyacrylamide and agar hydrogels that can maintain stable functionality even at an extremely low temperature of -30 °C. By incorporating MXenes as a catalyst to expedite free radical polymerization, we achieve outstanding mechanical and strain sensing properties at room temperature (a high response range of 1000%, a response signal linearity of 0.998, and a gauge factor (GF) value of 1.41). This sensing performance surpasses those reported for many other hydrogels. Importantly, we also observe that the stable micro-nanostructure in the hydrogel at an extreme temperature of approximately -30 °C results in exceptional strain-detection performance (a stable response range of up to 250%) with a linearity of 0.995 and a GF value of 1.25 due to its remarkably low freezing point (<-80 °C). These findings highlight the application of our hydrogel-based tactile sensor in low-temperature environments.

One-step fabrication of dual-network cellulose-based hydrogel sensors with high flexibility and conductivity under ZnCl2 solvent method for flexible sensing properties

Flexible smart sensing materials are gaining tremendous momentum in wearable and bionic smart electronics. To satisfy the growing demand for sustainability and eco-friendliness, biomass-based hydrogel sensors for green and biologically safe wearable sensors have attracted significant attention. In this work, we have prepared MCC/PAA/AgNWs/CNTs hydrogel sensors with excellent conductive sensing properties by a simple physical blending method. The ZnCl2 solvent system was used to dissolve the MCC, followed by introducing acrylic acid to polymerize under UV illumination. Subsequently, CNTs and AgNWs were introduced into the hydrogel network to obtain hydrogel with excellent conductive sensing and antibacterial properties. Here, the physical and chemical interactions between the components significantly improved the mechanical properties of the hydrogels, exhibiting good tensile strength (0.45 MPa), elongation at break (558 %) and adhesion properties. Hydrogel presented outstanding electrical conductivity and significantly elongation sensitive (GF = 4.73 when elongated 90-120 %). Additionally, the hydrogel was also found to have significant antimicrobial activity against both Escherichia coli and Staphylococcus aureus, and the antibacterial effect was almost 100 %. With high sensitivity, stability, and reproducibility, these hydrogel strain transducers can detect various human movements, including finger flexion, wrist movement, joint motion, and heartbeat.

Injectable platelet-mimicking silk protein-peptide conjugate microspheres for hemostasis modulation and targeted treatment of internal bleeding

Uncontrolled deep bleeding, commonly encountered in surgical procedures, combat injuries, and trauma, poses a significant threat to patient survival and recovery. The development of effective hemostatic agents capable of precisely targeting trauma sites in deep tissues and rapidly halt bleeding remains a considerable challenge. Drawing inspiration from the natural hemostatic cascade, we present platelet-like microspheres composed of silk fibroin (SF) and thrombus-targeting peptides, engineered to mimic natural platelets for rapid hemostasis in vivo. These peptide/SF hemostatic microspheres, formulated using a freezing self-assembly technology, closely resemble natural platelets in terms of size, shape, and zeta potential. Moreover, they exhibit favorable cytocompatibility, hemocompatibility, and anti-cell adhesion. Assessment of fibrin polymerization revealed that these hemostatic microspheres possessed enzymatic physiological functions, similar to activated platelets, facilitating platelet adhesion, fibrin binding, and wound-triggered hemostasis. Notably, these hemostatic microspheres rapidly target the bleeding site in vivo within 5 min, with minimal dispersion elsewhere, persisting after blood clot formation. Furthermore, these microspheres exhibit favorable metabolic kinetics, with 71% degradation occurring within one-day post-subcutaneous injection. Histological assessment revealed well-preserved organ structures and minimal inflammatory responses at 14 d post-injection, supporting their long-term biocompatibility. Importantly, they can be injected and targeted into damaged blood vessels, selectively binding to fibrin and forming blood clots within 2 min, resulting in a 74% reduction in bleeding volume compared to SF microspheres alone. Therefore, these injectable SF-based hemostatic microspheres emerge as promising candidates for future rapid hemostasis in tissue injuries.

Study on Highly Sensitive Capacitive Pressure Sensor Based on Silk Fibroin-Lignin Nanoparticles Hydrogel

Silk fibroin (SF) hydrogel has been proven to have excellent applications in the field of pressure sensors, but its sensing performance still needs improvement. A flexible hydrogel prepared from natural macromolecular materials was developed, and lignin nanoparticles (LNPs) were introduced during the preparation of the SF hydrogel. When LNPs account for 3% of SF, the sensing unit of the SF-LNPs3% hydrogel exhibits high stress sensitivity (1.32 kPa-1), fast response speed (<0.1 s), and superior cycle stability (≥8000 cycles). The sensor can detect human motion information, such as finger bending, elbow bending, and pulse signals. When worn at the vocal cord position, it can detect the peak value of the characteristic signal during the wearer speaks. This work demonstrates that the SF-LNPs3% hydrogel has high sensitivity and shows great potential in the field of pressure sensors.

Designing Silk Biomaterials toward Better Future Healthcare: The Development and Application of Silk-Based Implantable Electronic Devices in Clinical Diagnosis and Therapy

Implantable medical electronic devices (IMEDs) have attracted great attention and shown versatility for solving clinical problems ranging from real-time monitoring of physiological/ pathological states to electrical stimulation therapy and from monitoring brain cell activity to deep brain stimulation. The ongoing challenge is to select appropriate materials in target device configuration for biomedical applications. Currently, silk-based biomaterials have been developed for the design of diagnostic and therapeutic electronic devices due to their excellent properties and abundant active sites in the structure. Herein, the aim is to summarize the structural characteristics, physicochemical properties, and bioactivities of natural silk biomaterials as well as their derived materials, with a particular focus on the silk-based implantable biomedical electronic devices, such as implantable devices for invasive brain-computer interfaces, neural recording, and in vivo electrostimulation. In addition, future opportunities and challenges are also envisioned, hoping to spark the interests of researchers in interdisciplinary fields such as biomaterials, clinical medicine, and electronics.

