Tough adhesives for diverse wet surfaces

Molecular self-assembly strategy tuning a dry crosslinking protein patch for biocompatible and biodegradable haemostatic sealing

Uncontrolled haemorrhage is a leading cause of trauma-related fatalities, highlighting the critical need for rapid and effective haemostasis. Current haemostatic materials encounter limitations such as slow clotting and weak mechanical strength, while most of bioadhesives compromise their adhesion performance to wet tissues for biocompatibility and degradability. In this study, a molecular self-assembly strategy is proposed, developing a biocompatible and biodegradable protein-based patch with excellent adhesion performance. This strategy utilizes fibrinogen modified with hydrophobic groups to induce self-assembly into a hydrogel, which is converted into a dry patch. The protein patch enhances adhesion performance on the wet tissue through a dry cross-linking method and robust intra/inter-molecular interactions. This patch demonstrates excellent haemostatic efficacy in  both porcine oozing wound and porcine severe acute haemorrhage. It maintains biological functionality, and ensures sustained wound sealing while gradually degrading in vivo, making it a promising candidate for clinical tissue sealing applications.

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.

In Vitro Engineered ECM-incorporated Hydrogels for Osteochondral Tissue Repair: A Cell-Free Approach

Prevalence of osteoarthritis has been increasing in aging populations, which has necessitated the use of advanced biomedical treatments. These involve grafts or delivering drug molecules entrapped in scaffolds. However, such treatments often show suboptimal therapeutic effects due to poor half-life and off-target effects of drug molecules. As a countermeasure, a 3D printable robust hydrogel-based tissue-repair platform is developed containing decellularized extracellular matrix (dECM) from differentiated mammalian cells as the therapeutic cargo. Here, pre-osteoblastic and pre-chondrogenic murine cells are differentiated in vitro, decellularized, and incorporated into methacrylated gelatin (GelMA) solutions to form osteogenic (GelO) and chondrogenic (GelC) hydrogels, respectively. Integrating the bioactive dECM from differentiated cell sources allows GelO and GelC to induce differentiation in human adipose-derived stem cells (hASCs) toward osteogenic and chondrogenic lineages. Further, GelO and GelC can be covalently adhered using a carbodiimide coupling reaction, forming a multi-layered hydrogel with potential application as a bioactive osteochondral plug. The designed multi-layered hydrogel can also induce differentiation of hASCs in vitro. In conclusion, the bioactive dECM carrying 3D printed robust hydrogel offers a promising new drug and cell-free therapeutic strategy for bone and cartilage repair and future osteoarthritis management.

Integrating Hydrogels and Biomedical Plastics via In Situ Physical Entanglements and Covalent Bonding

Both rigid plastics and soft hydrogels find ample applications in engineering and medicine but bear their own disadvantages that limit their broader applications. Bonding these mechanically dissimilar materials may resolve these limitations, preserve their advantages, and offer new opportunities as biointerfaces. Here, a robust adhesion strategy is proposed to integrate highly entangled tough hydrogels and diverse plastics with high interfacial adhesion energy and strength. Systemic investigations on the effects of hydrogel monomer content and crosslink fraction revealed the significant contributions of both polymer physical entanglements and interfacial covalent bonding. This hybrid engineering strategy also enables the plastic-hydrogel composite to attenuate foreign body response caused by pristine rigid plastics in vivo in mice. This versatile materials engineering approach may be broadly applicable to other polymer-based devices commonly used in regenerative medicine and surgical robotics.

The adhesion mechanism of mucoadhesive tablets with dissimilar chain flexibility on viscoelastic hydrogels

Mucosal membranes with strong variability in their viscoelastic properties line numerous organs and are often targeted by mucoadhesive formulations, e.g., highly swellable hydroxypropylmethylcellulose (HPMC) and slightly cross-linked poly(acrylic acid) (PAA) tablets. Although the factors determining the strength of mucoadhesion are hierarchical and affected by both reversible and irreversible processes, the currently available strategies generally view mucoadhesion as the individual performance of the mucoadhesive excipient. We propose an integrated concept that considers the viscoelasticity and tensile properties of both the adhesive interphase and the bulk phases. To reduce the complexity of the mucosal membrane and eliminate the effect of specific macromolecular interactions, we studied the adhesion on mucosa-mimetic freeze/thawed (FT) poly(vinyl alcohol) (PVA) hydrogels. Their viscoelastic properties were controlled by the number of FT cycles and the polymer concentration. The adhesive strength of HPMC tablets displayed a pronounced dependence on the viscoelasticity of PVA gels, explained by the limited chain flexibility and interpenetration of HPMC, resulting in the formation of a thin the adhesive interphase compared to PAA. We recognized scaling laws between toughness and strength for tensile and adhesive properties as well as general correlations between viscoelastic and adhesive properties, which can aid the more rational design of both mucoadhesive formulations and mucosa-mimetic materials.

Dry Bondable Porous Silk Fibroin Films for Embedding Micropatterned Electronics in Hierarchical Silk Nacre

Future structural materials is not only be lightweight, strong, and tough, but also capable of integrating functions like sensing, adaptation, self-healing, deformation, and recovery as needed. Although bio-inspired materials are well developed, directly integrating microelectronic patterns into nacre-mimetic structures remains challenging, limiting the widespread application of electronic biomimetic materials. Here, an in situ freeze-drying method is reported for the successful preparation of porous silk fibroin materials that can achieve dry bonding. The in situ freeze-drying method preserves the structural integrity of the lyophilized membrane while reducing procedural steps, achieving control over pore gradient not feasible with traditional freeze-drying techniques. By leveraging their smooth surfaces and capacity to support heat transfer patterns, layer-by-layer assembly at a macroscopic scale is achieved. The material's excellent mechanical properties, controllable graded structure, and adjustable degradation behavior enable the construction of electronically functionalized hierarchical structures. Additionally, the dry-state, layer-by-layer bonding method for porous polymer films provides advantages in precision control, mechanical stability, functional versatility, hierarchical structuring, and scalability. It represents an innovative approach, offering multi-functional and customizable bulk materials, especially suited for biomedical applications. This work offers an effective pathway for developing high-performance and multifunctional biomimetic devices with controllable hierarchical structures.

