Tough Wet Adhesion of Hydrogen-Bond-Based Hydrogel with On-Demand Debonding and Efficient Hemostasis

Barnacle inspired strategy combined with solvent exchange for enhancing wet adhesion of hydrogels to promote seawater-immersed wound healing

Hydrogels are promising materials for wound protection, but in wet, or underwater environments, the hydration layer and swelling of hydrogels can seriously reduce adhesion and limit their application. In this study, inspired by the structural characteristics of strong barnacle wet adhesion and combined with solvent exchange, a robust wet adhesive hydrogel (CP-Gel) based on chitosan and 2-phenoxyethyl acrylate was obtained by breaking the hydration layer and resisting swelling. As a result, CP-Gel exhibited strong wet adhesion to various interfaces even underwater, adapted to joint movement and skin twisting, resisted sustained rushing water, and sealed damaged organs. More importantly, on-demand detachment and controllable adhesion were achieved by promoting swelling. In addition, CP-Gel with good biosafety significantly promotes seawater-immersed wound healing and is promising for use in water-contact wound care, organ sealing, and marine emergency rescue.

Injectable hemostatic hydrogel adhesive with antioxidant, antibacterial and procoagulant properties for hemorrhage wound management

In emergencies, uncontrolled severe bleeding can result in undesired complications and even death of the injured. Designing advanced hemostatic agents is a potential solution for emergency hemostasis, yet it remains challenging to realize the persistent adhesion in a wet wound environment. In this study, based on dynamic reversible Schiff base bond and photo-initiated double-bond polymerization, a novel injectable hemostatic hydrogel (L-COC) consisting of methacrylated carboxymethyl chitosan (CMCSMA), oxidized konjac glucomannan (OKGM) and (+)-catechin hydrate (CH) was synthesized for emergency hemostasis. To our delight, the incorporated CH imparted enhanced blood procoagulantion to the L-COC hydrogel by intensifying the hydrogel-red blood cell interactions. As a result, the hemostatic effect of the engineered L-COC hydrogel was significantly superior to that of fluid gelatin SurgifloTM for liver bleeding wounds in rats (Blood loss: 0.62 ± 0.11 g (L-COC), 0.90 ± 0.08 g (SurgifloTM); hemostasis time: 69.0 ± 2.9 s (L-COC), 84.0 ± 2.2 s (SurgifloTM)). With the favorable antioxidant and antibacterial activities, as well as multifunctional properties, the bio-adhesive L-COC hydrogel and the underlying design principles may facilitate further development of practical hemostatic hydrogels.

Epidermal growth factor-loaded, dehydrated physical microgel-formed adhesive hydrogel enables integrated care of wet wounds

Integrated wound care, a sequential process of promoting wound hemostasis, sealing, and healing, is of great clinical significance. However, the wet environment of wounds poses formidable challenges for integrated care. Herein, we developed an epidermal growth factor (EGF)-loaded, dehydrated physical microgel (DPM)-formed adhesive hydrogel for the integrated care of wet wounds. The DPMs were designed using the rational combination of hygroscopicity and reversible crosslinking of physical hydrogels. Unlike regular bioadhesives, which consider interfacial water as a barrier to adhesion, DPMs utilize water to form desirable adhesive structures. The hygroscopicity allowed the DPMs to absorb interfacial water and subsequently, the interfacial adhesion was realized by the interactions between tissue and DPMs. The reversible crosslinks further enabled DPMs to integrate into hydrogels (DPM-Gels), thus achieving wet adhesion. Importantly, the water-absorbing gelation mode of DPMs enabled facile loading of biologically active EGF to promote wound healing. We demonstrated that the DPM-Gels possessed wet tissue adhesive performance, with about 40 times the wet adhesive strength of fibrin glue and about 4 times the burst pressure of human blood pressure. Upon application at the injury site, the EGF-loaded DPM-Gels sequentially promoted efficient wound hemostasis, stable sealing, and quick healing, achieving integrated care of wet wounds.

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.

Facile Solvent-Free Fabrication of All-Small-Molecule Supramolecular Photothermal Bioadhesive for Sutureless Wound Closure

Achieving underwater adhesion possesses a significant challenge, primarily due to the presence of interfacial water, which restricts the potential applications of adhesives. In this study, we present a straightforward and environmentally friendly one-pot approach for synthesizing a solvent-free supramolecular TPFe bioadhesive composed of thioctic acid, proanthocyanidins, and FeCl3. The bioadhesive exhibits excellent biocompatibility and photothermal antibacterial properties and demonstrates effective adhesion on various substrates in both wet and dry environments. Importantly, the adhesive strength of this bioadhesive on steel exceeds 1.2 MPa and that on porcine skin exceeds 100 kPa, which is greater than the adhesive strength of most reported bioadhesives. In addition, the bioadhesive exhibits the ability to effectively halt bleeding, close wounds promptly, and promote wound healing in the rat skin wound model. Therefore, the TPFe bioadhesive has potential as a medical bioadhesive for halting bleeding quickly and promoting wound healing in the biomedical field. This study provides a new idea for the development of bioadhesives with firm wet adhesion.