Tough Polyurethane Hydrogels with a Multiple Hydrogen-Bond Interlocked Bicontinuous Phase Structure Prepared by In Situ Water-Induced Microphase Separation

Hydrogels with mechanical performances similar to load-bearing tissues are in demand for in vivo applications. In this work, inspired by the self-assembly behavior of amphiphilic polymers, polyurethane-based tough hydrogels with a multiple hydrogen-bond interlocked bicontinuous phase structure through in situ water-induced microphase separation strategy are developed, in which poly(ethylene glycol)-based polyurethane (PEG-PU, hydrophilic) and poly(ε-caprolactone)-based polyurethane (PCL-PU, hydrophobic) are blended to form dry films followed by water swelling. A multiple hydrogen bonding factor, imidazolidinyl urea, is introduced into the synthesis of the two polyurethanes, and the formation of multiple hydrogen bonds between PEG-PU and PCL-PU can promote homogeneous microphase separation for the construction of bicontinuous phase structures in the hydrogel network, by which the hydrogel features break strength of 12.9 MPa, fracture energy of 2435 J m-2, and toughness of 48.2 MJ m-3. As a biomedical patch, the outstanding mechanical performances can withstand abdominal pressure to prevent hernia formation in the abdominal wall defect model. Compared to the commercial PP mesh, hydrogel can prevent tissue/organ adhesion to reduce inflammatory responses and promote angiogenesis, thereby accelerating the repair of abdominal wall defects. This work may provide useful inspiration for researchers to design different gel materials through solvent-induced microphase separation.

Hygroscopic Nature of Lithium Ions: A Simple Key to Super Tough Atmosphere-Stable Hydrogel Electrolytes

Gel electrolytes have emerged as a versatile solution to address numerous limitations associated with liquid electrolytes in electrical energy storage (EES) devices, in terms of safety, flexibility, and affordability. Aqueous gel electrolytes, in particular, exhibit exceptional features by offering one of the highest ion solvation capacities and ionic conductivities. The two main challenges with hydrogel electrolytes are their easy freezing at subzero temperatures and rapid dehydration under open conditions, leading to the failure of the EES device. In response, we present an uncomplicated and quick-to-make hydrogel electrolyte system offering impressive mechanical properties (205.5 kPa tensile strength, 2880 kJ/m3 toughness, and 3030% strain at the break), along with antifreezing and antiflammability attributes. Notably, the hydrogel electrolyte demonstrates high ionic conductivity and superior performance in supercapacitor cells over a wide range of temperatures (-40 to 80 °C) and under various deformations. The hydrogel electrolyte maintains its capabilities under open conditions over an extended period of time, even at 50 °C, showcased by powering a wristwatch. The atmospheric stability of the hydrogel electrolyte demonstrated in this study introduces promising prospects for the future of EES devices spanning from production to end-user consumption.

A rapid and robust organ repair polyacrylamide/alginate adhesive hydrogel mediated via interfacial adhesion-trigger molecules

Adhesive hydrogels have been widely explored as tissue adhesives for wound sealing and repair. However, developing adhesive hydrogels with simple preparation techniques and strong adhesion to internal organs in a short time remains a challenge. In this study, we developed a strategy for robust and rapid tissue adhesion of internal organ sealing and repair by an interfacial adhesion-molecule triggered hydrogel system. In this system, polyphenol molecules act as adhesion-trigger reagents to achieve fast and strong adhesion of polyacrylamide/alginate hydrogels on the surface of wound tissue by rapidly forming abundant hydrogen bonds at the interface. The adhesion energy is significantly enhanced by 45 times under the mediation of polyphenol adhesion-trigger molecules, resulting in a robust (> 600 J m-2) tissue adhesion in just 30 s. This interfacial adhesion system demonstrates good biocompatibility, strong sealing performance on multiple organs (porcine heart, lung, stomach, and intestine), and excellent repair properties in gastric perforation wounds of rabbits in vivo. Moreover, immunocytochemical and transcriptomic analyses reveal that this interfacial adhesion system significantly promotes vascular regeneration and inhibits inflammatory responses during wound repairing. The proposed hydrogel provides a facile strategy for rapid and robust tissue adhesion, and shows potential applications in organ sealing and repair.

Fabrication of PHFPO Surface-Modified Conductive AgNWs/PNAGA Hydrogels with Enhanced Water Retention Capacity toward Highly Sensitive Strain Sensors

Conductive hydrogels, characterized by their unique features of flexibility, biocompatibility, electrical conductivity, and responsiveness to environmental stimuli, have emerged as promising materials for sensitive strain sensors. In this study, a facile strategy to prepare highly conductive hydrogels is reported. Through rational structural and synthetic design, silver nanowires (AgNWs) are incorporated into poly(N-acryloyl glycinamide) (PNAGA) hydrogels, achieving high electrical conductivity (up to 0.88 S m-1), significantly enhanced mechanical properties, and elevated deformative sensitivity. Furthermore, surface modification with polyhexafluoropropylene oxide (PHFPO) has substantially improved the water retention capacity and dressing comfort of this hydrogel material. Based on the above merits, these hydrogels are employed to fabricate highly sensitive wearable strain sensors which can detect and interpret subtle hand and finger movements and enable precise control of machine interfaces. The AgNWs/PNAGA based strain sensors can effectively sense finger motion, enabling the control of robotic fingers to replicate the human hand's gestures. In addition, the high deformative sensitivity and elevated water retention performance of the hydrogels makes them suitable for flow sensing. These conceptual applications demonstrate the potential of this conductive hydrogel in high-performance strain sensors in the future.

Recent advances in sustainable nature-based functional materials for biomedical sensor technologies

The lightweight, low-density, and low-cost natural polymers like cellulose, chitosan, and silk have good chemical and biodegradable properties due to their individually unique structural and functional elements. However, the mechanical properties of these polymers differ from each other. In this scenario, chitosan lacks good mechanical properties than cellulose and silk. The synthesis of nano natural polymer and reinforcement with suitable chemical compounds as the development of nanocomposite gives them promising multidisciplinary applications. Many kinds of research are already published with innovative bio-derived polymeric functional materials (Bd-PFM) applications. Most research interest is carried out on health concerns. Lots of attention has been paid to biomedical applications of Bd-PFM as biosensors. This review aims to provide a glimpse of the nanostructures Bd-PFM biosensors.