A natural gelatin/casein hydrogel with on-demand adhesion via chitosan solution for wound healing

Adhesive hydrogels have been widely studied as wound dressings due to their excellent biocompatibility and biological activity. However, most designed hydrogels still exist limitations including potentially toxic monomer, complex preparation process and non-degradable property. Here, a natural and degradable gelatin/casein hydrogel was prepared by enzymatic cross-linking. The hydrogel could adhere to tissue on-demand with the mediation of chitosan (CS) solution. It was found that the adhesion strength of hydrogel could be controlled by adjusting gelatin/casein ratio, EDC&NHS concentration, CS concentration, glycerol content and crosslinking degree. To further expand the applicability of hydrogels, the degradation and drug release rate of hydrogels could be modulated by changing transglutaminase (TG) concentration. Moreover, tetracycline hydrochloride (TH) was loaded into hydrogel as a drug model, which endowed hydrogel with good antibacterial properties. It was shown that the 0.03 % TH hydrogel had excellent blood compatibility and cell compatibility, and can promote the healing of infected wounds in mice. This research provides a new natural adhesive hydrogel for biomedical engineering field.

Recent Advances in Barnacle-Inspired Biomaterials in the Field of Biomedical Research

As a marine fouling organism, barnacles secrete a cement whose proteins self-assemble into stable nanofibers, conferring exceptional underwater adhesion and curing properties. The barnacle cement proteins (BCPs) are of significant interest in biomedicine due to their adhesiveness, water resistance, stability, and biocompatibility, making them ideal for developing novel biomaterials. Additionally, BCPs have wound-healing acceleration and antibacterial properties, offering new insights for antimicrobial biomaterial development. Recently, barnacle-inspired materials have seen extensive research and notable progress in biomedicine. As the understanding of barnacle cement and its adhesion mechanisms deepens, their medical applications are expected to expand. This review summarizes the latest advancements of barnacle biomimetic materials in biomedicine, including their use in adhesives, tissue engineering, drug delivery, and hemostasis, highlighting their characteristics, applications, and potential research directions, and providing a comprehensive reference for the field.

Hemostasis Strategies and Recent Advances in Hydrogels for Managing Uncontrolled Hemorrhage

Hemorrhage continues to pose a significant challenge in various medical contexts, underscoring the need for advanced hemostatic materials. Hemostatic hydrogels have gained recognition as innovative tools for addressing uncontrollable bleeding, attributed to their distinctive features including biological compatibility, tunable mechanical properties, and exceptional hemostatic performance. This review provides a comprehensive overview of hemostatic hydrogels that offer rapid and effective bleeding control. Particularly, this review focuses on hemostatic hydrogel design and associated hemostatic mechanisms. Additionally, recent advancements in the application of these materials are discussed in detail, especially in clinical trials. Finally, the challenges and potential advancements of hemostatic hydrogels are analyzed and assessed. This review seeks to emphasize the role of hydrogels in biomedical applications for hemorrhage control and provide perspectives on the innovation of clinically applicable hemostatic materials.

Development of Robust MWCNT Hydrogel Electrochemical Biosensor for Pyocyanin Detection by Phosphotungstic Acid Modification

The trace detection of pyocyanin (PCN) is crucial for infection control, and electrochemical sensing technology holds strong potential for application in this field. A pivotal challenge in utilizing carbon materials within electrochemical sensors lies in constructing carbon-based films with robust adhesion. To address this issue, a novel composite hydrogel consisting of multi-walled carbon nanotubes/polyvinyl alcohol/phosphotungstic acid (MWCNTs/PVA/PTA) was proposed in this study, resulting in the preparation of a highly sensitive and stable PCN electrochemical sensor. The sensor is capable of achieving stable and continuous detection of PCN within the range of 5-100 μM across a variety of complex electrolyte environments. The limit of detection (LOD) is as low as 1.67 μM in PBS solution, 2.71 μM in LB broth, and 3.63 μM in artificial saliva. It was demonstrated that the introduction of PTA can complex with PVA through hydrogen bonding to form a stabilized hydrogel architecture, effectively addressing issues related to inadequate film adhesion and unstable sensing characteristics observed with MWCNTs/PVA alone. By adjusting the content of PTA within the hydrogel, an increase followed by a subsequent decrease in sensing current response was observed, elucidating how PTA regulates the active sites and conductive network of MWCNTs on the sensor surface. This study provides a new strategy for constructing stable carbon-based electrochemical sensors and offers feasible assistance towards advancing PCN electrochemical sensors for practical applications.

Supramolecular adhesives inspired from adhesive proteins and nucleic acids: molecular design, properties, and applications

Bioinspired supramolecular adhesives have been recently emerging as novel functional materials, which have shown a wide range of applications in wearable sensors and tissue engineering such as tissue adhesives and wound dressings. In this review, we summarize and discuss two main types of biologically inspired supramolecular adhesives from adhesive proteins and nucleic acids. The widely studied catechol-based adhesives, that originated from adhesive proteins of marine organisms such as mussels, and recently emerging nucleobase-containing supramolecular adhesives are both introduced and discussed. Both bioinspired adhesives from nucleic acids and adhesive proteins involve multiple supramolecular interactions such as hydrogen bonding, hydrophobic interactions, π-π stacking, and so on. Several major types of these bioinspired adhesives are summarized, respectively, including polymer-based, hydrogel-based, and other types of adhesives. The novel molecular design and adhesion properties are focused on and highlighted for each type of bioinspired adhesive. In addition, the potential applications of these bioinspired supramolecular adhesives in different realms including tissue engineering and biomedical devices are discussed. This review concludes with issues and challenges in the area of the bioinspired adhesives, hopefully promoting further developments and broader applications of novel supramolecular adhesives.

