Highly conductive tissue-like hydrogel interface through template-directed assembly

Balancing performance and stability characteristics in organic electrochemical transistor

Nowadays organic electrochemical transistors (OECTs) are becoming a promising platform for bioelectronics and biosensing due to its biocompatibility, high sensitivity and selectivity, low driving voltages, high transconductance and flexibility. However, the existing problems associated with degradation processes within the OECT during long-term operation hinder their widespread implementation. Moreover, trade-offs often arise between OECT transconductance and speed, fast ion transport and electron mobility, electrochemical stability and sensitivity, cycling stability and signal amplification, and other metrics. Ensuring high performance characteristics and achieving enhanced stability in OECTs are distinct strategies that do not always align, as progress in one aspect often necessitates a trade-off with the other. This dynamic arises from the need to find a balance between reversible and irreversible processes in the behavior of OECT active layers, and providing simultaneously favorable conditions for ion and electron transport and their efficient charge coupling. This review article systematically summarizes the phenomenological and physical-chemical aspects associated with factors and mechanisms that determine both performance and long-term stability of OECT, paying special attention to the consideration of existing and promising approaches to extend the OECT lifespan, while maintaining (or even increasing) high effectiveness of its operation.

Human soft tissues-like PVA/cellulose hydrogels with multifunctional properties towards flexible electronics applications

Conductive hydrogels have attracted significant attention due to their exceptional flexibility, biocompatibility, and promising applications in flexible electronics. Inspired by human soft tissues, robust ionic conductive hydrogels were developed via constructing cellulose-reinforced polyvinyl alcohol networks and precise modulation of zinc ions. The hydrogel exhibits impressive mechanical behaviors (σ = 4.55 MPa, ε = 1293 %) and ionic conductivity as high as 1.17 S/m, ascribed from the multiscale interaction mechanism. These mechanisms include the formation of dense nanofiber networks and nanocrystalline domains, the effects of multiple metal coordination and hydrogen bonds, and the reinforcement of nanocellulose. Moreover, the hydrogel demonstrates a low strain detection limit of 1 % and shows great potential for applications in human health monitoring. Interestingly, based on the principle of Morse code, the hydrogel can be used for information transmission in hazardous environments for emergency signaling. More importantly, when used as an electrolyte in flexible zinc-ion battery, it significantly inhibits zinc dendrite growth and supports stable charge-discharge cycles, making it ideal for small flexible electronic devices. This work presents a biomimetic and sustainable strategy for the rapid fabrication of robust ionic conductive hydrogels, offering advanced applications in flexible electronics.

Conductive Hydrogels with Topographical Geometry and Mechanical Robustness for Enhanced Peripheral Nerve Regeneration

Nerve guidance conduits (NGCs) emerge as a promising solution for nerve regeneration; however, conventional NGCs fail to fulfill the requirements for peripheral nerve regeneration, which are subjected to periodical yet vigorous stretching, bending, and compression. Here, we developed a fatigue-resistant conductive hydrogel-based NGC by integrating topographical geometry, enhanced electroactivity, and superior fatigue resistance within one unit. The hydrogel, consisting of a PVA matrix with PEDOT:PSS as a conductive filler, features a topographical alignment that promotes axonal growth and achieves a fatigue threshold over 500 J/m2, making it well-suited for sciatic nerve repairing. Phase segregation of PEDOT chains enhances its electrical conductivity (>500 S/m) and mitigates the interfacial impedance mismatch, allowing for high-efficiency bioelectrical signal transmission. In vivo studies on a rat sciatic nerve injury model corroborate the accelerated peripheral nerve regeneration through improved motor function recovery and efficient electrophysiological signal transmission. These findings establish our hydrogel-based NGCs as a promising solution for high-efficiency nerve regeneration through the synergy of topographical, mechanical, and electrical engineering.

Bioelectric and physicochemical foundations of bioelectronics in tissue regeneration

Understanding and exploiting bioelectric signaling pathways and physicochemical properties of materials that interface with living tissues is central to advancing tissue regeneration. In particular, the emerging field of bioelectronics leverages these principles to develop personalized, minimally invasive therapeutic strategies tailored to the dynamic demands of individual patients. By integrating sensing and actuation modules into flexible, biocompatible devices, clinicians can continuously monitor and modulate local electrical microenvironments, thereby guiding regenerative processes without extensive surgical interventions. This review provides a critical examination of how fundamental bioelectric cues and physicochemical considerations drive the design and engineering of next-generation bioelectronic platforms. These platforms not only promote the formation and maturation of new tissues across neural, cardiac, musculoskeletal, skin, and gastrointestinal systems but also precisely align therapies with the unique structural, functional, and electrophysiological characteristics of each tissue type. Collectively, these insights and innovations represent a convergence of biology, electronics, and materials science that holds tremendous promise for enhancing the efficacy, specificity, and long-term stability of regenerative treatments, ushering in a new era of advanced tissue engineering and patient-centered regenerative medicine.

Hydrogel Adhesive Integrated-Microstructured Electrodes for Cuff-Free, Less-Invasive, and Stable Interface for Vagus Nerve Stimulation

Vagus nerve stimulation (VNS) is a recognized treatment for neurological disorders, yet the surgical procedure carries significant risks. During the process of isolating or cuffing the vagus nerve, there is a danger of damaging the nerve itself or the adjacent carotid artery or jugular vein. To minimize this risk, here we introduce a novel hydrogel adhesive-integrated and stretchable microdevice that provides a less invasive,  cuff-free option for interfacing with the vagus nerve. The device features a novel hydrogel adhesive formulation that enables crosslinking on biological tissue. The inclusion of kirigami structures within the thin-film microdevice creates space for uniform hydrogel-to-epineurium contact while accommodating the stiffness changes of the hydrogel upon hydration. Using a rodent model, we demonstrate a robust device adhesion on a partially exposed vagus nerve in physiological fluid even without the vagus nerve isolation and cuffing process. Our device elicted stable and clear evoked compound action potential (~1500 µV peak-to-peak) in C-fibers with a current amplitude of 0.4 mA. We believe this innovative platform provides a novel, less-risky approach to interface with fragile nerve and vascular structures during VNS implantation.

Nanoengineered and highly porous 3D chitosan-graphene scaffold for enhanced antibacterial activity and rapid hemostasis

Chitosan-based hydrogels have been utilized over the years as an efficient hemorrhage because of their biocompatibility and biodegradability nature. Here we have nanoengineered the polycationic peptide-conjugated graphene‑silver nanocomposite into the chitosan matrix as a 3D highly porous CGrSP scaffold to facilitate rapid hemostasis and prevent bacterial infection. This CGrSP scaffold interacted with blood cells and platelets, initiating the blood coagulation process by activating the plasmatic contact system. Notably, it reduced the activated Partial Thromboplastin Time (aPTT) and Prothrombin Time (PT), indicating that the scaffold promoted platelet activation associated with Factors XII and X, leading to fibrin formation and clot stabilization. In vitro studies showed that the CGrSP scaffold reduced whole blood clotting time by 87 % compared to the commercial dressing "QuikClot." Additionally, in vivo studies using rat-tail amputation and skin laceration models demonstrated a significant reduction in hemostatic time compared to both the chitosan scaffold (p-value<0.003) and "QuikClot" (p-value<0.01). Beyond its hemostatic properties, the CGrSP scaffold exhibited strong antibacterial activity, achieving a 5-log reduction against both Escherichia coli and Staphylococcus aureus. With its biodegradable nature, rapid hemostasis, and potential for tissue regeneration, the CGrSP scaffold presents a novel and safe therapeutic material.