Breathable, Adhesive, and Biomimetic Skin-Like Super Tattoo

Electronic tattoo, capable of imperceivably acquiring bio-electrical signals from the body, is broadly applied in healthcare and human-machine interface. Tattoo substrate, the foundation of electronic tattoo, is expected to be mechanically mimetic to skin, adhesive, and breathable, and yet remains highly challenging to achieve. Herein, the study mimics human skin and design a breathable, adhesive, and mechanically skin-like super tattoo substrate based on an ultra-thin film (≈2 µm). Similar to skin, super tattoo demonstrates strain-adaptive stiffening properties with high tear energy (5.4 kJ·m-2) and toughness (1.3 MJ·m-3). Superior to skin, it exhibits high adhesion, ionic conductivity, and permeability. A variety of conductive electrodes can be processed on it, showing the universality toward an ideal platform for electronic tattoo with stable and low contact impedance. Super tattoo-based electrodes can imperceivably and accurately monitor weak electromyography (EMG) of swallowing on the junction, providing effective guidance for rehabilitation training of dysphagia.

Enhancing regenerative medicine with self-healing hydrogels: A solution for tissue repair and advanced cyborganic healthcare devices

Considering the global burden related to tissue and organ injuries or failures, self-healing hydrogels may be an attractive therapeutic alternative for the future. Self-healing hydrogels are highly hydrated 3D structures with the ability to self-heal after breaking, this property is attributable to a variety of dynamic non-covalent and covalent bonds that are able to re-linking within the matrix. Self-healing ability specially benefits minimal invasive medical treatments with cell-delivery support. Moreover, those tissue-engineered self-healing hydrogels network have demonstrated effectiveness for myriad purposes; for instance, they could act as delivery-platforms for different cargos (drugs, growth factors, cells, among others) in tissues such as bone, cartilage, nerve or skin. Besides, self-healing hydrogels have currently found their way into new and novel applications; for example, with the development of the self-healing adhesive hydrogels, by merely aiding surgical closing processes and by providing biomaterial-tissue adhesion. Furthermore, conductive hydrogels permit the stimuli and monitoring of natural electrical signals, which facilitated a better fitting of hydrogels in native tissue or the diagnosis of various health diseases. Lastly, self-healing hydrogels could be part of cyborganics - a merge between biology and machinery - which can pave the way to a finer healthcare devices for diagnostics and precision therapies.

Silk fibroin increases the elasticity of alginate-gelatin hydrogels and regulates cardiac cell contractile function in cardiac bioinks

Silk fibroin (SF) is a natural protein extracted fromBombyx morisilkworm thread. From its common use in the textile industry, it emerged as a biomaterial with promising biochemical and mechanical properties for applications in the field of tissue engineering and regenerative medicine. In this study, we evaluate for the first time the effects of SF on cardiac bioink formulations containing cardiac spheroids (CSs). First, we evaluate if the SF addition plays a role in the structural and elastic properties of hydrogels containing alginate (Alg) and gelatin (Gel). Then, we test the printability and durability of bioprinted SF-containing hydrogels. Finally, we evaluate whether the addition of SF controls cell viability and function of CSs in Alg-Gel hydrogels. Our findings show that the addition of 1% (w/v) SF to Alg-Gel hydrogels makes them more elastic without affecting cell viability. However, fractional shortening (FS%) of CSs in SF-Alg-Gel hydrogels increases without affecting their contraction frequency, suggesting an improvement in contractile function in the 3D cultures. Altogether, our findings support a promising pathway to bioengineer bioinks containing SF for cardiac applications, with the ability to control mechanical and cellular features in cardiac bioinks.

Silk fibroin hydrogel adhesive enables sealed-tight reconstruction of meniscus tears

Despite orientationally variant tears of the meniscus, suture repair is the current clinical gold treatment. However, inaccessible tears in company with re-tears susceptibility remain unresolved. To extend meniscal repair tools from the perspective of adhesion and regeneration, we design a dual functional biologic-released bioadhesive (S-PIL10) comprised of methacrylated silk fibroin crosslinked with phenylboronic acid-ionic liquid loading with growth factor TGF-β1, which integrates chemo-mechanical restoration with inner meniscal regeneration. Supramolecular interactions of β-sheets and hydrogen bonds richened by phenylboronic acid-ionic liquid (PIL) result in enhanced wet adhesion, swelling resistance, and anti-fatigue capabilities, compared to neat silk fibroin gel. Besides, elimination of reactive oxygen species (ROS) by S-PIL10 further fortifies localized meniscus tear repair by affecting inflammatory microenvironment with dynamic borate ester bonds, and S-PIL10 continuously releases TGF-β1 for cell recruitment and bridging of defect edge. In vivo rabbit models functionally evidence the seamless and dense reconstruction of torn meniscus, verifying that the concept of meniscus adhesive is feasible and providing a promising revolutionary strategy for preclinical research to repair meniscus tears.

Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

An All-Protein Multisensory Highly Bionic Skin

To achieve a highly realistic robot, closely mimicking human skin in terms of materials and functionality is essential. This paper presents an all-protein silk fibroin bionic skin (SFBS) that emulates both fast-adapting (FA) and slow-adapting (SA) receptors. The mechanically different silk film and hydrogel, which exhibited skin-like properties, such as stretchability (>140%), elasticity, low modulus (<10 kPa), biocompatibility, and degradability, were prepared through mesoscopic reconstruction engineering to mimic the epidermis and dermis. Our SFBS, incorporating SA and FA sensors, demonstrated a highly sensitive (1.083 kPa-1) static pressure sensing performance (in vitro and in vivo), showed the ability to sense high-frequency vibrations (50-400 Hz), could discriminate materials and sliding, and could even identify the fine morphological differences between objects. As proof of concept, an SFBS-integrated rehabilitation glove was synthesized, which could help stroke patients regain sensory feedback. In conclusion, this work provides a practical approach for developing skin equivalents, prostheses, and smart robots.

Highly Robust Conductive Organo-Hydrogels with Powerful Sensing Capabilities Under Large Mechanical Stress

The low mechanical strength of conductive hydrogels (<1 MPa) has been a significant hurdle in their practical application, as they are prone to fracturing under complex conditions, limiting their effectiveness. Here, this work fabricates a strong and tough conductive hierarchical poly(vinyl alcohol) (PEDOT:PSS/PVA) organo-hydrogel (PPS organo-hydrogel) via a facile combining strategy of self-assembly and stretch training. With PVA/PEDOT:PSS microlayers and aligned PVA/PEDOT:PSS nanofibers, PVA and PEDOT:PSS nanocrystalline domains, and semi-interpenetrating polymer networks, PPS organo-hydrogels display outstanding mechanical performances (strength: 54.8 MPa, toughness: 153.97 MJ m-3 ). Additionally, PPS organo-hydrogels also exhibit powerful sensing capabilities (gauge factor (GF): 983) due to the aligned hierarchical structures and organic liquid phase of DMSO. Notably, with the synergy of such mechanical and sensing properties, organo-hydrogels can even detect objects as light as 1 gram, despite bearing a tensile strength of ≈23 MPa. By incorporating these materials into human-machine interfaces, such as controlling artificial arms for grabbing objects and monitoring sport behaviors in soccer training, this work has unlocked a new realm of possibilities for these high-performance hierarchical organo-hydrogels. This approach to designing hierarchical structures has the potential to lead to even more high-performance hydrogels in the future.