Recent Advances in Functional Hydrogel for Repair of Abdominal Wall Defects: A Review

The abdominal wall plays a crucial role in safeguarding the internal organs of the body, serving as an essential protective barrier. Defects in the abdominal wall are common due to surgery, infection, or trauma. Complex defects have limited self-healing capacity and require external intervention. Traditional treatments have drawbacks, and biomaterials have not fully achieved the desired outcomes. Hydrogel has emerged as a promising strategy that is extensively studied and applied in promoting tissue regeneration by filling or repairing damaged tissue due to its unique properties. This review summarizes the five prominent properties and advances in using hydrogels to enhance the healing and repair of abdominal wall defects: (a) good biocompatibility with host tissues that reduces adverse reactions and immune responses while supporting cell adhesion migration proliferation; (b) tunable mechanical properties matching those of the abdominal wall that adapt to normal movement deformations while reducing tissue stress, thereby influencing regulating cell behavior tissue regeneration; (c) drug carriers continuously delivering drugs and bioactive molecules to sites optimizing healing processes enhancing tissue regeneration; (d) promotion of cell interactions by simulating hydrated extracellular matrix environments, providing physical support, space, and cues for cell migration, adhesion, and proliferation; (e) easy manipulation and application in surgical procedures, allowing precise placement and close adhesion to the defective abdominal wall, providing mechanical support. Additionally, the advances of hydrogels for repairing defects in the abdominal wall are also mentioned. Finally, an overview is provided on the current obstacles and constraints faced by hydrogels, along with potential prospects in the repair of abdominal wall defects.

Advances of biological macromolecules hemostatic materials: A review

Achieving hemostasis is a necessary intervention to rapidly and effectively control bleeding. Conventional hemostatic materials currently used in clinical practice may aggravate the damage at the bleeding site due to factors such as poor adhesion and poor adaptation. Compared to most traditional hemostatic materials, polymer-based hemostatic materials have better biocompatibility and offer several advantages. They provide a more effective method of stopping bleeding and avoiding additional damage to the body in case of excessive blood loss. Various hemostatic materials with greater functionality have been developed in recent years for different organs using diverse design strategies. This article reviews the latest advances in the development of polymeric hemostatic materials. We introduce the coagulation cascade reaction after bleeding and then discuss the hemostatic mechanisms and advantages and disadvantages of various polymer materials, including natural, synthetic, and composite polymer hemostatic materials. We further focus on the design strategies, properties, and characterization of hemostatic materials, along with their applications in different organs. Finally, challenges and prospects for the application of hemostatic polymeric materials are summarized and discussed. We believe that this review can provide a reference for related research on hemostatic materials, contributing to the further development of polymer hemostatic materials.

A Photo-induced Cross-Linking Enhanced A and B Combined Multi-Functional Spray Hydrogel Instantly Protects and Promotes of Irregular Dynamic Wound Healing

Wounds in harsh environments can face long-term inflammation and persistent infection, which can slow healing. Wound spray is a product that can be rapidly applied to large and irregularly dynamic wounds, and can quickly form a protective film in situ to inhibit external environmental infection. In this study, a biodegradable A and B combined multi-functional spray hydrogel is developed with methacrylate-modified chitosan (CSMA1st) and ferulic acid (FA) as type A raw materials and oxidized Bletilla striata polysaccharide (OBSP) as type B raw materials. The precursor CSMA1st-FA/OBSP (CSOB-FA1st) hydrogel is formed by the self-cross-linking of dynamic Schiff base bonds, the CSMA-FA/OBSP (CSOB-FA) hydrogel is formed quickly after UV-vis light, so that the hydrogel fits with the wound. Rapid spraying and curing provide sufficient flexibility and rapidity for wounds and the hydrogel has good injectability, adhesive, and mechanical strength. In rats and miniature pigs, the A and B combined spray hydrogel can shrink wounds and promote healing of infected wounds, and promote the enrichment of fibrocyte populations. Therefore, the multifunctional spray hydrogel combined with A and B can protect irregular dynamic wounds, prevent wound infection and secondary injury, and be used for safe and effective wound treatment, which has a good prospect for development.