Lignosulfonate-enhanced dispersion and compatibility of liquid metal nanodroplets in PVA hydrogel for improved self-recovery and fatigue resistance in wearable sensors

Stretchable and resilient conductive hydrogels, incorporating flowable liquid metals (LM) into polyvinyl alcohol (PVA), have emerged as promising materials for wearable sensors due to their exceptional mechanical properties and sustainability. However, the fluidity and compatibility of LM with the hydrogel matrix limit the construction and performance of LM/PVA conductive hydrogels. This study aimed to develop a flexible, high-performance hydrogel for advanced wearable sensors by introducing LM nanoparticles encapsulated in sodium lignosulfonate (LS-LM) into the PVA matrix. The renewable natural macromolecule LS, rich in functional groups, enhanced the compatibility between LM and the PVA matrix. Moreover, LS formed a stable shell around the LM droplets, preventing rupture and leakage of LM, ensuring uniform dispersion within the hydrogel and significantly improving its durability by preventing phase separation. The optimized conductive lignosulfonate-liquid metal/polyvinyl alcohol hydrogel (LS-LM/PVA) exhibited a tensile stress of 1.60 MPa, a compressive strength of 0.53 MPa under 70 % strain, and electrical conductivity (4.87 S m-1). The hydrogel-based sensor demonstrated excellent sensitivity (GF = 2.40) and outstanding fatigue resistance (over 500 cycles). A Life Cycle Assessment (LCA) was conducted to evaluate the environmental impacts of LS-LM/PVA hydrogel production. The composite hydrogel-based sensor shows significant promise for advancing human motion tracking and information recognition.

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.

Supramolecular Conductive Hydrogels With Homogeneous Ionic and Electronic Transport

Mechanically resilient hydrogels with ion-electron mixed transport properties effectively bridge biology with electronics. An ideal bioelectronic interface can be realized through introducing electronically conductive polymers into supramolecular hydrogels. However, inhomogeneous morphologies of conducting polymers, such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have limited mechanical properties and ion-electron interactions. Here, supramolecular conductive hydrogels that possess homogeneous ionic and electronic transport are achieved. The materials demonstrate high toughness (620 kJ m-3), stretchability (>1000%), softness (10.5 kPa), and conductivity (5.8 S cm-1), which surpasses commonly used inhomogeneous PEDOT:PSS-based hydrogels. The homogeneous network leads to higher charge injection capacitance and lower skin impedance compared to commercial electrodes or commonly used inhomogeneous PEDOT:PSS conducting networks. This significant advance arises from the homogeneous incorporation of the hydrophilic self-doped conducting polymer S-PEDOT, which has polymerized within a supramolecular polymer network template mediated by high-binding affinity host-guest crosslinks. Furthermore, the compatibility of S-PEDOT with hydrophilic secondary networks enables the realization of fully dryable and reswellable electronic devices, facilitating reusability and improving their ease of handling. It is anticipated that achieving such material architectures will offer a promising new direction in future synthesis and implementation of conductive hydrogels in the field of bioelectronics.

Advancing Prosthetic Hand Capabilities Through Biomimicry and Neural Interfaces

Background and ObjectivesProsthetic hand development is undergoing a transformative phase, blending biomimicry and neural interface technologies to redefine functionality and sensory feedback. This article explores the symbiotic relationship between biomimetic design principles and neural interface technology (NIT) in advancing prosthetic hand capabilities.MethodsDrawing inspiration from biological systems, researchers aim to replicate the intricate movements and capabilities of the human hand through innovative prosthetic designs. Central to this endeavor is NIT, facilitating seamless communication between artificial devices and the human nervous system. Recent advances in fabrication methods have propelled brain-computer interfaces, enabling precise control of prosthetic hands by decoding neural activity.ResultsAnatomical complexities of the human hand underscore the importance of understanding biomechanics, neuroanatomy, and control mechanisms for crafting effective prosthetic solutions. Furthermore, achieving the goal of a fully functional cyborg hand necessitates a multidisciplinary approach and biomimetic design to replicate the body's inherent capabilities. By incorporating the expertise of clinicians, tissue engineers, bioengineers, electronic and data scientists, the next generation of the implantable devices is not only anatomically and biomechanically accurate but also offer intuitive control, sensory feedback, and proprioception, thereby pushing the boundaries of current prosthetic technology.ConclusionBy integrating machine learning algorithms, biomechatronic principles, and advanced surgical techniques, prosthetic hands can achieve real-time control while restoring tactile sensation and proprioception. This manuscript contributes novel approaches to prosthetic hand development, with potential implications for enhancing the functionality, durability, and safety of the prosthetic limb.

3D Bioprinting of Double-Layer Conductive Skin for Wound Healing

Conductive hydrogels are highly attractive in 3D bioprinting of tissue engineered scaffolds for skin injury repair. However, their application is limited by mismatched electrical signal conduction mode and poor printability. Herein, the 3D bioprinting-assisted fabrication of a double-layer ionic conductive skin scaffold using a newly designed ionic conductive biomimetic bioink (GHCM) is reported, which is composed of gelatin methacrylate (GelMA), oxidized hyaluronic acid (OHA), carboxymethyl chitosan (CMCS), and 2-methacryloyloxyethyl phosphorylcholine (MPC) for the treatment of full-thickness skin defects. The combination of rigid (GelMA) and dynamic (OHA-CMCS) polymer networks imparts GHCM bioink excellent reversible thixotropy, enabling good printability, and allowing the creation of skin-like constructs with high shape fidelity and cell activity by convenient one-step bioprinting. Moreover, the incorporation of zwitterionic MPC endows the bioink with electrical signaling pattern similar to that of natural skin tissue. By integrating human foreskin fibroblasts (HFF-1), human umbilical vein endothelial cells (HUVECs), and human immortalized keratinocytes (HaCaTs), a double-layer conductive skin scaffold comprising an epidermal layer and a vascularized dermal layer is created. In vivo experiments have demonstrated that the conductive skin scaffolds provide an appropriate conductive microenvironment for cellular signaling, growth, migration, and differentiation, ultimately accelerating the re-epithelialization, collagen deposition, and vascularization of skin wounds, which may represent a general and versatile strategy for precise engineering of electroactive tissues for regenerative medicine applications.

High-Fidelity, High-Conductivity and Multifunctional PEDOT:PSS Hydrogel for Efficient Electromagnetic Interference Shielding and Ultrafast Response Electrochromic Applications

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hydrogel are promising for applications in electromagnetic interference (EMI) shielding, energy storage, and electrochromic (EC) devices. However, challenges such as low conductivity at reduced solid content, limited charge storage, poor mechanical properties, and structural distortion during solvent treatment limit their high-performance applications. To address these issues, a high-fidelity, high-conductivity and multifunctional PEDOT:PSS hydrogel is developed by an ice crystal-assisted skeleton stacking and stepwise treatment strategy, achieving ultrahigh conductivity of 87,249 S m-1 at 5.8 wt% solid content. The PEDOT:PSS hydrogel also features a charge storage capacity of 35.66 mC cm-2 and a capacitance density of 587.6 mF cm-2. Additionally, The PEDOT:PSS hydrogel demonstrates exceptional EMI shielding effectiveness, achieving 81.2 dB, and also exhibits an ultrahigh specific surface shielding efficiency of 30,769.23 dB cm2 g-1. Notably, The PEDOT:PSS maintains high EMI shielding stability even after undergoing various harsh conditions. Using femtosecond laser direct writing, the highly stable all-solid-state EC reflective displays are developed with ultrafast response (<0.3 s) and superior durability.

