Tough, Anti-Swelling Supramolecular Hydrogels Mediated by Surfactant-Polymer Interactions for Underwater Sensors

Surfactant-Enhanced Anti-Swelling Hydrogel Flexible Sensor for Machine Learning-Assisted Underwater Gesture Recognition

Hydrogels hold great promise as flexible sensors. However, the development of hydrogel sensors with exceptional anti-swelling properties and stable performance remains a significant challenge. This study introduced a novel design strategy based on surfactant-assisted hydrophobic associative hydrogels. Through free-radical copolymerization in the presence of the surfactant cetyltrimethylammonium bromide, hydrogels with outstanding mechanical properties, excellent anti-swelling capabilities, and superior sensing performance are successfully fabricated. The resulting hydrogel demonstrated remarkable anti-swelling behavior (swelling ratio: -2.3%), exceptional mechanical strength (breaking strain: 3594.5%), and sustained fatigue resistance during repeated underwater loading-unloading cycles (1000 cycles). Integrating the hydrogel sensor with machine learning, precise and stable underwater gesture recognition and motion monitoring are achieved with an accuracy of 98.3%. This study provides a new perspective for advancing flexible underwater sensor technologies and underscores their broad potential in smart wearable devices, health monitoring, and underwater exploration.

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.

Environmentally tolerant multifunctional eutectogel for highly sensitive wearable sensors

Flexible hydrogel sensors have found extensive applications. However, the insufficient sensing sensitivity and the propensity to freeze at low temperatures restrict their use, particularly in frigid conditions. Herein, a multifunctional eutectogel with high transparency, anti-freezing, anti-swelling, adhesive, and self-healing properties is prepared by a one-step photopolymerization of acrylic acid and lauryl methacrylate in a binary solvent comprising water and deep eutectic solvent (DES). The results from the molecular dynamics simulations and density functional theory indicate that the hydrogen bonds between DES and water mixtures possess better stability than those between water molecules. On the other hand, DES breaks down hydrogen bonds in water, providing eutectogels with excellent anti-freezing even at -60 °C. Cetyltrimethylammonium bromide is incorporated to establish stable hydrophobic interactions and electrostatic attractions with polymer chains in the eutectogel network, resulting in superior mechanical (elongation at break of 2890%) and anti-swelling (only 2% swelling in water over 7 days) properties. The eutectogel-based strain sensors exhibit remarkable sensitivity, achieving a gauge factor of up to 15.4. The multifunctional eutectogel sensors can monitor motion and transmit encrypted information at low temperatures, demonstrating considerable potential for applications in flexible electronics within low-temperature environments.

A flexible wearable sensor based on the multiple interaction and synergistic effect of the hydrogel components with anti-freezing, low swelling for human motion detection and underwater communication

To meet the increasing demand for wearable sensor in special environment such as low temperature or underwater, a multifunctional ionic conducting hydrogel (Gel/PSAA-Al3+ hydrogel) with anti-freezing and low swelling for human motion detection and underwater communication was prepared using gelatin (Gel), [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA), acrylamide (AAm), acrylic acid (AAc), and AlCl3. Due to reversible hydrogen bonding, electrostatic interactions and metal coordination crosslinking between the polymer networks, the resulting Gel/PSAA-Al3+ hydrogels present low swelling property in water and exhibit large tensile properties (~1050 %), high tensile strength (~250 kPa) and excellent fatigue resistance. In addition, the hydration capacity of SBMA and AlCl3 endows the Gel/PSAA-Al3+ hydrogel fantastic anti-freezing (-31.58 °C) and water retention properties. Moreover, the electrostatic interaction between SBMA and AlCl3 due to the ion hopping mechanism endows the hydrogel with excellent ionic conductivity (6.38 mS/cm). The Gel/PSAA-Al3+ hydrogel sensors present good biocompatibility and provide a wide operating range (0 %-1050 %), fast response time (229 ms) and recovery time (248 ms), high sensitivity (GF = 1.61) and excellent stability for detecting large and small body movements. The Gel/PSAA-Al3+ hydrogel shows potential applications as a wearable sensor for communication at low temperature or underwater.

