Tannic Acid-Silver Dual Catalysis Induced Rapid Polymerization of Conductive Hydrogel Sensors with Excellent Stretchability, Self-Adhesion, and Strain-Sensitivity Properties

Recent advances in tannic acid-based gels: Design, properties, and applications

With the flourishing of mussel-inspired chemistry, the fast-growing development for environmentally friendly materials, and the need for inexpensive and biocompatible analogues to PDA in gel design, TA has led to its gradual emergence as a research focus due to its remarkable biocompatible, renewable, sustainable and particular physicochemical properties. As a natural building block, TA can be used as a substrate or crosslinker, ensuring versatile functional polymeric networks for various applications. In this review, the design of TA-based gels is summarized in detail (i.e., different interactions such as: metal coordination, electrostatic, hydrophobic, host-guest, cation-π and π-π stacking interactions, hydrogen bonding and various reactions including: phenol-amine Michael and Schiff base, phenol-thiol Michael addition, phenol-epoxy ring opening reaction, etc.). Subsequently, TA-based gels with a variety of functionalities, including mechanical, adhesion, conductive, self-healing, UV-shielding, anti-swelling, anti-freezing, shape memory, antioxidant, antibacterial, anti-inflammatory and responsive properties are introduced in detail. Then, a summary of recent developments in the use of TA-based gels is provided, including bioelectronics, biomedicine, energy, packaging, water treatment and other fields. Finally, the difficulties that TA-based gels are currently facing are outlined, and an original yet realistic viewpoint is provided in an effort to spur future development.

Multifunctional films based on tannic acid-coated cellulose nanocrystals and zinc-coating reinforced sodium carboxymethyl cellulose/polyvinyl alcohol for food active packaging

Multifunctional packaging materials made from biomass resources are key to achieving packaging storage and environmental friendliness. The aim of this study is to prepare high-performance cellulose-based packaging films to improve the high-value utilization of cellulose resources. In this paper, the blended films of sodium carboxymethyl cellulose (CMC) and poly(vinyl alcohol) (PVA) were used as the substrate and doped with tannic acid (TA)-coated cellulose nanocrystals (CNC@TA). Then, zinc ions (Zn2+) were decorated on the film surface by adsorption self-assembly. The modified films (Z-CPC@T5 films) were prepared with excellent mechanical properties (tensile strength and elongation at break of 73.85 MPa and 19.68 %, respectively). Meanwhile, the presence of CNC@TA provided the films with UV and oxidation resistance. In addition, the zinc coating formed on the film surface conferred water resistance, hydrophobicity, and structural stability (water contact angle up to 97.09°). The modified films also showed excellent antimicrobial and water-vapor barrier properties. The modified films preserved cherries for at least 16 days with a quality retention of 87.20 %. In addition, cytotoxicity tests confirmed the non-toxic properties of the modified films. Overall, this strategic fusion of internal and external dual crosslinking expanded the application potential of active packaging materials.

Concurrent effects and dynamic wetting abilities of nanometals anchored redox-active Janus nanoarchitectures on cotton fabric for sustainable catalysis and disinfection

Designing an ideal catalyst with antifouling performance and enhanced conversion efficiency can prevent microbial or dye contamination and protect the active phase of the catalysts at the triple-phase interface during disinfection processes. Herein, we developed an Lous-leaf-inspired nanometal anchored redox-active Janus nanoarchitecture with dynamic wetting abilities and synergistic catalytic/antibacterial performances. Specifically, the redox-active hydrophilic polydopamine (PDA) was used to mediate the localized self-assembly and nucleation of Ag on a cotton fabric without using other reductants. This catalyst coating features a superficial Janus nanoarchitecture and context-dependent hydrophobic surface, resulting in a charge- and/or air bubble-involved spontaneous wetting phenomenon for contamination droplet during catalytic reactions. Their synergistically enhanced catalytic degradation of industrial dyes and free radical scavenging abilities were validated. The PDA@Ag modified fabric exhibited excellent washing resistance, achieving >99 % antibacterial performance against E. coli after being washed 20 times. The proof-of-concept for an optimal catalyst and protective coating has been demonstrated with multiple anti-fouling strategies such as a self-cleaning/anti-adhesion surface, enhanced photothermal effect and antibacterial properties. Eventually, this rationally designed Janus nanoarchitecture interface was supposed to address the trade-off issues commonly encountered at the droplet-based triple-phase interfacial reaction with a dynamic changed active phase and excellent catalytic/antibacterial performances.

Multifunctional hydrogels for the healing of oral ulcers

Oral ulcers are one of the most common oral diseases in clinical practice. Its etiology is complex and varied. Due to the dynamic nature of the oral environment, the wound surface is painful due to contact and wear, which seriously affects the quality of life of patients. Oral ulcers are often treated with topical drug therapy. Studies have shown that functional hydrogels play a positive role in promoting wound healing, showing unique advantages in wound dressings. In this paper, the causes and healing characteristics of oral ulcers are discussed in depth, and then the common treatment methods for oral ulcers are summarized and compared. Finally, the potential of functional hydrogels in the treatment of oral ulcers is discussed and projected through a review of the literature in recent years.

Fast and Efficient Fabrication of Functional Electronic Devices through Grayscale Digital Light Processing 3D Printing

Fabricating polymeric composites with desirable characteristics for electronic applications is a complex and costly process. Digital light processing (DLP) 3D printing emerges as a promising technique for manufacturing intricate structures. In this study, polymeric samples are fabricated with a conductivity difference exceeding three orders of magnitude in various portions of a part by employing grayscale DLP (g-DLP) single-vat single-cure 3D printing deliberate resin design. This is realized through the manipulation of light intensity during the curing process. Specifically, the rational resin design with added lithium ions results in the polymer cured under the maximum UV-light intensity exhibiting higher electrical resistance. Conversely, sections that are only partially cured retains uncured monomers, serving as a medium that facilitates ion mobility, consequently leading to higher conductivity. The versatility of g-DLP allows precise control of light intensity in different regions during the printing process. This characteristic opens up possibilities for applications, notably the low-cost, facile, and rapid production of complex electrical circuits and sensors. The utilization of this technique makes it feasible to fabricate materials with tailored conductivity and functionality, providing an innovative pathway to advance the accelerated and facile creation of sophisticated electronic devices.

A tough, stretchable, adhesive and electroconductive polyacrylamide hydrogel sensor incorporated with sulfonated nanocellulose and carbon nanotubes

Recently, hydrogel sensors have been widely applied in wearable and portable electronics, but the low mechanical property, intolerance of fatigue, and low sensitivity and adhesion limit their further applications. In this study, sulfonated nanocellulose (SCNF) with dual functionality was blended into polyacrylamide (PAM) hydrogel matrix to reinforce the mechanical strength and facilitate the homogeneous dispersion of carbon nanotubes (CNTs). The SCNF-CNT/PAM hydrogel was designed through free radical polymerization to achieve commendable mechanical, electrical, and multifunctional properties. The environmental-friendly SCNF serves as bio-templates to facilitate the assembling of CNT into integrated SCNF-CNT structures with good dispersity, thus enabling the establishment of an integrated conducting and reinforcing network. The fabricated SCNF-CNT/PAM hydrogel exhibited outstanding compressive strength (∼0.45 MPa at 50 % strain), tensile strength (∼169.12 kPa), and antifatigue capacity under cyclic stretching and pressing. Furthermore, the multifunctional sensors assembled using this hydrogel demonstrated high strain sensitivity (gauge factor ~ 3.7 at 100-400 % strain) and effectively detected human motions. This design principle provides promising prospects for constructing next-generation multifunctional flexible sensors, and the integration of these distinctive properties enables the prepared composite hydrogels to find potential applications in various areas, such as implantable soft electronic devices, electronic skin, and human movement monitoring.