Novel Functional Dressing Materials for Intraoral Wound Care

Intraoral wounds represent a particularly challenging category of mucosal and hard tissue injuries, characterized by the unique structures, complex environment, and distinctive healing processes within the oral cavity. They have a common occurrence yet frequently inflict significant inconvenience and pain on patients, causing a serious decline in the quality of life. A variety of novel functional dressings specifically designed for the moist and dynamic oral environment have been developed and realized accelerated and improved wound healing. Thoroughly analyzing and summarizing these materials is of paramount importance in enhancing the understanding and proficiently managing intraoral wounds. In this review, the particular processes and unique characteristics of intraoral wound healing are firstly described. Up-to-date knowledge of various forms, properties, and applications of existing products are then intensively discussed, which are categorized into animal products, plant extracts, natural polymers, and synthetic products. To conclude, this review presents a comprehensive framework of currently available functional intraoral wound dressings, with an aim to provoke inspiration of future studies to design more convenient and versatile materials.

Remodelers of the vascular microenvironment: The effect of biopolymeric hydrogels on vascular diseases

Vascular disease is the leading health problem worldwide. Vascular microenvironment encompasses diverse cell types, including those within the vascular wall, blood cells, stromal cells, and immune cells. Initiation of the inflammatory state of the vascular microenvironment and changes in its mechanics can profoundly affect vascular homeostasis. Biomedical materials play a crucial role in modern medicine, hydrogels, characterized by their high-water content, have been increasingly utilized as a three-dimensional interaction network. In recent times, the remarkable progress in utilizing hydrogels and understanding vascular microenvironment have enabled the treatment of vascular diseases. In this review, we give an emphasis on the utilization of hydrogels and their advantages in the various vascular diseases including atherosclerosis, aneurysm, vascular ulcers of the lower limbs and myocardial infarction. Further, we highlight the importance and advantages of hydrogels as artificial microenvironments.

What else should hemostatic materials do beyond hemostasis: A review

Massive blood loss due to injury is the leading cause of prehospital deaths in disasters and emergencies. Hemostatic materials are used to realize rapid hemostasis and protect patients from death. Researchers have designed and developed a variety of hemostatic materials. However, in addition to their hemostatic effect, hemostatic materials must be endowed with additional functions to meet the practical application requirements in different scenarios. Here, strategies for modifications of hemostatic materials for use in different application scenarios are listed: effective positioning at the site of deep and narrow wounds to stop bleeding, resistance to high blood pressure and wound movement to maintain wound formation, rapid and easy removal from the wound without affecting further treatment after hemostasis is completed, and continued function when retained in the wound as a dressing (such as antibacterial, antiadhesion, tissue repair, etc.). The problems encountered in the practical use of hemostatic materials and the strategies and progress of researchers will be further discussed in this review. We hope to provide valuable references for the design of more comprehensive and practical hemostatic materials.

Mechanically Robust Hemostatic Hydrogel Membranes with Programmable Strain-Adaptive Microdomain Entanglement for Wound Treatment in Dynamic Tissues

Supramolecular hydrogels emerge as a promising paradigm for sutureless wound management. However, their translation is still challenged by the insufficient mechanical robustness in the context of complex wounds in dynamic tissues. Herein, we report a tissue-adhesive supramolecular hydrogel membrane based on biocompatible precursors for dressing wounds in highly dynamic tissues, featuring robust mechanical resilience through programmable strain-adaptive entanglement among microdomains. Specifically, the hydrogels are synthesized by incorporating a long-chain polyurethane segment into a Schiff base-ligated short-chain oxidized cellulose/quaternized chitosan network via acylhydrazone bonding, which readily establishes interpenetrating entangled microdomains in dynamic cross-linked hydrogel matrices to enhance their tear and fatigue resistance against extreme mechanical stresses. After being placed onto dynamic tissues, the hydrogel dressing could efficiently absorb blood to achieve rapid hemostasis. Moreover, metal ions released from ruptured erythrocytes could be scavenged by the Schiff base linkers to form additional ionic bonds, which would trigger the cross-linking of the short-chain components and establish abundant crystalline microdomains, eventually leading to the in situ stiffening of the hydrogels to endure heavy mechanical loads. Benefiting from its hemostatic capacity and strain adaptable mechanical performance, this hydrogel wound dressing shows promise for the clinical management of various traumatic wounds.