Self-encapsulated ionic fibers based on stress-induced adaptive phase transition for non-contact depth-of-field camouflage sensing

Ionically conductive fibers have promising applications; however, complex processing techniques and poor stability limit their practicality. To overcome these challenges, we proposed a stress-induced adaptive phase transition strategy to conveniently fabricate self-encapsulated hydrogel-based ionically conductive fibers (se-HICFs). se-HICFs can be produced simply by directly stretching ionic hydrogels with ultra-stretchable networks (us-IHs) or by dip-drawing from molten us-IHs. During this process, stress facilitated the directional migration and evaporation of water molecules in us-IHs, causing a phase transition in the surface layer of ionic fibers to achieve self-encapsulation. The resulting sheath-core structure of se-HICFs enhanced mechanical strength and stability while endowing se-HICFs with powerful non-contact electrostatic induction capabilities. Mimicking nature, se-HICFs were woven into spider web structures and camouflaged in wild environments to achieve high spatiotemporal resolution 3D depth-of-field sensing for different moving media. This work opens up a convenient route to fabricate stable functionalized ionic fibers.

Low-Hysteresis and High-Toughness Hydrogels Regulated by Porous Cationic Polymers: the Effect of Counteranions

Service life and range of polymer materials is heavily reliant on their elasticity and mechanical stability under long-term loading. Slippage of chain segments under load leads to significant hysteresis of the hydrogels, limiting its repeatability and mechanical stability. Achieving the desired elasticity exceeding that of rubber is a great challenge for hydrogels, particularly when subjected to large deformations. Here, low-hysteresis and high-toughness hydrogels were developed through controllable interactions of porous cationic polymers (PCPs) with adjustable counteranions, including reversible bonding of PCP frameworks/polymer segments (polyacrylamide, PAAm) and counteranions/PAAm. This strategy reduces chain segment slippage under load, endowing the PCP-based hydrogels (PCP-gels) with good elasticity under large deformations (7 % hysteresis at a strain ratio of 40). Furthermore, due to the enlarged chain segments entanglement by PCP, the PCP-gels exhibit large strain (13000 %), significantly enhanced toughness (68 MJ m-3 ), high fracture energy (43.1 kJ m-2 ), and fatigue resistance. The unique properties of these elastic PCP-gels have promising applications in the field of flexible sensors.

Multifunctional, Self-Adhesive MXene-Based Hydrogel Flexible Strain Sensors for Hand-Written Digit Recognition with Assistance of Deep Learning

The conductive hydrogel as a flexible sensor not only has certain mechanical flexibility but also can be used in the field of human health detection and human-computer interaction. Herein, by introduction of tannic acid (TA) with MXene into the polyacrylamide (PAM)/carboxymethyl chitosan (CMC) double-network hydrogel, a hydrogel with high stretchability, self-adhesion, and high sensitivity was prepared. CMC and PAM form a semi-interpenetrating double-network of high toughness and durability through electrostatic interactions and multiple hydrogen bonding networks. The abundant hydrophilic functional groups on TA and MXene form multiple hydrogen bonds simultaneously with the polymer network, ensuring high stretchability and sensitivity of the hydrogel. The hydrogel can display an accurate response to a variety of stimulus signals and can monitor both human joint movements and small physiological signal changes. It can also be combined with deep learning algorithms to classify handwritten digits with an accuracy rate of 98%. This work can promote the application of hydrogel sensors with durability and high sensitivity. The combination of algorithms and flexible sensors provides important ideas for the further development of flexible devices.

Multifunctional acetylated distarch phosphate based conducting hydrogel with high stretchability, ultralow hysteresis and fast response for wearable strain sensors

The rapid development of flexible sensors has greatly increased the demand for high-performance hydrogels. However, it remains a challenge to fabricate flexible hydrogel sensors with high stretching, low hysteresis, excellent adhesion, good conductivity, sensing characteristics and bacteriostatic function in a simple way. Herein, a highly conducting double network hydrogel is presented by incorporating lithium chloride (LiCl) into the hydrogel consisting of poly (2-acrylamide-2-methylpropanesulfonic acid/acrylamide/acrylic acid) (3A) network and acetylated distarch phosphate (ADSP). The addition of ADSP not only formed hydrogen bonds with 3A to improve the toughness of the hydrogel but also plays the role of "physical cross-linking" in 3A by "anchoring" the polymer molecular chains together. Tuning the composition of the hydrogel allows the attainment of the best functions, such as high stretchability (∼770 %), ultralow hysteresis (2.2 %, ε = 100 %), excellent electrical conductivity (2.9 S/m), strain sensitivity (GF = 3.0 at 200-500 % strain) and fast response (96 ms). Based on the above performance, the 3A/ADSP/LiCl hydrogel strain sensor can repeatedly and stably detect and monitor large-scale human movements and subtle sensing signals. In addition, the 3A/ADSP/LiCl hydrogel shows a good biocompatibility and bacteriostatic ability. This work provides an effective strategy for constructing the conductive hydrogels for wearable devices and flexible sensors.

Nanosized Silk-Magnesium Complexes for Tissue Regeneration

Metal ions provide multifunctional signals for cell and tissue functions, including regeneration. Inspired by metal-organic frameworks (MOFs), nanosized silk protein aggregates with a high negative charge density are used to form stable silk-magnesium ion complexes. Magnesium ions (Mg ions) are added directly to silk nanoparticle solutions, inducing gelation through the formation of silk-Mg coordination complexes. The Mg ions are released slowly from the nanoparticles through diffusion, with sustained release via tuning the degradation or dissolution of the nanosized silk aggregates. Studies in vitro reveal a dose-dependent influence of Mg ions on angiogenic and anti-inflammatory functions. Silk-Mg ion complexes in the form of hydrogels also stimulate tissue regeneration with a reduced formation of scar tissue in vivo, suggesting potential utility in tissue regeneration.

