Electroconductive, Adhesive, Non-Swelling, and Viscoelastic Hydrogels for Bioelectronics

Enhanced Electric Field via Multicomponent Synergistic Coordination on Single-Atom Catalysts for Antibiotic Mineralization in Water

The persistence of antibiotics in aquatic ecosystems poses significant long-term ecological and public health risks. Nevertheless, current remediation approaches face critical limitations, including high energy consumption and interference from dissolved organic carbon (DOC). To address these challenges, we developed an innovative N,O-doped graphene-like coordinated single-atom iron catalyst (FeSA-N,O-GO) using silkworm excrement as a sustainable precursor. The catalyst features robust Fe-π interactions between high-spin Fe atoms and the N,O-GO support, generating a substantial surface electric field (∼146 mV). This unique configuration enables FeSA-N,O-GO to achieve remarkable degradation efficiency for multiple antibiotics, including ciprofloxacin (CIP), tetracycline (TC), oxytetracycline (OTC), and norfloxacin (NFX), without requiring external peroxides or additional energy input. Complete antibiotic removal (100%) was attained within 90 min, accompanied by total organic carbon removal of 65.2-79.7%. Most importantly, the catalytic performance was further enhanced in DOC-containing real water samples, where synergistic coordination of DOC-antibiotics-H2O on FeSA-N,O-GO amplified the surface electric field effect. Our findings demonstrate the promising potential of harnessing electric energy generated through multicomponent coordination on single-atom catalysts for developing sustainable and cost-effective solutions for water micropollutant remediation.

Lattice Substitution Induced Fast Charge Transport in Integrated and Flexible Organic Photoelectrochemical Transistors for Portable Sensing

Timely and on-site biochemical monitoring is highly desired in healthcare systems and smart cities. However, known sensor technologies struggle to combine technologically relevant metrics of portability, sensitivity, and reliability due to their difficulties in integrating multiple functions (e.g., target recognition, signal amplification) into a single sensing platform. Here, an integrated and portable sensing device (named IP-OPECT) that combines photoelectrochemical sensor-gated organic electrochemical transistor units and hydrogel electrolyte designs is presented to address these challenges, enabling sensitive and on-site biochemical detection. Theoretical calculations reveal that elemental lattice substitution in the photoelectrode not only suppresses the recombination of photogenerated electron-hole pairs but also improves electron transfer kinetics, ultimately enhancing the optoelectronic properties. As a proof-of-concept, we demonstrate its use in detecting varying concentrations of ochratoxin A (OTA) from contaminated commercial coffee samples, achieving an accuracy of over 96.7%. Additionally, the intrinsically flexible hydrogel electrolyte covering the electrode surface ensures long-term operation of devices, with stable photocurrent signals under different bending angles (0-180°) and over 1000 bending cycles. These results further underscore the sensory capabilities and portability of as-prepared IP-OPECT sensing devices for OTA detection as well as other biochemicals requiring real-time and on-site monitoring within the IoT ecosystem.

Solvent Polarity-Induced Ultrahigh Strength Supramolecular Polyzwitterionic Organogels with Impact-Stiffening, Damping, and Anti-Freezing Properties

Widely used polyzwitterionic hydrogels usually suffer from significant mechanical loss, owing to the strong hydration of the zwitterionic groups. Herein, a novel solvent polarity-induced strategy is introduced for developing pure supramolecular polyzwitterionic organogels with ultrahigh strength by using a facile one-pot synthesis process. The mechanical properties of these polyzwitterionic organogels can be well-tuned by adjusting the polarity of dihydric alcohol solvents to regulate hydrogen bonding and dipole-dipole interactions between polymer chains in the organogel network. The polyzwitterionic organogels exhibit superior mechanical properties, including a tensile strength of 1.5 MPa, elongation at break of 669%, toughness of 3.2 MJ m- 3, and adhesive strength of 506 kPa. Additionally, the polyzwitterionic organogels display outstanding impact response performance (maximum strain-stiffening ratio of 140 times, and maximum impact-stiffening ratio of 450 times) and energy dissipation properties (energy dissipation ratio of above 60%, and maximum loss factor of 2.0 at 1 Hz), resulting from the presence of inter-molecular internal friction. Notably, the synergistic interactions between zwitterionic groups on polymer side chains and the organic solvents impart these organogels with mechanical flexibility and vibration absorption capabilities even in low-temperature environments. Furthermore, the polyzwitterionic organogels demonstrate flaw-insensitivity, self-healing ability, and water processability, broadening their applicability to more complex conditions.

Recent Progress of Soft and Bioactive Materials in Flexible Bioelectronics

Materials that establish functional, stable interfaces to targeted tissues for long-term monitoring/stimulation equipped with diagnostic/therapeutic capabilities represent breakthroughs in biomedical research and clinical medicine. A fundamental challenge is the mechanical and chemical mismatch between tissues and implants that ultimately results in device failure for corrosion by biofluids and associated foreign body response. Of particular interest is in the development of bioactive materials at the level of chemistry and mechanics for high-performance, minimally invasive function, simultaneously with tissue-like compliance and in vivo biocompatibility. This review summarizes the most recent progress for these purposes, with an emphasis on material properties such as foreign body response, on integration schemes with biological tissues, and on their use as bioelectronic platforms. The article begins with an overview of emerging classes of material platforms for bio-integration with proven utility in live animal models, as high performance and stable interfaces with different form factors. Subsequent sections review various classes of flexible, soft tissue-like materials, ranging from self-healing hydrogel/elastomer to bio-adhesive composites and to bioactive materials. Additional discussions highlight examples of active bioelectronic systems that support electrophysiological mapping, stimulation, and drug delivery as treatments of related diseases, at spatiotemporal resolutions that span from the cellular level to organ-scale dimension. Envisioned applications involve advanced implants for brain, cardiac, and other organ systems, with capabilities of bioactive materials that offer stability for human subjects and live animal models. Results will inspire continuing advancements in functions and benign interfaces to biological systems, thus yielding therapy and diagnostics for human healthcare.

Advances in Electrically Conductive Hydrogels: Performance and Applications

Electrically conductive hydrogels are highly hydrated 3D networks consisting of a hydrophilic polymer skeleton and electrically conductive materials. Conductive hydrogels have excellent mechanical and electrical properties and have further extensive application prospects in biomedical treatment and other fields. Whereas numerous electrically conductive hydrogels have been fabricated, a set of general principles, that can rationally guide the synthesis of conductive hydrogels using different substances and fabrication methods for various application scenarios, remain a central demand of electrically conductive hydrogels. This paper systematically summarizes the processing, performances, and applications of conductive hydrogels, and discusses the challenges and opportunities in this field. In view of the shortcomings of conductive hydrogels in high electrical conductivity, matchable mechanical properties, as well as integrated devices and machines, it is proposed to synergistically design and process conductive hydrogels with applications in complex surroundings. It is believed that this will present a fresh perspective for the research and development of conductive hydrogels, and further expand the application of conductive hydrogels.