Stimuli-Responsive Hydrogel Adhesives for Wound Closure and Tissue Regeneration

Sutures and staplers, as gold standards for clinical wound closure, usually cause secondary tissue injury and require professional technicians and equipment. The noninvasive hydrogel adhesives are used in various biomedical applications, such as wound closure, tissue sealing, and tissue regeneration, due to their remarkable properties. Recently-developed hydrogel adhesives, especially stimuli-responsive hydrogels, have shown great potential owing to their advantages in regulating their performance and functions according to the wound situations or external conditions, thus allowing the wounds to heal gradually. However, comprehensive summary on stimuli-responsive hydrogels as tissue adhesives is rarely reported to date. This review focuses on the advances in the design of various stimuli-responsive hydrogel adhesives over the past decade, including the systems responsive to pH, temperature, photo, and enzymes. Their potential biomedical applications, such as skin closure, cardiovascular and liver hemostasis, and gastrointestinal sealing, are emphasized. Meanwhile, the challenges and future development of stimuli-responsive hydrogel adhesives are discussed. This review aims to provide meaningful insights for the further design of next-generation of hydrogel adhesives for wound closure and tissue regeneration.

Recent research advances on polysaccharide-, peptide-, and protein-based hemostatic materials: A review

Hemorrhage is a potentially life-threatening emergency that can occur at any time or place. Whether traumatic, congenital, surgical, disease-related, or drug-induced, bleeding can lead to severe complications or death. Therefore, the development of efficient hemostatic materials is critical. However, the results and prognosis demonstrated by clinical means of hemostasis do not reach expectations. With the development of technology, novel hemostatic materials have been developed from polysaccharides (chitosan, hyaluronic acid, alginate, cellulose, cyclodextrins, starch, dextran, and carrageenan), peptides (self-assembling peptides), and proteins (silk fibroin, collagen, gelatin, keratin, and thrombin). These new materials exhibit high hemostatic efficacy due to the enhancement or interaction of various hemostatic mechanisms. The main forms include adhesives, sealants, bandages, hemostatic powders, and hemostatic sponges. This article introduces the clotting process and principles of hemostatic methods and reviews the research on polysaccharide-, peptide-, and protein-based hemostatic materials in the last five years. The design ideas and hemostatic principles of polysaccharide-, peptide-, and protein-based hemostatic materials are mainly introduced. Finally, we summarize material designs, advantages, disadvantages, and challenges regarding hemostatic materials.

Progress of polysaccharide-based tissue adhesives

Recently, polymer-based tissue adhesives (TAs) have gained the attention of scientists and industries as alternatives to sutures for sealing and closing wounds or incisions because of their ease of use, low cost, minimal tissue damage, and short application time. However, poor mechanical properties and weak adhesion strength limit the application of TAs, although numerous studies have attempted to develop new TAs with enhanced performance. Therefore, next-generation TAs with improved multifunctional properties are required. In this review, we address the requirements of polymeric TAs, adhesive characteristics, adhesion strength assessment methods, adhesion mechanisms, applications, advantages and disadvantages, and commercial products of polysaccharide (PS)-based TAs, including chitosan (CS), alginate (AL), dextran (DE), and hyaluronic acid (HA). Additionally, future perspectives are discussed.

Facile Bond Exchanging Strategy for Engineering Wet Adhesion and Antioxidant/Antibacterial Thin Layer over a Dynamic Hydrogel via the Carbon Dots Derived from Tannic Acid/ε-Polylysine

Adhesive hydrogels, playing an essential role in stretchable electronics, soft robotics, tissue engineering, and so forth, upon functioning often need to adhere to various substrates in wet conditions and simultaneously exhibit antibacterial/antioxidant properties while possessing the intrinsic stretchability and elasticity of the hydrogel network intact. Therefore, simple approaches to conveniently access adhesive hydrogels with multifunctional surfaces are being pursued. Herein, a facile strategy has been proposed to construct multifunctional adhesive hydrogels via surface engineering of a multifunctional carbon dot (CD)-decorated polymeric thin layer by dynamic bond exchange. By this strategy, a double cross-linked network hydrogel of polyacrylamide (PAM) and oxidized dextran (ODA) was engineered with a unique dense layer over the Schiff base hydrogel matrix by aqueous solution immersion of PA-120, versatile CDs derived from tannic acid (TA) and ε-polylysine (PL). Without any additional agents, the PA-120 CDs with residual polyphenolic/catechol and amine moieties were incorporated into the surface structure of the hydrogel network by the combined action of the Schiff base and hydrogen bonds to form a dense surface layer that can exhibit high wet adhesive performance via the amine-polyphenol/catechol pair. The armor-like dense architecture also endowed hydrogels with considerably enhanced tensile/compression properties and excellent antioxidant/antibacterial abilities. Besides, the single-sided modified Janus hydrogel and completely surface-modified hydrogel can be flexibly developed through this approach. This strategy will provide new insights into the preparation and application of surface-modified hydrogels featuring multiple functions and tunable interfacial properties.

