Self-healing conductive hydrogels: preparation, properties and applications

Recent advances in nanocellulose based hydrogels: Preparation strategy, typical properties and food application

Nanocellulose has been favored as one of the most promising sustainable nanomaterials, due to its competitive advantages and superior performances such as hydrophilicity, renewability, biodegradability, biocompatibility, tunable surface features, excellent mechanical strength, and high specific surface area. Based on the above properties of nanocellulose and the advantages of hydrogels such as high water absorption, adsorption, porosity and structural adjustability, nanocellulose based hydrogels integrating the benefits of both have attracted extensive attention as promising materials in various fields. In this review, the main fabrication strategies of nanocellulose based hydrogels are initially discussed in terms of different crosslinking methods. Then, the typical properties of nanocellulose based hydrogels are comprehensively summarized, including porous structure, swelling ability, adsorption, mechanical, self-healing, smart response performances. Especially, relying on these properties, the general application of nanocellulose based hydrogels in food field is also discussed, mainly including food packaging, food detection, nutrient embedding delivery, 3D food printing, and enzyme immobilization. Finally, the safety of nanocellulose based hydrogel is summarized, and the current challenges and future perspectives of nanocellulose based hydrogels are put forward.

Dynamic Metal-Coordinated Adhesive and Self-Healable Antifreezing Hydrogels for Strain Sensing, Flexible Supercapacitors, and EMI Shielding Applications

Dynamic metal-coordinated adhesive and self-healable hydrogel materials have garnered significant attention in recent years due to their potential applications in various fields. These hydrogels can form reversible metal-ligand bonds, resulting in a network structure that can be easily broken and reformed, leading to self-healing capabilities. In addition, these hydrogels possess excellent mechanical strength and flexibility, making them suitable for strain-sensing applications. In this work, we have developed a mechanically robust, highly stretchable, self-healing, and adhesive hydrogel by incorporating Ca2+-dicarboxylate dynamic metal-ligand cross-links in combination with low density chemical cross-links into a poly(acrylamide-co-maleic acid) copolymer structure. Utilizing the reversible nature of the Ca2+-dicarboxylate bond, the hydrogel exhibited a tensile strength of up to ∼250 kPa and was able to stretch to 15-16 times its original length. The hydrogel exhibited a high fracture energy of ∼1500 J m-2, similar to that of cartilage. Furthermore, the hydrogel showed good recovery, fatigue resistance, and fast self-healing properties due to the reversible Ca2+-dicarboxylate cross-links. The presence of Ca2+ resulted in a highly conductive hydrogel, which was utilized to design a flexible resistive strain sensor. This hydrogel can strongly adhere to different substrates, making it advantageous for applications in flexible electronic devices. When adhered to human body parts, the hydrogel can efficiently detect limb movements. The hydrogel also exhibited excellent performance as a solid electrolyte for flexible supercapacitors, with a capacitance of ∼260 F/g at 0.5 A/g current density. Due to its antifreezing and antidehydration properties, this hydrogel retains its flexibility at subzero temperatures for an extended period. Additionally, the porous network and high water content of the hydrogel impart remarkable electromagnetic attenuation properties, with a value of ∼38 dB in the 14.5-20.5 GHz frequency range, which is higher than any other hydrogel without conducting fillers. Overall, the hydrogel reported in this study exhibits diverse applications as a strain sensor, solid electrolyte for flexible supercapacitors, and efficient material for electromagnetic attenuation. Its multifunctional properties make it a promising candidate for use in various fields as a state-of-the-art material.

A Review of Conductive Hydrogel-Based Wearable Temperature Sensors

Conductive hydrogel has garnered significant attention as an emergent candidate for diverse wearable sensors, owing to its remarkable and tailorable properties such as flexibility, biocompatibility, and strong electrical conductivity. These attributes make it highly suitable for various wearable sensor applications (e.g., biophysical, bioelectrical, and biochemical sensors) that can monitor human health conditions and provide timely interventions. Among these applications, conductive hydrogel-based wearable temperature sensors are especially important for healthcare and disease surveillance. This review aims to provide a comprehensive overview of conductive hydrogel-based wearable temperature sensors. First, this work summarizes different types of conductive fillers-based hydrogel, highlighting their recent developments and advantages as wearable temperature sensors. Next, this work discusses the sensing characteristics of conductive hydrogel-based wearable temperature sensors, focusing on sensitivity, dynamic stability, stretchability, and signal output. Then, state-of-the-art applications are introduced, ranging from body temperature detection and wound temperature detection to disease monitoring. Finally, this work identifies the remaining challenges and prospects facing this field. By addressing these challenges with potential solutions, this review hopes to shed some light on future research and innovations in this promising field.

Natural Underwater Bioadhesive Offering Cohesion Modulation via Hydrogen Bond Disruptor: A Highly Injectable and in Vivo Stable Remedy for Gastric Ulcer Resolution

Injectable bioadhesives are attractive for managing gastric ulcers through minimally invasive procedures. However, the formidable challenge is to develop bioadhesives that exhibit high injectability, rapidly adhere to lesion tissues with fast gelation, provide reliable protection in the harsh gastric environment, and simultaneously ensure stringent standards of biocompatibility. Here, a natural bioadhesive with tunable cohesion is developed based on the facile and controllable gelation between silk fibroin and tannic acid. By incorporating a hydrogen bond disruptor (urea or guanidine hydrochloride), the inherent network within the bioadhesive is disturbed, inducing a transition to a fluidic state for smooth injection (injection force <5 N). Upon injection, the fluidic bioadhesive thoroughly wets tissues, while the rapid diffusion of the disruptor triggers instantaneous in situ gelation. This orchestrated process fosters the formed bioadhesive with durable wet tissue affinity and mechanical properties that harmonize with gastric tissues, thereby bestowing long-lasting protection for ulcer healing, as evidenced through in vitro and in vivo verification. Moreover, it can be conveniently stored (≥3 m) postdehydration. This work presents a promising strategy for designing highly injectable bioadhesives utilizing natural feedstocks, avoiding any safety risks associated with synthetic materials or nonphysiological gelation conditions, and offering the potential for minimally invasive application.

Conductive Gels for Energy Storage, Conversion, and Generation: Materials Design Strategies, Properties, and Applications

Gel-based materials have garnered significant interest in recent years, primarily due to their remarkable structural flexibility, ease of modulation, and cost-effective synthesis methodologies. Specifically, polymer-based conductive gels, characterized by their unique conjugated structures incorporating both localized sigma and pi bonds, have emerged as materials of choice for a wide range of applications. These gels demonstrate an exceptional integration of solid and liquid phases within a three-dimensional matrix, further enhanced by the incorporation of conductive nanofillers. This unique composition endows them with a versatility that finds application across a diverse array of fields, including wearable energy devices, health monitoring systems, robotics, and devices designed for interactive human-body integration. The multifunctional nature of gel materials is evidenced by their inherent stretchability, self-healing capabilities, and conductivity (both ionic and electrical), alongside their multidimensional properties. However, the integration of these multidimensional properties into a single gel material, tailored to meet specific mechanical and chemical requirements across various applications, presents a significant challenge. This review aims to shed light on the current advancements in gel materials, with a particular focus on their application in various devices. Additionally, it critically assesses the limitations inherent in current material design strategies and proposes potential avenues for future research, particularly in the realm of conductive gels for energy applications.

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.