Developing tough, fatigue-resistant and conductive hydrogels via in situ growth of metal dendrites

Developing hydrogels with high conductivity and toughness via a facile strategy is important yet challenging. Herein, we proposed a new strategy to develop conductive hydrogels by growing metal dendrites. Water-soluble Sn2+ ions were soaked into the gel and then converted to Sn dendrites via an electrochemical reaction; the excessive Sn2+ ions were finally removed by water dialysis, accompanied by dramatic shrinkage of the gel. Based on in situ transformation from metal ions to dendrites, the method integrated the advantages of ionic conductive fillers, such as LiCl (uniform dispersion), and electrical fillers, such as metal particles (high conductivity). Additionally, the morphology of metal dendrites combined advantages of 1D nanowires (large aspect ratio of the branches) and 2D nanosheets (large specific surface area of the skeleton). The strategy was found to be effective across diverse gel systems (non-ionic, anionic, cationic and zwitterionic). The dense, highly conductive and branched Sn dendrites not only formed a conductive pathway but also interacted with the polymer network to transfer stress and dissipate energy. The resultant gel exhibited a high conductivity of 12.5 S m-1, fracture energy of 1334.0 J m-2, and fatigue threshold of 720 J m-2. Additionally, the gel exhibited excellent sensitivity when used as a wearable strain sensor and bioelectrode. We believe this strategy offers new insights into the development of conductive hydrogels.

Mechanically Compatible Sealing of Hydrogel with Coherent Interface

Long-term operation of hydrogels relies on protective coatings to avoid water swelling or evaporation, but these protections often cause substantial decreases in overall softness and stretchability. Here, a mechanically compatible seal with a coherent interfacial design is developed to encapsulate hydrogels. This seal is made from polybutylene (PIB) and polypropylene-graft-maleic anhydride (PP-g-MAH) blended poly(styrene-isobutylene-styrene) (SIBS). The PIB oligomers soften the SIBS networks, while the MAH groups facilitate covalent bonding between the SIBS and hydrogel. The sealed hydrogel exhibits an elastic modulus of 24 kPa and an elongation at a break of >1000%, both comparable to those of the pristine hydrogel. The adhesion energy between the seal and hydrogel reached >140 J m-2 and can be further increased to >400 J m-2 by a thermal treatment. This tough interface, together with the intrinsically low water vapor transmission rate of SIBS, allows the sealed hydrogel to maintain its modulus and stretchability after 10 days of drying in air. The sealed hydrogel is chemically and mechanically stable under harsh conditions, including acidic/alkaline/salty solutions, high temperatures, and cyclic mechanical deformation. This strategy applies to various hydrogels with diverse compositions and structures, leading to orders of magnitude improvements in the longevity of hydrogel-based electronic devices.

Polydopamine-Mediated Nanofillers Reinforced Zwitterion Hydrogel Electrodes for Supercapacitors in Bioelectronics

Supercapacitors that can function when in direct contact with human tissue are of paramount importance for wearable bioelectronics but face mismatching with biological tissue and its movement. Herein, we developed a zwitterion hydrogel elastomer electrode-based all-hydrogel supercapacitor (AHSC) characterized by good energy storage properties, bioadhesion, body movement-matching mechanical properties, and biocompatibility. These functions were realized by integrating a [2-(methacryloyloxy)ethyl]dimethyl-(3-propylsulfonate)ammonium hydroxide (DMAPS) and hydroxyethyl acrylate (HEA)-copolymerized zwitterion hydrogel electrode (DMAPS-HEA) with redox-active nanofillers. This hydrogel electrode endowed AHSC with body movement-matching mechanical properties and biocompatibility. Redox-active nanofillers were designed with the structure of a reduced graphene oxide (rGO)-anchored cobalt/nickel bimetallic metal-organic framework (Co/Ni MOF) using polydopamine (PDA). The Co/Ni MOF contributes to the high energy storage performance. rGO enhances the conductivity, whereas PDA introduces catechol groups, contributing to the bioadhesion. This AHSC serves as a flexible alternative to traditional rigid and low-tissue-affinity power supply devices in bioelectronics.

Stretchable Polymer Hydrogels Based Flexible Triboelectric Nanogenerators for Self-Powered Bioelectronics

The rapid development of flexible electronics has led to unprecedented social and economic improvements. But conventional power devices cannot adapt to the advances of flexible electronics. Triboelectric nanogenerators (TENGs) have been used as robust power sources to transform ambient mechanical energy into electricity, thus meeting the power requirements of flexible electronics. Hydrogels are widely used for soft bioelectronics owing to the decent stretchability and biocompatibility. This Review presents the recent progress in the use of hydrogels for TENGs and self-powered hydrogel bioelectronics, including hydrogel synthesis, hydrogel TENGs fabrication, and their applications in wearable electricity generation, self-powered active sensing, and therapeutics. Hydrogel-enabled TENGs are emerging as a novel form of soft bioelectronics. We provided a critical analysis of hydrogel TENGs and insights into future opportunities and directions of this rapidly evolving field. These advancements will push the boundaries of hydrogel bioelectronics and contribute to the development of personalized healthcare solutions.

Acid-Induced in Situ Phase Separation and Percolation for Constructing Bi-Continuous Phase Hydrogel Electrodes With Motion-Insensitive Property

Conducting polymer hydrogels have gained attention in the bioelectronics field due to their unique combination of biocompatibility and customizable mechanical properties. However, achieving both excellent conductivity and mechanical strength in a hydrogel remains a significant challenge, primarily because of the inherent conflict between the hydrophobic nature of conducting polymers and the hydrophilic characteristics of hydrogels. To address this issue, this work proposes a simple one-step acid-induced approach that not only promotes the gelation of hydrophilic polymers but also facilitates the in situ phase separation of hydrophobic conducting polymers under mild conditions. This results in a distinctive bi-continuous phase structure with exceptional electrical property (906 mS cm-1) and mechanical performance (fracture strain of 1103%). The hydrogel forms robust percolating networks that maintain structural integrity under mechanical stress due to their entropic elasticity, providing remarkable strain insensitivity, low mechanical hysteresis, and an impressive resilience (95%). Electrodes fabricated from the conductive hydrogel exhibit stable and minimal interfacial contact impedance with skin (1-6 kilohms at 1-100 Hz) and significantly lower noise power (4.9 µV2). This work believes that the motion-insensitive characteristics and mechanical robustness of this hydrogel will enable efficient and reliable monitoring of biological signals, establishing a new benchmark in the bioelectronics.