Rapid Synthesis of Functions-Integrated Hydrogel as a Self-Powered Wound Dressing for Real-Time Drug Release and Health Monitoring

A bio-hydrogel is prepared via a low-cost and time-saving strategy and is studied as a self-powered wound dressing for precision medicine and health monitoring. Promoted by a dual self-catalytic pair composed of Fe3+ and catechol, gelation time is dramatically accelerated to 15 s and the hydrogel can be freely modeled at -18 °C without losing flexibility. As smart wound dressing, the required properties such as self-healing, self-adhesion, antibacterial, and sensing stability, are integrated into one hydrogel. TA@CNC offers abundant hydrogen bond and metal-ligand coordination which facilitate the hydrogel with a self-healing efficiency of 91.6%. Owing to the catechol in TA@CNC, hydrogel can adhere to multiple substrates including skin, and show good antibacterial activity. Inspired by a fruit battery, a self-powered wound dressing is fabricated, which exhibits excellent correlation and efficiency in real-time monitoring of body activity and drug release. In vivo experiments prove that efficient drug release of hydrogel dressing significantly accelerate wound healing. Additionally, the dressing exhibits excellent biocompatibility and has no negative impacts on organs. Herein, a smart wound dressing that is different from the traditional way is proposed. As a self-powered device, it can be integrated with wireless devices and is expected to participate in promising applications.

Advancements in Flexible Electronics Fabrication: Film Formation, Patterning, and Interface Optimization for Cutting-Edge Healthcare Monitoring Devices

Flexible electronics can seamlessly adhere to human skin or internal tissues, enabling the collection of physiological data and real-time vital sign monitoring in home settings, which give it the potential to revolutionize chronic disease management and mitigate mortality rates associated with sudden illnesses, thereby transforming current medical practices. However, the development of flexible electronic devices still faces several challenges, including issues pertaining to material selection, limited functionality, and performance instability. Among these challenges, the choice of appropriate materials, as well as their methods for film formation and patterning, lays the groundwork for versatile device development. Establishing stable interfaces, both internally within the device and in human-machine interactions, is essential for ensuring efficient, accurate, and long-term monitoring in health electronics. This review aims to provide an overview of critical fabrication steps and interface optimization strategies in the realm of flexible health electronics. Specifically, we discuss common thin film processing methods, patterning techniques for functional layers, interface challenges, and potential adjustment strategies. The objective is to synthesize recent advancements and serve as a reference for the development of innovative flexible health monitoring devices.

Environmentally stable and rapidly polymerized tin-tannin catalytic system hydroxyethyl cellulose hydrogel for wireless wearable sensing

In recent years, flexible sensors constructed mainly from hydrogels have played an indispensable role in several fields. However, the traditional hydrogel preparation process involves complex and time-consuming steps and the freezing or volatilization of water in the water gel in extreme environments greatly limits the further use of the sensor. Therefore, an ionic conductive hydrogel (SnHTD) was designed, which was composed of tannic acid (TA), metal ions Sn2+, hydroxyethyl cellulose (HEC), and acrylamide (AM) in a deep eutectic solvent (DES) and water binary solvent. It is worth noting that the gel time is shortened to less than 3 min by introducing the Sn-TA redox system. The addition of DES makes the hydrogel have a wide temperature tolerance range (-20 to 60 °C) and the ability to store for a long time (30 days). The introduction of HEC increased the tensile stress of hydrogel from 140.17 kPa to 219.89 kPa. Additionally, the hydrogel also has high conductivity, repeatable adhesion and UV shielding properties. In general, this research opens up a new way for room temperature polymerization of environmentally resistant hydrogel materials and effectively meets the growing demand for wireless wearable sensing.

Dually-crosslinked ionic conductive hydrogels reinforced through biopolymer gellan gum for flexible sensors to monitor human activities

Human-machine interactions, monitoring of health equipment, and gentle robots all depend considerably on flexible strain sensors. However, making strain sensors have better mechanical behavior and an extensive sensing range remains an urgent difficulty. In this study, poly acrylamide-co-butyl acrylate with gellan gum (poly(AAm-co-BA)@GG) hydrophobic association networks and intermolecular hydrogen bonding interactions are used to fabricate dual cross-linked hydrogels for wearable resistive-type strain sensors. This could be an acceptable way to minimize the limitations in hydrogels previously identified. The robust fracture strength (870 kPa) and exceptional stretchability (1297 %) of the hydrogel arise from the collaborative action of intermolecular hydrogen bonding and hydrophobic associations. It also demonstrates exceptional resilience to repeated cycles of uninterrupted stretching and relaxation, retaining its structural integrity. The response and restoration times are 110 and 120 ms respectively. Furthermore, a wide sensing range (0-900 %), notable sensitivity across various strain levels, and an impressive gauge factor (GF) of 31.51 with high durability were observed by the dual cross-linked (DC) hydrogel-based strain sensors. The measured conductivity of the hydrogel was 0.32 S/m which is due to the incorporation of NaCl. Therefore, the hydrogels can be tailored to function as wearable strain sensors that can detect subtle human gestures like speech patterns, distinguish between distinct words, and recognize vibrations of the larynx during drinking, as well as large joint motions like wrist, finger, and elbow. Furthermore, these hydrogels are capable of reliably distinguishing and reproducing various printed text. These findings imply that any electronic device that demands strain-sensing functionality might make use of these developed materials.

Bioinspired Self-Growing Layered Hydrogel Enabled by Catechol Chemistry-Mediated Interfacial Catalytic System

Tissue-inspired layered structural hydrogel has attracted increasing attention in artificial muscle, wound healing, wearable electronics, and soft robots. Despite numerous efforts being devoted to developing various layered hydrogels, the rapid and efficient preparation of layered hydrogels remains challenging. Herein, inspired by the self-growth concept of living organisms, an interfacial catalytic self-growth strategy based on catechol chemistry-mediated self-catalytic system of preparing layered hydrogels is demonstrated. Typically, the tannic acid-metal ion (e.g., TA-Fe3+) complex embedded in the hydrogel substrate would catalytically trigger rapid solid-liquid interfacial polymerization to grow the hydrogel layer without bulk solution polymerization. The self-growth process can be finely controlled by changing the growth time, the molar ratio of Fe3+/TA, and so on. The strategy is applicable to prepare various layered hydrogels as well as complex layered hydrogel patterns, allowing the customization of the physicochemical properties of the hydrogel. In addition, the self-adhesive layered hydrogel was prepared and can be utilized as a wearable strain sensor to monitor physiological activities and human motions. The demonstrated interfacial catalytic self-growth strategy will provide a route to design and fabricate layered hydrogel materials.