Advancements in Regenerative Hydrogels in Skin Wound Treatment: A Comprehensive Review

This state-of-the-art review explores the emerging field of regenerative hydrogels and their profound impact on the treatment of skin wounds. Regenerative hydrogels, composed mainly of water-absorbing polymers, have garnered attention in wound healing, particularly for skin wounds. Their unique properties make them well suited for tissue regeneration. Notable benefits include excellent water retention, creating a crucially moist wound environment for optimal healing, and facilitating cell migration, and proliferation. Biocompatibility is a key feature, minimizing adverse reactions and promoting the natural healing process. Acting as a supportive scaffold for cell growth, hydrogels mimic the extracellular matrix, aiding the attachment and proliferation of cells like fibroblasts and keratinocytes. Engineered for controlled drug release, hydrogels enhance wound healing by promoting angiogenesis, reducing inflammation, and preventing infection. The demonstrated acceleration of the wound healing process, particularly beneficial for chronic or impaired healing wounds, adds to their appeal. Easy application and conformity to various wound shapes make hydrogels practical, including in irregular or challenging areas. Scar minimization through tissue regeneration is crucial, especially in cosmetic and functional regions. Hydrogels contribute to pain management by creating a protective barrier, reducing friction, and fostering a soothing environment. Some hydrogels, with inherent antimicrobial properties, aid in infection prevention, which is a crucial aspect of successful wound healing. Their flexibility and ability to conform to wound contours ensure optimal tissue contact, enhancing overall treatment effectiveness. In summary, regenerative hydrogels present a promising approach for improving skin wound healing outcomes across diverse clinical scenarios. This review provides a comprehensive analysis of the benefits, mechanisms, and challenges associated with the use of regenerative hydrogels in the treatment of skin wounds. In this review, the authors likely delve into the application of rational design principles to enhance the efficacy and performance of hydrogels in promoting wound healing. Through an exploration of various methodologies and approaches, this paper is poised to highlight how these principles have been instrumental in refining the design of hydrogels, potentially revolutionizing their therapeutic potential in addressing skin wounds. By synthesizing current knowledge and highlighting potential avenues for future research, this review aims to contribute to the advancement of regenerative medicine and ultimately improve clinical outcomes for patients with skin wounds.

MXene/Silk Fibroin Strengthened PVA-Based Eutectogel with Excellent Self-Healing Ability and Environmental Adaptability: Design, Synthesis, and Sensing Application

A superelastic self-healing eutectogel was designed and prepared using poly (vinyl alcohol) (PVA) as the bulk skeleton material, while silk fibroin (SF) and two-dimensional (2D) MXene (Ti3C2TX) as reinforcing fillers. In brief, the eutectogel possesses a high tensile strength of 7.63 MPa, and its elongation at break reached 1115.2%, higher than most reported polymers (<1000%). In addition, the eutectogel-assembled sensor has a high ionic conductivity of 0.61 S/m and a high strain sensitivity of 5.17 kPa-1. Moreover, eutectogel shows excellent self-healing ability and can achieve self-healing quickly within 10 min, while its tensile strength and elongation at break can be restored to 84.7% and 97.4% of the initial levels. Besides, a stable electrical signal can be transmitted after 200 cycles at 30% strain. Finally, the eutectogel can withstand various environmental conditions, such as atmospheric or even vacuum evaporation and low-temperature freezing, while maintaining good mechanical and sensing performances. The assembled flexible sensors based on the eutectogel demonstrate their significant application prospects in wearable devices, especially human physiological monitoring.

Current status of development and biomedical applications of peptide-based antimicrobial hydrogels

Microbial contamination poses a serious threat to human life and health. Through the intersection of material science and modern medicine, advanced bionic hydrogels have shown great potential for biomedical applications due to their unique bioactivity and ability to mimic the extracellular matrix environment. In particular, as a promising antimicrobial material, the synthesis and practical biomedical applications of peptide-based antimicrobial hydrogels have drawn increasing research interest. The synergistic effect of peptides and hydrogels facilitate the controlled release of antimicrobial agents and mitigation of their biotoxicity while achieving antimicrobial effects and protecting the active agents from degradation. This review reports on the progress and trends of researches in the last five years and provides a brief outlook, aiming to provide theoretical background on peptide-based antimicrobial hydrogels and make suggestions for future related work.

Sandwich-Type Self-Healing Sensor with Multilevel for Motion Detection

Real-time detection of various parts of the human body is crucial in medical monitoring and human-machine technology. However, existing self-healing flexible sensing materials are limited in real-life applications due to the weak stability of conductive networks and difficulty in balancing stretchability and self-healing properties. Therefore, the development of wearable flexible sensors with high sensitivity and fast response with self-healing properties is of great interest. In this paper, a novel multilevel self-healing polydimethylsiloxane (PDMS) material is proposed for enhanced sensing capabilities. The PDMS was designed to have multiple bonding mechanisms including hydrogen bonding, coordination bonding, disulfide bonding, and local covalent bonding. To further enhance its sensing properties, modified carbon nanotubes (CNTs) were embedded within the PDMS matrix using a solvent etching technique. This created a sandwich-type sensing material with improved stability and sensitivity. This self-healing flexible sensing material (self-healing efficiency = 70.1% at 80 °C and 6 h) has good mechanical properties (stretchability ≈413%, tensile strength ≈0.69 MPa), thermal conductivity, and electrical conductivity. It has ultrahigh sensitivity, which makes it possible to be manufactured as a multifunctional flexible sensor.

Multifunctional self-healing hydrogels via nanoengineering of colloidal and polymeric cellulose

The unique features of self-healing hydrogels hold great potential for biomedical applications including injectable hydrogels for cancer treatment, procedures for tumor removal or resection. However, the fabrication of durable and multifunctional self-healing hydrogels composed of biocompatible, green building blocks via versatile synthetic methodology continues to pose a significant challenge. Here, we engineered dialdehyde cellulose (DAC, as a macromolecular bio-crosslinker), and electrosterically stabilized nanocrystalline cellulose (ENCC, as a ligand-targeted drug carrier) to facilitate a strategy for the construction of self-healing hydrogels. Benefiting from its high carboxyl group density, ENCC was functionalized with folic acid (FA) using a non-toxic DMTMM coupling agent and loaded with doxorubicin (DOX, a model drug) through electrostatic interactions. A natural self-healing hydrogel was prepared from carboxymethyl chitosan (CCTS) and DAC mixed with DOX-loaded FA-ENCC using dynamic Schiff-base and hydrogen linkages. A combination of active supramolecular and vital covalent junctions led to a soft (storage modulus ∼500 Pa) and durable material, with rapid (< 5 min) reconstruction of molecular structure from fractured and injected to intact forms. The DAC-CCTS hydrogel showed an appreciable loading capacity of ∼5 mg g-1. Biocompatibility of the hydrogels was evaluated using cell viability and metabolic activity assays, showing lower metabolic activity due to sustained release of its cargo. These materials offer a versatile, sustainable, and green platform for the efficient construction of hydrogels, based on macro- and nano-engineered cellulose, the most abundant and easily accessible biopolymer.