Skin-Inspired, Multifunctional, and 3D-Printable Flexible Sensor Based on Triple-Responsive Hydrogel for Signal Conversion in Skin Interface Electronics Health Management

Hydrogel-based flexible electronic components have become the optimal solution to address the rigidity problem of traditional electronics in health management. In this study, a multipurpose hydrogel is introduced, which is formed by combining a dual-network consisting of physical (chitosan, polyvinyl alcohol (PVA)) and chemical (poly(isopropyl acrylamide (NIPAM)-co-acrylamide (AM))) cross-linking, along with signal conversion fillers (eutectic gallium indium (EGaIn), Ti3C2 MXene, polyaniline (PANI)) for responding to external stimuli. Multiple sensing of dynamic and static signals is permissible for it. The strain sensor based on the hydrogel exhibits up to a 1000% resistance change within a 400% stretch range, and significant capacitance variations are observed upon touch. The temperature sensor yields a sensitivity of ≈-2.9% °C-1 at 20-40 °C and ≈65% °C-1 at 0-20 °C. The pH sensor responds with a sensitivity of near -13.68 mV pH-1. A paper-based triboelectric nanogenerator can be assembled to collect action energy at 83 mW m-2. The skin contact interface is kept in good condition owing to its 3D-printability, controllable antibacterial properties, along high cell survival rate. This multifunctional hydrogel holds promise in facilitating the integration of diagnosis and maintenance.

Confined Water Dynamics in Topological Networks Hydrogel for Aqueous Electrochemical Devices

The unique properties of confined water molecules within polymer networks have garnered extensive research interest in energy storage, catalysis, and sensing. Confined water molecules exhibit higher thermodynamic stability compared to free water, which reduces decomposition and evaporation of water in hydrogel electrolyte system. Herein, a facile strategy is developed to limit active water molecules in a hydrogel network via hydrogen bonding within a topological network. The design of this gel enhances hydrogen bonding between the gel network and water molecules, thereby improving stability by constructing interpenetrating networks. Using this design, the topological network gel is selected as the electrolyte for batteries, demonstrating an extended electrochemical window from 2.37 V with polyvinyl alcohol gel to 2.96 V, indicating superior confinement of water molecules by hydrogen bonds in the topological network. Additionally, batteries and capacitors assembled with the topological gel exhibit high-capacity retention rates of 94.25% after 20 000 cycles at a current density of 1.0 A g-1 and 87.63% after 10 000 cycles at a current density of 0.5 A g-1, respectively. This study demonstrates the feasibility of using a topological gel design to enhance gel electrolyte stability, offering a promising avenue for future research in regulating topological networks within gels for various applications.

Lessons from Nature: Advances and Perspectives in Bionic Microwave Absorption Materials

Inspired by the remarkable electromagnetic response capabilities of the complex morphologies and subtle microstructures evolved by natural organisms, this paper delves into the research advancements and future application potential of bionic microwave-absorbing materials (BMAMs). It outlines the significance of achieving high-performance microwave-absorbing materials through ingenious microstructural design and judicious composition selection, while emphasizing the innovative strategies offered by bionic manufacturing. Furthermore, this work meticulously analyzes how inspiration can be drawn from the intricate structures of marine organisms, plants, animals, and non-metallic minerals in nature to devise and develop BMAMs with superior electromagnetic wave absorption properties. Additionally, the paper provides an in-depth exploration of the theoretical underpinnings of BMAMs, particularly the latest breakthroughs in broadband absorption. By incorporating advanced methodologies such as simulation modeling and bionic gradient design, we unravel the scientific principles governing the microwave absorption mechanisms of BMAMs, thereby furnishing a solid theoretical foundation for understanding and optimizing their performance. Ultimately, this review aims to offer valuable insights and inspiration to researchers in related fields, fostering the collective advancement of research on BMAMs.

Functional Hydrogels for Aqueous Zinc-Based Batteries: Progress and Perspectives

Aqueous zinc batteries (AZBs) hold great potential for green grid-scale energy storage due to their affordability, resource abundance, safety, and environmental friendliness. However, their practical deployment is hindered by challenges related to the electrode, electrolyte, and interface. Functional hydrogels offer a promising solution to address such challenges owing to their broad electrochemical window, tunable structures, and pressure-responsive mechanical properties. In this review, the key properties that functional hydrogels must possess for advancing AZBs, including mechanical strength, ionic conductivity, swelling behavior, and degradability, from a perspective of the full life cycle of hydrogels in AZBs are summarized. Current modification strategies aimed at enhancing these properties and improving AZB performance are also explored. The challenges and design considerations for integrating functional hydrogels with electrodes and interface are discussed. In the end, the limitations and future directions for hydrogels to bridge the gap between academia and industries for the successful deployment of AZBs are discussed.

Design Strategy, On-Demand Control, and Biomedical Engineering Applications of Wet Adhesion

The adhesion of tissues to external devices is fundamental to numerous critical applications in biomedical engineering, including tissue and organ repair, bioelectronic interfaces, adhesive robotics, wearable electronics, biomedical sensing and actuation, as well as medical monitoring, treatment, and healthcare. A key challenge in this context is that tissues are typically situated in aqueous and dynamic environments, which poses a bottleneck to further advancements in these fields. Wet adhesion technology (WAT) presents an effective solution to this issue. In this review, we summarize the three major design strategies and control methods of wet adhesion, comprehensively and systematically introducing the latest applications and advancements of WAT in the field of biomedical engineering. First, single adhesion mechanism under the frameworks of the three design strategies is systematically introduced. Second, control methods for adhesion are comprehensively summarized, including spatiotemporal control, detachment control, and reversible adhesion control. Third, a systematic summary and discussion of the latest applications of WAT in biomedical engineering research and education were presented, with a particular focus on innovative applications such as tissue-electronic interface devices, ingestible devices, end-effector components, in vivo medical microrobots, and medical instruments and equipment. Finally, opportunities and challenges encountered in the design and development of wet adhesives with advanced adhesive performance and application prospects are discussed.

Engineering robust and transparent dual-crosslinked hydrogels for multimodal sensing without conductive additives

Conductive hydrogels are pivotal for the advancement of flexible sensors, electronic skin, and healthcare monitoring systems, facilitating transformative innovations. However, issues such as inadequate intrinsic compatibility, mismatched mechanical properties, and limited stability curtail their potential, resulting in compromised device efficacy and performance degradation. In this research, we engineered functional hydrogels featuring a dual-crosslinked network composed of (PA/PVA)-P(AM-AA) to address these challenges. This design eliminates the need for conductive additives, thereby enhancing intrinsic compatibility. Notably, the hydrogels exhibit exceptional mechanical properties, with high tensile strength (∼700 %), Young's modulus (∼5.33 MPa), increased strength (∼2.46 MPa) and toughness (∼6.59 MJ m-3). They also achieve a compressive strength of ∼7.33 MPa at 80 % maximal compressive strain and maintain about 89 % transparency. Moreover, flexible sensors derived from these hydrogels demonstrate enhanced multimodal sensing capabilities, including temperature, strain, and pressure detection, enabling precise monitoring of human movements. The integration of multiple hydrogels into a three-dimensional sensor array facilitates detailed spatial pressure distribution mapping. By strategically applying dual-crosslinked network engineering and eliminating conductive additives, we have streamlined the design and manufacturing of hydrogels to meet the rising demand for high-performance wearable sensors.