Induction of polymer-grafted cellulose nanocrystals in hydrogel nanocomposites to increase anti-swelling, mechanical properties and conductive self-recovery for underwater strain sensing

Anti-swelling conductive hydrogels with simultaneous high tensile strength (>1 MPa) and fast self-recovery are promising candidates for underwater strain sensing, but their preparation remains challenging. Herein, novel anti-swelling conductive nanocomposite hydrogels were fabricated based on poly(acrylamide-co-acrylic acid) (P(AM-co-AA)), polymer-grafted cellulose nanocrystals (CNCs) and Fe3+ ions through a strategy combining nano-reinforcing and multiple physical crosslinking. Due to the presence of interfacial H-bonds, polymer-grafted cellulose nanocrystals played important role in endowing hydrogels with anti-swelling capacity and enhanced mechanical performance. The obtained nanocomposite hydrogels exhibited relatively low swelling ratio (2.9-3.3 g/g), high tensile strength (>1.5 MPa), fast self-recovery (86 % recovery of hysteresis within 5 min) and conductivities of 0.0534-0.0593 S/m. The combination of excellent tensile properties and conductivity endowed the hydrogel-based strain sensors with good sensitivity (GF ≈ 0.8) and reliable cycling repeatability in 0-100 % strain range. Notably, the nanocomposite hydrogels can maintain their mechanical and sensing performance after soaking in water for 14 days, making them applicable for human motion detection both in air and underwater. Hence, this work provided a facile method to construct highly robust and anti-swelling CNC-reinforced conductive hydrogels, which have potential applications in underwater strain sensing and beyond.

Bridging papermaking and hydrogel production: Nanoparticle-loaded cellulosic hollow fibers with pitted walls as skeleton materials for multifunctional electromagnetic hydrogels

Electromagnetic hydrogels have attracted significant attention due to their vast potential in soft robotics, biomedical engineering, and energy harvesting. To facilitate future commercialization via large-scale industrial processes, we present a facile concept that utilizes the specialized knowledge of papermaking to fabricate hydrogels with multifunctional electromagnetic properties. The principles of papermaking wet end chemistry, which involves the handling of interactions among cellulosic fibers, fines, polymeric additives, and other components in aqueous systems, serves as a key foundation for this concept. Notably, based on these principles, the versatile use of chemical additives in combination with cellulosic materials enables the tailored design of various products. Our methodology exploits the unique hierarchically pitted and hollow tube-like structures of papermaking grade cellulosic fibers with discernible pits, enabling the incorporation of magnetite nanoparticles through lumen loading. By combining microscale softwood-derived cellulosic fibers with additives, we achieve dynamic covalent interactions that transform the cellulosic fiber slurry into an impressive hydrogel. The cellulosic fibers act as a skeleton, providing structural support within the hydrogel framework and facilitating the dispersion of nanoparticles. In accordance with our concept, the typical hydrogel exhibits combined attributes, including electrical conductivity, self-healing properties, pH responsiveness, and dynamic rheologic behavior. Our approach not only yields hydrogels with interesting properties but also aligns with the forefront of advanced cellulosic material applications. These materials hold the promise in remote strain sensing devices, electromagnetic navigation systems, contactless toys, and flexible electronic devices. The concept and findings of the current work may shed light on materials innovation based on traditional pulp and paper processes. Furthermore, the facile processes involved in hydrogel formation can serve as valuable tools for chemistry and materials education, providing easy demonstrations of principles for university students at different levels.

Multi-role conductive hydrogels for flexible transducers regulated by MOFs for monitoring human activities and electronic skin functions

Metal organic frameworks (MOFs) have garnered significant attention in the development of stretchable and wearable conductive hydrogels for flexible transducers. However, MOFs used in hydrogel networks have been hampered by low mechanical performance and poor dispersibility in aqueous solutions, which affect the performance of hydrogels, including low toughness, limited self-recovery, short working ranges, low conductivity, and prolonged response-recovery times. To address these shortcomings, a novel approach was adopted in which micelle co-polymerization was used for the ex situ synthesis of Zn-MOF-based hydrogels with exceptional stretchability, robust toughness, anti-fatigue properties, and commendable conductivity. This breakthrough involved the ex situ integration of Zn-MOFs into hydrophobically cross-linked polymer chains. Here the micelles of EHDDAB had two functions, first they uniformly dispersed the Zn-MOFs and secondly they dynamically cross-linked the polymer chains, profoundly influencing the mechanical characteristics of the hydrogels. The non-covalent synergistic interactions introduced by Zn-MOFs endowed the hydrogels with the capacity for high stretchability, high stress, rapid self-recovery, anti-fatigue properties, and conductivity, all achieved without external stimuli. Furthermore, hydrogels based on Zn-MOFs can serve as durable and highly sensitive flexible transducers, adept at detecting diverse mechanical deformations with swift response-recovery times and high gauge factor values. Consequently, these hydrogels can be tailored to function as wearable strain sensors capable of sensing significant human joint movements, such as wrist bending, and motions involving the wrist, fingers, and elbows. Similarly, they excel at monitoring subtle human motions, such as speech pronunciation, distinguishing between different words, as well as detecting swallowing and larynx vibrations during various activities. Beyond these applications, the hydrogels exhibit proficiency in distinguishing and reproducing various written words with reliability. The Zn-MOF-based hydrogels hold promising potential for development in electronic skin, medical monitoring, soft robotics, and flexible touch panels.

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

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

Rational Design of Dynamically Super-Tough and Super-Stretchable Hydrogels for Deformable Energy Storage Devices

Hydrogels possess unique polymer networks that offer flexibility/stretchability, high ionic conductivity, and resistance to electrolyte leakage, making them suitable for deformable energy storage devices. Endowing the mechanical functionality of the hydrogel electrolytes focus on either enhancing the stretchability or the toughness. However, the stretchability and the toughness are generally a trade-off that the stretchable gels are intrinsically prone to damage and sensitive to notches and cracks. Here, the regulating strategies on the hydrogel's mechanical properties are provided to develop the designated hydrogel electrolyte, where different polymeric network structures are constructed, including single network structures, semi-interpenetrating network structures, and interpenetrating dual-network structures. A comprehensive comparison of these polymer network structures is conducted to evaluate their mechanical stretchability and toughness. Designing super-tough and super-stretchable hydrogels based on specific application requirements can be realized by striking a balance by regulating the hydrogel structure. In specific, incorporating semi-interpenetrating networks significantly can enhance stretchability to achieve a break elongation up to 1300%, while the interpenetrating dual-networks can largely improve the toughness to realize the extraordinary fracture toughness of 6.843 kJ m-2. These findings offer valuable designing guidance for designated hydrogel electrolytes and the deformable zinc-silver battery is demonstrated with high mechanical stability and electrochemical performance.