Multifunctional Adhesive Hydrogels: From Design to Biomedical Applications

Adhesive hydrogels characterized by structural properties similar to the extracellular matrix, excellent biocompatibility, controlled degradation, and tunable mechanical properties have demonstrated significant potential in biomedical applications, including tissue engineering, biosensors, and drug delivery systems. These hydrogels exhibit remarkable adhesion to target substrates and can be rationally engineered to meet specific requirements. In recent decades, adhesive hydrogels have experienced significant advancements driven by the introduction of numerous multifunctional design strategies. This review initially summarizes the chemical bond-based design strategies for tissue adhesion, encompassing static covalent bonds, dynamic covalent bonds, and non-covalent interactions. Subsequently, the multiple functionalities imparted by these diverse design strategies, including highly stretchable and tough performances, responsiveness to microenvironments, anti-freezing/heating properties, conductivity, antibacterial activity, and hemostatic properties are discussed. In addition, recent advances in the biomedical applications of adhesive hydrogels, focusing on tissue repair, drug delivery, medical devices, and wearable sensors are reviewed. Finally, the current challenges are highlighted and future trends in this rapidly evolving field are discussed.

Robustly Injectable Tetra-PEG Hydrogel Sealants for Annulus Fibrosus Repair

Discectomy serves as the primary therapeutic approach for lumbar disc herniation, but the annular fibrosus defects after discectomy may lead to recurrence of disc herniation. Despite recent advances in bioinspired adhesives to seal the AF defect, the growing popularity of endoscopic discectomy has put forward high requirements for the tissue bioadhesives with rapid injectability, easy operation, and robust tissue adhesion in underwater environments. Herein, a rapidly in situ forming injectable tetra-PEG bioadhesive (ISG) comprising of FDA-approved tetra-armed poly (ethylene glycol) amine (tetra-PEG-NH2) and tetra-armed poly (ethylene glycol) succinimidyl glutarate (tetra-PEG-SG) for the sutureless closure of AF defects, is reported. Relying on quick ammonolysis reaction between N-hydroxysuccinimide (NHS)-ester of tetra-PEG-SG polymer and amine groups of tetra-PEG-NH2 polymer and tissue proteins, the uniform networks are formed within seconds with easy injection, efficient waterproofness, instant tissue adhesion, and durable compliance. The goat lumbar discectomy model was used to assess the effect of ISG hydrogels in vivo. The results reveal that the resultant ISG bioadhesive can effectively maintain the disc height, fuse with the host tissue, ameliorate IVD degeneration, and retain the initial biomechanics. Together, this study provides an efficient strategy of in situ injectable glue for the minimally invasive treatment of AF defects.

Development of a Janus tissue adhesive hemostatic patch based on hydrophobically-modified Alaska pollock gelatin

Uncontrollable hemorrhage from trauma and open surgery leads to a high percentage of death. Even though some patch-type hemostatic materials have been used in the clinic, sufficient tissue adhesion property and the management of tissue adhesion and anti-adhesion have been the challenges. In this report, we designed Janus tissue adhesive hemostatic patch, consisting of Alaska pollock gelatin (Org-ApGltn) as a support layer and decanoyl group-modified ApGltn (C10-ApGltn) with pentaerythritol poly(ethylene glycol) ether tetrasuccinimidyl glutarate (4S-PEG) as an adhesive layer, named as the C10-ApGltn patch. The C10-ApGltn patch adhered onto blood vessel surface by the activation 4S-PEG and hydrophobic groups in C10-ApGltn through the covalent bond formation and physical interaction. The burst strength of the C10-ApGltn patch was optimized in terms of the degree of substitution, the molecular weight of 4S-PEG, the concentration of C10-ApGltn, and the NHS/NH2 ratio. The optimized C10-ApGltn patch showed significantly higher burst strength with commercially available TachoSil®. The C10-ApGltn patch showed enzymatic degradability in a buffer solution with collagenase. In a rat liver hemorrhage model, the C10-ApGltn patch acted as a sealant on the hemorrhage site and exhibited competitive hemostatic property to TachoSil®.

Pullulan fermented by Aureobasidium melanogenum TZ-FC3 for the preparation of self-healing, adhesive, injectable and antibacterial pullulan/PVA/borax hydrogel

Natural polymer hydrogels, such as pullulan-based hydrogels, offer significant advantages over synthetic materials due to their thermal stability, film-forming capacity, solubility, adhesiveness, and antioxidant properties. In this study, the strain Aureobasidium melanogenum TZ-FC3, which produces a high level of pullulan, was successfully isolated from the mangrove ecosystems of Guangdong Province, China. 66.01 ± 1.10 g/L pullulan without melanin was produced by the TZ-FC3 strain within 120 h at flask level. Pullulan fermented by A. melanogenum TZ-FC3 was added to enhance the hydrogen bond network within the pullulan/PVA/borax hydrogels (P-2, P-3 and P-4 hydrogels) to improve mechanical strength and crosslinking density of PVA/borax hydrogel (P-1 hydrogel). Compared to the P-1 hydrogel, the P-2 hydrogel exhibited a 65.4 % increase in tensile strain, a remarkable 694.03 % increase in tensile strength and improved the degree of internal crosslinking. Additionally, the pullulan/PVA/borax hydrogels demonstrated excellent self-healing properties, adhesion, injectability, and antibacterial activity. The preparation process of pullulan/PVA/borax hydrogels is straightforward and effective, suggesting broad industrial applicability and underscoring their potential as next-generation materials for advanced healthcare solutions.

Sticky Science: Using Complex Coacervate Adhesives for Biomedical Applications

Tissue adhesives are used for various medical applications, including wound closure, bleeding control, and bone healing. Currently available options often show weak adhesion or cause adverse effects. Recently, there has been an increasing interest in complex coacervates as medical adhesives. Complex coacervates are formed by mixing oppositely charged macromolecules that associate and undergo liquid-liquid phase separation, in which the dense bottom phase is the complex coacervate. Complex coacervates are strong and often biocompatible, and show strong underwater adhesion. The properties of the resulting materials are tunable by intrinsic factors such as polymer chemistry, molecular weight, charge density, and topology of the macromolecules, as well as extrinsic factors such as temperature, pH, and salt concentration. Therefore, complex coacervates are interesting new candidates for medical adhesives. In this review, it is described how complex coacervates form and how different factors influence their behavior. Next, an overview of recent studies on complex coacervates in the context of medical adhesives is presented. The application of complex coacervates as hemostatic or embolic agents, skin or bone repair adhesives, and soft tissue sealants is discussed. Lastly, additional possibilities for utilizing these materials in the future are discussed.