Advanced function, design and application of skin substitutes for skin regeneration

The development of skin substitutes aims to replace, mimic, or improve the functions of human skin, regenerate damaged skin tissue, and replace or enhance skin function. This includes artificial skin, scaffolds or devices designed for treatment, imitation, or improvement of skin function in wounds and injuries. Therefore, tremendous efforts have been made to develop functional skin substitutes. However, there is still few reports systematically discuss the relationship between the advanced function and design requirements. In this paper, we review the classification, functions, and design requirements of artificial skin or skin substitutes. Different manufacturing strategies for skin substitutes such as hydrogels, 3D/4D printing, electrospinning, microfluidics are summarized. This review also introduces currently available skin substitutes in clinical trials and on the market and the related regulatory requirements. Finally, the prospects and challenges of skin substitutes in the field of tissue engineering are discussed.

ROS/pH dual responsive PRP-loaded multifunctional chitosan hydrogels with controlled release of growth factors for skin wound healing

Platelet-rich plasma (PRP) contains a variety of growth factors (GFs) and has been used in the treatment of a variety of diseases, including skin lesions. In particular, PRP with low immunogenicity will be more widely used. However, the explosive release of GFs limits its further application. In order to achieve controlled release of GFs, a multifunctional and reactive oxygen species (ROS)/pH dual responsive hydrogel was developed to load PRP derived from human cord blood for the treatment of skin wound healing. Based on the hydrogen bond and Schiff base interaction, carboxymethyl chitosan (CMCS), oxidized dextran (Odex) and oligomeric procyanidins (OPC) were crosslinked to form CMCS/Odex/OPC/PRP hydrogel with good injectability, self-healing, adhesion, ROS scavenging, antibacterial activity, controlled and sustained release of GFs. In vitro cell experiments suggested that this hydrogel possessed excellent biocompatibility and could promote the proliferation and migration of L929. In vivo healing of full-layer skin wounds further indicated that the prepared hydrogel could regulate inflammation and promote epithelialization, collagen deposition, and angiogenesis. In summary, this present study demonstrates that CMCS/Odex/OPC/PRP hydrogel may serve as a promising multifunctional dressing for skin wound healing.

Supramolecular hydrogels for wound repair and hemostasis

The unique network characteristics and stimuli responsiveness of supramolecular hydrogels have rendered them highly advantageous in the field of wound dressings, showcasing unprecedented potential. However, there are few reports on a comprehensive review of supramolecular hydrogel dressings for wound repair and hemostasis. This review first introduces the major cross-linking methods for supramolecular hydrogels, which includes hydrogen bonding, electrostatic interactions, hydrophobic interactions, host-guest interactions, metal ligand coordination and some other interactions. Then, we review the advanced materials reported in recent years and then summarize the basic principles of each cross-linking method. Next, we classify the network structures of supramolecular hydrogels before outlining their forming process and propose their potential future directions. Furthermore, we also discuss the raw materials, structural design principles, and material characteristics used to achieve the advanced functions of supramolecular hydrogels, such as antibacterial function, tissue adhesion, substance delivery, anti-inflammatory and antioxidant functions, cell behavior regulation, angiogenesis promotion, hemostasis and other innovative functions in recent years. Finally, the existing problems as well as future development directions of the cross-linking strategy, network design, and functions in wound repair and hemostasis of supramolecular hydrogels are discussed. This review is proposed to stimulate further exploration of supramolecular hydrogels on wound repair and hemostasis by researchers in the future.

Bioinspired Adhesive Antibacterial Hydrogel with Self-Healing and On-Demand Removability for Enhanced Full-Thickness Skin Wound Repair

Adhesive-caused injury is a great threat for extensive full-thickness skin trauma because extra-strong adhesion can incur unbearable pain and exacerbate trauma upon removal. Herein, inspired by the mussel, we designed and fabricated an adhesive antibacterial hydrogel dressing based on dynamic host-guest interaction that enabled on-demand stimuli-triggered removal to effectively care for wounds. In contrast with most hard-to-removable dressing, this adhesive antibacterial hydrogel exhibited strong adhesion property (85 kPa), which could achieve painless and noninvasive on-demand separation within 2 s through a host-guest competition mechanism (amantadine). At the same time, the hydrogel exhibited rapid self-healing properties, and the broken hydrogel could be completely repaired within 5 min. The hydrogel also had excellent protein adsorption properties, mechanical properties, antibacterial properties, and biocompatibility. This on-demand removal was facilitated by the introduction of amantadine as a competitive guest, without any significant adverse effects on cell activity (>90%) or wound healing (98.5%) in vitro. The full-thickness rat-skin defect model and histomorphological evaluation showed that the hydrogel could significantly promote wound healing and reduce scar formation by regulating inflammation, accelerating skin re-epithelialization, and promoting granulation tissue formation. These results indicate that the developed adhesive antibacterial hydrogel offers a promising therapeutic strategy for the healing of extensive full-layer skin injuries.