Facile preparation of highly adhesive yet ultra-strong poly (vinyl alcohol)/cellulose nanocrystals composite hydrogel enabled by multiple networks structure

Poly (vinyl alcohol) (PVA) hydrogel showed potential applications in bioengineering and wearable sensors fields. It is still a huge challenge to prepare highly adhesive yet strong poly (vinyl alcohol) hydrogel with good biocompatibility. Herein, we prepared a highly self-adhesive and strong poly (vinyl alcohol)/tannic acid@cellulose nanocrystals (PVA/TA@CNCs) composite hydrogel using TA@CNCs as functional nanofiller via facile freezing-thawing method. Multiple networks consisting of hydrogen bonding and coordination interactions endowed the hydrogel with high mechanical strength, excellent flexibility and fracture toughness with adequate energy dissipation mechanism and relatively dense network structure. The tensile strength of PVA/TA@CNCs hydrogel reached the maximum of 463 kPa, increasing by 367 % in comparison with pure PVA hydrogel (99 kPa), demonstrating the synergistic reinforcing and toughening effect of TA@CNCs. The hydrogel exhibited extremely high adhesion not only for various dry and wet substrates such as plastic, metal, Teflon, rubber, glass, leaf, but also sweaty human skin, showing good adhesion durability. The highest adhesion strength to silicone rubber, steel plate and pigskin could reach 197 kPa, 100 kPa and 46.9 kPa, respectively. Meanwhile the hydrogel had negligible cytotoxicity to cells and showed good biocompatibility.

Femtosecond Laser Fabrication of High-Linearity Liquid Metal-Based Flexible Strain Sensor

Liquid metal (LM) is widely used in flexible electronic devices due to its excellent metallic conductivity and ductility. However, the fabrication of LM flexible strain sensors with high sensitivity and linearity is still a huge challenge, since the resistance of LM does not change much with strain. Here, a highly sensitive and linear fully flexible strain sensor with a resistive sensing function is proposed. The sensor comprises an Fe-doped liquid metal (Fe-LM) electrode for enhanced performance. The design and manufacturing of flexible strain sensors are based on the technology of controlling surface wettability by femtosecond laser micro/nano-processing. A supermetalphobic microstructure is constructed on a polydimethylsiloxane (PDMS) substrate to achieve the selection adhesion of Fe-LM on the PDMS substrate. The Fe-LM-based flexible strain sensor has high sensitivity and linearity, a gauge factor (GF) up to 1.18 in the strain range of 0-100%, excellent linearity with an R2 of 0.9978, a fast response time of 358 ms, and an excellent durability of more than 2400 load cycles. Additionally, the successful monitoring of human body signals demonstrates the potential of our developed flexible strain sensor in wearable monitoring applications.

Lignin reinforced tough, adhesive, and recoverable protein organohydrogels for wearable strain sensing under sub-zero temperatures

Protein-based hydrogels with promising biocompatibility and biodegradability have attracted considerable interest in areas of epidermal sensing, whereas, which are still difficult to synchronously possess high mechanical strength, self-adhesion, and recoverability. Hence, the bio-polymer lignosulfonate-reinforced gluten organohydrogels (GOHLx) are fabricated through green and simple food-making processes and the following solvent exchange with glycerol/water binary solution. Ascribing to the uniform distribution of lignosulfonate in gluten networks, as well as the noncovalent interactions (e.g., H-bond) between them, the resultant GOHLx exhibit favorable conductivity (∼14.3 × 10-4 S m-1), toughness (∼711.0 kJ m-3), self-adhesion (a maximal lap-shear strength of ∼33.5 kPa), high sensitivity (GF up to ∼3.04), and durability (∼3000 cycles) toward shape deformation, which are suitable for the detection of both drastic (e.g., elbow and wrist bending) and subtle (e.g., swallowing and speaking) human movements even under -20 °C. Furthermore, the GOHLx is also biocompatible, degradable, and recoverable (by a simple kneading process). Thus, this work may pave a simple, green, and cheap way to prepare all-biomass-based, tough, sticky, and recoverable protein-based organohydrogels for epidermal strain sensing even in harsh environments.

Hemostatic Tranexamic Acid-Induced Fast Gelation and Mechanical Reinforcement of Polydimethylacrylamide/Carboxymethyl Chitosan Hydrogel for Hemostasis and Wound Healing

The combinational properties with excellent mechanical properties, adhesive performance, hemostatic ability, antibacterial action, and wound healing efficacy are highly desirable for injectable hydrogels' practical applications in hemorrhage control and wound closure, but designing one single hydrogel system integrating with such properties is still difficult. Herein, a simplified yet straightforward strategy is proposed to prepare an injectable and robust poly(N,N-dimethylacrylamide) (PDMAA)/carboxymethyl chitosan (CMCS) hydrogel induced by tranexamic acid (TXA). TXA not only promotes the rapid generation of free radicals but also introduces multiple hydrogen bonds into the hydrogel network. Moreover, as a common clinical hemostatic drug, TXA itself has excellent hemostatic effects. In addition, CMCS imparts sterilization and hemostasis effects to the hydrogel, thereby promoting wound healing. Besides, the amino and carboxyl groups on TXA molecules and the hydroxyl, amino, and carboxyl groups on CMCS molecules can form multiple hydrogen bonds with wet biological tissues, leading to good wet tissue adhesion of the hydrogel. As a result, the hydrogel with excellent mechanical properties (up to 1.83 MPa at 90% compression strain), adhesion performance (up to 18.7 kPa adhesion strength to porcine skin tissue), biocompatibility, hemostatic ability, antibacterial activity, and wound healing properties can be fabricated within several minutes. These combinational advantages enable the hydrogel to efficiently stop hemorrhage (blood loss amount: 110 mg; hemostasis time: 25 s) and promote the wound healing process (wound closure rate at 2 weeks: 83%), which can be verified using rat models of liver bleeding and infected full thickness skin defect. Overall, this facile strategy to design a hydrogel incorporating such unique advantages will greatly advance the hydrogel's clinical application in rapid hemostasis and wound healing.

Next-generation self-adhesive dressings: Highly stretchable, antibacterial, and UV-shielding properties enabled by tannic acid-coated cellulose nanocrystals

Hydrogels with excellent high-water uptake and flexibility have great potential for wound dressing. However, pure hydrogels without fiber skeleton faced poor water retention, weak fatigue resistance, and mechanical strength to hinder the development of the dressing as next-generation functional dressings. We prepared an ultrafast gelation (6 s) Fe3+/TA-CNC hydrogel (CTFG hydrogel) based on a self-catalytic system and bilayer self-assembled composites. The CTFG hydrogel has excellent flexibility (800% of strain), fatigue resistance (support 60% compression cycles), antibacterial, and self-adhesive properties (no residue or allergy after peeling off the skin). CTFG@S bilayer composites were formed after electrospun silk fibroin (SF) membranes were prepared and adhesive with CTFG hydrogels. The CTFG@S bilayer composites had significant UV-shielding (99.95%), tensile strain (210.9 KPa), and sensitive humidity-sensing properties. Moreover, the integrated structure improved the mechanical properties of electrospun SF membranes. This study would provide a promising strategy for rapidly preparing multifunctional hydrogels for wound dressing.