Bioadhesive First-Aid Patch with Rapid Hemostasis and High Toughness Designed for Sutureless Sealing of Acute Bleeding Wounds

The global military and civilian sectors express widespread concern over the significant hemorrhage associated with various acute wounds. Such bleedings lead to numerous casualties in military confrontations, traffic accidents, and surgical injuries. Consequently, the rapid control of the bleedings, particularly for extensive and pressurized wounds, is crucial in first-aid situations. In this work, a double-layered bioadhesive patch that combines a superabsorbent adhesive hydrogel with a highly tough antibacterial polyurethane film, which is called as Bio-Patch, is proposed. The Bio-Patch demonstrates superior mechanical strength and forms robust bioadhesion to acute bleeding wounds. Furthermore, the Bio-Patch enables protecting against external Gram-negative and Gram-positive bacteria. Thanks to the double-layered structures having synergistic functions of stable barrier and robust adhesion, the Bio-Patch provides optimal wound sealing (burst strength exceeding 310 mmHg) both in vitro and in vivo. It also demonstrates superior hemostatic effects (less than 30 s) in vivo. This offers promising opportunities for rapid control of extensive and pressurized hemorrhage in first-aid clinical scenarios.

Blacking any Material Surface by Amyloid-Fortified Carbon Coating Toward High-Performance Large-Scale Solar Steam Generation System

Enhancing the interfacial adhesion between carbon-based coatings and substrates through a simple method remains a challenge, mainly due to the intrinsic chemical inertness of carbon materials. Herein, a carbon nanosphere-based coating utilizing an amyloid-like protein aggregation strategy is developed, involving only the reaction of protein, reductant, and carbon nanospheres in an aqueous solution at room temperature. The resultant coating, enriched in amyloid-like protein structures, features both robust interfacial adhesion and high light absorption (≈98.5%) covering the entire UV/Vis to NIR regions. Adhesion energy between the coating and the glass exceeds 5436 J m-2, which is at least five times higher than those polymer-reinforced carbon-based coatings. Combining the strong adhesion and excellent photothermal conversion performance of this coating, a solar steam generation system is constructed with a water treatment capacity of 13.01 kg m-2 d-1, which is sufficient to provide daily supply for tens of people. Importantly, the photothermal conversion unit can be repeatedly cleaned and rolled up for storage, which is beneficial for the construction of portable devices. This work provides a facile and valuable method for preparing carbon-based coatings with strong interfacial adhesion, exhibiting great promise in energy conversion and storage, flexible wearable sensors, photothermal therapy, and so on.

A Pressure-Sensitive, Repositionable Bioadhesive for Instant, Atraumatic Surgical Application on Internal Organs

Pressure-sensitive adhesives are widely utilized due to their instant and reversible adhesion to various dry substrates. Though offering intuitive and robust attachment of medical devices on skin, currently available clinical pressure-sensitive adhesives do not attach to internal organs, mainly due to the presence of interfacial water on the tissue surface that acts as a barrier to adhesion. In this work, a pressure-sensitive, repositionable bioadhesive (PSB) that adheres to internal organs by synergistically combining the characteristic viscoelastic properties of pressure-sensitive adhesives and the interfacial behavior of hydrogel bioadhesives, is introduced. Composed of a viscoelastic copolymer, the PSB absorbs interfacial water to enable instant adhesion on wet internal organs, such as the heart and lungs, and removal after use without causing any tissue damage. The PSB's capabilities in diverse on-demand surgical and analytical scenarios including tissue stabilization of soft organs and the integration of bioelectronic devices in rat and porcine models, are demonstrated.

Penetration of Microplastics and Nanoparticles Through Skin: Effects of Size, Shape, and Surface Chemistry

Skin represents an effective barrier against the penetration of external agents into the human body. Nevertheless, recent research has shown that small particles, especially in the nanosized range, can not only penetrate through the skin but also work as vectors to transport active molecules such as contrast agents or drugs. This knowledge has opened new perspectives on nanomedicine and controlled drug delivery. On the other hand, micro- and nanoplastics represent a form of emerging pollutants, and their concentration in the environment has been reported to drastically increase in the last years. The possible penetration of these particles through the skin has become a major concern for human health. If the actual primary toxicity of these materials is still debated, their possible role in the transport of toxic molecules through the skin, originating as secondary toxicity, is surely alarming. In this review paper, we analyze and critically discuss the most recent scientific publications to underline how these two processes, (i) the controlled delivery of bioactive molecules by micro- and nano-structures and (ii) the unwanted and uncontrolled penetration of toxic species through the skin mediated by micro- and nanoparticles, are deeply related and their efficiency is strongly affected by the nature, size, and shape of the particles.

Nerve-Mimetic Adhesive Hydrogel Electroceuticals: Tailoring In Situ Physically Entangled Domains in Singular Polymers

Implantable electrochemicals stand out as promising candidates for resolving peripheral nerve injuries. However, challenges persist in designing bioelectronic materials that mimic tissue due to modulus matching, conformal adhesion, and immune responses. Herein, we present a nerve-mimicking design rationale for biocompatible hydrogel-based electroceuticals with a tissue-like modulus, robust and conformal tissue adhesion, exceptional mechanical toughness, and efficient stress dissipation. Inspired by the hierarchical structure of the peripheral nerve, the hydrogel substrate features a structurally gradient bilayer transitioning from a dense to a loose polymeric network, utilizing alginate functionalized with either photo-cross-linkable methacrylate or tissue-adhesive phenylborate. Due to the varying water affinity of the tethering groups, a physically entangled interfacial domain is in situ formed during dehydration of the pre-gel film, resulting in enhanced mechanical toughness and strong adhesion. The hydrogel electroceuticals, when integrated with conducting polymeric electrodes, locally stimulate nerve tissue, improving tissue regeneration in a crushed nerve injury model.