An Injectable Composite Hydrogel of Verteporfin-Bonded Carboxymethyl Chitosan and Oxidized Sodium Alginate Facilitates Scarless Full-Thickness Skin Regeneration

Full-thickness skin defect has always been a major challenge in clinics due to fibrous hyperplasia in the repair process. Hydrogel composite dressings loaded with anti-fibrotic drugs have been considered as a promising strategy for scarless skin regeneration. In this work, a hydrogel composite (VP-CMCS-OSA) of carboxymethyl chitosan (CMCS) and oxidized sodium alginate (OSA), with loading anti-fibrotic drug verteporfin (VP), is developed based on two-step chemical reactions. Verteporfin is bonded with carboxymethyl chitosan through EDC/NHS treatment to form VP-CMCS, and then VP-CMCS is crosslinked with oxidized sodium alginate by Schiff base reaction to form VP-CMCS-OSA hydrogel. The characterization by SEM, FTIR, and UV-Vis shows the microstructure and chemical bonding of VP-CMCS-OSA. VP-CMCS-OSA hydrogel demonstrates the properties of high tissue adhesion, strong self-healing, and tensile ability. In the full-thickness skin defect model, the VP-CMCS-OSA composite hydrogels hasten wound healing due to the synergistic effects of hydrogels and verteporfin administration. The histological examination reveals the regular collagen arrangement and more skin appendages after VP-CMCS-OSA composite hydrogel treatment, indicating the full-thickness skin regeneration without potential scar formation. The outcomes suggest that the verteporfin-loaded composite hydrogel could be a potential method for scarless skin regeneration.

Fabrication of anti-freezing and self-healing nanocomposite hydrogels based on phytic acid and cellulose nanocrystals for high strain sensing applications

For hydrogel-based flexible sensors, it is a challenge to enhance the stability at sub-zero temperatures while maintaining good self-healing properties. Herein, an anti-freezing nanocomposite hydrogel with self-healing properties and conductivity was designed by introducing cellulose nanocrystals (CNCs) and phytic acid (PA). The CNCs were grafted with polypyrrole (PPy) by chemical oxidation, which were used as the nanoparticle reinforcement phase to reinforce the mechanical strength of hydrogels (851.8%). PA as a biomass material could form strong hydrogen bond interactions with H2O molecules, endowing hydrogels with prominent anti-freezing properties. Based on the non-covalent interactions, the self-healing rate of the hydrogels reached 92.9% at -15 °C as the content of PA was 40.0 wt%. Hydrogel-based strain sensors displayed high sensitivity (GF = 0.75), rapid response time (350 ms), good conductivity (3.1 S m-1) and stability at -15 °C. Various human movements could be detected by using them, including small (smile and frown) and large changes (elbow and knee bending). This work provides a promising method for the development of flexible wearable sensors that work stably in frigid environments.

Antibacterial hyaluronic acid hydrogels with enhanced self-healing properties via multiple dynamic bond crosslinking

The self-healing hydrogel offering intrinsic antibacterial activity is often required for the treatment of wounds because it can provide effective wound protection and prevent wound infection. Herein, antibacterial hyaluronic acid hydrogels with enhanced self-healing performances are prepared by multiple dynamic-bond crosslinking between aldehyde hyaluronic acid, 3, 3'- dithiobis (propionyl hydrazide) and fungal-sourced quaternized chitosan. Due to the formation of these different types of reversible interactions e.g. hydrazone bonds, disulfide bonds, and electrostatic interactions, the hyaluronic acid hydrogels can gel rapidly and exhibit excellent self-healing ability, which can heal completely within 1 h. Furthermore, the hydrogels show good antibacterial activity against E. coli and S. aureus with an inhibition ratio of ~100 % and above 75 %, respectively. Additionally, the hydrogels are cytocompatible, which makes them the potential for biomedical applications e.g. cell culture, tissue engineering, and wound dressing.

3D printable, stretchable, anti-freezing and rapid self-healing organogel-based sensors for human motion detection

Self-healing hydrogels have promising applications in sensors and wearable devices. However, self-healing hydrogels prepared with water as the dispersion medium inevitably freeze at sub-zero temperature, resulting in a loss of the self-healing and sensing ability. The black phosphorene / ethylene glycol / polyvinyl alcohol / sodium tetraborate / sodium alginate (BP/EG-SPB) organogels were prepared by 3D printing technology and solvent displacement method. The organogel exhibits high stretchability (1900 % strain), excellent self-healing property (25 s) and outstanding anti-freezing property (lower than -120 °C freezing point). Furthermore, the organogel can rapidly self-healed (150 s) at a low temperature (-80 °C) without any external stimulation. Additionally, this organogel-based flexible sensor possesses excellent sensitivity (gauge factor: 28.66 at 1900 % strain) and fast response capability, allowing for effective detection of human motion. This work provides a novel method for preparing multifunctional organogel-based sensors for use in harsh climates.

Protein/polysaccharide-based hydrogels loaded probiotic-mediated therapeutic systems: A review

The natural characteristics of protein/polysaccharide-based hydrogels, as a potential drug delivery platform, have attracted extensive attention. Probiotics have attracted renewed interest in drug research because of their beneficial effects on host health. The idea of using probiotics loaded on protein/polysaccharide-based hydrogels as potential drugs to treat different diseases has been put forward and shows great prospects. Based on this, in this review, we highlight the design strategy of hydrogels loaded probiotic-mediated therapy systems and review the potential diseases that have been proved to be treatable in the laboratory, including promoting wound healing and improving intestinal health and vaginal health, and discuss the challenges existing in the current design.

Bioadhesive Technology Platforms

Bioadhesives have emerged as transformative and versatile tools in healthcare, offering the ability to attach tissues with ease and minimal damage. These materials present numerous opportunities for tissue repair and biomedical device integration, creating a broad landscape of applications that have captivated clinical and scientific interest alike. However, fully unlocking their potential requires multifaceted design strategies involving optimal adhesion, suitable biological interactions, and efficient signal communication. In this Review, we delve into these pivotal aspects of bioadhesive design, highlight the latest advances in their biomedical applications, and identify potential opportunities that lie ahead for bioadhesives as multifunctional technology platforms.

Conductive, water-retaining and knittable hydrogel fiber from xanthan gum and aniline tetramer modified-polysaccharide for strain and pressure sensors

Herein, we explored strategies for defoaming and controllable adjustment of spinnable and mechanical properties of polyanion polysaccharide-based hydrogels to fabricate conductive, water-retaining, and knittable hydrogel fibers for next-generation flexible electronics. Xanthan gum (XG) and aniline tetramer modified-polysaccharide (TMAT38) were crosslinked with sodium trimetaphosphate (STMP) and subsequently by Fe3+/Fe2+ ions coordination to prepare conductive and spinnable hydrogels. Polypropylene glycol was introduced as chemical antifoam, and solvent displacement method was adopted to improve mechanical and water-retaining properties. The glycerol-immersed XG5-TMAT38-STMP-Fe3+/CA-PPG hydrogel exhibited conductivity of 3.55×10-3-27.30×10-3 S/cm, storage modulus at linear viscoelastic region of 573 Pa-1717 Pa and self-healing percentage of 100 %-108 %. The 2 h glycerol-immersed hydrogel fibers with good flexibility, moisture retention and freezing tolerance were ready to bend and knit into fabrics. The hydrogel fiber braid possessed better conductivity, reliability and durability than the single hydrogel fiber as strain sensors. The hydrogel fiber fabric perceived tiny vibration triggered by swallowing, speaking and writing with good sensitivity and reproducibility. Furthermore, a three-component model was developed to evaluate response sensitivity and recoverability of the hydrogel fiber fabric-based pressure sensors, which facilitated understanding transient response of polymer-based hydrogel strain and pressure sensors.