Tough and On-Demand Detachable Wet Tissue Adhesive Hydrogel Made from Catechol Derivatives with a Long Aliphatic Side Chain

Wet adhesion is critical in cases of wound closure, but it is usually deterred by the hydration layer on tissues. Inspired by dopamine-mediated underwater adhesion in mussel foot proteins, wet tissue adhesives containing catechol with 2-3 carbons side chains are reported mostly. To make wet adhesion of this type of adhesives much tougher, catechol derivatives with a long aliphatic side chain (≈10 atoms length) are synthesized. Then, a series of strong wet tissue adhesive hydrogels are prepared through photoinduced copolymerization of acrylic acid with synthetic monomers. The adhesive hydrogel has a high cohesion strength, that is, tensile strength and strain, and toughness of ≈1800 kPa, ≈540%, and ≈4100 kJ m-3 , respectively. Its interfacial toughness on wet and underwater porcine skin is respectively ≈1300 and ≈1100 J m-2 , and its adhesion strength to wet porcine skin is ≈153 kPa. These values are much higher than those of dopamine-based adhesives in the same conditions, demonstrating that the long aliphatic side chain on catechol can greatly improve the wet tissue-adhesion. Additionally, the tough interfacial adhesion can be broken on demand with 5 wt.% aqueous urea solution. This adhesive hydrogel is highly promising in safe wound closure.

Facilely Prepared Thirsty Granules Arouse Tough Wet Adhesion on Overmoist Wounds for Hemostasis and Tissue Repair

Bioadhesives have been widely used in hemostasis and tissue repair, but the overmoist and wet nature of wound surface (due to the presence of blood and/or wound exudate) has led to poor wet adhesion of bioadhesives, which interrupts the continuous care of wounds. Here, a thirsty polyphenolic silk granule (Tan@SF-pwd-hydro), which absorbs blood and exudate to self-convert to robust bioadhesives (Tan@SF-gel-hydro) in situ, was facilely developed in this study for enhanced wet adhesion toward hemostasis and tissue repair. Tan@SF-pwd-hydro could shield wounds' wetness and immediately convert itself to Tan@SF-gel-hydro to seal wounds for hemorrhage control and wound healing. The maximum adhesiveness of Tan@SF-gel-hydro over wet pigskin was as high as 59.8 ± 2.1 kPa. Tan@SF-pwd-hydro is a promising transformative dressing for hemostasis and tissue repair since its hemostatic time was approximately half of that of the commercial hemostatic product, CeloxTM, and its healing period was much shorter than that of the commercial bioadhesive product, TegadermTM. This pioneering study utilized adverse wetness over wounds to arouse robust adhesiveness by converting thirsty granules to bioadhesives in situ, creatively turning adversity into opportunities. The facile fabrication approach also offers new perspectives for manufacturing sustainability of biomaterials.

Reversible fluorescent adhesives based on covalent adaptable networks with dynamic AIE crosslinking: in situ visualization of adhesion capability

A new family of reversible fluorescent adhesives based on Diels-Alder covalent adaptable networks with dynamic AIE crosslinks was developed. The accurate intrinsic correlation between the emission behavior, cross-linking state, and adhesion capability can be established, enabling the in situ visualization of adhesion capacity.

Double-Cross-Linked Hydrogel with Long-Lasting Underwater Adhesion: Enhancement of Maxillofacial In Situ and Onlay Bone Retention

Bone retention is a usual clinical problem existing in a lot of maxillofacial surgeries involving bone reconstruction and bone transplantation, which puts forward the requirements for bone adhesives that are stable, durable, biosafe, and biodegradable in wet environment. To relieve the suffering of patients during maxillofacial surgery with one-step operation and satisfying repair, herein, we developed a double-cross-linked A-O hydrogel named by its two components: [(3-Aminopropyl) methacrylamide]-co-{[Tris(hydroxymethyl) methyl] acrylamide} and oxidated methylcellulose. With excellent bone adhesion ability, it can maintain long-lasting stable underwater bone adhesion for over 14 days, holding a maximum adhesion strength of 2.32 MPa. Schiff-base reaction and high-density hydrogen bonds endow the hydrogel with strong cohesion and adhesion performance as well as maneuverable properties such as easy formation and injectability. A-O hydrogel not only presents rarely reported long-lasting underwater adhesion of hard tissue but also owns inherent biocompatibility and biodegradation properties with a porous structure that facilitates the survival of bone graft. Compared to the commercial cyanoacrylate adhesive (3 M Vetbond Tissue Adhesive), the A-O hydrogel is confirmed to be safer, more stable, and more effective in calvarial in situ bone retention model and onlay bone retention model of rat, providing a practical solution for the everyday scenario of clinical bone retention.