One-pot rapid preparation of long-term antioxidant and antibacterial biomedical gels based on lipoic acid and eugenol for accelerating cutaneous wound healing

The complex battlefield environment often puts great pressure on the treatment of open wounds caused by burns and trauma, which cannot heal for a long time due to the lack of medical resources. Once wounds are not sutured and severely infected, they can lead to infective endocarditis, sepsis, and even death. Therefore, it is urgent to develop advanced dressings to replace sutures and antibiotics, which can quickly seal wounds and maintain long-term stability of antibacterial and antioxidant properties. In this study, novel biobased antibacterial adhesive gels composed of natural small molecule lipoic acid and eugenol were prepared via a one-pot solvent-free reaction for wound management. The gels were crosslinked via the ring-opening polymerization of lipoic acid and hydrogen bond interaction. Due to its structure feature, the PLA-E1 gel displayed excellent flexibility, transparency, self-healing and adhesiveness. The gel system showed long-term high antioxidant activity (95%) after exposure to air at room temperature for one year. Meanwhile, the reactive oxygen species (ROS) scavenging efficacy was kept around 52%. Both trauma and burn in vivo experiments demonstrated that the PLA-E1 gel could accelerate wound healing through antibacterial, antioxidant, angiogenic and tissue regenerative effects, indicating the potential applications of cutaneous wound healing on the battlefield.

Design Strategy, On-Demand Control, and Biomedical Engineering Applications of Wet Adhesion

The adhesion of tissues to external devices is fundamental to numerous critical applications in biomedical engineering, including tissue and organ repair, bioelectronic interfaces, adhesive robotics, wearable electronics, biomedical sensing and actuation, as well as medical monitoring, treatment, and healthcare. A key challenge in this context is that tissues are typically situated in aqueous and dynamic environments, which poses a bottleneck to further advancements in these fields. Wet adhesion technology (WAT) presents an effective solution to this issue. In this review, we summarize the three major design strategies and control methods of wet adhesion, comprehensively and systematically introducing the latest applications and advancements of WAT in the field of biomedical engineering. First, single adhesion mechanism under the frameworks of the three design strategies is systematically introduced. Second, control methods for adhesion are comprehensively summarized, including spatiotemporal control, detachment control, and reversible adhesion control. Third, a systematic summary and discussion of the latest applications of WAT in biomedical engineering research and education were presented, with a particular focus on innovative applications such as tissue-electronic interface devices, ingestible devices, end-effector components, in vivo medical microrobots, and medical instruments and equipment. Finally, opportunities and challenges encountered in the design and development of wet adhesives with advanced adhesive performance and application prospects are discussed.

Sustained Release of Curcumin from Cur-LPs Loaded Adaptive Injectable Self-Healing Hydrogels

Biological tissue defects are typically characterized by various shaped defects, and they are prone to inflammation and the excessive accumulation of reactive oxygen species. Therefore, it is still urgent to develop functional materials which can fully occupy and adhere to irregularly shaped defects by injection and promote the tissue repair process using antioxidant and anti-inflammatory mechanisms. Herein, in this work, phenylboronic acid modified oxidized hyaluronic acid (OHAPBA) was synthesized and dynamically crosslinked with catechol group modified glycol chitosan (GCHCA) and guar gum (GG) into a hydrogel loaded with curcumin liposomes (Cur-LPs) which were relatively uniformly distributed around 180 nm. The hydrogel possessed rapid gelation within 30 s, outstanding injectability and tissue-adaptive properties with self-healing properties, and the ability to adhere to biological tissues and adapt to tissue movement. Moreover, good biocompatibility and higher DPPH scavenging efficiency were illustrated in the hydrogel. And a more sustainable release of curcumin from Cur-LPs-loaded hydrogels, which could last for 10 days, was achieved to improve the bioavailability of curcumin. Finally, they might be injected to fully occupy and adhere to irregularly shaped defects and promote the tissue repair process by antioxidant mechanisms and the sustained release of curcumin for anti-inflammation. And the hydrogel would have potential application as candidates in tissue defect repair.

Hydrogels in wearable neural interfaces

The integration of wearable neural interfaces (WNIs) with the human nervous system has marked a significant progression, enabling progress in medical treatments and technology integration. Hydrogels, distinguished by their high-water content, low interfacial impedance, conductivity, adhesion, and mechanical compliance, effectively address the rigidity and biocompatibility issues common in traditional materials. This review highlights their important parameters-biocompatibility, interfacial impedance, conductivity, and adhesiveness-that are integral to their function in WNIs. The applications of hydrogels in wearable neural recording and neurostimulation are discussed in detail. Finally, the opportunities and challenges faced by hydrogels for WNIs are summarized and prospected. This review aims to offer a thorough examination of hydrogel technology's present landscape and to encourage continued exploration and innovation. As developments progress, hydrogels are poised to revolutionize wearable neural interfaces, offering significant enhancements in healthcare and technological applications.

Graphical Abstract

Self-Reinforcing Ionogel Bioadhesive Interface for Robust Integration and Monitoring of Bioelectronic Devices with Hard Tissues

Integrating bioelectronic devices with hard tissues, such as bones and teeth, is essential for advancing diagnostic and therapeutic technologies. However, stable and durable adhesion in dynamic, moist environments remains challenging. Traditional bioadhesives often fail to maintain strong bonds, especially when interfacing with metal electrodes and hard tissues. This study introduces a self-reinforcing ionogel bioadhesive interface (IGBI) combining silk fibroin and calcium ions, designed to provide robust and conductive integration of bioelectronic devices with hard tissues. The IGBI exhibits strong adhesion (up to 186 J m-2) and undergoes mechanical self-reinforcement through a structural transition in silk fibroin under physiological conditions. In vivo experiments demonstrate the IGBI's effectiveness in repairing bone defects and reimplanting teeth, with the added capability of wireless, real-time monitoring of bone healing. This approach allows for continuous tracking of tissue regeneration without a second invasive surgery for device removal. The IGBI represents a significant advancement in bioelectronic integration, offering a durable and versatile solution for challenging environments. Such unique self-reinforcing properties make the IGBI a promising material for biomedical applications where traditional adhesives are insufficient.