Electrostatically Enhanced Biomimetic Asymmetric Hydrogel with a Dung Beetle-Inspired Pattern for Internal Trauma Sealing

Herein, a biologically asymmetric adhesion-patterned hydrogel induced by the dung beetle surface was proposed for internal trauma sealing. The electrostatic interaction-enhanced dual networks endowed the hydrogel patch with superior mechanical performance, thus achieving a favorable sealing ability. Poly(acrylic acid) (pAA), chitooligosaccharide (COS), and gelatin were used as the composition of our hydrogel system. Concurrently, the bionic raised structure enabled a significant adhesion drop effect. The surface waviness function, fitted to the curved bumps, showed the design direction of the patterned bumps, which was indicative of subsequent research. Also, the microparticle deposition method could exert a synergistic effect with the patterned surface, which together contributed to the asymmetry of the adhesive hydrogel patch. Following simulation experiments such as in vitro bursting tests, we conducted a rat gastric trauma model to validate the application potential of this bionic asymmetric patterned patch. The asymmetric adhesion hydrogel patch had an excellent sealing effect, antiadhesive properties, and operability and was expected to have a promising application prospect, providing a strategy for the design of subsequent in vivo trauma-sealing biomaterials.

Bioinspired Antiswelling Hydrogel Sensors with High Strength and Rapid Self-Recovery for Underwater Information Transmission

Hydrogel-based sensors typically demonstrate conspicuous swelling behavior in aqueous environments, which can severely compromise the mechanical integrity and distort sensing signals, thereby considerably constraining their widespread applicability. Drawing inspiration from the multilevel heterogeneous structures in biological tissues, an antiswelling hydrogel sensor endowed with high strength, rapid self-recovery, and low swelling ratio was fabricated through a water-induced phase separation and coordination cross-linking strategy. A dense heterogeneous architecture was developed by the integration of "rigid" quadridentate carboxyl-Zr4+ coordination bonds and "soft" hydrophobic unit-rich regions featuring π-π stacking and cation-π interactions into the hydrogels. This unique structural design facilitated the progressive breaking of cross-links within the hydrogel network from "soft" to "rigid" under external loads, effectively dissipating energy and thereby imparting the hydrogels with exceptional mechanical characteristics, evidenced by a strength of 1.42 MPa, and complete self-recovery within 3 min. Simultaneously, the "rigid" and "soft" dynamic interactions synergistically conferred augmented elastic retractive forces on the hydrogel network by enhancing cross-linking density, thereby providing the hydrogels with prominent antiswelling capabilities in water (with a swelling ratio of only -2.49%), in solutions with diverse pH (1-9), and in seawater. Moreover, the hydrogels manifested favorable strain-sensitivity (gauge factor up to 2.45) and frequency response by virtue of the collaborative contribution of dynamic ions (Cl- and Zr4+). Consequently, the hydrogels were utilized to assemble underwater sensors with the capacity to transmit information using Morse code. This bioinspired design methodology achieved the desired integration of the mechanical, swelling-resistant, and sensing performance within the hydrogels, thereby contributing innovative insights toward the advancement of underwater sensor technology.

Multifunctional Conductive Hydrogel for Sensing Underwater Applications and Wearable Electroencephalogram Recording

Flexible electronics have been rapidly advancing and have garnered significant interest in monitoring physiological activities and health conditions. However, flexible electronics are prone to detachment in humid environments, so developing human-friendly flexible electronic devices that can effectively monitor human movement under various aquatic conditions and function as flexible electrodes remains a significant challenge. Here, we report a strongly adherent, self-healing, and swelling-resistant conductive hydrogel formed by combining the dual synergistic effects of hydrogen bonding and dipole-dipole interactions. The hydrogel has a commendable linear operating range (∼200% strain, GF = 1.44), stability of electrical signals for 200 cycles, excellent conductivity (2.18 S m-1), self-healing properties (∼30 min), and durable underwater adhesion stability. The conductive hydrogel can be developed into a flexible electronic sensor for detecting motion signals, such as joint flexion and swallowing, as well as for real-time underwater communication using Morse code. Additionally, the integration of this polymer with a low contact impedance facilitates real-time, high-fidelity detection of electroencephalogram (EEG) signals, serving as a flexible electrode. It is believed that our hydrogel will have good prospects in future wearable electronics.

A Wet-Adhesion and Swelling-Resistant Hydrogel for Fast Hemostasis, Accelerated Tissue Injury Healing and Bioelectronics

Hydrogel bioadhesives with adequate wet adhesion and swelling resistance are urgently needed in clinic. However, the presence of blood or body fluid usually weakens the interfacial bonding strength, and even leads to adhesion failure. Herein, profiting from the unique coupling structure of carboxylic and phenyl groups in one component (N-acryloyl phenylalanine) for interfacial drainage and matrix toughening as well as various electrostatic interactions mediated by zwitterions, a novel hydrogel adhesive (PAAS) is developed with superior tissue adhesion properties and matrix swelling resistance in challenging wet conditions (adhesion strength of 85 kPa, interfacial toughness of 450 J m-2, burst pressure of 514 mmHg, and swelling ratio of <4%). The PAAS hydrogel can not only realize fast hemostasis of liver, heart, artery rupture, and sealing of pulmonary air-leakage but also accelerate the recovery of stomach and liver defects in rat, rabbit, and pig models. Moreover, PAAS hydrogel can precisely and durably monitor various physiological activities (pulse, electrocardiogram, and electromyogram) even under humid environments (immersion in water for 3 days), and can be employed for the evaluation of in vivo sealing efficiency for artery rupture. The work provides a promising hydrogel adhesive for clinical hemostasis, tissue injury repair, and bioelectronics.