Super-Tough, Non-Swelling Zwitterionic Hydrogel Sensor Based on the Hofmeister Effect for Potential Motion Monitoring of Marine Animals

Hydrogel-based electronic devices in aquatic environments have sparked widespread research interest. Nevertheless, the challenge of developing hydrogel electronics underwater has not been profoundly surmounted because of the fragility and swelling of hydrogels in aquatic environments. In this work, a zwitterionic double network hydrogel comprised of polyvinyl alcohol (PVA), poly(sulfobetaine methacrylate) (PSBMA), and sulfuric acid (H2SO4) demonstrates super-tough and non-swelling performance. The Hofmeister effect of H2SO4 and PSBMA induces the PVA chains to form numerous nanocrystalline domains, which serve as the primary physical crosslinking points and provide effective energy dissipation. H2SO4 induces a strong salting-out effect to facilitate PVA crystallization and the formation of a dense and stable network structure that inhibits swelling. The resulting hydrogel exhibits an ultra-high toughness of 4.61 MJ m-3, non-swelling, and long-term stability for up to a month in pure water and seawater. Based on this, a hydrogel-based seawater strain sensor has been developed to monitor the underwater movements of marine animal models. Reliable and stable sensing performance ensures real-time collection of underwater motion signals, despite the impacts of water flow and the interference of ions. This study provides a facile approach to designing super-tough and non-swelling hydrogels and further expands the application of underwater electronic devices.

Highly Sensitive, Degradable, and Rapid Self-Healing Hydrogel Sensor with Semi-Interpenetrating Network for Recognition of Micro-Expressions

Flexible conductive hydrogels have revolutionized the lives and are widely applied in health monitoring and wearable electronics as a new generation of sensing materials. However, the inherent low mechanical strength, sensitivity, and lack of rapid self-healing capacity results in their short life, poor detection accuracy, and environmental pollution. Inspired by the molecular structure of bone and its chemical characteristics, a novel fully physically cross-linked conductive hydrogel is fabricated by the introduction of nanohydroxyapatite (HAp) as the dynamic junction points. In detail, the dynamically cross-linked network, including multiple physical interactions, provides it with rapid self-healing ability and excellent mechanical properties (elongation at break (>1200%), tensile strength (174kPa), and resilience (92.61%)). Besides, the ions (Cl-, Li+, Ca2+) that move freely within the system impart outstanding electrical conductivity (2.46 ± 0.15 S m-1), high sensitivity (gauge factor, GF>8), good antifreeze (-40.2 °C), and humidity properties. The assembled sensor can be employed to sensitively detect various large human motions and subtle changes in behavior (facial expressions, speech recognition). Meanwhile, the hydrogel sensor can also degrade in phosphate-buffered saline solution without causing any environmental pollution. Therefore, the designed hydrogels may become a promising candidate material in the future potential applications for smart wearable sensors and electronic skin.

Flexible Artificial Tactility with Excellent Robustness and Temperature Tolerance Based on Organohydrogel Sensor Array for Robot Motion Detection and Object Shape Recognition

Hydrogel-based flexible artificial tactility is equipped to intelligent robots to mimic human mechanosensory perception. However, it remains a great challenge for hydrogel sensors to maintain flexibility and sensory performances during cyclic loadings at high or low temperatures due to water loss or freezing. Here, a flexible robot tactility is developed with high robustness based on organohydrogel sensor arrays with negligent hysteresis and temperature tolerance. Conductive polyaniline chains are interpenetrated through a poly(acrylamide-co-acrylic acid) network with glycerin/water mixture with interchain electrostatic interactions and hydrogen bonds, yielding a high dissipated energy of 1.58 MJ m-3, and ultralow hysteresis during 1000 cyclic loadings. Moreover, the binary solvent provides the gels with outstanding tolerance from -100 to 60 °C and the organohydrogel sensors remain flexible, fatigue resistant, conductive (0.27 S m-1), highly strain sensitive (GF of 3.88) and pressure sensitive (35.8 MPa-1). The organohydrogel sensor arrays are equipped on manipulator finger dorsa and pads to simultaneously monitor the finger motions and detect the pressure distribution exerted by grasped objects. A machine learning model is used to train the system to recognize the shape of grasped objects with 100% accuracy. The flexible robot tactility based on organohydrogels is promising for novel intelligent robots.

Neural interfaces: Bridging the brain to the world beyond healthcare

Neural interfaces, emerging at the intersection of neurotechnology and urban planning, promise to transform how we interact with our surroundings and communicate. By recording and decoding neural signals, these interfaces facilitate direct connections between the brain and external devices, enabling seamless information exchange and shared experiences. Nevertheless, their development is challenged by complexities in materials science, electrochemistry, and algorithmic design. Electrophysiological crosstalk and the mismatch between electrode rigidity and tissue flexibility further complicate signal fidelity and biocompatibility. Recent closed-loop brain-computer interfaces, while promising for mood regulation and cognitive enhancement, are limited by decoding accuracy and the adaptability of user interfaces. This perspective outlines these challenges and discusses the progress in neural interfaces, contrasting non-invasive and invasive approaches, and explores the dynamics between stimulation and direct interfacing. Emphasis is placed on applications beyond healthcare, highlighting the need for implantable interfaces with high-resolution recording and stimulation capabilities.

Extracellular Matrix-Surrogate Advanced Functional Composite Biomaterials for Tissue Repair and Regeneration

Native tissues, comprising multiple cell types and extracellular matrix components, are inherently composites. Mimicking the intricate structure, functionality, and dynamic properties of native composite tissues represents a significant frontier in biomaterials science and tissue engineering research. Biomimetic composite biomaterials combine the benefits of different components, such as polymers, ceramics, metals, and biomolecules, to create tissue-template materials that closely simulate the structure and functionality of native tissues. While the design of composite biomaterials and their in vitro testing are frequently reviewed, there is a considerable gap in whole animal studies that provides insight into the progress toward clinical translation. Herein, we provide an insightful critical review of advanced composite biomaterials applicable in several tissues. The incorporation of bioactive cues and signaling molecules into composite biomaterials to mimic the native microenvironment is discussed. Strategies for the spatiotemporal release of growth factors, cytokines, and extracellular matrix proteins are elucidated, highlighting their role in guiding cellular behavior, promoting tissue regeneration, and modulating immune responses. Advanced composite biomaterials design challenges, such as achieving optimal mechanical properties, improving long-term stability, and integrating multifunctionality into composite biomaterials and future directions, are discussed. We believe that this manuscript provides the reader with a timely perspective on composite biomaterials.