A Mechanically Resilient and Tissue-Conformable Hydrogel with Hemostatic and Antibacterial Capabilities for Wound Care

Hydrogels are used in wound dressings because of their tissue-like softness and biocompatibility. However, the clinical translation of hydrogels remains challenging because of their long-term stability, water swellability, and poor tissue adhesiveness. Here, tannic acid (TA) is introduced into a double network (DN) hydrogel consisting of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) to realize a tough, self-healable, nonswellable, conformally tissue-adhesive, hemostatic, and antibacterial hydrogel. The TA within the DN hydrogel forms a dynamic network, enabling rapid self-healing (within 5 min) and offering effective energy dissipation for toughness and viscoelasticity. Furthermore, the hydrophobic moieties of TA provide a water-shielding effect, rendering the hydrogel nonswellable. A simple chemical modification to the hydrogel further strengthens its interfacial adhesion with tissues (shear strength of ≈31 kPa). Interestingly, the TA also can serve as an effective hemostatic (blood-clotting index of 58.40 ± 1.5) and antibacterial component, which are required for a successful wound dressing. The antibacterial effects of the hydrogel are tested against Escherichia coli and Staphylococcus aureus. Finally, the hydrogel is prepared in patch form and applied to a mouse model to test in vivo biocompatibility and hemostatic performances.

Silk-based conductive materials for smart biointerfaces

Silk-based conductive materials are widely used in biointerface applications, such as artificial epidermal sensors, soft and implantable bioelectronics, and tissue/cell scaffolds. Such biointerface materials require coordinated physicochemical, biological, and mechanical properties to meet current practical needs and future sophisticated demands. However, it remains a challenge to formulate silk-based advanced materials with high electrical conductivity, good biocompatibility, mechanical robustness, and in some cases, tissue adhesion ability without compromising other physicochemical properties. In this review, we highlight recent progress in the development of functional conductive silk-based advanced materials with different morphologies. Then, we reviewed the advanced paradigms of these silk materials applied as wearable flexible sensors, implantable electronics, and tissue/cell engineering with perspectives on the application challenges. Silk-based conductive materials can serve as promising building blocks for biomedical devices in personalized healthcare and other fields of bioengineering.

Using Wool Keratin as a Structural Biomaterial and Natural Mediator to Fabricate Biocompatible and Robust Bioelectronic Platforms

The design and fabrication of biopolymer-incorporated flexible electronics have attracted immense interest in healthcare systems, degradable implants, and electronic skin. However, the application of these soft bioelectronic devices is often hampered by their intrinsic drawbacks, such as poor stability, inferior scalability, and unsatisfactory durability. Herein, for the first time, using wool keratin (WK) as a structural biomaterial and natural mediator to fabricate soft bioelectronics is presented. Both theoretical and experimental studies reveal that the unique features of WK can endow carbon nanotubes (CNTs) with excellent water dispersibility, stability, and biocompatibility. Therefore, well-dispersed and electroconductive bio-inks can be prepared via a straightforward mixing process of WK and CNTs. The as-obtained WK/CNTs inks can be directly exploited to design versatile and high-performance bioelectronics, such as flexible circuits and electrocardiogram electrodes. More impressively, WK can also be a natural mediator to connect CNTs and polyacrylamide chains to fabricate a strain sensor with enhanced mechanical and electrical properties. With conformable and soft architectures, these WK-derived sensing units can be further assembled into an integrated glove for real-time gesture recognition and dexterous robot manipulations, suggesting the great potential of the WK/CNT composites for wearable artificial intelligence.

Intrinsically Nonswellable Multifunctional Hydrogel with Dynamic Nanoconfinement Networks for Robust Tissue-Adaptable Bioelectronics

Developing bioelectronics that retains their long-term functionalities in the human body during daily activities is a current critical issue. To accomplish this, robust tissue adaptability and biointerfacing of bioelectronics should be achieved. Hydrogels have emerged as promising materials for bioelectronics that can softly adapt to and interface with tissues. However, hydrogels lack toughness, requisite electrical properties, and fabrication methodologies. Additionally, the water-swellable property of hydrogels weakens their mechanical properties. In this work, an intrinsically nonswellable multifunctional hydrogel exhibiting tissue-like moduli ranging from 10 to 100 kPa, toughness (400-873 J m-3 ), stretchability (≈1000% strain), and rapid self-healing ability (within 5 min), is developed. The incorporation of carboxyl- and hydroxyl-functionalized carbon nanotubes (fCNTs) ensures high conductivity of the hydrogel (≈40 S m-1 ), which can be maintained and recovered even after stretching or rupture. After a simple chemical modification, the hydrogel shows tissue-adhesive properties (≈50 kPa) against the target tissues. Moreover, the hydrogel can be 3D printed with a high resolution (≈100 µm) through heat treatment owing to its shear-thinning capacity, endowing it with fabrication versatility. The hydrogel is successfully applied to underwater electromyography (EMG) detection and ex vivo bladder expansion monitoring, demonstrating its potential for practical bioelectronics.

Recent Progress in Self-Healable Hydrogel-Based Electroluminescent Devices: A Comprehensive Review

Flexible electronics have gained significant research attention in recent years due to their potential applications as smart and functional materials. Typically, electroluminescence devices produced by hydrogel-based materials are among the most notable flexible electronics. With their excellent flexibility and their remarkable electrical, adaptable mechanical and self-healing properties, functional hydrogels offer a wealth of insights and opportunities for the fabrication of electroluminescent devices that can be easily integrated into wearable electronics for various applications. Various strategies have been developed and adapted to obtain functional hydrogels, and at the same time, high-performance electroluminescent devices have been fabricated based on these functional hydrogels. This review provides a comprehensive overview of various functional hydrogels that have been used for the development of electroluminescent devices. It also highlights some challenges and future research prospects for hydrogel-based electroluminescent devices.