Electrically conductive "SMART" hydrogels for on-demand drug delivery

In the current transformative era of biomedicine, hydrogels have established their presence in biomaterials due to their superior biocompatibility, tuneability and resemblance with native tissue. However, hydrogels typically exhibit poor conductivity due to their hydrophilic polymer structure. Electrical conductivity provides an important enhancement to the properties of hydrogel-based systems in various biomedical applications such as drug delivery and tissue engineering. Consequently, researchers are developing combinatorial strategies to develop electrically responsive "SMART" systems to improve the therapeutic efficacy of biomolecules. Electrically conductive hydrogels have been explored for various drug delivery applications, enabling higher loading of therapeutic cargo with on-demand delivery. This review emphasizes the properties, mechanisms, fabrication techniques and recent advancements of electrically responsive "SMART" systems aiding on-site drug delivery applications. Additionally, it covers prospects for the successful translation of these systems into clinical research.

Strong and anti-swelling nanofibrous hydrogel composites inspired by biological tissue for amphibious motion sensors

Conductive hydrogel-based sensors are increasingly favored for flexible electronics due to their skin-like characteristics. However, conventional hydrogels suffer from significant swelling in humid environments and poor mechanical properties which largely restrict their applications in wearable electronic devices, especially for underwater sensing. Herein, drawing inspiration from the extracellular matrix (ECM) structure, a TPU-PVAc@AgNPs/MXene nanofibrous hydrogel composite (TPAMH) with excellent mechanical robustness and anti-swelling properties is developed. The TPAMH nanofibrous hydrogel composite is created by integrating the silver nanoparticles (AgNPs) and MXene nanosheets into an interwoven network comprising of stiff TPU nanofibers as the fibril scaffold and formic acid-crosslinked PVA hydrogel fibers as the elastic matrix (PVAc). Benefiting from the unique ECM structure, the obtained nanofibrous hydrogel composites exhibit exceptional tensile strength (4.47 MPa), remarkable elongation at break (621%), excellent anti-swelling properties, and high detection sensitivity (maximum gauge factor = 105.02), which are sufficient to monitor body motions in both air and water environments effectively. They can detect large strain movements of fingers, elbows, wrists, and knees, as well as small strain physiological signals such as frown, smile, and pulse beats, with high accuracy. Particularly noteworthy is their ability to accurately identify underwater multidirectional motions and facilitate underwater smart alarms using Morse code.

Mechanical and Dimensional Stability of Gelatin-Based Hydrogels Through 3D Printing-Facilitated Confined Space Assembly

Hydrogels have emerged as promising biomaterials for tissue regeneration; yet, their inherent swelling can cause deformation and reduced mechanical properties, posing challenges for practical applications in biomedical engineering. Traditional methods to reduce hydrogel swelling often involve complex synthesis procedures with limited flexibility. Inspired by nature's efficient designs, we present here the approach to improve hydrogel performance using 3D printing-assisted microstructure engineering. By utilizing polymerization-induced phase separation of hydrogel from copolymerization of gelatin methacrylate and hydroxyethyl methacrylate (poly(GelMA-co-HEMA)) in the confined space during vat photopolymerization (VPP) 3D printing, we replicate the cuttlebone-like microstructure of hydrogels with enhanced mechanical properties and swelling resistance. We demonstrate here a 4-fold increase in elastic modulus compared to bulk polymerization of poly(GelMA-co-HEMA), together with improved mechanical and dimensional stability. This method offers promising opportunities for practical biomedical and tissue engineering applications, overcoming previous limitations in the design and performance.