Robust but On-Demand Detachable Wet Tissue Adhesive Hydrogel Enhanced with Modified Tannic Acid

Adhesives with robust but readily detachable wet tissue adhesion are of great significance for wound closure. Polyelectrolyte complex adhesive (PECA) is an important wet tissue adhesive. However, its relatively weak cohesive and adhesive strength cannot satisfy clinical applications. Herein, modified tannic acid (mTA) with a catechol group, a long alkyl hydrophobic chain, and a phenyl group was prepared first, and then, it was mixed with acrylic acid (AA) and polyethylenimine (PEI), followed by UV photopolymerization to make a wet tissue adhesive hydrogel with tough cohesion and adhesion strength. The hydrogel has a strong wet tissue interfacial toughness of ∼1552 J/m2, good mechanical properties (∼7220 kPa cohesive strength, ∼873% strain, and ∼33,370 kJ/m3 toughness), and a bursting pressure of ∼1575 mmHg on wet porcine skin. The hydrogel can realize quick and effective adhesion to various wet biological tissues including porcine skin, liver, kidney, and heart and can be changed easily with triggering urea solution to avoid tissue damage or uncomfortable pain to the patient. This biosafe adhesive hydrogel is very promising for wound closure and may provide new ideas for the design of robust wet tissue adhesives.

In situ generation of bioadhesives using dry tannic silk particles: a wet-adhesion strategy relying on removal of hydraulic layer over wet tissues for wound care

Tissue adhesives have been widely used in biomedical applications. However, the presence of a hydrated layer on the surface of wet tissue severely hinders their adhesion capacities, resulting in ineffective wound treatment. To address this issue, a dry particle dressing (plas@SF/tann-hydro-pwd) capable of removing the hydrated layer and converting in situ to bioadhesives (plas@SF/tann-hydro-gel) was fabricated via simple physical mixing based on the hydrophobic-hydrogen bonding synergistic effect and Schiff-base reaction. It was found that the plas@SF/tann-hydro-gel bioadhesive, which was changed from plas@SF/tann-hydro-pwd dressing by adsorption of water, exhibited good wet adhesion to diverse biological tissues. In addition, the wet adhesion qualities of the plas@SF/tann-hydro-gel adhesive was studied under a variety of demanding conditions, including a wide range of temperatures, varying pH levels, highly concentrated salt solutions, and simulated fluids. Experiments on animals had showed that the adhesive plas@SF/tann-hydro-gel has superior wet adhesion qualities and superior wound healing properties compared to the commercial product Tegaderm™. This study develops a new wet-adhesion technique employing dry particle dressing to eliminate the hydrated layer over wet tissues for the in situ creation of gel bioadhesives for wound healing.

Self-healing, self-adhesive, stretchable and flexible conductive hydrogels for high-performance strain sensors

Conductive hydrogels have been widely studied for their potential application as wearable sensors due to their flexibility and biocompatibility. However, the simultaneous incorporation of excellent stretchability, toughness, conductivity, self-healing, and adhesion via a simple method remains a great challenge. Herein, a multifunctional hydrogel with self-adhesion, self-healing, conductivity, and mechanical properties was fabricated by ionic cross-linking of chitosan (CS), the acrylic acid (AA) polymer, and tea polyphenols (TPs) in the presence of graphitized carbon nanotubes (CNTs) in this work. The resultant hydrogel has unique self-healing properties (94.11% for strain self-healing and 90.60% for stress self-healing) and mechanical properties. The fracture stress was 0.075 MPa when the strain was 1184%, and the toughness reached 0.48 MJ m^-3. The synergistic effect of free ions and CNTs endows the hydrogel with an excellent electrical conductivity (6.67 S m^-1). Moreover, the hydrogel can adhere to various organic and inorganic materials. It exhibits repeatable self-adhesion to human skin and can be peeled off completely without any residual, irritation or allergic reactions. Additionally, the hydrogel also has good strain sensitivity and exhibits stable output signals in motion monitoring of the human body as a biosensor. Therefore, this work provides a new prospect for the design of multifunctional hydrogels for their potential applications in wearable biosensors.

Dynamic Hydrogels with Viscoelasticity and Tunable Stiffness for the Regulation of Cell Behavior and Fate

The extracellular matrix (ECM) of natural cells typically exhibits dynamic mechanical properties (viscoelasticity and dynamic stiffness). The viscoelasticity and dynamic stiffness of the ECM play a crucial role in biological processes, such as tissue growth, development, physiology, and disease. Hydrogels with viscoelasticity and dynamic stiffness have recently been used to investigate the regulation of cell behavior and fate. This article first emphasizes the importance of tissue viscoelasticity and dynamic stiffness and provides an overview of characterization techniques at both macro- and microscale. Then, the viscoelastic hydrogels (crosslinked via ion bonding, hydrogen bonding, hydrophobic interactions, and supramolecular interactions) and dynamic stiffness hydrogels (softening, stiffening, and reversible stiffness) with different crosslinking strategies are summarized, along with the significant impact of viscoelasticity and dynamic stiffness on cell spreading, proliferation, migration, and differentiation in two-dimensional (2D) and three-dimensional (3D) cell cultures. Finally, the emerging trends in the development of dynamic mechanical hydrogels are discussed.