Scalable Manufacturing of Environmentally Stable All-Solid-State Plant Protein-Based Supercapacitors with Optimal Balance of Capacitive Performance and Mechanically Robust

The development of advanced biomaterial with mechanically robust and high energy density is critical for flexible electronics, such as batteries and supercapacitors. Plant proteins are ideal candidates for making flexible electronics due to their renewable and eco-friendly natures. However, due to the weak intermolecular interactions and abundant hydrophilic groups of protein chains, the mechanical properties of protein-based materials, especially in bulk materials, are largely constrained, which hinders their performance in practical applications. Here, a green and scalable method is shown for the fabrication of advanced film biomaterials with high mechanical strength (36.3 MPa), toughness (21.25 MJ m^-3 ), and extraordinary fatigue-resistance (213 000 times) by incorporating tailor-made core-double-shell structured nanoparticles. Subsequently, the film biomaterials combine to construct an ordered, dense bulk material by stacking-up and hot-pressing techniques. Surprisingly, the solid-state supercapacitor based on compacted bulk material shows an ultrahigh energy density of 25.8 Wh kg^-1 , which is much higher than those previously reported advanced materials. Notably, the bulk material also demonstrates long-term cycling stability, which can be maintained under ambient condition or immersed in H₂ SO₄ electrolyte for more than 120 days. Thus, this research improves the competitiveness of protein-based materials for real-world applications such as flexible electronics and solid-state supercapacitors.

Press-N-Go On-Skin Sensor with High Interfacial Toughness for Continuous Healthcare Monitoring

On-skin electronic sensors are demanded for healthcare monitoring such as the continuous recording of biopotential and motion signals from patients. However, the mechanical mismatches and poor interface adhesion at the skin/sensor interfaces always cause high interfacial impedance and artifacts, frequent interfacial failure, and unexpected depletion of the device, which significantly limit the performance of the sensors. We here develop an on-skin sensor based on a conductive pressure-sensitive tape, which is assembled from supramolecular dual-cross-linked hydrogel composites. Both covalent and noncovalent cross-links in the hydrogel networks could harvest high flexibility, pressure-sensitive adhesion, and high interfacial toughness altogether, enabling a convenient "Press-N-Go" application of the sensor on human skin without additional pre/post-treatment on the skin or the senor. The high conformability and low resistivity of the tape can sustainably lower the interfacial impedance and thus improve signal quality in various measurement conditions. Our design provides a feasible path to develop interface-toughened on-skin electronics, which is desired in dynamic human-machine interfaces.

A universal interface for plug-and-play assembly of stretchable devices

Stretchable hybrid devices have enabled high-fidelity implantable^1-3 and on-skin^4-6 monitoring of physiological signals. These devices typically contain soft modules that match the mechanical requirements in humans^7,8 and soft robots^9,10, rigid modules containing Si-based microelectronics^11,12 and protective encapsulation modules^13,14. To make such a system mechanically compliant, the interconnects between the modules need to tolerate stress concentration that may limit their stretching and ultimately cause debonding failure^15-17. Here, we report a universal interface that can reliably connect soft, rigid and encapsulation modules together to form robust and highly stretchable devices in a plug-and-play manner. The interface, consisting of interpenetrating polymer and metal nanostructures, connects modules by simply pressing without using pastes. Its formation is depicted by a biphasic network growth model. Soft-soft modules joined by this interface achieved 600% and 180% mechanical and electrical stretchability, respectively. Soft and rigid modules can also be electrically connected using the above interface. Encapsulation on soft modules with this interface is strongly adhesive with an interfacial toughness of 0.24 N mm^-1. As a proof of concept, we use this interface to assemble stretchable devices for in vivo neuromodulation and on-skin electromyography, with high signal quality and mechanical resistance. We expect such a plug-and-play interface to simplify and accelerate the development of on-skin and implantable stretchable devices.

Water-Modulated Biomimetic Hyper-Attribute-Gel Electronic Skin for Robotics and Skin-Attachable Wearables

Electronic skin (e-skin), mimicking the physical-chemical and sensory properties of human skin, is promising to be applied as robotic skins and skin-attachable wearables with multisensory functionalities. To date, most e-skins are dedicated to sensory function development to mimic human skins in one or several aspects, yet advanced e-skin covering all the hyper-attributes (including both the sensory and physical-chemical properties) of human skins is seldom reported. Herein, a water-modulated biomimetic hyper-attribute-gel (Hygel) e-skin with reversible gel-solid transition is proposed, which exhibits all the desired skin-like physical-chemical properties (stretchability, self-healing, biocompatibility, biodegradability, weak acidity, antibacterial activities, flame retardance, and temperature adaptivity), sensory properties (pressure, temperature, humidity, strain, and contact), function reconfigurability, and evolvability. Then the Hygel e-skin is applied as an on-robot e-skin and skin-attached wearable to demonstrate its highly skin-like attributes in capturing multiple sensory information, reconfiguring desired functions, and excellent skin compatibility for real-time gesture recognition via deep learning. This Hygel e-skin may find more applications in advanced robotics and even skin-replaceable artificial skin.

Assessment of a Bionic Broach Implanted with Nylon Fibers

The optimization of a broach surface is of great significance to improve the cutting performance of the tool. However, the traditional optimization method (surface texture, coating, etc.) destroys the stress distribution of the tool and reduces the service life of the tool. To avoid these problems, four kinds of flocking surfaces (FB1, FB2, FB3, and FB4), imitating the biological structure of Daphniphyllum calycinum Benth (DCB), were fabricated on the rake face of the broach by electrostatic flocking. The broaching experiment, wettability, and spreading experiment were then conducted. Moreover, the mathematical model of the friction coefficient of the bionic broach was built. The effect of broaches with different flocking surfaces on the broaching force, chip morphology, and surface quality of workpieces was studied. The results indicate that the flocked broaches (FB) with good lubricity and capacity of microchips removal (CMR) present a smaller cutting force (F_c) and positive pressure (F_t) compared to the unflocked broach (NB), and reduce the friction coefficient (COF). The chip curl was decreased, and the shear angle was increased by FB, which were attributed to the function of absorbing lubricant, storing, and sweeping microchips. Its vibration suppression effect enhanced the stability in the broaching process and improved the surface quality of the workpiece. More importantly, the FB2 with the most reasonable fluff area and spacing exhibited the best cutting performance. The experimental conclusions and methods of this paper can provide a new research idea for functional structure tools.