A highly sensitive and anti-freezing conductive strain sensor based on polypyrrole/cellulose nanofiber crosslinked polyvinyl alcohol hydrogel for human motion detection

Electro-conductive hydrogels emerge as a stretchable conductive materials with diverse applications in the synthesis of flexible strain sensors. However, the high-water content and low cross-links density cause them to be mechanically destroyed and freeze at subzero temperatures, limiting their practical applications. Herein, we report a one-pot strategy by co-incorporating cellulose nanofiber (CNF), Poly pyrrole (PPy) and glycerol with polyvinyl alcohol (PVA) to prepare hydrogel. The addition of PPy endowed the hydrogel with good conductivity (∼0.034 S/m) compared to the no PPy@CNF group (∼0.0095 S/m), the conductivity was increased by 257.9 %. The hydrogel exhibits comparable ionic conductivity at -18 °C as it does at room temperature. It's attributed to the glycerol as a cryoprotectant and the formation of hydrated [Zn(H2O)n]2+ ions via strong interaction between Zn2+ and water molecules. Moreover, the cellulose nanofiber intrinsically assembled into unique hierarchical structures allow for strong hydrogen bonds between adjacent cellulose and PPy polymer chains, greatly improve the mechanical strength (stress∼0.65 MPa, strain∼301 %) and excellent viscoelasticity (G'max ∼ 82.7 KPa). This novel PPy@CNF-PVA hydrogel exhibits extremely high Gauge factor (GF) of 2.84 and shows excellent sensitivity, repeatability and stability. Therefore, the hydrogel can serve as reliable and stable strain sensor which shows excellent responsiveness in human activities monitoration.

Developing conductive hydrogels for biomedical applications

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

Tannic acid-coated cellulose nanocrystal-reinforced transparent multifunctional hydrogels with UV-filtering for wearable flexible sensors

Ionically conductive hydrogels are an ideal alternative material for applications in wearable flexible sensors to monitor human health. However, producing hydrogels with both high sensitivity and excellent versatility is difficult, and their transparency and UV-blocking properties are significantly limited. Here, with mussel- and gecko-inspired biomimicry, all-biomass-based hydrogels (OGTCGs) with self-adhesive, self-healing, transparent, UV-filtering, frost-resistant, environmentally stable, antibacterial, and biocompatible properties were designed and constructed via a simple one-step approach with a water/glycerol system and borax added without any crosslinker using synergistic dynamic covalent and noncovalent chemistry. The transparency of the OGTCG hydrogel reached 81.06 %, while the added tannic acid-coated cellulose nanocrystal (TA@CNC) induced a UV-blocking effect. The OGTCG hydrogel exhibited a high toughness (218.67 kPa) and modulus (100.32 kPa) reinforced by TA@CNC. The OGTCG hydrogel showed good self-healing abilities with an efficiency of over 90 % after 6 h. In a binary solvent system, the OGTCG hydrogel had environmental stability, as illustrated by density functional theory (DFT), greatly broadening its application range. Moreover, it had an electrical conductivity of 2.3 mS cm-1 and a sensitivity of 3.97. Therefore, with its rapid response and real-time monitoring capabilities, the OGTCG hydrogel shows great potential for applications in monitoring human health.

Multi-characteristic tannic acid-reinforced polyacrylamide/sodium carboxymethyl cellulose ionic hydrogel strain sensor for human-machine interaction

Big data and cloud computing are propelling research in human-computer interface within academia. However, the potential of wearable human-machine interaction (HMI) devices utilizing multiperformance ionic hydrogels remains largely unexplored. Here, we present a motion recognition-based HMI system that enhances movement training. We engineered dual-network PAM/CMC/TA (PCT) hydrogels by reinforcing polyacrylamide (PAM) and sodium carboxymethyl cellulose (CMC) polymers with tannic acid (TA). These hydrogels possess exceptional transparency, adhesion, and remodelling features. By combining an elastic PAM backbone with tunable amounts of CMC and TA, the PCT hydrogels achieve optimal electromechanical performance. As strain sensors, they demonstrate higher sensitivity (GF = 4.03), low detection limit (0.5 %), and good linearity (0.997). Furthermore, we developed a highly accurate (97.85 %) motion recognition system using machine learning and hydrogel-based wearable sensors. This system enables contactless real-time training monitoring and wireless control of trolley operations. Our research underscores the effectiveness of PCT hydrogels for real-time HMI, thus advancing next-generation HMI systems.

Compressible, anti-freezing, and ionic conductive cellulose/polyacrylic acid composite hydrogel prepared via AlCl3/ZnCl2 aqueous system as solvent and catalyst

From the perspective of environmental sustainability, introducing cellulose into ionic conductive hydrogel is an inevitable trend for the development of flexible conductive materials. We report a double-network cellulose/polyacrylic acid (Cel/PAA) composite hydrogel based on the dissolving of cellulose by AlCl3/ZnCl2 aqueous system. The Cel/PAA composite hydrogel consists of rigid cellulose chains and flexible polyacrylic acid, which synergistically realize the improvement of the mechanical properties. The AlCl3/ZnCl2 aqueous system not only serves as the green solvent for cellulose, but also the Al3+ and Zn2+ metal ions can be served as a catalyst to activate the initiator for polymerization of acrylic acid. Compared with pure cellulose hydrogel, the compression strain of the Cel/PAA composite hydrogel was significantly improved to 80 %, and its conductivity increased by 28.1 %. In addition, its compression stress was enhanced over 2 times than pure PAA hydrogel. The Cel/PAA composite hydrogel exhibits excellent anti-freezing (-45 °C), weight retention (90 %), and conductivity (2.70 S/m) properties, still maintaining transparency and storage stability in the extreme environment. This work presents a facile strategy to develop an ionic conductive cellulose-based composite hydrogel with good conductivity and mechanical properties, which shows potential for the application fields of flexible sensors and 3D-printing functional materials.

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

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

One-Pot Preparation of Skin-Inspired Multifunctional Hybrid Hydrogel with Robust Wound Healing Capacity

Bioinspired hydrogels have demonstrated multiple superiorities over traditional wound dressings for wound healing applications. However, the fabrication of bioinspired hydrogel-based wound dressings with desired functionalities always requires multiple successive steps, time-consuming processes, and/or sophisticated protocols, plaguing their clinical applications. Here, a facile one-pot strategy is developed to prepare a skin-inspired multifunctional hydrogel within 30 min by incorporating elastin (an essential functional component of the dermal extracellular matrix), tannic acid, and chitosan into the covalently cross-linked poly(acrylamide) network through noncovalent interactions. The resulting hydrogel exhibits a Young's modulus (ca. 36 kPa) comparable to that of human skin, a high elongation-at-break (ca. 1550%), a satisfactory tensile strength (ca. 61 kPa), and excellent elastic self-restorability, enabling the hydrogel to synchronously and conformally deform with human skin when used as wound dressings. Importantly, the hydrogel displays a self-adhesive property to skin tissues with an appropriate bonding strength (ca. 55 kPa measured on intact porcine skin), endowing the hydrogel with the ability to rapidly self-adhere to intact human skin, sealing the wound surface and also easily being removed without residue left or trauma caused to the skin. The hydrogel also possesses remarkable antibacterial activity, antioxidant capability, and hemocompatibility. All of these collective beneficial properties enable the hydrogel to significantly accelerate the wound healing process, outperforming the commercial wound dressings.