Polyacrylamide/sodium alginate double network hydrogel with easily repairable superhydrophobic surface for strain sensor resistant to fluid interference

Constructing an easily repairable hydrophobic layer on the hydrogel surface that confers resistance to liquid interference remains a great challenge for hydrogel strain sensors. In this paper, superhydrophobic hydrogel sensors were prepared by driving hydrophobic organically modified silica (o-SiO2) nanoparticles to the surface of polyacrylamide/sodium alginate (PAM/SA) double network hydrogels by a weak ultrasonic field in o-SiO2/cyclohexane dispersion. The hydroxyl groups present on the surface of o-SiO2 are able to form hydrogen bonds with hydrogels, which in turn form a strong surface hydrophobic layer on its surface. The sensor exhibits superhydrophobic properties for different types of liquids, such as acids, salt solutions, etc., even in the stretched state. The broken o-SiO2 layers can be repaired by immersing in the o-SiO2/cyclohexane dispersion. The SA significantly improved the mechanical properties as well as the strain response sensitivity of the hydrogels. The hydrogel sensor is characterized by low hysteresis to strain, wide detection range (0-894 %), low detection limit (1 %), high sensitivity (GF = 4.8), and good cyclic stability. The superhydrophobic surface allows the sensor to exhibit excellent anti-liquid interference. Salt solution droplets, prolonged contact with salt solution, and even short-term water immersion will not affect the sensor's response to strain. Moreover, repairing the broken hydrophobic layer enables the sensor to restore its resistance to liquid interference. The prepared hydrogel can be used for human motion monitoring in complex scenarios, including exercise sweating, rain, and short-time exposure to water.

Antifreezing/Antiswelling Hydrogels: Synthesis Strategies and Applications as Flexible Motion Sensors

Hydrogels are excellent materials for fabricating flexible electronic devices, such as flexible sensors. However, obtaining hydrogels with superior swelling capacity and good hydrophilicity suitable for use under extreme environments, such as cold and underwater conditions, is still challenging due to the occurrence of freezing and excessive swelling. Alternatively, hydrogels with antifreezing and antiswelling capacities exhibit minimal changes in their physical and chemical properties under extreme conditions with retained original performance, such as mechanical properties, conductivity, and adhesiveness, making them suitable for various applications. Accordingly, various multifunctional antifreezing/antiswelling hydrogels meeting practical application requirements have been developed thanks to the advancement of hydrogel technology. Examples include flexible sensors for monitoring various motion signals, such as changes during sports events. However, comprehensive reviews describing these hydrogels in terms of synthesis and application in sensors are still lacking. Herein, the design and synthetic strategies of antifreezing/antiswelling hydrogels reported in recent years are comprehensively analyzed along with their mechanisms and applications in flexible motion sensors. This review aims to provide a comprehensive understanding of the research of antifreezing/antiswelling hydrogels and offer valuable insights for researchers engaged in the development of advanced materials suitable for practical applications.

High-strength fibrous sensors with an enhanced aggregate state for biomechanical monitoring of the Achilles tendon

Continuous monitoring of biomechanical signals generated from the injured Achilles tendon is essential for the deep understanding of the recovery or rehabilitation process, thus decreasing the risk of secondary injuries. With tissue-like components and adjustable properties, hydrogel-based biomechanical sensors are considered promising materials for human motion detection. However, existing hydrogels are characterized by inferior mechanical properties with strength and modulus typically lower than 1 MPa, as well as poor stability under physiological conditions, which hampers their applications in implantable devices. Moreover, acquiring the stress signal from collected electrical signals remains challenging. Herein, based on the regulation of polymer aggregation, a high-strength fibrous sensor composed of polyvinyl alcohol (PVA) and reduced graphene oxide (rGO) for in vivo monitoring is prepared through a two-step procedure, including freeze-thaw and freeze-soak. Benefiting from the synergy of crystallization, Hofmeister effect and nanocomposite, the hydrogel fibers feature high tensile strength (8.34 ± 0.66 MPa) and elastic modulus (1.15 ± 0.10 MPa). Meanwhile, the removal of salt ions during fabrication improves the water content (69.18 ± 1.47%) and anti-swelling performance of such fibers and minimizes side effects after implantation. It is demonstrated that the fibrous sensor could record the relative resistance changes upon stretching with ideal sensitivity (GF = 1.57) and convert them into bearing stress through formula derivation and calculations. In vitro and in vivo assays further confirm its feasibility for real-time monitoring of joint motion, providing important references for medical diagnosis and treatment.

Recent advances in hydrogel-based flexible strain sensors for harsh environment applications

Flexible strain sensors are broadly investigated in electronic skins and human-machine interaction due to their light weight, high sensitivity, and wide sensing range. Hydrogels with unique three-dimensional network structures are widely used in flexible strain sensors for their exceptional flexibility and adaptability to mechanical deformation. However, hydrogels often suffer from damage, hardening, and collapse under harsh conditions, such as extreme temperatures and humidity levels, which lead to sensor performance degradation or even failure. In addition, the failure mechanism in extreme environments remains unclear. In this review, the performance degradation and failure mechanism of hydrogel flexible strain sensors under various harsh conditions are examined. Subsequently, strategies towards the environmental tolerance of hydrogel flexible strain sensors are summarized. Finally, the current challenges of hydrogel flexible strain sensors in harsh environments are discussed, along with potential directions for future development and applications.