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.

Photopatternable PEDOT:PSS Hydrogels for High-Resolution Photolithography

Conducting polymer hydrogels have been extensively explored toward diverse applications like bioelectronics and soft robotics. However, the fabrication resolution of conducting polymer hydrogels by typical techniques, including ink-jet printing, 3D-printing, etc., has been generally limited to >10 µm, significantly restricting rapid innovations and broad applications of conducting polymer hydrogels. To address this issue, a photosensitive biphasic conducting polymer hydrogel (PB-CH) is rationally designed and synthesized, comprising poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the conductive phase and a light-sensitive matrix as the mechanical phase. The formation of phase-separated structures within PB-CH preserves the integrity of the conductive channels during the photoinitiated cross-linking. This minimizes the conductivity loss, a common limitation in similar materials. Remarkably, the resultant PB-CH exhibits a combination of excellent electrical conductivity (≈30 S cm-1), robust mechanical performance (tensile strain up to 50%), and high photopatternability. A detailed investigation of the photolithography process identifies key technological parameters that enable high-resolution patterning of 5 µm. By simultaneously maintaining processability, conductivity, and mechanical flexibility, this PB-CH represents an ideal candidate for advanced flexible electronic applications, offering a new technique to fabricating high-performance conducting polymer hydrogels.

Soft, stretchable conductive hydrogels for high-performance electronic implants

Conductive hydrogels are emerging as promising materials for electronic implants owing to their favorable mechanical and electrical properties. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hydrogels are particularly attractive, but their preparation often requires toxic additives. Here, we introduced a nutritive sweetener, d-sorbitol, as a nontoxic additive to create soft and stretchable PEDOT:PSS conductive hydrogels. These hydrogels exhibit mechanical properties comparable with biological tissues, reducing adverse immune responses. The hydrogels can be patterned on elastic substrates using a simple, low-cost micromolding technique to fabricate soft and stretchable implantable devices for electrical stimulation and recording. The hydrogel electrodes show much lower electrochemical impedance and higher charge storage and injection capacity compared to platinum electrodes. In addition, the properties of hydrogels and devices remain stable after long-term storage and exposure to extreme conditions. We demonstrate the use of soft hydrogel-based electronic devices for effective electrical stimulation and high-quality electrical recordings in live animal models.

Regulation of the gelatin helix-to-coil transition through chain confinements at the polymer-protein interface and protein-protein interface

Gelatin is an essential material widely used in biomedical applications due to its characteristic temperature responsivity-helix-to-coil transition. However, the current helix-to-coil transition is limited by its single-step behavior and the difficulty in designing a specific onset temperature. In this study, we investigated the fundamentals of the helix-to-coil transition with a focus on gelatin chain mobility. We observed distinctive kinetics of the helix-to-coil transition, which is resilient and can actuate in multiple steps or with a controllable onset point. This was achieved by confining the gelatin chain with a hydrophilic polymer or gelatin itself. The confinement approach serves two purposes: first, it prevents excessive mobility of the generated coils, maintaining physical resilience after the helix-to-coil transition; second, the interfacial confinement between the polymer and gelatin, referred to as polymer-protein interface confinement, restricts the helix-to-coil transition, resulting in a multistep transition process. Additionally, strong confinement at the interface between gelatins of different origins, that is protein-protein interface confinement, shifts the onset temperature to a higher point. This fundamental comprehension of helix-to-coil transition could contribute to broadening the biomedical application potential of gelatin materials. STATEMENT OF SIGNIFICANCE: Gelatin is essential in biomedical applications due to its characteristic temperature responsivity-helix-to-coil transition. Herein, we fundamentally investigated the distinctive kinetics of the helix-to-coil transition, which is resilient and can actuate in multiple steps or with a controllable onset point. This was achieved by confining the gelatin chain with a hydrophilic polymer or gelatin itself. The gelatin chain confinement prevents excessive mobility of the generated coils, maintaining physical resilience after the helix-to-coil transition. The interfacial confinement between the polymer and gelatin restricts the helix-to-coil transition, resulting in a multistep transition process. Additionally, strong confinement at the interface between gelatins of different origins shifts the onset temperature to a higher point.

Intrinsically Adhesive and Conductive Hydrogel Bridging the Bioelectronic-Tissue Interface for Biopotentials Recording

Achieving high-quality biopotential signal recordings requires soft and stable interfaces between soft tissues and bioelectronic devices. Traditional bioelectronics, typically rigid and dependent on medical tape or sutures, lead to mechanical mismatches and inflammatory responses. Existing conducting polymer-based bioelectronics offer tissue-like softness but lack intrinsic adhesion, limiting their effectiveness in creating stable, conductive interfaces. Here, we present an intrinsically adhesive and conductive hydrogel with a tissue-like modulus and strong adhesion to various substrates. Adhesive catechol groups are incorporated into the conductive poly(3,4-ethylenedioxythiophene) (PEDOT) hydrogel matrix, which reduces the PEDOT size and improves dispersity to form a percolating network with excellent electrical conductivity and strain insensitivity. This hydrogel effectively bridges the bioelectronics-tissue interface, ensuring pristine signal recordings with minimal interference from bodily movements. This capability is demonstrated through comprehensive in vivo experiments, including electromyography and electrocardiography recordings on both static and dynamic human skin and electrocorticography on moving rats. This hydrogel represents a significant advancement for bioelectronic interfaces, facilitating more accurate and less intrusive medical diagnostics.

Bioinspired Super-Robust Conductive Hydrogels for Machine Learning-Assisted Tactile Perception System

Conductive hydrogels have attracted significant attention due to exceptional flexibility, electrochemical property, and biocompatibility. However, the low mechanical strength can compromise their stability under high stress, making the material susceptible to fracture in complex or harsh environments. Achieving a balance between conductivity and mechanical robustness remains a critical challenge. In this study, super-robust conductive hydrogels were designed and developed with highly oriented structures and densified networks, by employing techniques such as stretch-drying-induced directional assembly, salting-out, and ionic crosslinking. The hydrogels showed remarkable mechanical property (tensile strength: 17.13-142.1 MPa; toughness: 50 MJ m- 3), high conductivity (30.1 S m-1), and reliable strain sensing performance. Additionally, it applied this hydrogel material to fabricate biomimetic electronic skin device, significantly improving signal quality and device stability. By integrating the device with 1D convolutional neural network algorithm, it further developed a real-time material recognition system based on triboelectric and piezoresistive signal collection, achieving a classification accuracy of up to 99.79% across eight materials. This study predicted the potential of the high-performance conductive hydrogels for various applications in flexible smart wearables, the Internet of Things, bioelectronics, and bionic robotics.