Injectable organo-hydrogels influenced by click chemistry as a paramount stratagem in the conveyor belt of pharmaceutical revolution

The field of injectable hydrogels has demonstrated a paramount headway in the myriad of biomedical applications and paved a path toward clinical advancements. The innate superiority of hydrogels emerging from organic constitution has exhibited dominance in overcoming the bottlenecks associated with inorganic-based hydrogels in the biological milieu. Inorganic hydrogels demonstrate various disadvantages, including limited biocompatibility, degradability, a cumbersome synthesis process, high cost, and ecotoxicity. The excellent biocompatibility, eco-friendliness, and manufacturing convenience of organo-hydrogels have demonstrated to be promising in therapizing biomedical complexities with low toxicity and augmented bioavailability. This report manifests the realization of biomimetic organo-hydrogels with the development of bioresponsive and self-healing injectable organo-hydrogels in the emerging pharmaceutical revolution. Furthermore, the influence of click chemistry in this regime as a backbone in the pharmaceutical conveyor belt has been suggested to scale up production. Moreover, we propose an avant-garde design stratagem of developing a hyaluronic acid (HA)-based injectable organo-hydrogel via click chemistry to be realized for its pharmaceutical edge. Ultimately, injectable organo-hydrogels that materialize from academia or industry are required to follow the standard set of rules established by global governing bodies, which has been delineated to comprehend their marketability. Thence, this perspective narrates the development of injectable organo-hydrogels via click chemistry as a prospective elixir to have in the arsenal of pharmaceuticals.

A Skin-Inspired Multifunctional Conductive Hydrogel with High Stretchable, Adhesive, Healable, and Decomposable Properties for Highly Sensitive Dual-Sensing of Temperature and Strain

Developing smart hydrogels with excellent physicochemical properties and multi-sensing capabilities for various simulation of human skin's functions still remains a great challenge. Here, based on simple and convenient one-step covalent cross-linking method enhanced by dynamic RS-Ag interactions, a skin-inspired multifunctional conductive hydrogel with desirable physicochemical properties (including high stretchability, self-adhesion, self-healing, decomposition and removability) is developed for highly sensitive dual-sensing of temperature and strain. Benefiting from the synergistic action of multiple hydrogen bonds, RS-Ag bonds and S-S bonds, the gel exhibited a novel thermosensitive mechanism. The prepared hydrogels exhibited extremely high mechanical properties (maximum tensile strength of 0.35 MPa, elongation at break nearly 1800%, compressive stress over 4.43 MPa), excellent self-healing (96.82% (stress), 88.45% (temperature), 73.89% (mechanical property)), decomposition (the molecular weight after decomposition is below 700) and self-adhesion (enhanced contact with the material interface). In addition, this conductive hydrogel could also simultaneously achieve highly sensitive temperature-sensing (TCR: 10.89) and stress-sensing (GF: 1.469). As a proof-to-concept, the hydrogel displayed superior capability for simulation of human skin to perception of touch, pressure and ambient temperature simultaneously, indicating promising applications in the fields of wearable devices, personal health care, and human-machine interfaces.

Physical dynamic double-network hydrogels as dressings to facilitate tissue repair

Double-network hydrogels can be tuned to have high mechanical strength, stability, elasticity and bioresponsive properties, which can be combined to create self-healing, adhesive and antibacterial wound dressings. Compared with single-network hydrogel, double-network hydrogel shows stronger mechanical properties and better stability. In comparison with chemical bonds, the cross-linking in double networks makes them more flexible than single-network hydrogels and capable of self-healing following mechanical damage. Here, we present the stepwise synthesis of physical double-network hydrogels where hydrogen bonds and coordination reactions provide self-healing, pH-responsive, tissue-adhesive, antioxidant, photothermal and antibacterial properties, and can be removed on demand. We then explain how to carry out physical, chemical and biological characterizations of the hydrogels for use as wound dressings, yet the double-network hydrogels could also be used in different applications such as tissue engineering scaffolds, cell/drug delivery systems, hemostatic agents or in flexible wearable devices for monitoring physiological and pathological parameters. We also outline how to use the double-network hydrogels in vivo as wound dressings or hemostatic agents. The synthesis of the ureido-pyrimidinone-modified gelatin, catechol-modified polymers and the hydrogels requires 84 h, 48 h and 1 h, respectively, whereas the in vivo assays require 3.5 weeks. The procedure is suitable for users with expertise in biomedical polymer materials.

Conductive polymers for cardiac tissue engineering and regeneration

Cardiovascular diseases, such as myocardial infarction, are considered a significant global burden and the leading cause of death. Given the inability of damaged cardiac tissue to self-repair, cell-based tissue engineering and regeneration may be the only viable option for restoring normal heart function. To maintain the normal excitation-contraction coupling function of cardiac tissue, uniform electronic and ionic conductance properties are required. To transport cells to damaged cardiac tissues, several techniques, including the incorporation of cells into conductive polymers (CPs) and biomaterials, have been utilized. Due to the complexity of cardiac tissues, the success of tissue engineering for the damaged heart is highly dependent on several variables, such as the cell source, growth factors, and scaffolds. In this review, we sought to provide a comprehensive overview of the electro CPs and biomaterials used in the engineering and regeneration of heart tissue.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Designing self-healing hydrogels for biomedical applications

Self-healing hydrogels have emerged as the most promising alternatives to conventional brittle hydrogels used in the biomedical field due to the features of long-term stability and durability. However, the incompatibility between the fast self-healing property and enough mechanical strength of hydrogels remains a challenge. Therefore, hydrogels that possess not only mechanical toughness but also autonomous self-healing capacity are sought after. This review presents a comprehensive summary of the latest self-healing mechanisms. Specifically, we review various systems based on dynamic bonds, ranging from dynamic covalent bonds to non-covalent bonds. Additionally, this review presents different characterization methods for self-healing hydrogels, and also highlights their potential applications in the biomedical field, such as tissue engineering, drug delivery, cell therapy, and wound dressing. Furthermore, this review aims to provide valuable guidance for constructing diverse self-healing hydrogels with tailored functions.

Smart and versatile biomaterials for cutaneous wound healing

The global increase of cutaneous wounds imposes huge health and financial burdens on patients and society. Despite improved wound healing outcomes, conventional wound dressings are far from ideal, owing to the complex healing process. Smart wound dressings, which are sensitive to or interact with changes in wound condition or environment, have been proposed as appealing therapeutic platforms to effectively facilitate wound healing. In this review, the wound healing processes and features of existing biomaterials are firstly introduced, followed by summarizing the mechanisms of smart responsive materials. Afterwards, recent advances and designs in smart and versatile materials of extensive applications for cutaneous wound healing were submarined. Finally, clinical progresses, challenges and future perspectives of the smart wound dressing are discussed. Overall, by mapping the composition and intrinsic structure of smart responsive materials to their individual needs of cutaneous wounds, with particular attention to the responsive mechanisms, this review is promising to advance further progress in designing smart responsive materials for wounds and drive clinical translation.