Study on the discoloration phenomenon caused by iron ion oxidation in Boston ivy pads and its effect on adhesion force

Boston ivy has received much attention from researchers owing to its exceptional climbing abilities. However, many aspects of their adhesion behavior remain unresolved. Our research has discovered a phenomenon of oxidation and discoloration in Boston ivy pads, which leads to a significant decrease in adhesion force. In this study, we conducted a comprehensive investigation into the oxidation discoloration phenomenon. Through XPS analysis, we confirmed that the transition from Fe2+ to Fe3+ in the pad is the primary cause of the oxidation discoloration reaction. Furthermore, by conducting in situ adhesion testing using AFM, we observed a decrease in adhesion during the oxidation of iron ions. The magnitude of adhesion is closely related to the amount of pyrocatechol. Following the oxidation reaction, iron ions chelate with more pyrocatechol, resulting in a decrease in the available pyrocatechol content for adhesion. To validate this mechanism, we designed and prepared a biomimetic composite adhesion surface of a PDMS hydrogel. This composite surface improved oxidation resistance through the hydrogel, demonstrating improved adhesion performance. These findings offer promising prospects for the application of bionic materials in various fields.

Advances in arthropod-inspired bionic materials for wound healing

Arthropods contain lots of valuable bionic information from the composition to the special structure of the body. In particular, the rapid self-healing ability and antibacterial properties are amazing. Biomimetic materials for arthropods have been helpful methods for wound management. Here, we have identified four major dimensions needed to create biomimetic materials for arthropods, including ingredient, behavior, structure and internal reaction. According to different dimensions, we classify and introduce the reported arthropod biomimetic materials. Antibacterial, hemostatic and healing promotion are the main functions of the active compositions of arthropods developed by humans, and most of them play a drug effect. We believe that an ideal biomimetic material of arthropod should have the effect on promoting wound healing through the advantages of structure and composition. The special macroscopic and microscopic structure of the epidermis may provide good mechanical support for biomimetic materials. The drug release regularity in the bionic materials can be referred to the aggressive and secretory behavior of arthropods. The synthesis of substances in arthropods is also noteworthy, and we can learn these special reactions to complete the fast preparation of materials. Arthropod-inspired bionic materials have broad innovation and application prospects in the field of wound repair.

Enhancing under-urine adhesion and bladder adaptation of silk fibroin hydrogels with tea polyphenols for hemorrhagic cystitis

Hemorrhagic Cystitis (HC) presents a significant therapeutic challenge due to the dynamic fluid environment and cyclical mechanical stress within the bladder. Tissue-adhesive hydrogels have shown promise in treating HC; however, maintaining strong adhesion and mechanical integrity under these fluctuating conditions remains a critical obstacle. Herein, we designed a robust bladder-adhesive hydrogel by leveraging the affinity of tea polyphenols (TP) for damaged tissues and their ability to rapidly enhance the stability of photo-crosslinked silk fibroin methylacryloyl (SFMA) through abundant hydrogen bonding. The resulting SFMA/TP hydrogel could withstand high compressive and tensile loads while maintaining efficient under-urine adhesion, achieving up to 15.1 kPa to adapt to the dynamic mechanical environment of the bladder. Furthermore, urea dissociation disrupted hydrogen bonding, enabling the SFMA/TP hydrogels to exhibit urea-responsiveness and effective biodegradation both in vitro and in vivo within the bladder. In a rat model of cyclophosphamide-induced HC, this under-urine hydrogel adhesive demonstrated superior hemostatic effects and promoted healing by modulating inflammation, enhancing neovascularization, and facilitating smooth muscle formation. Overall, this bladder-adaptive hydrogel adhesive represents a minimally invasive therapeutic option for HC by offering targeted and sustained treatment within the bladder environment.

Design and Improvement of Bone Adhesive in response to Clinical Needs

Fracture represents one of the most common diagnoses in contemporary medical practice, with the majority of cases traditionally addressed through metallic device fixation. However, this approach is marred by several drawbacks, including prolonged operative durations, considerable expenses, suboptimal applicability to comminuted fractures, increased infection risks, and the inevitable requirement for secondary surgery. The inherent advantages of bone adhesives in these fields have garnered the attention of orthopedic surgeons, who have commenced utilizing biocompatible and biodegradable bone adhesives to bond and stabilize bone fragments. Regrettably, the current bone adhesives generally exhibit insufficient adhesive strength in vivo environments, and it is desirable for them to possess effective osteogenesis to facilitate fracture healing. Consequently, aligning bone adhesives with practical clinical demands remains a significant hurdle, which has catalyzed a surge in research endeavors. Within this review, the conceptual framework, characteristics, and design ideas of bone adhesives based on clinical needs are delineated. Recent advancements in this domain, specifically focusing on the enhancement of two pivotal characteristics-adhesive strength and osteogenic potential are also reviewed. Finally, a prospective analysis of the future advancements in bone adhesives, offering new insights into solutions for diverse clinical problems is presented.

An Environmentally Stable, Biocompatible, and Multilayered Wound Dressing Film with Reversible and Strong Adhesion

Reversible adhesives for wound care improve patient experiences by permitting reuse and minimizing further tissue injury. Existing reversible bandages are vulnerable to water and can undergo unwanted deformation during removal and readdressing procedures. Here, a biocompatible, multilayered, reversible wound dressing film that conforms to skin and is waterproof is designed. The inner layer is capable of instant adhesion to various substrates upon activation of the dynamic boronic ester bonds by water; intermediate hydrogel layer and outer silicone backing layer can enhance the dressing's elasticity and load distribution for adhesion, and the silicone outer layer protects the dressing from exposure to water. The adhesive layer is found to be biocompatible with mouse skin. Skin injuries on the mouse skin heal more rapidly with the film compared to no dressing controls. Evaluations of the film on skin of freshly euthanized minipigs corroborate the findings in the mouse model. The film remains attached to skins without delamination despite subjecting to various degrees of deformation. Exposure to water softens the film to allow removal from the skin without pulling any hair off. The multilayered design considers soft mechanics in each layer and will offer new insights to improve wound dressing performance and patient comfort.