Multidimensional Integrated Architectonics for Hierarchical Hydrogels with Enhanced Thermal Conductivity for Effective Burn Healing

To be the wound dressings that alleviate progressive thermal injury, hydrogels require to have high thermal conductivity, excellent mechanical properties, and outstanding biocompatibility, which are hard to achieve simultaneously. Inspired by the multiscale structure of spider silk, we developed a hierarchical hydrogel architecture. This architecture includes molecular-scale hydrogen bonds between oxygen plasma-treated graphene fibers (OPGFs) and poly(vinyl alcohol) (PVA), as well as within PVA molecular chains. It also features nanometer-scale ordered crystalline domains in the PVA matrix and micrometer-scale highly oriented OPGFs. The hydrogel was prepared using OPGFs assisted by a magnetic field, directional freezing, and salting-out treatment. This hierarchical structure reduces the interfacial thermal resistance between OPGFs and PVA, as proven by molecular dynamics. Finite element simulations show that the improvement in the hydrogel's thermal conductivity during the salting-out process is primarily due to the increase in PVA's intrinsic thermal conductivity. Additionally, the oriented OPGFs create directional thermal pathways. PVA/OPGFs hydrogels have a thermal conductivity of 1.71 W/(m·K), 322% higher than that of pristine PVA hydrogel. PVA/OPGFs hydrogels exhibit robust mechanical properties and good biocompatibility, indicating their vast potential as burn wound dressings. In vivo tests show that PVA/OPGFs hydrogels quickly reduce thermal damage in burn wounds and accelerate healing. Furthermore, the PVA/OPGFs hydrogels demonstrated good electrical conductivity and outstanding sensing capabilities, indicating that they can monitor motion under stress and serve as electrical stimulation carriers, showing great potential for integrated therapeutic monitoring in intelligent wound dressings. The strategy of constructing multiscale structures expands the potential applicability of hydrogels in biothermal management.

Biointerface engineering of flexible and wearable electronics

Biointerface sensing is a cutting-edge interdisciplinary field that merges conceptual and practical aspects. Wearable bioelectronics enable efficient interaction and close contact with biological components such as tissues and organs, paving the way for a wide range of medical applications, including personal health monitoring and medical intervention. To be applicable in real-world settings, the patches must be stable and adhere to the skin without causing discomfort or allergies in both wet and dry conditions, as well as other desirable features such as being ultra-soft, thin, flexible, and stretchable. Biosensors have emerged as promising tools primarily used to directly detect biological and electrophysiological signals, enhancing the efficacy of personalized medical treatments and enabling accurate tracking of human well-being. This review highlights the engineering of skin-tissue surfaces/interfaces and their interactions with wearable patches, aiming for both a broad and in-depth understanding of the mechanical and physicochemical properties required for the advancement of flexible and wearable skin patches. Specifically, the advantages of flexible bioelectronics and sensors with optimized surface geometry for long-term diagnosis are discussed. This insight aims to guide the future development of functional materials that can interact with human tissue in a controlled manner. Finally, we provide perspectives on the challenges and potential applications of biointerface engineering in wearable devices.

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.

Conductive polyacrylamide/pullulan/ammonium sulfate hydrogels with high toughness, low-hysteresis and tissue-like modulus as flexible strain sensors

Conductive hydrogels have great potential for applications in flexible wearable sensors due to the combination of biocompatibility, mechanical flexibility and electrical conductivity. However, constructing conductive hydrogels with high toughness, low hysteresis and skin-like modulus simultaneously remains challenging. In the present study, we prepared a tough and conductive polyacrylamide/pullulan/ammonium sulfate hydrogel with a semi-interpenetrating network. Ammonium sulfate promoted the formation of low-energy-dissipating motifs between polymer chains, reinforcing the gel matrix and resulting in excellent mechanical properties, including a high stretchability of 2063 %, a high strength of 890 kPa, and a high toughness of 4268 kJ/m3. The hydrogen bonds formed within the network endowed the gels with low-hysteresis under deformation. The unique semi-interpenetrating network structure provided the gels with a tissue-like low modulus. Additionally, the resulting hydrogels exhibited a high conductivity of 2.39 S/m and excellent anti-freezing properties, making them suitable for flexible strain sensors. These sensors demonstrated high sensitivity over a broad strain window of 0.1-1500 %, enabling the detection of various human motions and the recognition of different languages. These findings emphasize the potential of the composite hydrogels as wearable strain sensors for flexible devices.

Tissue-Adhesive and Stiffness-Adaptive Neural Electrodes Fabricated Through Laser-Based Direct Patterning

Recently, implantable devices for treating peripheral nerve disorders have demonstrated significant potential as neuroprosthetics for diagnostics and electrical stimulation. However, the mechanical mismatch between these devices and nerves frequently results in tissue damage and performance degradation. Although advances are made in stretchable electrodes, challenges, including complex patterning techniques and unstable performance, persist. Herein, an efficient method for developing a tissue-adhesive, stiffness-adaptive peripheral neural interface (TA-SA-PNI) is presented employing mechanically and electrically stable ultrathin conductive micro/nanomembrane bilayer (UC-MNB) electrodes. A direct laser-patterning technique is utilized to anchor the UC-MNB, comprising a conductive Cu micromembrane encapsulated by a biocompatible Au nanomembrane, onto a tough self-healing polymer (T-SHP) substrate using the thermoplastic properties of T-SHP. The UC-MNB with a wavy structure exhibited strain-insensitive performance under strains of up to 60%. Furthermore, its dynamic stress-relaxation properties enable stiffness adaptation, potentially minimizing chronic nerve compression. Finally, the phenylboronic acid-conjugated alginate (Alg-BA) adhesive layer offers stable tissue adhesion and ionic conductivity, optimizing the TA-SA-PNI for seamless integration into neural applications. Leveraging these advantages, in vivo demonstrations of bidirectional neural pathways are successfully conducted, featuring stable measurements of sensory neural signals and feedback electrical stimulation of the sciatic nerves of rats.

Recent Development of Fibrous Hydrogels: Properties, Applications and Perspectives

Fibrous hydrogels (FGs), characterized by a 3D network structure made from prefabricated fibers, fibrils and polymeric materials, have emerged as significant materials in numerous fields. However, the challenge of balancing mechanical properties and functions hinders their further development. This article reviews the main advantages of FGs, including enhanced mechanical properties, high conductivity, high antimicrobial and anti-inflammatory properties, stimulus responsiveness, and an extracellular matrix (ECM)-like structure. It also discusses the influence of assembly methods, such as fiber cross-linking, interfacial treatments of fibers with hydrogel matrices, and supramolecular assembly, on the diverse functionalities of FGs. Furthermore, the mechanisms for improving the performance of the above five aspects are discussed, such as creating ion carrier channels for conductivity, in situ gelation of drugs to enhance antibacterial and anti-inflammatory properties, and entanglement and hydrophobic interactions between fibers, resulting in ECM-like structured FGs. In addition, this review addresses the application of FGs in sensors, dressings, and tissue scaffolds based on the synergistic effects of optimizing the performance. Finally, challenges and future applications of FGs are discussed, providing a theoretical foundation and new insights for the design and application of cutting-edge FGs.