High-Strength, Antiswelling Directional Layered PVA/MXene Hydrogel for Wearable Devices and Underwater Sensing

Hydrogel sensors are widely utilized in soft robotics and tissue engineering due to their excellent mechanical properties and biocompatibility. However, in high-water environments, traditional hydrogels can experience significant swelling, leading to decreased mechanical and electrical performance, potentially losing shape, and sensing capabilities. This study addresses these challenges by leveraging the Hofmeister effect, coupled with directional freezing and salting-out techniques, to develop a layered, high-strength, tough, and antiswelling PVA/MXene hydrogel. In particular, the salting-out process enhances the self-entanglement of PVA, resulting in an S-PM hydrogel with a tensile strength of up to 2.87 MPa. Furthermore, the S-PM hydrogel retains its structure and strength after 7 d of swelling, with only a 6% change in resistance. Importantly, its sensing performance is improved postswelling, a capability rarely achievable in traditional hydrogels. Moreover, the S-PM hydrogel demonstrates faster response times and more stable resistance change rates in underwater tests, making it crucial for long-term continuous monitoring in challenging aquatic environments, ensuring sustained operation and monitoring.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Carbon Quantum Dot-Functionalized Dermis-Derived Transparent Electronic Skin for Multimodal Human Motion Signal Monitoring and Construction of Self-Powered Triboelectric Nanogenerator

Electronic skin (e-skin) is considered as a highly promising interface for human-computer interaction systems and wearable electronic devices. Through elaborate design and assembly of various materials, it possesses multiple characteristics similar to human skin, including remarkable flexibility, stretchability, sensitivity to temperature and humidity, biocompatibility, and efficient interfacial ion/electron transport capabilities. Here, we innovatively integrate multifunctional carbon quantum dots (CQDs), which exhibit conductivity, antibacterial properties, ultraviolet absorption, and fluorescence emission, with poly(acrylic acid) and glycerin (Gly) into a three-dimensional network structure of natural goatskin collagen fibers. Through a top-down design strategy enhanced by hydrogen bond reconstruction, we successfully fabricated a novel transparent e-skin (PAC-eSkin). This e-skin exhibited significant tensile properties (4.94 MPa of tensile strength and 263.42% of a maximum breaking elongation), while also possessing Young's modulus similar to human skin (2.32 MPa). It is noteworthy that the functionalized CQDs used was derived from discarded goat hair, and the addition of Gly gave PAC-eSkin excellent antifreezing and moisturizing properties. Due to the presence of ultrasmall CQDs, which creates efficient ion/electron transport channels within PAC-eSkin, it could rapidly sense human motion and physiological signals (with a gauge factor (GF) of 1.88). Furthermore, PAC-eSkin had the potential to replace traditional electrode patches for real-time monitoring of electrocardiogram, electromyogram, and electrooculogram signals, with a higher SNR (signal-to-noise ratio) of 25.1 dB. Additionally, the customizable size and shape of PAC-eSkin offer vast possibilities for the construction of single-electrode triboelectric nanogenerator systems. We have reason to believe that the design and development of this transparent e-skin based on CQDs-functionalized dermal collagen matrices can pave a new way for innovations in human-computer interaction interfaces and their sensing application in diverse scenarios.

Multifunctional Conductive Hydrogel Interface for Bioelectronic Recording and Stimulation

The past few decades have witnessed the rapid advancement and broad applications of flexible bioelectronics, in wearable and implantable electronics, brain-computer interfaces, neural science and technology, clinical diagnosis, treatment, etc. It is noteworthy that soft and elastic conductive hydrogels, owing to their multiple similarities with biological tissues in terms of mechanics, electronics, water-rich, and biological functions, have successfully bridged the gap between rigid electronics and soft biology. Multifunctional hydrogel bioelectronics, emerging as a new generation of promising material candidates, have authentically established highly compatible and reliable, high-quality bioelectronic interfaces, particularly in bioelectronic recording and stimulation. This review summarizes the material basis and design principles involved in constructing hydrogel bioelectronic interfaces, and systematically discusses the fundamental mechanism and unique advantages in bioelectrical interfacing with the biological surface. Furthermore, an overview of the state-of-the-art manufacturing strategies for hydrogel bioelectronic interfaces with enhanced biocompatibility and integration with the biological system is presented. This review finally exemplifies the unprecedented advancement and impetus toward bioelectronic recording and stimulation, especially in implantable and integrated hydrogel bioelectronic systems, and concludes with a perspective expectation for hydrogel bioelectronics in clinical and biomedical applications.

Annular Conductive Hydrogel-Mediated Wireless Electrical Stimulation for Augmenting Neurogenesis

Electrical stimulation (ES) has a remarkable capacity to regulate neuronal differentiation and neurogenesis in the treatment of various neurological diseases. However, wired devices connected to the stimulating electrode and the mechanical mismatch between conventional rigid electrodes and soft tissues restrict their motion and cause possible infections, thereby limiting their clinical utility. An approach integrating the advantages of wireless techniques and soft hydrogels provides new insights into ES-induced nerve regeneration. Herein, a flexible and implantable wireless ES-responsive electrode based on an annular gelatin methacrylate-polyaniline (Gel/Pani) hydrogel is fabricated and used as a secondary coil to achieve wireless ES via electromagnetic induction in the presence of a primary coil. The Gel/Pani hydrogels exhibit favorable biocompatibility, biodegradability, conductivity, and compression resistance. The annular electrode of the Gel/Pani conductive hydrogel (AECH) supports neural stem cell growth, while the applied wireless ES facilitates neuronal differentiation and the formation of functional neural networks in vitro. Furthermore, AECH is implanted in vivo in rats with ischemic stroke and the results reveal that AECH-mediated wireless ES significantly ameliorates brain impairment and neurological function by activating endogenous neurogenesis. This novel flexible hydrogel system addresses wireless stimulation and implantable technical challenges, holding great potential for the treatment of neurodegenerative diseases.

A comprehensive exploration of hydrogel applications in multi-stage skin wound healing

Hydrogels, as an emerging biomaterial, have found extensive use in the healing of wounds due to their distinctive physicochemical structure and functional properties. Moreover, hydrogels can be made to match a range of therapeutic requirements for materials used in wound healing through specific functional modifications. This review provides a step-by-step explanation of the processes involved in cutaneous wound healing, including hemostasis, inflammation, proliferation, and reconstitution, along with an investigation of the factors that impact these processes. Furthermore, a thorough analysis is conducted on the various stages of the wound healing process at which functional hydrogels are implemented, including hemostasis, anti-infection measures, encouraging regeneration, scar reduction, and wound monitoring. Next, the latest progress of multifunctional hydrogels for wound healing and the methods to achieve these functions are discussed in depth and categorized for elucidation. Finally, perspectives and challenges associated with the clinical applications of multifunctional hydrogels are discussed.

Degradable and Recyclable Hydrogels for Sustainable Bioelectronics

Hydrogel bioelectronics has been widely used in wearable sensors, electronic skin, human-machine interfaces, and implantable tissue-electrode interfaces, providing great convenience for human health, safety, and education. The generation of electronic waste from bioelectronic devices jeopardizes human health and the natural environment. The development of degradable and recyclable hydrogels is recognized as a paradigm for realizing the next generation of environmentally friendly and sustainable bioelectronics. This review first summarizes the wide range of applications for bioelectronics, including wearable and implantable devices. Then, the employment of natural and synthetic polymers in hydrogel bioelectronics is discussed in terms of degradability and recyclability. Finally, this work provides constructive thoughts and perspectives on the current challenges toward hydrogel bioelectronics, providing valuable insights and guidance for the future evolution of sustainable hydrogel bioelectronics.