Histidine-Triggered GO Hybrid Hydrogels for Microfluidic 3D Printing

Graphene oxide (GO) hydrogels have provided tremendous opportunities in designing and fabricating complex constructs for diverse applications, while their 3D printing without photocuring is still a challenging task due to their low viscosity, uncontrollable gelation, and low interfacial tension. Here, we report a histidine-assisted printing strategy to prepare GO hybrid hydrogels through the microfluidic 3D printing technique. We found that the GO additive could significantly hamper the Knoevenagel condensation (KC) reaction between benzaldehyde and cyanoacetate group-functionalized polymers to form a hydrogel, while these GO mixed solutions were rapidly solidified into a hydrogel when histidine was added. This fascinating phenomenon enabled us to prepare low-viscosity GO mixed polymer solutions as printable inks and generate hydrogel microfibers in histidine solutions. The hydrogel fibers could support cell survival and be further constructed into complex 3D structures through microfluidic 3D printing techniques. Moreover, due to the addition of GO, the microfibers exhibited excellent electrical conductivity and could sense the motion changes and convert these stimuli as electrical resistance signals. This strategy adds an option for the design and application of 3D printable aqueous GO inks in many fields.

Outstanding Bio-Tribological Performance Induced by the Synergistic Effect of 2D Diamond Nanosheet Coating and Silk Fibroin

Due to their excellent biocompatibility, outstanding mechanical properties, high strength-to-weight ratio, and good corrosion resistance, titanium (Ti) alloys are extensively used as implant materials in artificial joints. However, Ti alloys suffer from poor wear resistance, resulting in a considerably short lifetime. In this study, we demonstrate that the chemical self-assembly of novel two-dimensional (2D) diamond nanosheet coatings on Ti alloys combined with natural silk fibroin used as a novel lubricating fluid synergistically results in excellent friction and wear performance. Linear-reciprocating sliding tests verify that the coefficient of friction and the wear rate of the diamond nanosheet coating under silk fibroin lubrication are reduced by 54 and 98%, respectively, compared to those of the uncoated Ti alloy under water lubrication. The lubricating mechanism of the newly designed system was revealed by a detailed analysis of the involved microstructural and chemical changes. The outstanding tribological behavior was attributed to the establishment of artificial joint lubrication induced by the cross binding between the diamond nanosheets and silk fibroin. Additionally, excellent biocompatibility of the lubricating system was verified by cell viability, which altogether paves the way for the application of diamond coatings in artificial Ti joint implants.

Development of 3D printable graphene oxide based bio-ink for cell support and tissue engineering

Tissue engineered constructs can serve as in vitro models for research and replacement of diseased or damaged tissue. As an emerging technology, 3D bioprinting enables tissue engineering through the ability to arrange biomaterials and cells in pre-ordered structures. Hydrogels, such as alginate (Alg), can be formulated as inks for 3D bioprinting. However, Alg has limited cell affinity and lacks the functional groups needed to promote cell growth. In contrast, graphene oxide (GO) can support numerous cell types and has been purported for use in regeneration of bone, neural and cardiac tissues. Here, GO was incorporated with 2% (w/w) Alg and 3% (w/w) gelatin (Gel) to improve 3D printability for extrusion-based 3D bioprinting at room temperature (RT; 25°C) and provide a 3D cellular support platform. GO was more uniformly distributed in the ink with our developed method over a wide concentration range (0.05%-0.5%, w/w) compared to previously reported GO containing bioink. Cell support was confirmed using adipose tissue derived stem cells (ADSCs) either seeded onto 3D printed GO scaffolds or encapsulated within the GO containing ink before direct 3D printing. Added GO was shown to improve cell-affinity of bioinert biomaterials by providing more bioactive moieties on the scaffold surface. 3D cell-laden or cell-seeded constructs showed improved cell viability compared to pristine (without GO) bio-ink-based scaffolds. Our findings support the application of GO for novel bio-ink formulation, with the potential to incorporate other natural and synthetic materials such as chitosan and cellulose for advanced in situ biosensing, drug-loading and release, and with the potential for electrical stimulation of cells to further augment cell function.

Coacervation-Based Method for Constructing a Multifunctional Strain-Stiffening Crystalline Polyvinylamine Hydrogel

Strain-stiffening hydrogels are essential in the development of ionic skin, as human skin possesses a strain-stiffening property for self-protection. Semicrystalline polymers such as poly(vinyl alcohol) (PVA) have been widely investigated to fabricate strain-stiffening hydrogels via freeze-thaw cycling or chemical cross-linking but with limited adjustable properties. Compared with PVA, polyvinylamine (PVAm) has a higher reactive activity, making it easier to achieve multifunctionalities including strain-stiffening in a PVAm hydrogel. However, the amine moieties in the backbone tend to be ionized and form strong ionic hydrogen bonds with water, resulting in difficulties in forming crystalline hydrogels by conventional methods. Herein, a one-pot method to induce crystallinity and achieve multifunctional hydrogel is devised via coacervation of PVAm. Different from a published coacervation method to fabricate hydrogels with various properties via noncovalent interactions between different chemicals, coacervation occurs between PVAm to form aggregated and loose PVAm in our devised system. Such a strategy lowers the amine-water binding energy in the polymer-dense phase to achieve crystallinity and subsequently the strain-stiffening property; meanwhile, self-healability, self-adhesion, and ionic conductivity can be realized in the polymer-loose phase. The obtained hydrogel integrates stretchability (∼1300% elongation), toughness (227 kPa), the strain-stiffening property (∼10 times increase), self-adhesion (90 J m^-2), self-healability (∼80% healing efficiency in toughness), and ionic conductivity (0.22 mS m^-1). This convenient strategy will open a new horizon to design multifunctional skin-mimic materials.

Crack-Based Core-Sheath Fiber Strain Sensors with an Ultralow Detection Limit and an Ultrawide Working Range

With the booming development of flexible wearable sensing devices, flexible stretchable strain sensors with crack structure and high sensitivity have been widely concerned. However, the narrow sensing range has been hindering the development of crack-based strain sensors. In addition, the existence of the crack structure may reduce the interface compatibility between the elastic matrix and the sensing material. Herein, to overcome these problems, integrated core-sheath fibers were prepared by coaxial wet spinning with partially added carbon nanotube sensing materials in thermoplastic polyurethane elastic materials. Due to the superior interface compatibility and the change in the conductive path during stretching, the fiber strain sensor exhibits excellent durability (5000 tensile cycles), high sensitivity (>10⁴), large stretchability (500%), a low detection limit (0.01%), and a fast response time of ∼60 ms. Based on these outstanding strain sensing performances, the fiber sensor is demonstrated to detect subtle strain changes (e.g., pulse wave and swallowing) and large strain changes (e.g., finger joint and wrist movement) in real time. Moreover, the fabric sensor woven with the core-sheath fibers has an excellent performance in wrist bending angle detection, and the smart gloves based on the fabric sensors also show exceptional recognition ability as a wireless sign language translation device. This integrated strategy may provide prospective opportunities to develop highly sensitive strain sensors with durable deformation and a wide detection range.