Recent Advances in Nanomaterial-Based Biosignal Sensors

Recent research for medical fields, robotics, and wearable electronics aims to utilize biosignal sensors to gather bio-originated information and generate new values such as evaluating user well-being, predicting behavioral patterns, and supporting disease diagnosis and prevention. Notably, most biosignal sensors are designed for body placement to directly acquire signals, and the incorporation of nanomaterials such as metal-based nanoparticles or nanowires, carbon-based or polymer-based nanomaterials-offering stretchability, high surface-to-volume ratio, and tunability for various properties-enhances their adaptability for such applications. This review categorizes nanomaterial-based biosignal sensors into three types and analyzes them: 1) biophysical sensors that detect deformation such as folding, stretching, and even pulse, 2) bioelectric sensors that capture electric signal originating from human body such as heart and nerves, and 3) biochemical sensors that catch signals from bio-originated fluids such as sweat, saliva and blood. Then, limitations and improvements to nanomaterial-based biosignal sensors is depicted. Lastly, it is highlighted on deep learning-based signal processing and human-machine interface applications, which can enhance the potential of biosignal sensors. Through this paper, it is aim to provide an understanding of nanomaterial-based biosignal sensors, outline the current state of the technology, discuss the challenges that be addressed, and suggest directions for development.

One-Step Electrochemical Modification of PEDOT:PSS/PBNPs Hybrid Hydrogel on the Screen-Printed Electrode Surface for Highly Sensitive Detection of Creatinine

Creatinine (CRE) is frequently measured in clinical practice due to its recognized significance as a pivotal biomarker across a spectrum of renal and cardiovascular disorders. However, the rapid and accurate detection of CRE for assessing kidney and muscle functions remains challenging. Here, we prepared the poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hydrogel uniformly loaded with Prussian blue nanoparticles (PBNPs) via a one-step metal-assisted electrochemical modification method on the screen-printed electrode for ultrasensitive CRE detection. The conductive, porous PEDOT:PSS/PBNPs hydrogel provided a confined space that facilitated highly efficient biocatalytic cascade reactions of creatinine amidohydrolase, creatine amidinohydrolase (Cl), and sarcosine oxidase, enabling the CRE detection with a high sensitivity (40.2 μA mM-1 cm-2), a wide linear detection range (20-600 μM), and a low detection limit (8.3 μM). What is more, we developed an integrated platform utilizing a differential strategy to eliminate the interference from endogenous creatine (CR), employing a dual-channel working electrode for independent CR and CRE detection, along with modules for signal processing and wireless communication. The differential method and system were validated in simulated blood, the detection error was reduced from 41.1% to 8.89% after applying the differential method, and the recoveries ranged from 89.5% to 107.8%, with errors remaining below 12%. This PEDOT:PSS/PBNPs hydrogel CRE biosensor, based on one-step modification method, offered a promising strategy for precise assessment of kidney and muscle health in both clinical and at-home settings.

Nerve-Mimetic Adhesive Hydrogel Electroceuticals: Tailoring In Situ Physically Entangled Domains in Singular Polymers

Implantable electrochemicals stand out as promising candidates for resolving peripheral nerve injuries. However, challenges persist in designing bioelectronic materials that mimic tissue due to modulus matching, conformal adhesion, and immune responses. Herein, we present a nerve-mimicking design rationale for biocompatible hydrogel-based electroceuticals with a tissue-like modulus, robust and conformal tissue adhesion, exceptional mechanical toughness, and efficient stress dissipation. Inspired by the hierarchical structure of the peripheral nerve, the hydrogel substrate features a structurally gradient bilayer transitioning from a dense to a loose polymeric network, utilizing alginate functionalized with either photo-cross-linkable methacrylate or tissue-adhesive phenylborate. Due to the varying water affinity of the tethering groups, a physically entangled interfacial domain is in situ formed during dehydration of the pre-gel film, resulting in enhanced mechanical toughness and strong adhesion. The hydrogel electroceuticals, when integrated with conducting polymeric electrodes, locally stimulate nerve tissue, improving tissue regeneration in a crushed nerve injury model.

Polyvinyl alcohol/sodium alginate-based conductive hydrogels with in situ formed bimetallic zeolitic imidazolate frameworks towards soft electronics

Bimetallic zeolitic imidazolate frameworks (BZIFs) have received enormous attention due to their unique physi-chemical properties, but are rarely reported for electrically conductive hydrogel (ECH) applications arising from low intrinsic conductivity and poor dispersion. Herein, we propose an innovative strategy to prepare highly conductive and mechanically robust ECHs by in situ growing Ni/Co-BZIFs within the polyvinyl alcohol/sodium alginate dual network (PZPS). 2-methylimidazole (MeIM) ligands copolymerize with pyrrole monomers, enhancing the electrical conductivity; meanwhile, MeIM ligands act as anchor points for in-situ formation of BZIFs, effectively avoiding phase-to-phase interfacial resistance and ensuring a uniform distribution in the hydrogel network. Due to the synergism of Ni/Co-BZIFs, the PZPS hydrogel exhibits a high areal capacitance of 630.3 mF·cm-2 at a current density of 0.5 mA·cm-2, promising for flexible energy storage devices. In addition, PZPS shows excellent mechanical strength and toughness (with an ultimate tensile strength of 405.0 kPa and a toughness of 784.2 kJ·m-3 at an elongation at break of 474.0 %), a high gauge factor of up to 4.18 over an extremely wide stress range of 0-42 kPa when used as flexible wearable strain/pressure sensors. This study provides new insights to incorporating highly conductive and uniformly dispersed ZIFs into hydrogels for flexible wearable electronics.

Hydrogels in wearable neural interfaces

The integration of wearable neural interfaces (WNIs) with the human nervous system has marked a significant progression, enabling progress in medical treatments and technology integration. Hydrogels, distinguished by their high-water content, low interfacial impedance, conductivity, adhesion, and mechanical compliance, effectively address the rigidity and biocompatibility issues common in traditional materials. This review highlights their important parameters-biocompatibility, interfacial impedance, conductivity, and adhesiveness-that are integral to their function in WNIs. The applications of hydrogels in wearable neural recording and neurostimulation are discussed in detail. Finally, the opportunities and challenges faced by hydrogels for WNIs are summarized and prospected. This review aims to offer a thorough examination of hydrogel technology's present landscape and to encourage continued exploration and innovation. As developments progress, hydrogels are poised to revolutionize wearable neural interfaces, offering significant enhancements in healthcare and technological applications.

Graphical Abstract

Thermal and light-driven soft actuators based on a conductive polypyrrole nanofibers integrated poly(N-isopropylacrylamide) hydrogel with intelligent response

The development of soft hydrogel actuators with outstanding mechanical properties, fast actuation speed, and available quantification of self-sensing actuation remains a challenging endeavor. In this work, dopamine-decorated polypyrrole nanofibers (DAPPy) were introduced into the polyethylene glycol diacrylate (PEGDA)-crosslinked poly(N-isopropyl acrylamide) network to generate a stretchable, NIR-responsive, and strain sensitive DAPPy/PNIPAM hydrogel layer. Besides, this active layer was combined with the passive ligninsulfonate sodium/polyacrylamide (LS/PAAM) to give DAPPy/PNIPAM//LS/PAAM bilayer hydrogel actuator, which exhibits ultrafast thermo-responsive actuation (19°/s) and underwater grasping and lifting performance. Moreover, the DAPPy/PNIPAM layer has excellent electrical conductivity (0.29 S/m) and thermal conversion ability (10.8 °C/min), which enable such a conductive hydrogel to act as a highly sensitive strain and temperature sensor with real-time resistance change in response to tensile strain (gauge factor up to 3.4), applied pressure, temperature, and remote NIR light irradiation. More importantly, the bilayer hydrogel actuator can integrate both actuation and self-sensing functions through the bending angle-surface temperature-relative resistance change relationship of the photothermal process. With excellent mechanical actuation and self-sensing ability, the resulting bilayer hydrogel showed a promising application potential as soft biomimetic actuating materials and soft intelligent actuators.