Polyurethanes Made with Blends of Polycarbonates with Different Molecular Weights Showing Adequate Mechanical and Adhesion Properties and Fast Self-Healing at Room Temperature

Dynamic non-covalent interactions between polycarbonate soft segments have been proposed for explaining the intrinsic self-healing of polyurethanes synthesized with polycarbonate polyols (PUs) at 20 °C. However, these self-healing PUs showed insufficient mechanical properties, and their adhesion properties have not been explored yet. Different PUs with self-healing at 20 °C, acceptable mechanical properties, and high shear strengths (similar to the highest ones reported in the literature) were synthesized by using blends of polycarbonate polyols of molecular weights 1000 and 2000 Da (CD1000 + CD2000). Their structural, thermal, rheological, mechanical, and adhesion (single lap-shear tests) properties were assessed. PUs with higher CD1000 polyol contents exhibited shorter self-healing times and dominant viscous properties due to the higher amount of free carbonate groups, significant carbonate-carbonate interactions, and low micro-phase separation. As the CD2000 polyol content in the PUs increased, slower kinetics and longer self-healing times and higher mechanical and adhesion properties were obtained due to a dominant rheological elastic behavior, soft segments with higher crystallinities, and greater micro-phase separation. All PUs synthesized with CD1000 + CD2000 blends exhibited a mixed phase due to interactions between polycarbonate soft segments of different lengths which favored the self-healing and mobility of the polymer chains, resulting in increased mechanical properties.

External Bleeding and Advanced Biomacromolecules for Hemostasis

Hemorrhage is a significant medical problem that has been an active area of research over the past few decades. The human body has a complex response to bleeding that leads to blood clot formation and hemostasis. Many biomaterials based on various biomacromolecules have been developed to either accelerate or improve the body's natural response to bleeding. This review examines the mechanisms of hemostasis, types of bleeding, and the in vitro or in vivo models and techniques used to study bleeding and hemostatic materials. It provides a detailed overview of the diverse hemostatic materials, including those that are highly absorbent, wet adhesives, and those that accelerate the biochemical cascade of blood clotting. These materials are currently marketed, under preclinical testing, or being researched. In exploring the latest advancements in hemostatic technologies, this paper highlights the potential of these materials to significantly improve bleeding control in clinical and emergency situations.

An integrally formed Janus supramolecular bio-gel with intelligent adhesion for multifunctional healthcare

Despite the rapid development of Janus adhesive hydrogels, most of them still entail complex fabrication processes and have the inherent flaws, such as fragility and instability, thereby restricting their biomedical applications. In this study, a novel Janus bio-gel with strong mechanical and intelligent adhesion functions is facilely fabricated through a gravity-driven settlement strategy, employing poly-cyclodextrin microspheres (PCDMs). This strategy takes advantage of the sedimentation behavior of PCDMs with various diameters to establish structural disparities on both sides of the Janus bio-gel, thereby resolving multiple predicaments including the tedious synthesis steps and poor bonding of multilayer hydrogels. Owing to the multiple dynamic interactions between polymers and PCDMs, the Janus supramolecular bio-gel demonstrates outstanding mechanical toughness (1.97 MJ/m3) and elongation rate (≈800 %). More attractively, the resulting Janus bio-gel exhibits remarkable adhesiveness (316.4 J/m2 for interfacial toughness) and adhesive differences that are exceed 50 times between the two surfaces. Furthermore, the Janus supramolecular bio-gel also has excellent antibacterial properties, biocompatibility, environmental stability, and multiple monitoring functions, accelerating wound stably healing and monitoring physiologic parameters on the skin. This strategy provides a straightforward and promising approach to directly achieve multifunctional integration for smart health management.

High strength "breathable" glycosilicone/Aloe vera polysaccharide-based gel dressing for efficient wound repair

Medical wound dressings are effective in protecting wounds, maintaining moisture, creating an optimal healing environment and accelerating wound healing. However, their deficiencies in mechanical properties, adhesion and prevention of adhesion to the wound bed have been identified as limiting factors for their therapeutic efficacy in wound healing. To address these issues, we prepared glycosilicone gel dressings consisting of hydrophobic polysiloxanes and highly hydrophilic polysaccharides via ester exchange and silicone hydrogen addition reactions. Silicone gel dressings exhibit skin-like "respiratory" properties, with good permeability to O2 and CO2. Additionally, elongation and other important parameters are similar to those of the skin, which provides a foundation for the application of silicone gels in the field of wound dressings. The introduction of Aloe vera polysaccharide (AP) results in the glycosilicone gel exhibiting certain mechanical properties, including a tensile strength of 0.35 MPa and an adhesion force of 10 N/m. Furthermore, a mouse model of total skin defect demonstrated that the wound healing rate of the mice on the 12th day was 98 %, which effectively promotes wound healing. Consequently, the glycosilicone gel is anticipated to be an optimal wound dressing.

A hemoadhican-based sponge with anti-heparin interference for treating uncontrollable massive hemorrhage

Hemoadhican (HD) is an exopolysaccharide with a branched structure that has been reported for its high hemostatic ability. In this study, a HD-based hemostatic sponge was prepared through ultrasonic dissolution and freeze-drying without using any cross-linking agent. The sponge could spontaneously cross-link using hydrogen bonds to form adhesive mud within 3 s upon contact with blood. This sponge-mud mixture adhered tightly to the wound tissue, forming a pressure-resistant physical barrier that captures and locks in blood cells and platelets. Simultaneously, the hydrophobic methyl groups of HD sponges repel blood inwardly, effectively sealing the wound. The brush-like structure of HD molecules was suspected to penetrate wet tissues through topological entanglement, thereby enhancing wet adhesion. Compared with gauze and gelatin sponges, HD sponges achieved more effective hemostasis in animal models using rat and rabbit femoral arteries. In particular, HD sponges showed excellent hemostasis in heparin-induced hemorrhage models in mice and pigs. The in vivo experiment demonstrated the excellent biosafety of the HD sponge. Conclusively, the HD sponge is a safe and efficient rapid hemostatic material that is expected to become an alternative material for clinical hemostatic procedures.