Fabrication of Flexible Wearable Mechanosensors Utilizing Piezoelectric Hydrogels Mechanically Enhanced by Dipole-Dipole Interactions

Conductive hydrogels have been increasingly employed to construct wearable mechanosensors due to their excellent mechanical flexibility close to that of soft tissues. In this work, piezoelectric hydrogels are prepared through free radical copolymerization of acrylamide (AM) and acrylonitrile (AN) and further utilized in assembling flexible wearable mechanosensors. Introduction of the polyacrylonitrile (PAN) component in the copolymers endows the hydrogels with excellent piezoelectric properties. Meanwhile, significant enhancement of mechanical properties has been accessed by forming dipole-dipole interactions, which results in a tensile strength of 0.51 MPa. Flexible wearable mechanosensors are fabricated by utilizing piezoelectric hydrogels as key signal converting materials. Self-powered piezoelectric pressure sensors are assembled with a sensitivity (S) of 0.2 V kPa-1. Additionally, resistive strain sensors (gauge factor (GF): 0.84, strain range: 0-250%) and capacitive pressure sensors (S: 0.23 kPa-1, pressure range: 0-8 kPa) are fabricated by utilizing such hydrogels. These flexible wearable mechanosensors can monitor diverse body movements such as joint bending, walking, running, and stair climbing. This work is anticipated to offer promising soft materials for efficient mechanical-to-electrical signal conversion and provides new insights into the development of various wearable mechanosensors.

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

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

Wearable sensor supports in-situ and continuous monitoring of plant health in precision agriculture era

Plant health is intricately linked to crop quality, food security and agricultural productivity. Obtaining accurate plant health information is of paramount importance in the realm of precision agriculture. Wearable sensors offer an exceptional avenue for investigating plant health status and fundamental plant science, as they enable real-time and continuous in-situ monitoring of physiological biomarkers. However, a comprehensive overview that integrates and critically assesses wearable plant sensors across various facets, including their fundamental elements, classification, design, sensing mechanism, fabrication, characterization and application, remains elusive. In this study, we provide a meticulous description and systematic synthesis of recent research progress in wearable sensor properties, technology and their application in monitoring plant health information. This work endeavours to serve as a guiding resource for the utilization of wearable plant sensors, empowering the advancement of plant health within the precision agriculture paradigm.

Recent Progress of Anti-Freezing, Anti-Drying, and Anti-Swelling Conductive Hydrogels and Their Applications

Hydrogels are soft-wet materials with a hydrophilic three-dimensional network structure offering controllable stretchability, conductivity, and biocompatibility. However, traditional conductive hydrogels only operate in mild environments and exhibit poor environmental tolerance due to their high water content and hydrophilic network, which result in undesirable swelling, susceptibility to freezing at sub-zero temperatures, and structural dehydration through evaporation. The application range of conductive hydrogels is significantly restricted by these limitations. Therefore, developing environmentally tolerant conductive hydrogels (ETCHs) is crucial to increasing the application scope of these materials. In this review, we summarize recent strategies for designing multifunctional conductive hydrogels that possess anti-freezing, anti-drying, and anti-swelling properties. Furthermore, we briefly introduce some of the applications of ETCHs, including wearable sensors, bioelectrodes, soft robots, and wound dressings. The current development status of different types of ETCHs and their limitations are analyzed to further discuss future research directions and development prospects.

Generation of a photothermally responsive antimicrobial, bioadhesive gelatin methacryloyl (GelMA) based hydrogel through 3D printing for infectious wound healing

Recently, photothermal nanomaterials has attracted enormous interests owing to their enhanced therapeutic effects and less adverse effects in the treatment of infectious diseases. Herein, this work presents a photothermally responsive antimicrobial, bioadhesive hydrogel through three dimensions (3D) printing technology for treatment the wound infection. The hydrogel is based on a visible-light-activated naturally derived polymer (GelMA), GelMA grafted with dopamine (GelMA-DA) and the polydopamine coated reduced graphene oxide (rGO@PDA), which can provide the multifunctional such as photothermal antibacterial, antioxidant, conductivity, adhesion and hemostasis performance to accelerate wound healing. The developed hydrogel shown the excellent adhesion capability to adhere the in vitro physiological tissues and glass surface. Moreover, the fabricated hydrogel also exhibited excellent cytocompatibility to L929 cells which is a vital biofunction for efficiently promoting cell proliferation and migration in vitro. The hydrogel also showed remarkable photothermally responsive antimicrobial capability against two strains (99.3 % antibacterial ratio for E. coli and 98.6 % antibacterial ratio for S. aureus). Furthermore, it could support the wound repair and regeneration of S. aureus infected full-thickness wound defects in rats. Overall, the 3D printed hydrogel could be used as a photothermal platform for the development of more effective therapies against the infected wound.