Copolymer-grafted cellulose nanocrystal induced nanocomposite hydrogels with enhanced strength, high elasticity and adhesiveness for flexible strain and pressure sensors

Recently, the application of cellulose nanocrystals (CNCs) in the field of hydrogel sensors has attracted much attention. However, it remains challenging to construct CNC-reinforced conductive hydrogels with a combination of enhanced strength, low hysteresis, high elasticity and remarkable adhesiveness. Herein, we present a facile method to prepare conductive nanocomposite hydrogels with the above-mentioned properties by reinforcing chemically crosslinked poly(acrylic acid) (PAA) hydrogel with rational-designed copolymer-grafted CNCs. The copolymer-grafted CNCs interact with PAA matrix to form carboxyl-amide conventional hydrogen bonds and carboxyl-amino ionic hydrogen bonds, among which the ionic hydrogen bonds with rapid recovery capability are critical to the low hysteresis and high elasticity of hydrogel. The introduction of copolymer-grafted CNCs endowed the hydrogels with enhanced tensile/compressive strength, high resilience (>95 %) during tensile cyclic loading, rapid self-recovery during compressive cyclic loading and improved adhesiveness. Thanks to the high elasticity and durability of hydrogel, the assembled hydrogel sensors exhibited good cycling repeatability and durability in detecting various strains, pressures and human motions. The hydrogel sensors also showed satisfying sensitivity. Hence, the proposed preparation method and the obtained CNC-reinforced conductive hydrogels would open new avenues in flexible strain and pressure sensors for human motion detection and beyond.

Tannic acid-Fe3+ dual catalysis induced rapid polymerization of injectable poly(lysine) hydrogel for infected wound healing

Infected wounds are difficult to heal because they are vulnerable to bacterial attacks, inflammatory responses, and oxidative stress. To promote the healing of infected wounds, we developed an injectable dual-network hydrogel TFAEP (TA-Fe, APS, EPL-GMA, PVA) based on ε-poly-l-lysine-graft-glycidyl methacrylate (EPL-GMA), polyvinyl alcohol (PVA), and tannic acid-iron (TA-Fe). TA-Fe formed a stable redox pair, which acted as a dual-autocatalytic system to activate ammonium persulfate, generate free radicals, and subsequently induce EPL-GMA polymerization. Then PVA formed hydrogen bonds with TA molecules. Here, TA-Fe not only simulated peroxidase to convert H2O2 into hydroxyl radicals (OH), but also exhibited good near-infrared photothermal conversion efficiency, which all endowed the hydrogel with excellent antibacterial ability. In addition, the hydrogel could remove excessive reactive oxygen species and reactive nitrogen species, alleviating oxidative stress and reducing inflammation response due to the presence of TA molecules. Moreover, the hydrogel showed good injectability and tissue adhesion, ensuring the close adhesion of the hydrogel to the wound and achieving the maximum function. In vivo experiments demonstrated that the hydrogel promoted infected wound healing by accelerating epidermal regeneration, promoting angiogenesis and collagen deposition, and facilitating the expression of anti-inflammatory factors.

A Strong and Double-sided Self-Adhesive Hydrogel Sensor

Flexible self-adhesive hydrogel sensors are attracting considerable concerns in recent years. However, creating a self-adhesive hydrogel sensor with excellent mechanical properties remains to be challenging. Herein, a double-sided self-adhesive hydrogel capable of strain sensor with high strength is demonstrated by penetration strategy. The middle poly(acrylic acid)-polyacrylamide/Fe3+ (PAA-PAM/Fe3+ ) tough layer endows the double-sided self-adhesive hydrogel with high mechanical properties, while the bilateral poly[2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide-polyacrylamide (PSBMA-PAM) adhesive layers are used to ensure excellent adhesiveness on diverse substrates. The tough layer of the double-sided self-adhesive hydrogel sensor shows a strong interface bonding force against the adhesive layer. The double-sided self-adhesive hydrogel sensor enables excellent adhesiveness on diverse substrates. More importantly, it can accurately detect different strains and human motions as a self-adhesive hydrogel strain sensor. This work manifests a new route of structural design to develop a self-adhesive hydrogel sensor with excellent mechanical properties that is suitable for a wide range of applications.

Improving the mechanical properties of 3D printed GelMA composite hydrogels by tannic acid

The lack of advanced biomaterials is a major challenge in bio‐printing. Gelatin‐methacryloyl (GelMA) hydrogel, as one of the most commonly used biomaterials in 3D printing, has limited the applications of medicine because of its low mechanical properties. In this study, to enhance the mechanical strength of GelMA hydrogels, we prepared a composite hydrogel based on F127 diacrylate (F127DA) and GelMA, followed by lyophilization and tannic acid (TA) treatment. In this composite hydrogel, the F127DA could self‐assemble into nanomicelles as crosslinking centers for monomer polymerization, which provides additional energy dissipation in hydrogels due to the synergistic deformation of micelles and internal rearrangement of physical binding. After lyophilization of the composite hydrogel, the porous hydrogel was formed. The subsequent treatment of TA could diffuse into the inner of the hydrogel and react with the hydrogel via hydrogen bonds, resulting in the significant enhancement of mechanical properties. The maximum tensile deformation of the obtained hydrogel was about 11 times higher than that of GelMA. This work demonstrates a method to enhance the mechanical properties of 3D‐printed GelMA hydrogel with promising application in bioprinting.

Recent advances in cellulose nanocrystals-based antimicrobial agents

Over the past five years, there has been growing interest in the design of modified cellulose nanocrystals (CNCs) as nanoscale antimicrobial agents in potential end-user applications such as food preservation/packaging, additive manufacturing, biomedical and water purification. The interest of applying CNCs-based antimicrobial agents arise due to their abilities to be derived from renewable bioresources and their excellent physicochemical properties including rod-like morphologies, large specific surface area, low toxicity, biocompatibility, biodegradability and sustainability. The presence of ample surface hydroxyl groups further allows easy chemical surface modifications for the design of advanced functional CNCs-based antimicrobial materials. Furthermore, CNCs are used to support antimicrobial agents that are subjected to instability issues. The current review summarizes recent progress in CNC-inorganic hybrid-based materials (Ag and Zn nanoparticles, other metal/metal oxide) and CNC-organic hybrid-based materials (polymers, chitosan, simple organic molecules). It focuses on their design, syntheses and applications with a brief discussion on their probable modes of antimicrobial action whereby the roles of CNCs and/or the antimicrobial agents are highlighted.