Chitosan-based mussel-inspired hydrogel for rapid self-healing and high adhesion of tissue adhesion and wound dressings

The hydrogel wound dressing with self-healing and adhesive property can provide better protection to the wound and prolong the service life of the material. Inspired by mussels, a high-adhesion, injectable, self-healing and antibacterial hydrogel was designed in this study. The lysine (Lys) and the catechol compound 3,4-dihydroxyphenylacetic acid (DOPAC) were grafted onto chitosan (CS). The presence of catechol group endows the hydrogel strong adhesion and antioxidation. In the experiment of wound healing in vitro, the hydrogel can adhere to the wound surface and promote wound heal. In addition, it has been proved the hydrogel has good antibacterial properties against S. aureus and E. coli. The treatment of CLD hydrogel, the degree of wound inflammation was significantly alleviated. The levels of TNF-α, IL-1β, IL-6 and TGF-β1 were reduced from 39.8379 %, 31.6768 %, 32.1015 % and 38.4911 % to 18.5931 %, 12.2275 %, 13.0524 % and 16.9959 %, respectively. And the levels of PDGFD and CD31 were increased from 35.6054 %, 21.7394 % to 51.8555 %, 43.9326 %, respectively. These results indicated that the CLD hydrogel has a good ability to promote angiogenesis, thickening of skin and epithelial structures.

Intrinsically Stretchable Conductive Self-Healable Organogels for Strain, Pressure, Temperature, and Humidity Sensing

Intrinsically stretchable conductive self-healable organogels containing poly(lipoic acid), Al3+ ion, tannic acid, and reduced graphene oxide are produced in this report. These noncovalent networks interlocked through physical (hydrogen and coordination) bonds offered high stretchabilities and mechanical strengths as well as fast self-healing behaviors. The optimum organogel-based sensor showed outstanding pressure sensitivities (0.94 kPa-1 up to 10 and 1.07 kPa-1 for 10-50 kPa) and high strain responses (corresponding gauge factors of 1.1 and 0.4 for 0-50 and 50-100% stretching ratios). This organogel also revealed high stabilities at ambient atmosphere due to the presence of binary solvents of dimethyl sulfoxide and glycerol. Additionally, this stretchable thermistor displayed remarkable two-stage sensitivities of -2.6 and -0.4%/°C ranging over 0-30 and 30-80 °C, respectively. Besides, the signal variations of water droplet addition and removal with different temperatures were recorded by the organogel sensor to elucidate the practical applicabilities as a temperature sensor. Moreover, the organogel was utilized to demonstrate humidity sensing, where individual sensitivities of 0.89 and 0.55 were obtained in the respective relative humidity ranges of 10-30 and 40-90%. In the meanwhile, the sensor device illustrated distinct humidity signals during respiration monitoring of nose and mouth breathing detection. Accordingly, these quad-functional sensor applications in strain, pressure, temperature, and humidity detection enable this gel to act as a promising material for future multifunctional flexible electronics.

Conducting polymer-based nanostructured materials for brain-machine interfaces

As scientists discovered that raw neurological signals could translate into bioelectric information, brain-machine interfaces (BMI) for experimental and clinical studies have experienced massive growth. Developing suitable materials for bioelectronic devices to be used for real-time recording and data digitalizing has three important necessitates which should be covered. Biocompatibility, electrical conductivity, and having mechanical properties similar to soft brain tissue to decrease mechanical mismatch should be adopted for all materials. In this review, inorganic nanoparticles and intrinsically conducting polymers are discussed to impart electrical conductivity to systems, where soft materials such as hydrogels can offer reliable mechanical properties and a biocompatible substrate. Interpenetrating hydrogel networks offer more mechanical stability and provide a path for incorporating polymers with desired properties into one strong network. Promising fabrication methods, like electrospinning and additive manufacturing, allow scientists to customize designs for each application and reach the maximum potential for the system. In the near future, it is desired to fabricate biohybrid conducting polymer-based interfaces loaded with cells, giving the opportunity for simultaneous stimulation and regeneration. Developing multi-modal BMIs, Using artificial intelligence and machine learning to design advanced materials are among the future goals for this field. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease.

Electrochemical Wearable Biosensors and Bioelectronic Devices Based on Hydrogels: Mechanical Properties and Electrochemical Behavior

Hydrogel-based wearable electrochemical biosensors (HWEBs) are emerging biomedical devices that have recently received immense interest. The exceptional properties of HWEBs include excellent biocompatibility with hydrophilic nature, high porosity, tailorable permeability, the capability of reliable and accurate detection of disease biomarkers, suitable device-human interface, facile adjustability, and stimuli responsive to the nanofiller materials. Although the biomimetic three-dimensional hydrogels can immobilize bioreceptors, such as enzymes and aptamers, without any loss in their activities. However, most HWEBs suffer from low mechanical strength and electrical conductivity. Many studies have been performed on emerging electroactive nanofillers, including biomacromolecules, carbon-based materials, and inorganic and organic nanomaterials, to tackle these issues. Non-conductive hydrogels and even conductive hydrogels may be modified by nanofillers, as well as redox species. All these modifications have led to the design and development of efficient nanocomposites as electrochemical biosensors. In this review, both conductive-based and non-conductive-based hydrogels derived from natural and synthetic polymers are systematically reviewed. The main synthesis methods and characterization techniques are addressed. The mechanical properties and electrochemical behavior of HWEBs are discussed in detail. Finally, the prospects and potential applications of HWEBs in biosensing, healthcare monitoring, and clinical diagnostics are highlighted.

Hydrogel Bioelectronics for Health Monitoring

Hydrogels are considered an ideal platform for personalized healthcare due to their unique characteristics, such as their outstanding softness, appealing biocompatibility, excellent mechanical properties, etc. Owing to the high similarity between hydrogels and biological tissues, hydrogels have emerged as a promising material candidate for next generation bioelectronic interfaces. In this review, we discuss (i) the introduction of hydrogel and its traditional applications, (ii) the work principles of hydrogel in bioelectronics, (iii) the recent advances in hydrogel bioelectronics for health monitoring, and (iv) the outlook for future hydrogel bioelectronics' development.

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

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

Tough, Healable, and Sensitive Strain Sensor Based on Multiphysically Cross-Linked Hydrogel for Ionic Skin

Ion conductive hydrogels (ICHs) have attracted great interest in the application of ionic skin because of their superior characteristics. However, it remains a challenge for ICHs to achieve balanced properties of high strength, large fracture strain, self-healing and freezing tolerance. In this study, a strong, stretchable, self-healing and antifreezing ICH was demonstrated by rationally designing a multiphysically cross-linked network structure consisting of the hydrophobic association, metal-ion coordination and chain entanglement among poly(acrylic acid) (PAA) polymer chains. The deliberately designed Brij S 100 acrylate (Brij-100A) micelle cross-linker can effectively dissipate energy and endow hydrogels with desirable stretchability. The self-healing ability of hydrogels originates from the reversible hydrophobic association in micelles and Fe³⁺-COO^- coordination. After the addition of NaCl, the chain-entangled physical network caused by the salting-out effect can both enhance mechanical strength and promote electron transport. With the synergy of hydrophobic association, mental-ligand coordination and chain entanglement, the PAA/Brij-100A/Fe³⁺/NaCl (PAA/BA/Fe³⁺/NaCl) hydrogels exhibited a high tensile strain of 1140%, a tensile strength of 0.93 MPa and a toughness of 3.48 MJ m^-3. Besides, the PAA/BA/Fe³⁺/NaCl hydrogels exhibited a high conductivity of 0.43 S m^-1 and good freezing resistance. The ionic skin based on the PAA/BA/Fe³⁺/NaCl hydrogels showed high sensitivity (GF = 5.29), wide strain range (0-950%), fast response time (220 ms) and good stability. Also, the self-healing ability of the ionic skin can significantly prolong its service time, and the antifreezing property can broaden its applicable temperature. This study offers new insight into the design of multifunctional ionic skin for wearable electronics.