Hydrophobic deep eutectic solvent-based eutectogels for underwater sensing

Eutectogels derived from deep eutectic solvents (DESs) exhibit great potential for the fabrication of flexible sensors. However, the hygroscopicity of eutectogels hinders their applications in underwater sensing. In this work, a hydrophobic eutectogel with exceptional long-term underwater stability is produced through one-step polymerization of lauryl methacrylate and glycidyl methacrylate in a hydrophobic DES. The hydrophobic gel network and hydrophobic DES fulfill the eutectogel with outstanding water resistance (water contact angle > 110°) and excellent mechanical properties in an aqueous environment, thus leading to extraordinary durability (over 1000 testing cycles). Additionally, based on this eutectogel, underwater strain and pressure sensors with high sensitivity, rapid responsiveness, and superior durability were fabricated for accurate real-time monitoring of human activity. Furthermore, it has been demonstrated that the eutectogel sensor can transmit information through Morse code, performing as a wearable underwater communicator. This research provides an exemplary way for a demonstration method of hydrophobic eutectogel for durable underwater applications.

Functional adhesive hydrogels for biological interfaces

Hydrogel adhesives are extensively employed in biological interfaces such as epidermal flexible electronics, tissue engineering, and implanted device. The development of functional hydrogel adhesives is a critical, yet challenging task since combining two or more attributes that seem incompatible into one adhesive hydrogel without sacrificing the hydrogel's pristine capabilities. In this Review, we highlight current developments in the fabrication of functional adhesive hydrogels, which are suitable for a variety of application scenarios, particularly those that occur underwater or on tissue/organ surface conditions. The design strategies for a multifunctional adhesive hydrogel with desirable properties including underwater adhesion, self-healing, good biocompatibility, electrical conductivity, and anti-swelling are discussed comprehensively. We then discuss the challenges faced by adhesive hydrogels, as well as their potential applications in biological interfaces. Adhesive hydrogels are the star building blocks of bio-interface materials for individualized healthcare and other bioengineering areas.

Biomedical applications of supramolecular hydrogels with enhanced mechanical properties

Supramolecular hydrogels bound by hydrogen bonding, host-guest, hydrophobic, and other non-covalent interactions are among the most attractive biomaterials available. Supramolecular hydrogels have attracted extensive attention due to their inherent dynamic reversibility, self-healing, stimuli-response, excellent biocompatibility, and near-physiological environment. However, the inherent contradiction between non-covalent interactions and mechanical strength makes the practical application of supramolecular hydrogels a great challenge. This review describes the mechanical strength of hydrogels mediated by supramolecular interactions, and focuses on the potential strategies for enhancing the mechanical strength of supramolecular hydrogels and illustrates their applications in related fields, such as flexible electronic sensors, wound dressings, and three-dimensional (3D) scaffolds. Finally, the current problems and future research prospects of supramolecular hydrogels are discussed. This review is expected to provide insights that will motivate more advanced research on supramolecular hydrogels.

A Facile Strategy to Construct Anti-Swelling, Antibacterial, and Antifogging Coatings for Protection of Medical Goggles

During the COVID-19 (Corona Virus Disease 2019) pandemic, traditional medical goggles are not only easy to attach bacteria and viruses in long-term exposure, but easy to fogged up, which increases the risk of infection and affects productivity. Bacterial adhesion and fog can be significantly inhibited through the hydrogel coatings, owing to super hydrophilic properties. On the one hand, hydrogel coatings are easy to absorb water and swell in wet environment, resulting in reduced mechanical properties, even peeling off. On the other hand, the hydrogel coatings don't have intrinsic antibacterial properties, which still poses a potential risk of bacterial transmission. Herein, an anti-swelling and antibacterial hydrogel coating is synthesized by 2-hydroxyethyl methacrylate (HEMA), acrylamide (AM), dimethylaminoethyl acrylate bromoethane (IL-Br), and poly(sodium-p-styrenesulfonate) (PSS). Due to the self-driven entropy reduction effect of polycation and polyanion, an ion cross-linking network is formed, which endows the hydrogel coating with excellent antiswelling performance. Moreover, because of the synergistic effect of highly hydrated surfaces and the active bactericidal effect from quaternary ammonium cations, the hydrogel coating exhibits outstanding antifouling performances. This work develops a facile strategy to fabricate anti-swelling, antifouling, and antifogging hydrogel coatings for the protection of medical goggles, and also for biomedical and marine antifouling fields.