Guar gum reinforced conductive hydrogel for strain sensing and electronic devices

Hydrophobically associated conductive hydrogels got great attention due to their excellent properties like stretchability, energy dissipation mechanism, and strain sensor. But hydrophobically associated hydrogels have poor mechanical properties and time response to external stimuli. To enhance the mechanical properties and response to stimuli, Acrylamide- co-Butyl acrylate/Gum based conductive hydrogels were prepared. SDS works as a cross-linker and micelle-forming agent while NaCl makes hydrogel as conductive. The results show that our % strain sensing reached up to 400 %, and fracture stress and fracture strain reached to 0.5 MPa and 401 % respectively. Besides this, it's having an excellent response to continuous stretching and unstretching multiple cycles without any fracture up to 180 s and an excellent time response of 190 s. The conductivity of the hydrogel was 0.20 Sm-1. The hydrophobic hydrogels showed a clear and quick response to human motions like finger, wresting, writing, speaking, etc. Interestingly, our prepared hydrogels can detect the mood of the human face. Similarly, the hydrogels were found efficient in bridging the surface of electronic devices with human skin. This indicates that our prepared hydrogels can monitor human body motion and will replace the existing materials used in strain sensors in the near future.

High-Performing Conductive Hydrogels for Wearable Applications

Conductive hydrogels have gained significant attention for their extensive applications in healthcare monitoring, wearable sensors, electronic devices, soft robotics, energy storage, and human-machine interfaces. To address the limitations of conductive hydrogels, researchers are focused on enhancing properties such as sensitivity, mechanical strength, electrical performance at low temperatures, stability, antibacterial properties, and conductivity. Composite materials, including nanoparticles, nanowires, polymers, and ionic liquids, are incorporated to improve the conductivity and mechanical strength. Biocompatibility and biosafety are emphasized for safe integration with biological tissues. Conductive hydrogels exhibit unique properties such as stretchability, self-healing, wet adhesion, anti-freezing, transparency, UV-shielding, and adjustable mechanical properties, making them suitable for specific applications. Researchers aim to develop multifunctional hydrogels with antibacterial characteristics, self-healing capabilities, transparency, UV-shielding, gas-sensing, and strain-sensitivity.

Super-Rapid In Situ Formation of a Silver Ion-Induced Supramolecular Hydrogel with Efficient Antibacterial Activity for Root Canal Disinfection

Supramolecular hydrogels prepared using low-molecular-weight gelators have attracted considerable attention for biomedical applications. However, in situ supramolecular hydrogels are limited in terms of their prolonged gelation time and/or unstable nature at high temperatures. In this study, we constructed a stable supramolecular Ag-isoG hydrogel through super-rapid in situ formation, wherein hydrogelation process occurred instantaneously upon mixing isoG and Ag⁺ within 1 s under ambient conditions. Interestingly, unlike most nucleoside-based supramolecular hydrogels, this Ag-isoG hydrogel remains stable even at a high temperature (100 °C). Moreover, the as-designed hydrogel demonstrated significant antibacterial activity against Staphylococcus aureus and the oral bacterium Streptococcus mutans owing to the strong chelating ability of Ag ions, and the hydrogel exhibited relatively low cytotoxicity in root canal and an easy removal feature by saline. The hydrogel was then applied to a root canal infection model, which demonstrated strong antibacterial activity against Enterococcus faecalis, with performance even better than that of the regular calcium hydroxide paste. This feature makes the Ag-isoG hydrogel a prospective alternative material as intracanal medicaments for root canal treatment.

High-stretchable, self-healing, self-adhesive, self-extinguishing, low-temperature tolerant starch-based gel and its application in stimuli-responsiveness

Starch with active hydroxyl groups is one of the most attractive carbohydrates for the preparation of gels in recent years. However, the mechanical properties, self-healing properties, self-adhesion properties, especially low-temperature resistance are generally unsatisfactory for current starch-based gels. Based on that, a multiple network structure of amylopectin-carboxymethyl cellulose-polyacrylamide (ACP) gel was prepared by a "cooking" method. Tannic acid (TA) was used to construct multiple hydrogen bonds among molecular chains. ACP gel demonstrates high elongation at break (1090 %) and strength, self-healing performance and adhesion behavior, extraordinary low-temperature resistance (-80 °C) and self-extinguishing. As a sensor device, ACP gel can effectively monitor human movements and microscopic expression changes and achieve real-time monitoring under harsh conditions (After multiple cutting-healing steps, under low-temperature conditions, even a month later). Additionally, ACP gel could be served to detect temperature changes with a wide operating range and a high sensitivity of 33 %·°C^-1, which is promising to monitor the changes in temperature. More interestingly, ACP gel can even monitor the cooking process and breathing frequency with fast response, implying applications in food processing, disease diagnosis and medical treatment. This study provides new opportunities for the design and fabrication of carbohydrate-based gels with multiple performance and multifunctional electronic devices.

Repair of degenerative nucleus pulposus by polyphenol nanosphere-encapsulated hydrogel gene delivery system

Intervertebral disc degeneration (IDD) progresses due to local inflammatory response, gradually unbalanced anabolic/catabolic activity, and progressive functional impairment within the nucleus pulposus. Antagomir-21, a cholesterol-modified miRNA-21 inhibitor, has potential extracellular matrix (ECM) regenerative ability, but its application for IDD is limited by inadequate local delivery systems. An injectable hydrogel gene delivery system encapsulating a modified tannic acid nanoparticles (TA NPs) vector was engineered for on-demand and sustained delivery of antagomir-21 into the nucleus pulposus. After nucleus pulposus cell uptake, antagomir-21 was released from TA NPs and regulated the ECM metabolic balance by inhibiting the MAPK/ERK signaling pathway. TA NPs scavenged intracellular ROS and reduced inflammation by downregulating TNF-α expression. In vivo, synergistic anti-inflammatory effects and ECM regeneration effectively promoted therapeutic efficacy against IDD. This hydrogel gene delivery system represents a creative, promising strategy for IDD repair.

Combined Catalysis: A Powerful Strategy for Engineering Multifunctional Sustainable Lignin-Based Materials

The production and engineering of sustainable materials through green chemistry will have a major role in our mission of transitioning to a more sustainable society. Here, combined catalysis, which is the integration of two or more catalytic cycles or activation modes, provides innovative chemical reactions and material properties efficiently, whereas the single catalytic cycle or activation mode alone fails in promoting a successful reaction. Polyphenolic lignin with its distinctive structural functions acts as an important template to create materials with versatile properties, such as being tough, antimicrobial, self-healing, adhesive, and environmentally adaptable. Sustainable lignin-based materials are generated by merging the catalytic cycle of the quinone-catechol redox reaction with free radical polymerization or oxidative decarboxylation reaction, which explores a wide range of metallic nanoparticles and metal ions as the catalysts. In this review, we present the recent work on engineering lignin-based multifunctional materials devised through combined catalysis. Despite the fruitful employment of this concept to material design and the fact that engineering has provided multifaceted materials able to solve a broad spectrum of challenges, we envision further exploration and expansion of this important concept in material science beyond the catalytic processes mentioned above. This could be accomplished by taking inspiration from organic synthesis where this concept has been successfully developed and implemented.