Biomimetic Microstructured Antifatigue Fracture Hydrogel Sensor for Human Motion Detection with Enhanced Sensing Sensitivity

Antifatigue fracture performance and high sensing sensitivity are key characteristics for hydrogel sensors used in flexible electronic applications. Herein, inspired by human muscle tissues and epidermal skin tissues, an effective and straightforward strategy is proposed to fabricate hydrogel sensors for detecting human motion with antifatigue fracture performance and high sensing sensitivity. The crystalline regions and orientation along the stretching direction of cellulose nanofiber@carbon nanotube nanohybrids in the hydrogels provide antifatigue fracture performance (the crack does not expand after 2000 stretching cycles, and the fatigue threshold was calculated to be 187 J/m2), which protects hydrogels from severe damage during long-term use. In addition, the microstructured surfaces of the hydrogels with a random height distribution increase the contact area and improve the response to weak stimuli, resulting in a sensing sensitivity of 1.11 kPa-1, 18 times higher than that of a flat hydrogel. This sensing sensitivity is higher than those of most of the hydrogel-based pressure sensors that have been reported earlier. By integrating antifatigue fracture performance and enhanced sensing sensitivity, biomimetic microstructured hydrogel sensors show great potential for use in future flexible electronic applications.

Highly Conformal Polymers for Ambulatory Electrophysiological Sensing

Stable ambulatory electrophysiological sensing is widely utilized for smart e-healthcare monitoring, clinical diagnosis of cardiovascular diseases, treatment of neurological diseases, and intelligent human-machine interaction. As the favorable signal interaction platform of electrophysiological sensing, the conformal property of on-skin electrodes is an extremely crucial factor that can affect the stability of long-term ambulatory electrophysiological sensing. From the perspective of materials, to realize conformal contact between electrodes and skin for stable sensing, highly conformal polymers are strongly demanding and attracting ever-growing attention. In this review, we focused on the recent progress of highly conformal polymers for ambulatory electrophysiological sensing, including their synthetic methods, conformal property, and potential applications. Specifically, three main types of highly conformal polymers for stable long-term electrophysiological signals monitoring were proposed, including nature silk fibroin based conformal polymers, marine mussels bio-inspired conformal polymers, and other conformal polymers such as zwitterionic polymers and polyacrylamide. Furthermore, the future challenges and opportunities of preparing highly conformal polymers for on-skin electrodes were also highlighted. This article is protected by copyright. All rights reserved.

Detachable Soft Actuators with Tunable Stiffness Based on Wire Jamming

The integration of variable stiffness materials and structures into soft robots is a popular trend, allowing soft robots to switch between soft and rigid states in different situations. This concept combines the advantages of rigid mechanisms and soft robots, resulting in not only excellent flexibility but also tunable stiffness for high load capacity and fast and precise operation. Here, a stiffness-tunable soft actuator based on wire/fiber jamming structure is proposed, where the fiber-reinforced soft actuator is responsible for the bending motion, and the jamming structure acts as a stiffness-tunable layer controlled by vacuum pressure. The primary design objective of this study is to fabricate a jamming structure with wide-range stiffness, universal adaptability and high dexterity. Thus, the behaviors of wire/fiber jamming structures with different layouts, materials and wire arrangements are analyzed, and a theoretical model is developed to predict the effect of geometric parameters. Experimental characterizations show that the stiffness can be significantly enhanced in the bending direction, while the stiffness is smaller in the torsion direction. Additionally, by integrating Velcro strips into the design, a quick and detachable scheme for the stiffness-tunable soft actuator is achieved. Application examples exhibit high load capacity and good shape adaptability.

Mxene reinforced supramolecular hydrogels with high strength, stretchability and reliable conductivity for sensitive strain sensors

Conductive hydrogels used as electronics have received much attention due to their great flexibility and stretchability. However, the fabrication of ideal conductive hydrogels fulfilling with excellent mechanical properties and outstanding sensitivity remains a great challenge until now. Moreover, high sensitivity and broad linearity range are pivotal for the feasibility and accuracy of hydrogel sensors. In this study, a conductive supramolecular hydrogel was engineered by directly mixing the aqueous dispersion of MXene with the precursor of N-acryloyl glycinamide (NAGA) monomer and then rapidly photo cross-linked by UV irradiation. The resultant PNAGA/MXene hydrogel-sensors exhibited high mechanical strength (4.8 MPa), great stretchability (630%), and excellent durability. The conductive hydrogel-based sensor presented excellent conductivity (17.3 S·m^-1 ) and a wide scope of linear dependence of sensitivity on strain (0-125%, gauge factor = 2.05). It displayed reliable detection of various motions, including repeated subtle movements and large strain. It was also showed good degradation in vitro and antifouling capability. This work may provide a simple and promising platform for engineering conductive supramolecular hydrogels with integrated high performance aiming for smart wearable electronics, electronic skin, soft robots, and human-machine interfacing. This article is protected by copyright. All rights reserved.

Stress Dissipation Encoded Silk Fibroin Electrode for the Athlete-Beneficial Silk Bioelectronics

The kinetic body motions have guided the core-shell fabrics of wearable bioelectronics to be elastoplastic. However, the polymeric electrodes follow the trade-off relationship between toughness and stretchability. To this end, the stress dissipation encoded silk fibroin electrode is proposed as the core electrode of wearable bioelectronics. Significantly, the high degree of intrinsic stress dissipation is realized via an amino acid crosslink. The canonical phenolic amino acid (i.e., tyrosine) of silk fibroin is engineered to bridge the secondary structures. A sufficient crosslink network is constructed when tyrosine is exposed near the amorphous strand. The stress dissipative tyrosine crosslink affords 12.5-fold increments of toughness (4.72 to 58.9 MJ m^-3 ) and implements the elastoplastic silk fibroin. The harmony of elastoplastic core electrodes with shell fabrics enables the wearable bioelectronics to employ mechanical performance (elastoplasticity of 750 MJ m^-3 ) and stable electrical response. The proposed wearable is capable of assisting the effective workouts via triboelectricity. In principle, active mobility with suggested wearables potentially relieves muscular fatigues and severe injuries during daily fitness.