Strong, tough and environment-tolerant organohydrogels for flaw-insensitive strain sensing

Conductive organohydrogels are promising for strain sensing, while their weak mechanical properties, poor crack propagation resistance and unstable sensing signals during long-term use have seriously limited their applications as high-performance strain sensors. Here, we propose a facile method, i.e., solvent exchange assisted hot-pressing, to prepare strong yet tough, transparent and anti-fatigue ionically conductive organohydrogels (ICOHs). The densified polymeric network and improved crystallinity endow ICOHs with excellent mechanical properties. The tensile strength, toughness, fracture energy and fatigue threshold of ICOHs can reach 36.12 ± 4.15 MPa, 54.57 ± 2.89 MJ m-3, 43.44 ± 8.54 kJ m-2 and 1212.86 ± 57.20 J m-2, respectively, with a satisfactory fracture strain of 266 ± 33%. In addition, ICOH strain sensors with freezing and drying resistance exhibit excellent cycling stability (10 000 cycles). More importantly, the fatigue resistance allows the notched strain sensor to work normally with no crack propagation and output stable and reliable sensing signals. Overall, the unique flaw-insensitive strain sensing makes ICOHs promising in the field of wearable and durable electronics.

Unlocking Spatial Surface Energy in Porous Skeletons: a Pathway to Bridging Electronic Circuits from 2D to 3D Architectures

Conventional approaches to creating high-resolution electric circuits face challenges such as the requirement for skilled personnel and expensive equipment. In response, we propose an innovative strategy that leverages a photochemically modified porous polymer skeleton for in-situ circuit fabrication. By developing maskless surface energy manipulation that guides PEDOT:PSS-based conductive ink deposition, electric circuits with high precision, density, stability and adaptability are effortlessly engineered within or atop the porous skeleton, enabling transitions between 2D and 3D circuit configurations. This process simplifies prototyping while significantly reducing costs and maintaining efficiency, promising advancements across various technological sectors.

Highly deformable bi-continuous conducting polymer hydrogels for electrochemical energy storage

Conducting polymer hydrogels with inherent flexibility, ionic conductivity and environment friendliness are promising materials in the fields of energy storage. However, a trade-off between mechanical and electrochemical properties has limited the development of flexible/stretchable conducting polymer hydrogel electrodes, owing to the intrinsic conflict among mechanical and electrical phases. Here, we report a reliable design to enable conducting polymer with both exceptional mechanical and electrical/electrochemical performance through the construction of bi-continuous conducting polymer crosslinked network. The resultant bi-continuous conducting polymer hydrogels (BCPH) demonstrate significantly improved mechanical and electrochemical properties compared to the conventional conducting polymer hydrogel (CPH) electrode. BCPH presents a high specific capacitance of 715 F g-1 at 0.5 A/g, a high mechanical strength (∼1 MPa) and a large stretchability (∼300%). Enabled by such intrinsically deformability and electrochemical properties, we further demonstrate its utility in flexible solid-state supercapacitor (FSSC), which exhibits an outstanding specific capacitance of 760 mF cm-2 at 2 mA cm-2, excellent electrochemical stability with 81% capacitance retention after 5000 charge/discharge cycles, and superior bending cycle stability. This simple and scalable strategy provides a platform for the fabrication of high-performance conducting hydrogel electrodes for various wearable electronic equipment.

Advances of conductive hydrogel designed for flexible electronics: A review

In recent years, there has been considerable attention devoted to flexible electronic devices within the realm of biomedical engineering. These devices demonstrate the capability to accurately capture human physiological signals, thereby facilitating efficient human-computer interaction, and providing a novel approach of flexible electronics for monitoring and treating related diseases. A notable contribution to this domain is the emergence of conductive hydrogels as a novel flexible electronic material. Renowned for their exceptional flexibility, adjustable electrical conductivity, and facile processing, conductive hydrogels have emerged as the preferred material for designing and fabricating innovative flexible electronic devices. This paper provides a comprehensive review of the recent advancements in flexible electronic devices rooted in conductive hydrogels. It offers an in-depth exploration of existing synthesis strategies for conductive hydrogels and subsequently examines the latest progress in their applications, including flexible neural electrodes, sensors, energy storage devices and soft robots. The analysis extends to the identification of technological challenges and developmental opportunities in both the synthesis of new conductive hydrogels and their application in the dynamic field of flexible electronics.

PEDOT-based stretchable optoelectronic materials and devices for bioelectronic interfaces

The rapid development of wearable and implantable electronics has enabled the real-time transmission of electrophysiological signals in situ, thus allowing the precise monitoring and regulation of biological functions. Devices based on organic materials tend to have low moduli and intrinsic stretchability, making them ideal choices for the construction of seamless bioelectronic interfaces. In this case, as an organic ionic-electronic conductor, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has low impedance to offer a high signal-to-noise ratio for monitoring bioelectrical signals, which has become one of the most promising conductive polymers. However, the initial conductivity and stretchability of pristine PEDOT:PSS are insufficient to meet the application requirements, and there is a trade-off between their improvement. In addition, PEDOT:PSS has poor stability in aqueous environments due to the hygroscopicity of the PSS chains, which severely limits its long-term applications in water-rich bioelectronic interfaces. Considering the growing demands of multi-function integration, the high-resolution fabrication of electronic devices is urgent. It is a great challenge to maintain both electrical and mechanical performance after miniaturization, particularly at feature sizes below 100 μm. In this review, we focus on the combined improvement in the conductivity and stretchability of PEDOT:PSS, as well as the corresponding mechanisms in detail. Also, we summarize the effective strategies to improve the stability of PEDOT:PSS in aqueous environments, which plays a vital role in long-term applications. Finally, we introduce the reliable micropatterning technologies and PEDOT:PSS-based stretchable optoelectronic devices applied at bio-interfaces.

Myelin Sheath-Inspired Hydrogel Electrode for Artificial Skin and Physiological Monitoring

Significant advancements in hydrogel-based epidermal electrodes have been made in recent years. However, inherent limitations, such as adaptability, adhesion, and conductivity, have presented challenges, thereby limiting the sensitivity, signal-to-noise ratio (SNR), and stability of the physiological-electrode interface. In this study, we propose the concept of myelin sheath-inspired hydrogel epidermal electronics by incorporating numerous interpenetrating core-sheath-structured conductive nanofibers within a physically cross-linked polyelectrolyte network. Poly(3,4-ethylenedioxythiophene)-coated sulfonated cellulose nanofibers (PEDOT:SCNFs) are synthesized through a simple solvent-catalyzed sulfonation process, followed by oxidative self-polymerization and ionic liquid (IL) shielding steps, achieving a low electrochemical impedance of 42 Ω. The physical associations within the composite hydrogel network include complexation, electrostatic forces, hydrogen bonding, π-π stacking, hydrophobic interaction, and weak entanglements. These properties confer the hydrogel with high stretchability (770%), superconformability, self-adhesion (28 kPa on pigskin), and self-healing capabilities. By simulating the saltatory propagation effect of the nodes of Ranvier in the nervous system, the biomimetic hydrogel establishes high-fidelity epidermal electronic interfaces, offering benefits such as low interfacial contact impedance, significantly increased SNR (30 dB), as well as large-scale sensor array integration. The advanced biomimetic hydrogel holds tremendous potential for applications in electronic skin (e-skin), human-machine interfaces (HMIs), and healthcare assessment devices.