Water-Resistant Conductive Gels toward Underwater Wearable Sensing

Conductive gels are developing vigorously as superior wearable sensing materials due to their intrinsic conductivity, softness, stretchability, and biocompatibility, showing a great potential in many aspects of lives. However, compared to their wide application on land, it is significant yet rather challenging for traditional conductive gels to realize sensing application under water. The swelling of gels and the loss of conductive components in the aqueous environment, resulted from the diffusion across the interface, lead to structural instability and sensing performance decline. Fortunately, great efforts are devoted to improving the water resistance of conductive gels and employing them in the field of underwater wearable sensing in recent years, and some exciting achievements are obtained, which are of great significance for promoting the safety and efficiency of underwater activities. However, there is no review to thoroughly summarize the underwater sensing application of conductive gels. This review presents a brief overview of the representative design strategies for developing water-resistant conductive gels and their diversified applications in the underwater sensing field as wearable sensors. Finally, the ongoing challenges for further developing water-resistant conductive gels for underwater wearable sensing are also discussed along with recommendations for the future.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

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

Pathways toward wearable and high-performance sensors based on hydrogels: toughening networks and conductive networks

Wearable hydrogel sensors provide a user-friendly option for wearable electronics and align well with the existing manufacturing strategy for connecting and communicating with large numbers of Internet of Things devices. This is attributed to their components and structures, which exhibit exceptional adaptability, scalability, bio-compatibility, and self-healing properties, reminiscent of human skin. This review focuses on the recent research on principal structural elements of wearable hydrogels: toughening networks and conductive networks, highlighting the strategies for enhancing mechanical and electrical properties. Wearable hydrogel sensors are categorized for an extensive exploration of their composition, mechanism, and design approach. This review provides a comprehensive understanding of wearable hydrogels and offers guidance for the design of components and structures in order to develop high-performance wearable hydrogel sensors.

Extreme-environment-adapted eutectogel mediated by heterostructure for epidermic sensor and underwater communication

In recent years, gel-based ion conductor has been widely considered in wearable electronics because of the favorable flexibility and conductivity. However, it is of vital importance, yet rather challenging to adapt the gel for underwater and dry conditions. Herein, an anti-swelling and anti-drying, intrinsic conductor eutectogel is designed via a one-step radical polymerization of acrylic acid and 2, 2, 2‑trifluoroethyl methacrylate in binary deep eutectic solvents (DESs) medium. On the one hand, the synergistic effects of hydrophilic/hydrophobic heteronetworks can elicit the integrity stability of eutectogel in liquid environment. It is proved that both the mechanical property and conductivity are maintained after immersing in different salt, alkaline and acid solution and organic solvent for one month. On the other hand, the eutectogel inherits well conductivity (93 mS/m), anti-drying and antibacterial properties from DESs. Based on the above features, the resulting eutectogel can be assembled as smart sensor for stable information transmission in air and underwater with fast response time (1 s), high sensitivity (Gauge factor = 1.991) and long-time reproducibility (500 cycles, 70 % strain). Considering the simple preparation and integration of multiple functions, the binary cooperative complementary principle can provide insights into the development of next-generation conductive soft materials.

Anti-swellable cellulose hydrogel for underwater sensing

Underwater sensing is of great significance in ocean exploration by divers to monitor their movements and keep in touch with the shore. However, unique sensors are required to apply in the marine environment that is quite different from the land circumstance. Herein, we reported a cellulose-skeleton-based composite hydrogel that is constraint to expand underwater under the effect of hydrogen bonds (H-bonds) and features advantages of high swelling resistance, structural durability, mechanical robustness, medium flexibility, high gauge factor (2.33) and long-term stability in water as a highly efficient wearable underwater sensor. This cellulose-based anti-swellable underwater hydrogel sensor showed tremendous potentials in underwater sensing applications for posture monitoring, communication, and marine biological research, etc.

Wet-Adhesive Multifunctional Hydrogel with Anti-swelling and a Skin-Seamless Interface for Underwater Electrophysiological Monitoring and Communication