Cyclodextrin regulated natural polysaccharide hydrogels for biomedical applications-a review

Cyclodextrin and its derivative (CDs) are natural building blocks for linking with other components to afford functional biomaterials. Hydrogels are polymer network systems that can form hydrophilic three-dimensional network structures through different cross-linking methods and are developing as potential materials in biomedical applications. Natural polysaccharide hydrogels (NPHs) are widely adopted in biomedical field with good biocompatibility, biodegradability, low cytotoxicity, and versatility in emulating natural tissue properties. Compared with conventional NPHs, CD regulated natural polysaccharide hydrogels (CD-NPHs) maintain good biocompatibility, while improving poor mechanical qualities and unpredictable gelation times. Recently, there has been increasing and considerable usage of CD-NPHs while there is still no review comprehensively introducing their construction, classification, and application of these hydrogels from the material point of view regarding biomedical fields. To draw a complete picture of the current and future development of CD-NPHs, we systematically overview the classification of CD-NPHs, and provide a holistic view on the role of CD-NPHs in different biomedical fields, especially in drug delivery, wound dressing, cell encapsulation, and tissue engineering. Moreover, the current challenges and prospects of CD-NPHs are discussed rationally, providing an insight into developing vibrant fields of CD-NPHs-based biomedicine, and facilitating their translation from bench to clinical medicine.

Human Albumin-Based Hydrogels for Their Potential Xeno-Free Microneedle Applications

Nowadays, hydrogels-based microneedles (MNs) have attracted a great interest owing to their outstanding qualities for biomedical applications. For the fabrication of hydrogels-based microneedles as tissue engineering scaffolds and drug delivery carriers, various biomaterials have been tested. They are required to feature tunable physiochemical properties, biodegradability, biocompatibility, nonimmunogenicity, high drug loading capacity, and sustained drug release. Among biomaterials, human proteins are the most ideal biomaterials for fabrication of hydrogels-based MNs; however, they are mechanically weak and poorly processible. To the best of the knowledge, there are no reports of xeno-free human protein-based MNs so far. Here, human albumin-based hydrogels and microneedles for tissue engineering and drug delivery by using relatively new processible human serum albumin methacryloyl (HSAMA) are engineered. The resultant HSAMA hydrogels display tunable mechanical properties, biodegradability, and good biocompatibility. Moreover, the xeno-free HSAMA microneedles display a sustained drug release profile and significant mechanical strength to penetrate the model skin. In vitro, they also show good biocompatibility and anticancer efficacy. Sustainable processible human albumin-based biomaterials may be employed as a xeno-free platform in vivo for tissue engineering and drug delivery in clinical trials in the future.

Vitrimer ionogels towards sustainable solid-state electrolytes

The growing demand for flexible, stretchable, and wearable devices has boosted the development of ionogels used as polymer electrolytes. Developing healable ionogels based on vitrimer chemistry is a promising approach to improve their lifetimes as these materials are usually subjected to repeated deformation during functioning and are susceptible to damage. In this work, we reported in the first place the preparation of polythioether vitrimer networks based on the not extensively studied associative S-transalkylation exchange reaction using thiol-ene Michael addition. Thanks to the exchange reaction of sulfonium salts with thioether nucleophiles, these materials demonstrated vitrimer properties such as healing and stress relaxation. The fabrication of dynamic polythioether ionogels was then demonstrated by loading 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM triflate) within the polymer network. The resulting ionogels exhibited Young's modulus of 0.9 MPa and ionic conductivities in the order of 10^-4 S cm^-1 at room temperature. It has been found that adding ionic liquids (ILs) changes the dynamic properties of the systems, most likely due to a dilution effect of the dynamic functions by the IL but also due to a screening effect of the alkyl sulfonium OBrs-couple by the ions of the IL itself. To the best of our knowledge, these are the first vitrimer ionogels based on an S-transalkylation exchange reaction. While the addition of ILs resulted in less efficient dynamic healing at a given temperature, these ionogels can provide materials with more dimensional stability at application temperatures and can potentially pave the way for the development of tunable dynamic ionogels for flexible electronics with a longer lifespan.

Enhancing Conductivity and Self-Healing Properties of PVA/GEL/OSA Composite Hydrogels by GO/SWNTs for Electronic Skin

The use of flexible, self-healing conductive hydrogels as a type of typical electronic skin with the function of transmitting sensory signals has attracted wide attention in the field of biomaterials. In this study, composite hydrogels based on polyvinyl alcohol (PVA), gelatin (GEL), oxidized sodium alginate (OSA), graphene oxide (GO), and single-walled carbon nanotubes (SWNTs) were successfully prepared. The hydrogen and imine bonding of the composite hydrogels gives them excellent self-healing properties. Their self-healing properties restore 68% of their breaking strength and over 95% of their electrical conductivity. The addition of GO and SWNTs enables the PGO-GS hydrogels to achieve a compressive modulus and conductivity of 42.2 kPa and 29.6 mS/m, which is 8.2 times and 1.5 times that of pure PGO, respectively. Furthermore, the PGO-GS hydrogels can produce profound feedback signals in response to deformation caused by external forces and human movements such as finger flexion and speech. In addition, the PGO-GS hydrogels exhibit superior biocompatibility compared to PGO. All of these results indicate that the PGO-GS hydrogels have great potential with respect to future applications in the field of electronic skin.

Rapid Preparation of Antifreezing Conductive Hydrogels for Flexible Strain Sensors and Supercapacitors

Conductive hydrogels have shown great promise in flexible electronics, but their practical applications may be impeded by the time-consuming and energy-consuming polymerization process. We proposed a sodium lignosulfonate-Fe (SLS-Fe) strategy to address this challenge and took advantage of carboxymethyl cellulose (CMC) and poly(acrylic acid) to prepare the CMC/PAA/Fe3+/LiCl interpenetrating conductive hydrogels with good self-healing properties, antifreezing properties, and a 6-fold increase in conductivity in this study. The hydrogel-based flexible strain sensors demonstrated a broad detection range (400%), high sensitivity (GF = 6.19 at 200-400%), and human motion detection capability. The hydrogel-based supercapacitor exhibited a single-electrode specific capacitance of 122.36 F g-1 which successfully powered LEDs. Furthermore, the supercapacitor showed a single-electrode specific capacitance of 83.16 F g-1 at -23 °C (68% of the one exhibited at 25 °C). Therefore, the multifunctional performance of the CMC/PAA/Fe3+/LiCl hydrogel is anticipated to play an exemplary role in a new generation of flexible electronics.

Conductive, self-healing, and antibacterial Ag/MXene-PVA hydrogel as wearable skin-like sensors

The rapid development of flexible electronic technology has led to the in-depth study of flexible wearable sensors to achieve accurate sensing under different external stimuli. However, it is still a huge challenge to develop hydrogel-based wearable skin-like sensors with super ductility, high sensitivity, and self-healing properties. Herein, the Ti₃C₂ type of MXene was synthesized, and the Ag/MXene nanocomplexes were incorporated into polyvinyl alcohol-borax matrix to construct a novel composite hydrogel as the multifunctional nanofillers, which could bring both improved properties and novel functionalities. The Ag/MXene-Poly (vinyl alcohol) (PVA) hydrogel displayed integrated merits of highly strain sensitive (GF = 3.26), self-healing (within 10 min, 91% healing efficiency), and excellent antibacterial activity. The hydrogel could be assembled into a wearable skin-like sensor to monitor human movement, including large deformations (finger, elbow, wrist, and knee bending) and tiny deformations (mouth's movement and throat vocalization) in real time. Therefore, this work shed a new light on the development of flexible wearable skin-like sensors for the personalized healthcare monitoring, human-machine interfaces, and artificial intelligence.