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.

Introductory Review of Soft Implantable Bioelectronics Using Conductive and Functional Hydrogels and Hydrogel Nanocomposites

Interfaces between implantable bioelectrodes and tissues provide critical insights into the biological and pathological conditions of targeted organs, aiding diagnosis and treatment. While conventional bioelectronics, made from rigid materials like metals and silicon, have been essential for recording signals and delivering electric stimulation, they face limitations due to the mechanical mismatch between rigid devices and soft tissues. Recently, focus has shifted toward soft conductive materials, such as conductive hydrogels and hydrogel nanocomposites, known for their tissue-like softness, biocompatibility, and potential for functionalization. This review introduces these materials and provides an overview of recent advances in soft hydrogel nanocomposites for implantable electronics. It covers material strategies for conductive hydrogels, including both intrinsically conductive hydrogels and hydrogel nanocomposites, and explores key functionalization techniques like biodegradation, bioadhesiveness, injectability, and self-healing. Practical applications of these materials in implantable electronics are also highlighted, showcasing their effectiveness in real-world scenarios. Finally, we discuss emerging technologies and future needs for chronically implantable bioelectronics, offering insights into the evolving landscape of this field.

Probing the ionic activation enthalpies in anionic polysaccharide xerogel-based single ion conductor for temperature sensing

Ionic charge transport in polymer-based solid electrolytes is significantly affected by thermal perturbations, facilitating the detection of temperature variations. However, the impact of ionic interactions and molecular arrangements in polymeric single-ion conductors (SICs) has not been thoroughly investigated for temperature sensing. By probing the effect of the associated energies for ionic interactions and polymeric rearrangements, the thermal sensing characteristics of alginate have been studied. For the first time, alginate SIC interacting with multivalent ions (viz., Na+, Ca2+ and Fe3+) to form xerogel has been exploited as a temperature-sensing layer by fabricating a xerogel-based ionic thermistor (xIT) as a temperature sensor. The xIT has demonstrated stable functioning from 25 to 70 °C and unveiled enhanced sensing abilities in the physiological state of the human body (35-40 °C), exhibiting a monotonic linear response, high sensitivity (-3.77 % °C-1), and high accuracy (0.1 °C). The sensing characteristic is observed due to the inward ionic flux under thermal and electrical perturbations. The concentration of ionic charge carriers and ionic drift are assumed to be Arrhenius-activated processes. A general microscopic model of ion transport within polysaccharides has been elucidated via hopping mechanisms, and the effects of the associated activation energies on temperature sensitivity have been explained.

Zwitterionic cellulose nanofibers-based hydrogels with high toughness, ionic conductivity, and healable capability in cryogenic environments

Extreme environmental conditions often lead to irreversible structural failure and functional degradation in hydrogels, limiting their service life and applicability. Achieving high toughness, self-healing, and ionic conductivity in cryogenic environments is vital to broaden their applications. Herein, we present a novel approach to simultaneously enhance the toughness, self-healing, and ionic conductivity of hydrogels, via inducing non-freezable water within the zwitterionic cellulose-based hydrogel skeleton. This approach enables resulting hydrogel to achieve an exceptional toughness of 10.8 MJ m-3, rapid self-healing capability (98.9 % in 30 min), and high ionic conductivity (2.9 S m-1), even when subjected to -40 °C, superior to the state-of-the-art hydrogels. Mechanism analyses reveal that a significant amount of non-freezable water with robust electrostatic interactions is formed within zwitterionic cellulose nanofibers-modified polyurethane molecular networks, imparting superior freezing tolerance and versatility to the hydrogel. Importantly, this strategy harnesses the non-freezable water molecular state of the zwitterionic cellulose nanofibers network, eliminating the need for additional antifreeze and organic solvents. Furthermore, the dynamic Zn coordination within these supramolecular molecule chains enhances interfacial interactions, thereby promoting rapid subzero self-healing and exceptional mechanical strength. Demonstrating its potential, this hydrogel can be used in smart laminated materials, such as aircraft windshields.

Highly-stable, injectable, conductive hydrogel for chronic neuromodulation

Electroceuticals, through the selective modulation of peripheral nerves near target organs, are promising for treating refractory diseases. However, the small sizes and the delicate nature of these nerves present challenges in simplifying the fixation and stabilizing the electrical-coupling interface for neural electrodes. Herein, we construct a robust neural interface for fine peripheral nerves using an injectable bio-adhesive hydrogel bioelectronics. By incorporating a multifunctional molecular regulator during network formation, we optimize the injectability and conductivity of the hydrogel through fine-tuning reaction kinetics and multi-scale interactions within the conductive network. Meanwhile, the mechanical and electrical stability of the hydrogel is achieved without compromising its injectability. Minimal tissue damage along with low and stable impedance of the injectable neural interface enables chronic vagus neuromodulation for myocardial infarction therapy in the male rat model. Our highly-stable, injectable, conductive hydrogel bioelectronics are readily available to target challenging anatomical locations, paving the way for future precision bioelectronic medicine.

Highly Conductive and Stretchable Hydrogel Nanocomposite Using Whiskered Gold Nanosheets for Soft Bioelectronics

The low electrical conductivity of conductive hydrogels limits their applications as soft conductors in bioelectronics. This low conductivity originates from the high water content of hydrogels, which impedes facile carrier transport between conductive fillers. This study presents a highly conductive and stretchable hydrogel nanocomposite comprising whiskered gold nanosheets. A dry network of whiskered gold nanosheets is fabricated and then incorporated into the wet hydrogel matrices. The whiskered gold nanosheets preserve their tight interconnection in hydrogels despite the high water content, providing a high-quality percolation network even under stretched states. Regardless of the type of hydrogel matrix, the gold-hydrogel nanocomposites exhibit a conductivity of ≈520 S cm-1 and a stretchability of ≈300% without requiring a dehydration process. The conductivity reaches a maximum of ≈3304 S cm-1 when the density of the dry gold network is controlled. A gold-adhesive hydrogel nanocomposite, which can achieve conformal adhesion to moving organ surfaces, is fabricated for bioelectronics demonstrations. The adhesive hydrogel electrode outperforms elastomer-based electrodes in in vivo epicardial electrogram recording, epicardial pacing, and sciatic nerve stimulation.

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.