Bioelectronic Neural Interfaces: Improving Neuromodulation Through Organic Conductive Coatings

Integration of bioelectronic devices in clinical practice is expanding rapidly, focusing on conditions ranging from sensory to neurological and mental health disorders. While platinum (Pt) electrodes in neuromodulation devices such as cochlear implants and deep brain stimulators have shown promising results, challenges still affect their long-term performance. Key among these are electrode and device longevity in vivo, and formation of encapsulating fibrous tissue. To overcome these challenges, organic conductors with unique chemical and physical properties are being explored. They hold great promise as coatings for neural interfaces, offering more rapid regulatory pathways and clinical implementation than standalone bioelectronics. This study provides a comprehensive review of the potential benefits of organic coatings in neuromodulation electrodes and the challenges that limit their effective integration into existing devices. It discusses issues related to metallic electrode use and introduces physical, electrical, and biological properties of organic coatings applied in neuromodulation. Furthermore, previously reported challenges related to organic coating stability, durability, manufacturing, and biocompatibility are thoroughly reviewed and proposed coating adhesion mechanisms are summarized. Understanding organic coating properties, modifications, and current challenges of organic coatings in clinical and industrial settings is expected to provide valuable insights for their future development and integration into organic bioelectronics.

The Unprecedented Biodegradable Polyzwitterion: A Removal-Free Patch for Accelerating Infected Diabetic Wound Healing

Zwitterionic polymers have emerged as an important class of biomaterials to construct wound dressings and antifouling coatings over the past decade due to their excellent hydrophilicity. However, all the reported zwitterionic polymers as wound dressings are nondegradable because of noncleavable carbon─carbon bonding backbones, and must be removed periodically after treatment to avoid hypoxia in the wound, thus leading to potential secondary injury. In this work, a biodegradable polyzwitterion patch is fabricated for the first time by ring-opening polymerization of carboxybetaine dithiolane (CBDS), which is self-crosslinked via inter-amide hydrogen bonds and zwitterionic dipole-dipole interactions on the side chains. The unprecedented polyCBDS (PCBDS) patch demonstrates enough ductility owing to the intermolecular physical interactions to fully cover irregular wounds, also showing excellent biodegradability and antifouling performance resulted from the existence of disulfide bonds and carboxybetaine groups. Besides, the PCBDS degradation-induced released CBDS owns potent antioxidant and antibacterial activities. This PCBDS patch is used as a diabetic wound dressing, inhibiting bacterial adhesion on the external surface, and its degradation products can exactly kill bacteria and scavenge excessive reactive oxygen species (ROS) at the wound site to regulate local microenvironment, including regulation of cytokine express and macrophage polarization, accelerating infected diabetic wound repair, and also avoiding the potential secondary injury.

Low hysteresis zwitterionic supramolecular polymer ion-conductive elastomers with anti-freezing properties, high stretchability, and self-adhesion for flexible electronic devices

The fabrication of stretchable ionic conductors with low hysteresis and anti-freezing properties to enhance the durability and reliability of flexible electronics even at low temperatures remains an unmet challenge. Here, we report a facile strategy to fabricate low hysteresis, high stretchability, self-adhesion and anti-freezing zwitterionic supramolecular polymer ion-conductive elastomers (ICEs) by photoinitiated polymerization of aqueous precursor solutions containing a newly designed zwitterionic monomer carboxybetaine ureido acrylate (CBUIA) followed by solvent evaporation. The resultant poly(carboxybetaine ureido acrylate) (PCBUIA) ICEs are highly stretchable and self-adhesive owing to the presence of strong hydrogen bonds between ureido groups and dipole-dipole interactions of zwitterions. The zwitterion groups on the polymer side chains and loaded-lithium chloride endow PCBUIA ICEs with excellent anti-freezing properties, demonstrating mechanical flexibility and ionic transport properties even at a low temperature (-20 °C). Remarkably, the PCBUIA ICEs demonstrate a low hysteresis (≈10%) during cyclic mechanical loading-unloading (≤500%), and are successfully applied as wearable strain sensors and triboelectric nanogenerators (TENGs) for energy harvesting and human motion monitoring. In addition, the PCBUIA ICE-based TENG was used as a wireless sensing terminal for Internet of Things smart devices to enable wireless sensing of finger motion state detection.

Physical cues of scaffolds promote peripheral nerve regeneration

The effective treatment of long-gap peripheral nerve injury (PNI) remains a challenge in clinical settings. The autograft, the gold standard for the long-gap PNI therapy, has several limitations, including a limited supply of donor nerve, size mismatch between the donor and recipient sites, functional loss at the donor site, neuroma formation, and the requirement for two operations. With the increasing abundance of biocompatible materials with adjustable structures and properties, tissue engineering provides a promising avenue for bridging peripheral nerve gaps and addressing the above issues of autograft. The physical cues provided by tissue engineering scaffolds, essential for regulating the neural cell fate and microenvironments, have received considerable research attention. This review elaborates on three major physical cues of tissue engineering scaffolds for peripheral nerve regeneration: topological structure, mechanical support, and electrical stimulation. These three aspects are analogs to Lego bricks, wherein different combinations result in diverse functions. Innovative and more effective bricks, along with multi-level and all-around integration, are expected to provide new advances in tissue engineering for peripheral nerve generation.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

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.

Hydrophobic Cross-Linked Chains Regulate High Wet Tissue Adhesion Hydrogel with Toughness, Anti-hydration for Dynamic Tissue Repair

Hydrogel adhesion materials are widely reported for tissue engineering repair applications, however, wet tissue surface moisture can reduce the wet-adhesion properties and mechanical strength of hydrogels limiting their application. Here, anti-hydration gelatin-acrylic acid-ethylene dimethacrylate (GAE) hydrogels with hydrophobic cross-linked chains are constructed. The prepared GAE hydrogel is soaked in PBS (3 days) with a volume change of 0.6 times of the original and the adhesive strength, Young's modulus, toughness, and burst pressure are maintained by ≈70% of the original. A simple and universal method is used to introduce hydrophobic chains as cross-linking points to prepare hydrogels with anti-hydration, toughness, and high wet state adhesion. The hydrophobic cross-linked chains not only restrict the movement of molecular chains but also hinder the intrusion of water molecules. Antihydration GAE hydrogels exhibit good biocompatibility, slow drug release, and dynamic oral wet-state tissue repair properties. Therefore, the anti-hydration hydrogel has excellent toughness, wet tissue adhesion properties, and good prospects for biological applications.

All-dielectric absorber based on nano-graphite sheets/ionogels with configurable absorbing band

With the increasing complexity of the electromagnetic environment, there is a growing demand for the manipulation of electromagnetic waves, leading to the rapid development in configurable microwave absorbers. All-dielectric absorbers offer broadband and high-intensity absorption effects in microwave absorption and shielding. However, they face a significant challenge: their performance is not adjustable once the design is completed. In this study, we propose a solution to this problem by creating all-dielectric absorbers with flexibly configurable absorbing properties. We achieve this by designing a composite material of ionogels/nano-graphite sheets into compressible deformable absorbing units that can be molded into different shapes using 3D printing modes. The plasticity allows us to change the performance of the all-dielectric absorber, including the microwave absorption intensity, absorption peak, frequency bandwidths, and wide-angle absorption performance. With this approach, we can flexibly manipulate electromagnetic waves using all-dielectric absorbers through different plasticity models.