High-Sensitivity Wearable Sensor Based On a MXene Nanochannel Self-Adhesive Hydrogel

To address the shortcomings of traditional filler-based wearable hydrogels, a new type of nanochannel hydrogel sensor is fabricated in this work through a combination of the unique structure of electrospun fiber textile and the properties of a double network hydrogel. Unlike the traditional Ti₃C₂T_x MXene-based hydrogels, the continuously distributed Ti₃C₂T_x MXene in the nanochannels of the hydrogel forms a tightly interconnected structure similar to the neuron network. As a result, they have more free space to flip and perform micromovements, which allows one to significantly increase the electrical conductivity and sensitivity of the hydrogel. According to the findings, the Ti₃C₂T_x MXene nanochannel hydrogel has excellent mechanical properties as well as self-adhesion and antifreezing characteristics. The hydrogel sensor successfully detects different human motions and physiological signals (e.g., low pulse signals) with high stability and sensitivity. Therefore, the proposed Ti₃C₂T_x MXene-based hydrogel with a unique structure and properties is very promising in the field of flexible wearable devices.

Room Temperature Ca2+-Initiated Free Radical Polymerization for the Preparation of Conductive, Adhesive, Anti-freezing and UV-Blocking Hydrogels for Monitoring Human Movement

In recent years, conductive hydrogels have received increasing attention as wearable electronics due to the electrochemical properties of conductive polymers combined with the softness of hydrogels. However, conventional hydrogels are complicated to prepare, require high temperature or UV radiation to trigger monomer polymerization, and are frozen at low temperatures, which seriously hinder the application of flexible wearable devices. In this paper, a conductive sensor integrating mechanical properties, adhesion, UV shielding, anti-dehydration, and anti-freeze was prepared based on Ca²⁺-initiated radical polymerization at room temperature using the synergy of sodium lignosulfonate, acrylamide (AM), and calcium chloride (CaCl₂). Metal ions can activate ammonium persulfate to generate free radicals that allow rapid gelation of AM monomers at room temperature without external stimuli. Due to ionic cross-linking and non-covalent interaction, the hydrogels have good tensile properties (1153% elongation and 168 kPa tensile strength), high toughness (758 KJ·m^-3), excellent adhesive properties (48.5 kPa), high ionic conductivity (7.2 mS·cm^-1), and UV resistance (94.4%). CaCl₂ can inhibit ice nucleation, so that the hydrogels have anti-dehydration and frost resistance properties and even at -80 °C can maintain flexibility, high conductivity, and adhesion. Assembled into a flexible sensor, it can sense various large and small movements such as compression, bending, and talking, which is a flexible sensing material with wide application prospects.

Cellulose nanocrystals boosted hydrophobic association in dual network polymer hydrogels as advanced flexible strain sensor for human motion detection

Conductive hydrogels attract the attention of researchers worldwide, especially in the field of flexible sensors like strain and pressure. These flexible materials have potential applications in the field of electronic skin, soft robotics, energy storage, and human motion detection. However, its practical application is limited due to low stretchability, high hysteresis energy, low conductivity, long-range strain sensitivity, and high response time. It's still a challenging job to endow all these properties in a single hydrogel network. In the present work, cellulose nano crystals (CNCs) reinforced hydrophobically associated gels were developed using APS as a source of radical polymerization, acrylamide and lauryl methacrylate were used as a monomer. CNCs reinforced the hydrophobically associated hydrogels through hydrogen bonding to retain the hydrogel's network structure. Hydrogels consist of dual crosslinking, which demonstrate exceptional mechanical performance (fracture stress and strain, toughness, and Young's modulus). The low hysteresis energy (10.9 kJm^-3) and high conductivity (22.97 mS/cm) make the hydrogels a strong candidate for strain sensors with high sensitivity (GF = 19.25 at 700% strain) and a fast response time of 200 ms. Cyclic performance was also investigated up to 300 continuous cycles. After 300 cycles, the hydrogels were still stable and no considerable change was observed. These hydrogels are capable of sensing different human motions like wrist, finger bending, and neck (up-down and straight and right/left motion of neck). The hydrogels also demonstrate changes in current in response to swallowing, different speaking words, and writing different alphabets. These results suggest that our prepared materials can sense different small and large human motions, and also could be used in any electronic device where strain sensing is required.

Low-Swelling Adhesive Hydrogel with Rapid Hemostasis and Potent Anti-Inflammatory Capability for Full-Thickness Oral Mucosal Defect Repair

Full-thickness oral mucosal defects are accompanied by significant blood loss and frequent infections. Instead of conventional therapies that separate hemostasis and anti-inflammation in steps, emerging hydrogels can integrate multiple functions for the successive process after defect including hemostasis/inflammatory phase, proliferative phase, and remodeling phase. However, these functions can be easily compromised by rapid swelling and degradation of hydrogels in wet oral environment. Herein, a low-swelling adhesive hydrogel with rapid hemostasis and potent anti-inflammatory capability was developed using a dual cross-linking strategy as well as a safe and facile fabrication method. It was double cross-linked hydrogel consisting of gelatin methacrylate (GelMA), nanoclay, and tannic acid (TA) (referred to as GNT). GNT hydrogel exhibited low-swelling (one-eighth of that of GelMA), excellent stretchability (211.86%), and good adhesive properties (5 times the adhesive strength of GelMA). Physicochemical characterization illuminated the close interactions among the three components. A systematic investigation of the therapeutic effects of GNT hydrogels was performed. In vitro and in vivo experimental results demonstrated the potent hemostatic property and excellent antibacterial and anti-inflammatory effects of GNT hydrogels. The RNA sequencing analysis results for rat full-thickness oral mucosal samples showed that GNT reduced inflammation levels by down-regulating the expression of multiple inflammation-related pathways, including TNF and IL-17 pathways. It also enhanced the expression levels of tissue regeneration-related genes and thus accelerated defective mucosal repair. More importantly, the therapeutic effects of GNT were superior to those of a commercial oral tissue repair membrane when applied for full-thickness oral mucosal defect repair in rabbits. In summary, the prepared low-swelling adhesive GNT hydrogel with rapid hemostasis and potent anti-inflammatory is a promising therapy for full-thickness mucosal defect in the moist and dynamic oral environment.

Design strategies for adhesive hydrogels with natural antibacterial agents as wound dressings: Status and trends

The wound healing process is usually susceptible to different bacterial infections due to the complex physiological environment, which significantly impairs wound healing. The topical application of antibiotics is not desirable for wound healing because the excessive use of antibiotics might cause bacteria to develop resistance and even the production of super bacteria, posing significant harm to human well-being. Wound dressings based on adhesive, biocompatible, and multi-functional hydrogels with natural antibacterial agents have been widely recognized as effective wound treatments. Hydrogels, which are three-dimensional (3D) polymer networks cross-linked through physical interactions or covalent bonds, are promising for topical antibacterial applications because of their excellent adhesion, antibacterial properties, and biocompatibility. To further improve the healing performance of hydrogels, various modification methods have been developed with superior biocompatibility, antibacterial activity, mechanical properties, and wound repair capabilities. This review summarizes hundreds of typical studies on various ingredients, preparation methods, antibacterial mechanisms, and internal antibacterial factors to understand adhesive hydrogels with natural antibacterial agents for wound dressings. Additionally, we provide prospects for adhesive and antibacterial hydrogels in biomedical applications and clinical research.