Bioinspired Polyacrylic Acid-Based Dressing: Wet Adhesive, Self-Healing, and Multi-Biofunctional Coacervate Hydrogel Accelerates Wound Healing

Polyacrylic acid (PAA) and its derivatives are commonly used as essential matrices in wound dressings, but their weak wet adhesion restricts the clinical application. To address this issue, a PAA-based coacervate hydrogel with strong wet adhesion capability is fabricated through a facile mixture of PAA copolymers with isoprenyl oxy poly(ethylene glycol) ether and tannic acid (TA). The poly(ethylene glycol) segments on PAA prevent the electrostatic repulsion among the ionized carboxyl groups and absorbed TA to form coacervates. The absorbed TA provides solid adhesion to dry and wet substrates via multifarious interactions, which endows the coacervate with an adhesive strength to skin of 23.4 kPa and 70% adhesion underwater. This coacervate achieves desirable self-healing and extensible properties suitable for frequently moving joints. These investigations prove that the coacervate has strong antibacterial activity, facilitates fibroblast migration, and modulates M1/M2 polarization of macrophages. In vivo hemorrhage experiments further confirm that the coacervate dramatically shortens the hemostatic time from hundreds to tens of seconds. In addition, full-thickness skin defect experiments demonstrate that the coacervate achieves the best therapeutic effect by significantly promoting collagen deposition, angiogenesis, and epithelialization. These results demonstrate that a PAA-based coacervate hydrogel is a promising wound dressing for medical translation.

Functional hydrogels for the repair and regeneration of tissue defects

Tissue defects can be accompanied by functional impairments that affect the health and quality of life of patients. Hydrogels are three-dimensional (3D) hydrophilic polymer networks that can be used as bionic functional tissues to fill or repair damaged tissue as a promising therapeutic strategy in the field of tissue engineering and regenerative medicine. This paper summarises and discusses four outstanding advantages of hydrogels and their applications and advances in the repair and regeneration of tissue defects. First, hydrogels have physicochemical properties similar to the extracellular matrix of natural tissues, providing a good microenvironment for cell proliferation, migration and differentiation. Second, hydrogels have excellent shape adaptation and tissue adhesion properties, allowing them to be applied to a wide range of irregularly shaped tissue defects and to adhere well to the defect for sustained and efficient repair function. Third, the hydrogel is an intelligent delivery system capable of releasing therapeutic agents on demand. Hydrogels are capable of delivering therapeutic reagents and releasing therapeutic substances with temporal and spatial precision depending on the site and state of the defect. Fourth, hydrogels are self-healing and can maintain their integrity when damaged. We then describe the application and research progress of functional hydrogels in the repair and regeneration of defects in bone, cartilage, skin, muscle and nerve tissues. Finally, we discuss the challenges faced by hydrogels in the field of tissue regeneration and provide an outlook on their future trends.

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 pH-Sensitive, Stretchable, Antibacterial Artificial Tongue Based on MXene Cross-Linked Ionogel

Dehydration has always been a confusing problem for a hydrogel exposed to air, which restricts its application in practical detection. While an ionogel with unique properties can lock water molecules efficiently due to its low vapor pressure, the design and development of an ionogel with excellent water-locking properties and stability to achieve satisfactory detection are thus highly desirable. In this work, a pH-sensitive, stretchable, antibacterial, and stable ionogel artificial tongue was fabricated through dual cross-linking. The artificial tongue consisted of MXene as a cross-linking agent, phenol red as an indicator, gelatin, N-(2-hydroxyethyl) acrylamide, and 1-butyl-3-methylimidazolium chloride as the three-dimensional polymer network. A small quantity of MXene was used as a cross-linker the first time to promote the initiator in the system to generate free radicals, accelerating the reaction process and the multilevel linkage among the polymers. Meanwhile, the indicator phenol red was introduced into the ionogel successfully for the first time, showing great stability in 5 weeks. After evaluation by the Brand-Altman analysis, it was found that the method of pH measurement using ionogels had good consistency with the pH meter measurement method. Additionally, the prepared ionogel presented excellent water retention, mechanical stretchability, antibacterial property, and stability, as well as good test results in the test of artificial simulated saliva, demonstrating great potential in clinical applications.