Developing conductive hydrogels for biomedical applications

Conductive hydrogels have attracted copious attention owing to their grateful performances, such as similarity to biological tissues, compliance, conductivity and biocompatibility. A diversity of conductive hydrogels have been developed and showed versatile potentials in biomedical applications. In this review, we highlight the recent advances in conductive hydrogels, involving the various types and functionalities of conductive hydrogels as well as their applications in biomedical fields. Furthermore, the current challenges and the reasonable outlook of conductive hydrogels are also given. It is expected that this review will provide potential guidance for the advancement of next-generation conductive hydrogels.

Preparation of Tough and Adhesive PVA/P(AM-AMPS)/Glycerol/Laponite/Na2SO4 Organohydrogels for All-Solid-State Supercapacitors and Self-Powered Wearable Strain Sensors

Hydrogel electrolytes are ideal for flexible wearable electronic devices because of their high ionic conductivity, flexibility, and biocompatibility. However, some problems, such as poor mechanical properties, low conductivity, and lack of adhesivity, are encountered in the process of hydrogel preparation and application, which restrict the further development of hydrogel electrolytes. In this study, PVA was used as the first network, and P(AM-co-AMPS) as the second network to prepare a double-network hydrogel electrolyte. Laponite and Na2SO4 were introduced into the hydrogel during hydrogel formation as the nanofiller and salt with the salting-out effect to enhance its mechanical properties. The hydrogel electrolyte with high toughness (1663 kJ·m-3), adhesivity (77 kPa), and ionic conductivity (1.7 S·m-1) was obtained. In addition, the hydrogel electrolyte also has excellent antifatigue performance. In the 10 consecutive tensile cycles, the tensile strength does not decay. Due to the high adhesivity of the hydrogel electrolyte, a symmetrical all-solid-state supercapacitor was assembled with a tight interface between the hydrogel electrolyte and the AC/CNT composite electrode. The supercapacitor has a high specific capacitance of 186.1 mF·cm-2 at the current density of 1 mA·cm-2. In addition, the capacitor has good flexibility and can withstand bending at various angles. The hydrogel electrolyte also has excellent strain sensing performance, with an ultrafast tensile response time (0.17 s) and high sensitivity factor (GF = 10.01). Finally, the self-powered sensor system composed of a supercapacitor as the power supply device and hydrogel electrolyte as the sensing part was obtained and applied to human motion monitoring, which provides a potential application in the integrated flexible electronic system.

Oxidized sodium alginate/polyacrylamide hydrogels adhesive for promoting wheat growth

Chemical modification of sodium alginate (SA) polymer chains can increase its functional group species. Sodium periodate (SP) was usually used to oxidize the hydroxyl groups on the chain of SA to aldehyde groups, the preparation of oxidized sodium alginate (OSA) using SP is not only complicated, also limits the variety of functional groups on the chain of OSA. By contrast, we have developed an innovative strategy for OSA, in which ammonium persulfate (APS) was used to oxidize SA, providing a clear elucidation of the oxidizing process and mechanism. OSA/PAM hydrogels were synthesized using OSA, the hydrogels possess excellent adhesion properties to various non-metallic and metallic substrates. Tensile and compression tests show that the cross-linked OSA/PAM hydrogels have superior mechanical properties. We exploit OSA/PAM hydrogels as soil adhesive and water-retaining agents for wheat growth. OSA/PAM hydrogels significantly improve the survival time of wheat grown in brown loam soil under a water-shortage environment, and slow down the wilting of wheat in a water-shortage environment and prolong the survival time of wheat in sandy soils. Our trials should make hydrogels important for wheat cultivation in brown loam soils and the development of desert areas.

Conductive and antibacterial dual-network hydrogel for soft bioelectronics

Conductive hydrogels have shown significant potential for use in soft bioelectronics due to their unique similarities to biological tissue, including high water content, low modulus, and conductivity. However, their high water content makes them susceptible to absorbing microorganisms and promoting bacterial growth, which can trigger an immune response. Besides, the adhesion and biocompatibility of the hydrogel are not satisfactory, seriously limiting the conductive hydrogel's high-performance applications in human healthcare monitoring. Herein, the problem is addressed by introducing borax through a swelling and a semi-dehydration method into the interpenetrated network of a polyvinyl alcohol and poly(acrylic acid) hydrogel. The hydrogel exhibits both outstanding antibacterial (>99.99% toward E. coli and S. aureus) activity and high ionic conductivity, in addition to tissue-like softness, strong wet-tissue adhesion (600 J m-2 for skin), environmental stability, and excellent biocompatibility. Furthermore, the as-prepared hydrogel can serve as a biosensing conductor, showing high-quality recording and monitoring of real-time tiny yet complex muscle movements during speaking and realizing neuromodulation through low-current electronic stimulation (40 μA) of a rat's nerve. Simultaneously, the hydrogel also exhibits the capacity to accelerate wound healing. Therefore, the proposed antibacterial conductive hydrogel is a safer option for next-generation bioelectronic materials in human healthcare.

Injectable Antiswelling and High-Strength Bioactive Hydrogels with a Wet Adhesion and Rapid Gelling Process to Promote Sutureless Wound Closure and Scar-free Repair of Infectious Wounds

Developing injectable antiswelling and high-strength bioactive hydrogels with wet tissue adhesiveness and a rapid gelling process to meet the requirements for rapid hemostasis, sutureless wound closure, and scar-free repair of infected skin wounds continues to have ongoing challenges. Herein, injectable, antibacterial, and antioxidant hydrogel adhesives based on poly(citric acid-co-polyethylene glycol)-g-dopamine and amino-terminated Pluronic F127 (APF) micelles loaded with astragaloside IV (AS) are prepared. The H2O2/horseradish peroxidase (HRP) system is used to cause cross-linking of the hydrogel network through oxidative coupling between catechol groups and chemical cross-linking between the catechol group and the amino group. The hydrogels exhibit a rapid gelling process, high mechanical strength, an antiswelling effect, good antioxidant property, H2O2 release behavior, and degradability. In addition, the hydrogels present good wet tissue adhesiveness, high bursting pressure, excellent antibacterial activity, long-term sustained release of AS, and good biocompatibility. The hydrogels perform good hemostasis on mouse liver, rat liver, and rabbit femoral vein bleeding models and achieve much better closure and healing of skin incisions than biomedical glue and surgical sutures. Furthermore, the hydrogel dressing significantly improved the scar-free repair of MRSA-infected full thickness skin defect wounds by modulating inflammation, regulating the ratio of collagen I/III, and improving the vascularization and granulation tissue formation. Thus, AS-loaded hydrogels show huge potential as multifunctional dressings for in vivo hemostasis, sutureless wound closure, and scar-free repair of infected skin wounds.

