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

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.