A stable and seamless adhesion between the human skin and the hydrogel-based electronic skin is necessary for accurate sensing and human health monitoring in aquatic environments. Despite achieving significant progress in this field, it remains a great challenge to design skin-interfaced conductive hydrogels with high electrical conductivity, stablility, and seamless underwater adhesion to skin. Herein, a skin-inspired conductive multifunctional hydrogel is proposed, which has a wet-adhesive/hydrophilic and a non-adhesive/hydrophobic bilayer structure. The hydrogel shows high stretchability (∼2400%) and an ultra-low modulus (4.5 kPa), which facilitate the conformal and seamless attachment of the hydrogel to the skin with reduced motion artifacts. Owing to synergistic physical and chemical interactions, this hydrogel can achieve reliable underwater adhesion and display remarkable underwater adhesion strength (388.1 kPa) to porcine skin. In addition, MXene has been employed to obtain high electrical conductivity, create a route for stable electron transport, and reinforce mechanical properties. The hydrogel also possesses self-healing ability, a low swelling ratio (∼3.8%), biocompatibility, and specific adhesion to biological tissues in water. Facilitated with these advantages, the hydrogel-based electrodes achieve reliable electrophysiological signal detection in both air and wet conditions and demonstrate a higher signal-to-noise ratio (28.3 dB) than that of commercial Ag/AgCl gel electrodes (18.5 dB). Also, the hydrogel can be utilized as a strain sensor with high sensitivity for underwater communication. This multifunctional hydrogel improves the stability of the skin-hydrogel interface in aquatic environments and is expected to be promising for the next-generation bio-integrated electronics.

A novel triple-network hydrogel based on borate ester groups: from structural modulation to rapid wound hemostasis

Supramolecular hydrogels have received widespread attention due to their soft texture, strong hygroscopicity, and good biocompatibility. These materials have become particularly attractive for sensing, tissue engineering, fluorescence encoding, wound healing, etc. Inspired by the assembly of G-quadruplexes, we choose the boric acid/polyvinyl alcohol/guanosine system to construct a novel triple-network supramolecular hydrogel "tri-BA@PVA/G" via non-covalent cross-linking, and the effect of the concentration of each component on the hydrogel stability was systematically revealed at the same time. Then, the biocompatibility, shape adaption and optical information storage capacity, the rapid hemostatic ability and the ability of the hydrogel to promote wound healing were confirmed both in vitro and in vivo. These results that predict the properties and reveal prospective applications in the field of wound hemostasis have a certain guiding significance for the subsequent preparation of borate-based triple network hydrogels which can be used as wound hemostatic materials.

Ultrasonic-Induced Synthesis of Underwater Adhesive and Antiswelling Hydrogel for Strain Sensor

To perceive the human body's multienvironmental mobility, intelligent flexible electronic equipment with an underwater motion monitoring function has potential research value in the field of intelligent detection. Hydrogels are widely used in the field of flexible electronics for their unique three-dimensional polymer networks. Due to the instinctive hydrophilicity of hydrogels, the swelling of hydrogels underwater and the formation of hydration coating on the surface become the primary obstacles to underwater applications. Herein, a hydrogel sensor that can achieve underwater utilization was prepared through copolymerization between hydrophobic and hydrophilic polymer monomers. The synergistic impact of electrostatic interaction, metal coordination, and hydrogen bonding ensured the hydrogel's remarkable underwater adhesive ability to a variety of substrates. The hydrophobic micelles and self-hydrophobization process induced from ultrasonic dispersion in the polymer matrix gave an outstanding hydrophobic performance (water contact angle of 130.4°) and antiswelling property (swelling ratio of 26% after 72 h of immersion), presenting unprecedented underwater adaptability. The above-mentioned hydrogel could be assembled into a flexible hydrogel sensor with satisfactory sensitivity (gauge factor of 0.44), ultrafast response rate (106 ms), and excellent cyclic stability, demonstrating accurate monitoring of complex human motions in water and air.

Supramolecular Hydrogel Dressing: Effect of Lignin on the Self-Healing, Antibacterial, Antioxidant, and Biological Activity Improvement

The functionalization and performance improvement of supramolecular hydrogels are very important for their application in the wound dressing field. Inspired by the role of lignin in plant cell walls, sulfonated lignin is introduced into the supramolecular hydrogel to improve functionality, mechanical strength, and biological activity. According to the chemical structure characteristics of the sulfonated lignin and the requirements for wound dressing, a novel polymer system is designed and successfully synthesized to cooperate with the sulfonated lignin to form the supramolecular hydrogel dressings. The introduction of the sulfonated lignin can effectively improve the mechanical strength, self-healing property, antioxidant activity, and biological activity of the obtained supramolecular hydrogel dressings. In the rat wound healing model experiment, the supramolecular hydrogel dressings can maintain the moist environment on the wound surface, clean up the excretion of wound tissue, promote wound healing, and reduce the occurrence of inflammation. This supramolecular hydrogel dressing shows obvious potential for wound management and treatment by a facile and effective approach and has great promise for long-term application of wound dressings. This strategy for designing polymers according to the chemical structure characteristics of the sulfonated lignin and the application requirements has reference value for further development of biomass-based compound materials.