Highly Stretchable, Self-Healing, and Low Temperature Resistant Double Network Hydrogel Ionic Conductor as Flexible Sensor and Quasi-Solid Electrolyte

With the rapid development of flexible energy storage and wearable strain sensing, conductive hydrogels are attracting attention as electrolyte materials for flexible strain sensors and flexible supercapacitors due to their excellent flexibility and wetting properties. In this work, antifreezing hydrogels with high stretchability, adhesion, and conductivity are designed and prepared by introducing phosphoric acid solutions into polyacrylamide and chitosan systems. The multifunctional hydrogel samples prepared by this method can be used as both quasi-solid electrolytes and wearable strain sensors. The hydrogel-based supercapacitor shows a charge/discharge efficiency of 99.67% and a capacitance retention of 98.85% after 10 000 cycles charge/discharge tests at -30 °C. The tiny characteristic heartbeat wave forms are detected by the hydrogel as a flexible strain sensor. It is foreseeable that PCP multifunctional hydrogel can be a promising flexible material for a new generation of flexible sensors and flexible energy storage devices in a certain range of temperatures.

Advances and Progress in Self-Healing Hydrogel and Its Application in Regenerative Medicine

A hydrogel is a three-dimensional structure that holds plenty of water, but brittleness largely limits its application. Self-healing hydrogels, a new type of hydrogel that can be repaired by itself after external damage, have exhibited better fatigue resistance, reusability, hydrophilicity, and responsiveness to environmental stimuli. The past decade has seen rapid progress in self-healing hydrogels. Self-healing hydrogels can automatically self-repair after external damage. Different strategies have been proposed, including dynamic covalent bonds and reversible noncovalent interactions. Compared to traditional hydrogels, self-healing gels have better durability, responsiveness, and plasticity. These features allow the hydrogel to survive in harsh environments or even to be injected as a drug carrier. Here, we summarize the common strategies for designing self-healing hydrogels and their potential applications in clinical practice.

Mussel-Inspired Adhesive, Antibacterial, and Stretchable Composite Hydrogel for Wound Dressing

Hydrogels have attracted extensive attention in the field of biomedicine because of their similar structure to extracellular matrix (ECM) and good biocompatibility. However, the adhesiveness, mechanical properties, and antibacterial properties of conventional hydrogels are not satisfactory. In this study, multifunctional chitosan/polydopamine/polyacrylamide (CS/PDA/PAM) hydrogels are prepared through a nature-inspired strategy. The catechol group of polydopamine (PDA) component endows CS/PDA/PAM hydrogels with tissue adhesion and self-healing properties. The introduction of chitosan (CS) not only greatly improves antibacterial ability, but also enhances the mechanical properties of CS/PDA/PAM hydrogels. Skin wound healing experiments show that CS/PDA/PAM hydrogels could accelerate skin tissue regeneration and promote wound healing. Therefore, CS/PDA/PAM hydrogels have great potential in the application of new wound dressings.

Ultrafast Self-Healing, Highly Stretchable, Adhesive, and Transparent Hydrogel by Polymer Cluster Enhanced Double Networks for Both Strain Sensors and Environmental Remediation Application

Stretchable, healable, biocompatible, and conductive hydrogels are one of the promising candidates for both wearable electronics and environmental remediation applications. To date, the design of hydrogels that integrate ultrafast self-healing with high efficiency (seconds), high stretchability, and biocompatibility and reversibility into one system is not an easy task. Herein, we demonstrate a general oxidation approach to accelerate the hydrogelation of hPEI-based double network gels via the generation of fluorescent polymer clusters at room temperature or triggered by the heating-cooling process. The resulting ohPEI hydrogel has the merit of biocompatibility over most reported hPEI hydrogels for strain sensors. It shows a high conductivity (1.3 S/m), an ultrafast self-healing ability (<3 s, 98% healing efficiency within 60 s), a high stretchability (∼1850 and ∼7000% in deformation), and reversible adhesivity on various material surfaces. The excellent performance of the hydrogel is ascribed to the cooperative and hierarchical interactions of four types of dynamic combinations, including the reversible borate bond, hydrogen bonding, electrostatic interaction, and polymer cluster interactions. The reversible fabrication process by the one-spot method (just by simple mixing of the components) and superior properties of the hydrogel make it an ideal candidate for a wearable strain sensor to monitor human motions and physiological activities. Moreover, it is also a good hydrogel absorbent for phase separation absorption of volatile organic compounds with a high capacity (for acetone: 4.75 g g^-1), reusability, and an easy handling process.

Engineering functional natural polymer-based nanocomposite hydrogels for wound healing

Skin injury occurs due to acute trauma, chronic trauma, infection, and surgical intervention, which can result in severe dysfunction and even death in humans. Therefore, clinical intervention is critical for the treatment of skin wounds. One idealized method is to use wound dressings to protect skin wounds and promote wound healing. Among these wound dressings, nanocomposite natural polymer hydrogels (NNPHs) are multifunctional wound dressings for wound healing. The combination of nanomaterials and natural polymer hydrogels avoids the shortcomings of a single component. Moreover, nanomaterials could provide improved antibacterial, anti-inflammatory, antioxidant, stimuli-responsive, electrically conductive and mechanical properties of hydrogels to accelerate wound healing. This review focuses on recent advancements in NNPHs for skin wound healing and repair. Initially, the functions and requirements of NNPHs as wound dressings were introduced. Second, the design, preparation and capacities of representative NNPHs are classified based on their nanomaterial. Third, skin wound repair applications of NNPHs have been summarized based on the types of wounds. Finally, the potential issues of NNPHs are discussed, and future research is proposed to prepare idealized multifunctional NNPHs for skin tissue regeneration.

Nanomaterial-Based Electrically Conductive Hydrogels for Cardiac Tissue Repair

Cardiovascular disease is one of major causes of deaths, and its incidence has gradually increased worldwide. For cardiovascular diseases, several therapeutic approaches, such as drugs, cell-based therapy, and heart transplantation, are currently employed; however, their therapeutic efficacy and/or practical availability are still limited. Recently, biomaterial-based tissue engineering approaches have been recognized as promising for regenerating cardiac function in patients with cardiovascular diseases, including myocardial infarction (MI). In particular, materials mimicking the characteristics of native cardiac tissues can potentially prevent pathological progression and promote cardiac repair of the heart tissues post-MI. The mechanical (softness) and electrical (conductivity) properties of biomaterials as non-biochemical cues can improve the cardiac functions of infarcted hearts by mitigating myocardial cell death and subsequent fibrosis, which often leads to cardiac tissue stiffening and high electrical resistance. Consequently, electrically conductive hydrogels that can provide mechanical strength and augment the electrical activity of the infarcted heart tissue are considered new functional materials capable of mitigating the pathological progression to heart failure and stimulating cardiac regeneration. In this review, we highlight nanomaterial-incorporated hydrogels that can induce cardiac repair after MI. Nanomaterials, including carbon-based nanomaterials and recently discovered two-dimensional nanomaterials, offer great opportunities for developing functional conductive hydrogels owing to their excellent electrical conductivity, large surface area, and ease of modification. We describe recent results using nanomaterial-incorporated conductive hydrogels as cardiac patches and injectable hydrogels for cardiac repair. While further evaluations are required to confirm the therapeutic efficacy and toxicity of these materials, they could potentially be used for the regeneration of other electrically active tissues, such as nerves and muscles.