High-Performance Organic-Inorganic Hybrid Conductive Hydrogels for Stretchable Elastic All-Hydrogel Supercapacitors and Flexible Self-Powered Integrated Systems

Conductive polymer hydrogels exhibit unique electrical, electrochemical, and mechanical properties, making them highly competitive electrode materials for stretchable high-capacity energy storage devices for cutting-edge wearable electronics. However, it remains extremely challenging to simultaneously achieve large mechanical stretchability, high electrical conductivity, and excellent electrochemical properties in conductive polymer hydrogels because introducing soft insulating networks for improving stretchability inevitably deteriorates the connectivity of rigid conductive domain and decreases the conductivity and electrochemical activity. This work proposes a distinct confinement self-assembly and multiple crosslinking strategy to develop a new type of organic-inorganic hybrid conductive hydrogels with biphase interpenetrating cross-linked networks. The hydrogels simultaneously exhibit high conductivity (2000 S m-1), large stretchability (200%), and high electrochemical activity, outperforming existing conductive hydrogels. The inherent mechanisms for the unparalleled comprehensive performances are thoroughly investigated. Elastic all-hydrogel supercapacitors are prepared based on the hydrogels, showing high specific capacitance (212.5 mF cm-2), excellent energy density (18.89 µWh cm-2), and large deformability. Moreover, flexible self-powered luminescent integrated systems are constructed based on the supercapacitors, which can spontaneously shine anytime and anywhere without extra power. This work provides new insights and feasible avenues for developing high-performance stretchable electrode materials and energy storage devices for wearable electronics.

Highly conductive, stretchable, and biocompatible graphene oxide biocomposite hydrogel for advanced tissue engineering

The importance of hydrogels in tissue engineering cannot be overemphasized due to their resemblance to the native extracellular matrix. However, natural hydrogels with satisfactory biocompatibility exhibit poor mechanical behavior, which hampers their application in stress-bearing soft tissue engineering. Here, we describe the fabrication of a double methacrylated gelatin bioink covalently linked to graphene oxide (GO) via a zero-length crosslinker, digitally light-processed (DLP) printable into 3D complex structures with high fidelity. The resultant natural hydrogel (GelGOMA) exhibits a conductivity of 15.0 S m-1as a result of the delocalization of theπ-orbital from the covalently linked GO. Furthermore, the hydrogel shows a compressive strength of 1.6 MPa, and a 2.0 mm thick GelGOMA can withstand a 1.0 kg ms-1momentum. The printability and mechanical strengths of GelGOMAs were demonstrated by printing a fish heart with a functional fluid pumping mechanism and tricuspid valves. Its biocompatibility, electroconductivity, and physiological relevance enhanced the proliferation and differentiation of myoblasts and neuroblasts and the contraction of human-induced pluripotent stem cell-derived cardiomyocytes. GelGOMA demonstrates the potential for the tissue engineering of functional hearts and wearable electronic devices.

Recent Advances of Stretchable Nanomaterial-Based Hydrogels for Wearable Sensors and Electrophysiological Signals Monitoring

Electrophysiological monitoring is a commonly used medical procedure designed to capture the electrical signals generated by the body and promptly identify any abnormal health conditions. Wearable sensors are of great significance in signal acquisition for electrophysiological monitoring. Traditional electrophysiological monitoring devices are often bulky and have many complex accessories and thus, are only suitable for limited application scenarios. Hydrogels optimized based on nanomaterials are lightweight with excellent stretchable and electrical properties, solving the problem of high-quality signal acquisition for wearable sensors. Therefore, the development of hydrogels based on nanomaterials brings tremendous potential for wearable physiological signal monitoring sensors. This review first introduces the latest advancement of hydrogels made from different nanomaterials, such as nanocarbon materials, nanometal materials, and two-dimensional transition metal compounds, in physiological signal monitoring sensors. Second, the versatile properties of these stretchable composite hydrogel sensors are reviewed. Then, their applications in various electrophysiological signal monitoring, such as electrocardiogram monitoring, electromyographic signal analysis, and electroencephalogram monitoring, are discussed. Finally, the current application status and future development prospects of nanomaterial-optimized hydrogels in wearable physiological signal monitoring sensors are summarized. We hope this review will inspire future development of wearable electrophysiological signal monitoring sensors using nanomaterial-based hydrogels.

Stretchable Electrodes for Interconnects in Soft Electronics

Soft electronics have significantly enhanced user convenience and data accuracy in wearable devices, implantable devices, and human-machine interfaces. However, a persistent challenge in their development has been the disconnection between the rigid and soft components of devices due to the substantial difference in modulus and stretchability. To address this issue, establishing a durable and flexible connection that smoothly links components of varying stiffness to signal-capturing sections with a lower stiffness is essential. In this study, we developed a novel stretchable interconnect that strongly adheres to various materials, facilitating electrical connections effortlessly by applying minimal finger pressure. Capable of stretching up to 1000% while maintaining electrical integrity, this interconnect proves its applicability across multiple domains, including electrocardiogram (ECG), electromyography (EMG), and stretchable light-emitting diode (LED) circuits. Its versatility is further demonstrated through its compatibility with various manufacturing techniques such as 3D printing, painting, and spin coating, highlighting its adaptability in soft electronics.

A Highly Stretchable, Conductive, and Transparent Bioadhesive Hydrogel as a Flexible Sensor for Enhanced Real-Time Human Health Monitoring

Real-time continuous monitoring of non-cognitive markers is crucial for the early detection and management of chronic conditions. Current diagnostic methods are often invasive and not suitable for at-home monitoring. An elastic, adhesive, and biodegradable hydrogel-based wearable sensor with superior accuracy and durability for monitoring real-time human health is developed. Employing a supramolecular engineering strategy, a pseudo-slide-ring hydrogel is synthesized by combining polyacrylamide (pAAm), β-cyclodextrin (β-CD), and poly 2-(acryloyloxy)ethyltrimethylammonium chloride (AETAc) bio ionic liquid (Bio-IL). This novel approach decouples conflicting mechano-chemical effects arising from different molecular building blocks and provides a balance of mechanical toughness (1.1 × 106 Jm-3), flexibility, conductivity (≈0.29 S m-1), and tissue adhesion (≈27 kPa), along with rapid self-healing and remarkable stretchability (≈3000%). Unlike traditional hydrogels, the one-pot synthesis avoids chemical crosslinkers and metallic nanofillers, reducing cytotoxicity. While the pAAm provides mechanical strength, the formation of the pseudo-slide-ring structure ensures high stretchability and flexibility. Combining pAAm with β-CD and pAETAc enhances biocompatibility and biodegradability, as confirmed by in vitro and in vivo studies. The hydrogel also offers transparency, passive-cooling, ultraviolet (UV)-shielding, and 3D printability, enhancing its practicality for everyday use. The engineered sensor demonstratesimproved efficiency, stability, and sensitivity in motion/haptic sensing, advancing real-time human healthcare monitoring.

Interface-Mediated Neurogenic Signaling: The Impact of Surface Geometry and Chemistry on Neural Cell Behavior for Regenerative and Brain-Machine Interfacing Applications

Nanomaterial advancements have driven progress in central and peripheral nervous system applications such as tissue regeneration and brain-machine interfacing. Ideally, neural interfaces with native tissue shall seamlessly integrate, a process that is often mediated by the interfacial material properties. Surface topography and material chemistry are significant extracellular stimuli that can influence neural cell behavior to facilitate tissue integration and augment therapeutic outcomes. This review characterizes topographical modifications, including micropillars, microchannels, surface roughness, and porosity, implemented on regenerative scaffolding and brain-machine interfaces. Their impact on neural cell response is summarized through neurogenic outcome and mechanistic analysis. The effects of surface chemistry on neural cell signaling with common interfacing compounds like carbon-based nanomaterials, conductive polymers, and biologically inspired matrices are also reviewed. Finally, the impact of these extracellular mediated neural cues on intracellular signaling cascades is discussed to provide perspective on the manipulation of neuron and neuroglia cell microenvironments to drive therapeutic outcomes.