Injectable tissue prosthesis for instantaneous closed-loop rehabilitation

To construct tissue-like prosthetic materials, soft electroactive hydrogels are the best candidate owing to their physiological mechanical modulus, low electrical resistance and bidirectional stimulating and recording capability of electrophysiological signals from biological tissues1,2. Nevertheless, until now, bioelectronic devices for such prostheses have been patch type, which cannot be applied onto rough, narrow or deep tissue surfaces3-5. Here we present an injectable tissue prosthesis with instantaneous bidirectional electrical conduction in the neuromuscular system. The soft and injectable prosthesis is composed of a biocompatible hydrogel with unique phenylborate-mediated multiple crosslinking, such as irreversible yet freely rearrangeable biphenyl bonds and reversible coordinate bonds with conductive gold nanoparticles formed in situ by cross-coupling. Closed-loop robot-assisted rehabilitation by injecting this prosthetic material is successfully demonstrated in the early stage of severe muscle injury in rats, and accelerated tissue repair is achieved in the later stage.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

In situ molecular permeation of liquid cationic polymers into solid anionic polymer films enabling self-adaptive adhesion of hydrogel biosensors

Self-adaptive adhesion is essential for hydrogel sensors. However, the traditional protocol involves covering a pre-prepared hydrogel sensor on a tested surface. As a result, the sensor cannot achieve self-adaptive adhesion owing to an air-layer hindrance between the sensor and tested surface, which inevitably leads to the loss of critical biological signals. To address the issue of air-layer hindrance, this work proposes an in situ permeation method that enables the self-adaptive adhesion of hydrogel biosensors on various surfaces. After applying a liquid solution of poly(methacrylamido propyl trimethyl ammonium chloride-co-acrylamide) (poly(MPTAC-co-AM)) on the testing surface, a thin film of poly(acrylic aminoethane sulfonic acid-co-acrylamide) (poly(AASA-co-AM)) is applied, where the electrostatic interaction between -SO3- and -Me3N+ facilitates rapid permeation of the solution into the solid film, leading to the formation of a hydrogel layer in situ. The coating of liquid poly(MPTAC-co-AM) sweeps away the air layer and works as a natural glue, enabling a strong bonding interaction between the hydrogel layer and the tested surface. Such a hydrogel layer is very thin (microscale), and can retain its self-adaptive adhesion even with deformation of the tested surface. When it is applied on the surface of an active frog heart, the weak heartbeats can be transduced to electrical signals. Moreover, this self-adaptive adhesion can work on both soft and hard surfaces including biological tissues, metals, rubbers, ceramics, and glass. Therefore, this in situ permeation method enables the hydrogel layer to detect weak dynamic changes on various soft and hard surfaces, which might offer a new pathway for physiological signal monitoring.

Bioadhesive polymer semiconductors and transistors for intimate biointerfaces

The use of bioelectronic devices relies on direct contact with soft biotissues. For transistor-type bioelectronic devices, the semiconductors that need to have direct interfacing with biotissues for effective signal transduction do not adhere well with wet tissues, thereby limiting the stability and conformability at the interface. We report a bioadhesive polymer semiconductor through a double-network structure formed by a bioadhesive brush polymer and a redox-active semiconducting polymer. The resulting semiconducting film can form rapid and strong adhesion with wet tissue surfaces together with high charge-carrier mobility of ~1 square centimeter per volt per second, high stretchability, and good biocompatibility. Further fabrication of a fully bioadhesive transistor sensor enabled us to produce high-quality and stable electrophysiological recordings on an isolated rat heart and in vivo rat muscles.

Mimosa-Inspired Stimuli-Responsive Curling Bioadhesive Tape Promotes Peripheral Nerve Regeneration

Trauma often results in peripheral nerve injuries (PNIs). These injuries are particularly challenging therapeutically because of variable nerve diameters, slow axonal regeneration, infection of severed ends, fragility of the nerve tissue, and the intricacy of surgical intervention. Surgical suturing is likely to cause additional damage to peripheral nerves. Therefore, an ideal nerve scaffold should possess good biocompatibility, diameter adaptability, and a stable biological interface for seamless biointegration with tissues. Inspired by the curl of Mimosa pudica, this study aimed to design and develop a diameter-adaptable, suture-free, stimulated curling bioadhesive tape (SCT) hydrogel for repairing PNI. The hydrogel is fabricated from chitosan and acrylic acid-N-hydroxysuccinimide lipid via gradient crosslinking using glutaraldehyde. It closely matches the nerves of different individuals and regions, thereby providing a bionic scaffold for axonal regeneration. In addition, this hydrogel rapidly absorbs tissue fluid from the nerve surface achieving durable wet-interface adhesion. Furthermore, the chitosan-based SCT hydrogel loaded with insulin-like growth factor-I effectively promotes peripheral nerve regeneration with excellent bioactivity. This procedure for peripheral nerve injury repair using the SCT hydrogel is simple and reduces the difficulty and duration of surgery, thereby advancing adaptive biointerfaces and reliable materials for nerve repair.

Current Progress in Conductive Hydrogels and Their Applications in Wearable Bioelectronics and Therapeutics

Wearable bioelectronics and therapeutics are a rapidly evolving area of research, with researchers exploring new materials that offer greater flexibility and sophistication. Conductive hydrogels have emerged as a promising material due to their tunable electrical properties, flexible mechanical properties, high elasticity, stretchability, excellent biocompatibility, and responsiveness to stimuli. This review presents an overview of recent breakthroughs in conductive hydrogels, including their materials, classification, and applications. By providing a comprehensive review of current research, this paper aims to equip researchers with a deeper understanding of conductive hydrogels and inspire new design approaches for various healthcare applications.

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.

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.

Highly Stretchable, Self-Adhesive, Antidrying Ionic Conductive Organohydrogels for Strain Sensors

As flexible wearable devices, hydrogel sensors have attracted extensive attention in the field of soft electronics. However, the application or long-term stability of conventional hydrogels at extreme temperatures remains a challenge due to the presence of water. Antifreezing and antidrying ionic conductive organohydrogels were prepared using cellulose nanocrystals and gelatin as raw materials, and the hydrogels were prepared in a water/glycerol binary solvent by a one-pot method. The prepared hydrogels were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The mechanical properties, electrical conductivity, and sensing properties of the hydrogels were studied by means of a universal material testing machine and LCR digital bridge. The results show that the ionic conductive hydrogel exhibits high stretchability (elongation at break, 584.35%) and firmness (up to 0.16 MPa). As the binary solvent easily forms strong hydrogen bonds with water molecules, experiments show that the organohydrogels exhibit excellent freezing and drying (7 days). The organohydrogels maintain conductivity and stable sensitivity at a temperature range (-50 °C-50 °C) and after long-term storage (7 days). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 6.47 (strain, 0-400%) could detect human motions. Therefore, multifunctional organohydrogel wearable sensors with antifreezing and antidrying properties have promising potential for human body monitoring under a broad range of environmental conditions.