Biocompatible and Printable Ionotronic Sensing Materials Based on Silk Fibroin and Soluble Plant-Derived Polyphenols

The emergence of ionotronic materials has been recently exploited for interfacing electronics and biological tissues, improving sensing with the surrounding environment. In this paper, we investigated the synergistic effect of regenerated silk fibroin (RS) with a plant-derived polyphenol (i.e., chestnut tannin) on ionic conductivity and how water molecules play critical roles in regulating ion mobility in these materials. In particular, we observed that adding tannin to RS increases the ionic conductivity, and this phenomenon is accentuated by increasing the hydration. We also demonstrated how silk-based hybrids could be used as building materials for scaffolds where human fibroblast and neural progenitor cells can highly proliferate. Finally, after proving their biocompatibility, RS hybrids demonstrate excellent three-dimensional (3D) printability via extrusion-based 3D printing to fabricate a soft sensor that can detect charged objects by sensing the electric fields that originate from them. These findings pave the way for a viable option for cell culture and novel sensors, with the potential base for tissue engineering and health monitoring.

Controlling mechanical properties of ultrahigh molecular weight ion gels by chemical structure of ionic liquids and monomers

A new class of ion gels, termed ultrahigh molecular weight (UHMW) gels, formed by physical entanglement of ultrahigh molecular weight polymers in ionic liquids, are synthesised using facile one step radical polymerisation with significantly low initiator conditions, and exhibit superior mechanical characteristics such as stretchability, recyclability, and room temperature self-healing ability. In this study, UHMW gels are synthesised using various combinations of monomer and IL structures, and the effect of their chemical structures on the physicochemical properties of UHMW gels are thoroughly investigated. UHMW polymers are prepared in situ for all combinations of ILs and monomers used in this study, indicating the wide applicability of this fabrication strategy. The structure-property relationships between chemical structures and mechanical properties of UHMW gels are investigated in detail. Furthermore, the differences in self-healing efficiency of UHMW gels depending on the chemical structure is discussed in terms of individual polymer conformation and polymer-polymer interaction based on molecular dynamics simulations.

Acetylated Distarch Phosphate-Mediated Tough and Conductive Hydrogel for Antibacterial Wearable Sensors

Conductive, stretchable, and flexible hydrogel wearable sensors have attracted extensive attention in the fields of artificial intelligence and electronic equipment. However, it is an enormous challenge to fabricate conductive hydrogel sensors with biocompatibility, antibacterial properties, and toughness. Here, a highly conductive hydrogel with excellent toughness, good biocompatibility, and strong antibacterial properties was prepared by incorporating acetylated distarch phosphate (ADSP) into poly(vinyl alcohol) (PVA)/polyhexamethylene biguanide hydrochloride (PHMG). The addition of ADSP not only ionized sodium ions to make the hydrogel conductive but also provided abundant hydroxyl groups to form hydrogen bonds with PVA to improve the toughness of the hydrogel. Furthermore, PHMG endowed the hydrogel with antibacterial properties toward E. coli (Escherichia coli, Gram-negative bacteria) and S. aureus (Staphylococcus aureus, Gram-positive bacteria). Meanwhile, the hydrogel was implanted in mice for 14 days, and the surrounding tissue remained in good condition. More importantly, the hydrogel could detect ECG signals and electrical signals under different actions. This study affords a novel approach for exploiting wearable sensors with antibacterial properties and biocompatibility.

Triple crosslinking conductive hydrogels with digitally printable and outstanding mechanical stability for high-resolution conformable bioelectronics

Soft, conductive, and stretchable hydrogels offer a broad variety of applications, including skin-interfacing electrodes, biomonitoring patches, and electrostimulation. Despite rapid developments over the last decades, a combination of good electrical and mechanical properties, low-cost fabrication, and biocompatibility is yet to be demonstrated. Also, the current methods for deposition and patterning of these hydrogels are manual, and there is a need toward autonomous and digital fabrication techniques. In this work, we demonstrate a novel Gallium (Ga) embedded sodium-alginate-polyacrylamide-LAPONITE® (Ga-SA-PAAM-La) hydrogel, that is ultra-stretchable (Maximum strain tolerance of∼985%), tough (toughness ∼30 kJ m^-3), bio-adhesive (adhesion energy ∼216 J m^-2), conductive, and digitally printable. Ga nanoparticles are used as radical initiators. By adjusting the sonication parameters, we control the solution viscosity and curing time, thus allowing us to prepare pre-polymers with the desired properties for casting, or digital printing. These hydrogels benefit from a triple-network structure due to the role of Ga droplets as crosslinkers besides BIS (N,N'-methylene-bis-acrylamide) and LAPONITE®, thus resulting in tough composite hydrogels. The inclusion of LAPONITE® into the hydrogel network improved its electrical conductivity, adhesion, digital printability, and its mechanical properties, (>6× compared to the same hydrogel without LAPONITE®). As electrodes in the electrocardiogram, the signal-to-noise ratio was surprisingly higher than the medical-grade Ag/AgCl electrodes, which are applied for monitoring muscles, heart, respiration, and body joint angle through EMG, ECG, and bioimpedance measurements. The results obtained prove that such digitally printed conductive and tough hydrogels can be used as potential electrodes and sensors in practical applications in the next generation of printed wearable computing devices.

Flexible Actuator Based on Conductive PAM Hydrogel Electrodes with Enhanced Water Retention Capacity and Conductivity

Conductive polyacrylamide (PAM) hydrogels with salts that act as electrolytes have been used as transparent electrodes with high elasticity in flexible electronic devices. Different types and contents of raw materials will affect their performance in all aspects. We tried to introduce highly hydratable salts into PAM hydrogels to improve their water retention capacity. Different salts can improve the water retention capacity of PAM hydrogels to a certain extent. In particular, PAM hydrogels containing higher concentrations of lithium chloride (LiCl) and calcium chloride (CaCl₂) showed an extremely strong water retention capacity and could retain about 90% and more than 98% of the initial water in the experimental environment at a temperature of 25 °C and a relative humidity of 60% RH, respectively. In addition, we conducted electrical conductivity tests on these PAM hydrogels with different salts. The PAM hydrogels containing LiCl also show outstanding conductivity, and the highest conductivity value can reach up to about 8 S/m. However, the PAM hydrogels containing CaCl₂, which also performed well in terms of their water retention capacity, were relatively common in terms of their electrical conductivity. On this basis, we attempted to introduce single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), and graphene (GO) electronic conductors to enhance the electrical conductivity of the PAM hydrogels containing LiCl. The conductivity of the PAM hydrogels containing LiCl was improved to a certain extent after the addition of these electronic conductors. The highest electrical conductivity was about 10 S/m after we added the SWCNTs. This experimental result indicates that these electronic conductors can indeed enhance the electrical conductivity of PAM hydrogels to a certain extent. After a maximum of 5000 repeated tensile tests, the conductive hydrogel samples could still maintain their original morphological characteristics and conductivity. This means that these conductive hydrogel samples have a certain degree of system reliability. We made the PAM conductive hydrogels with high water retention and good conductivity properties into thin electrodes and applied them to an electric response flexible actuator with dielectric elastomer as the functional material. This flexible actuator can achieve a maximum area strain of 18% under an external voltage of 10 kV. This new composite hydrogels with high water retention and excellent conductivity properties will enable more possibilities for the application of hydrogels.