N-succinyl chitosan-oxidized hyaluronic acid-calcium chloride hydrogel as hemostatic agent

This study aims to develop an effective hemostatic agent in the management of irregular and deep wounds that can accelerate the hemostatic process. The background revealed the importance of rapid treatment of bleeding, with data showing a significant risk of death from blood loss. Current treatments use conventional hemostatic dressings, but they are less effective on irregular surgical wounds. Several studies have developed chitosan, hyaluronic acid, and CaCl2-based hydrogels that have hemostatic, regenerative, and antibacterial potential. However, there is still a need to develop hydrogels that are thermally stable, biocompatible, and able to accelerate the hemostatic process. This research will synthesize self-healing hydrogels by modifying the structure of chitosan and hyaluronic acid, using a certain ratio of ingredients. The research procedure was carried out with the preparation of N-succinyl chitosan (NSC) and oxidized hyaluronic acid (OHA) as the main ingredients which were then added with CaCl2 to produce self-healing injectable hydrogel. First, NSC and OHA were dissolved in phosphate buffer solution (pH = 7.4 PBS) to obtain 60 mg/mL NSC and OHA solution respectively. Calcium chloride was then dissolved in water to obtain 120 mg/mL CaCl2 solution. Then NSC-OHA-CaCl2-based hydrogels were synthesized through rapid and full solution mixing above room temperature with the composition of (1-1-0.1; 1-1-0.2; and 1-1-0.3). The targeted findings of this research are sample characterization results that explain and prove the best NSC-OHA-CaCl2 composition variation that can be used as a hemostatic agent for irregular and deep wounds. The results of the analysis obtained FTIR test data with the formation of C = N functional groups in the four samples; blood clotting time test for sample K0, K1, K2, and K3 with time 4.6, 3.33, 2.66, and 1 s; MTT assay with cell viability percentage of 77.82% for sample K0, 84.18% for sample K1, 89.30% for sample K2, and 89.50% for sample K3; hemolysis index percentage of 0.373% for sample K0, 0.555% for sample K1, 0.625% for sample K2, and 0.201% for sample K3; Viscosity test obtained data of 13 dPa s for sample K0, 15 dPa s for sample K1, 16 dPa s for sample K2, and 18 dPa. The injectability test yielded an injectability percentage of 96.84% for sample K0, 95.03% for sample K1, 94.78% dPa s for sample K2, and 94.61% for sample K3; the DSC test results of the four samples obtained a transition peak at the exothermic peak of 62.27°C for sample K0, 70.23°C for sample K1, 73.77°C for sample K2, and 74.49°C for sample K3; and the characteristic graph of the TGA test results, the weight profile of the hydrogel during heating which showed a mass change of 21.64 mg in sample K0, 16.89 mg in sample K1, 15.37 mg in sample K2, and 11.43 mg in sample K3 (°C).

A rapid and robust organ repair polyacrylamide/alginate adhesive hydrogel mediated via interfacial adhesion-trigger molecules

Adhesive hydrogels have been widely explored as tissue adhesives for wound sealing and repair. However, developing adhesive hydrogels with simple preparation techniques and strong adhesion to internal organs in a short time remains a challenge. In this study, we developed a strategy for robust and rapid tissue adhesion of internal organ sealing and repair by an interfacial adhesion-molecule triggered hydrogel system. In this system, polyphenol molecules act as adhesion-trigger reagents to achieve fast and strong adhesion of polyacrylamide/alginate hydrogels on the surface of wound tissue by rapidly forming abundant hydrogen bonds at the interface. The adhesion energy is significantly enhanced by 45 times under the mediation of polyphenol adhesion-trigger molecules, resulting in a robust (> 600 J m-2) tissue adhesion in just 30 s. This interfacial adhesion system demonstrates good biocompatibility, strong sealing performance on multiple organs (porcine heart, lung, stomach, and intestine), and excellent repair properties in gastric perforation wounds of rabbits in vivo. Moreover, immunocytochemical and transcriptomic analyses reveal that this interfacial adhesion system significantly promotes vascular regeneration and inhibits inflammatory responses during wound repairing. The proposed hydrogel provides a facile strategy for rapid and robust tissue adhesion, and shows potential applications in organ sealing and repair.

Lignin-modified cellulose nanofibers hydrogel under adjustable binary solvent systems with excellent adhesion, self-healing and anti-freeze properties

Hydrogels with remarkable flexibility have gained popularity as materials for current research. However, the unfavorable properties of short-term adhesion, susceptibility to damage, and freezing in low-temperature presented by conventional hydrogels have become bottlenecks for further applications. In this work, an anti-freezing hydrogel with excellent mechanical, adhesion, and self-healing properties were developed by constructing a persistent semiquinone/quinone-catechol redox equilibrium environment. The introduction of lignin-modified cellulose nanofibers (LCNFs) significantly improved the overall mechanical properties of the material, driven by strong hydrogen bond interactions. This enhancement was evident in the tensile properties (97.74 ± 1.72 kPa, 783 %) and compression properties (> 90 %). Within the internal network of the gel, the synergistic action of lignin and ammonium persulfate resulted in the production of catechol, which imparted the gel with excellent adhesion properties (28.26 ± 2.13 KPa) and broad adhesion applicability. In addition, the incorporation of ethylene glycol (EG) positively contributed to the strengthening of the gel while endowed with tunable anti-freezing properties. Given the exceptional advantages of the prepared hydrogels, they were used to assemble flexible strain sensors with outstanding sensitivity for monitoring human motions.

Robust tetra-armed poly (ethylene glycol)-based hydrogel as tissue bioadhesive for the efficient repair of meniscus tears

Repair and preservation of the injured meniscus has become paramount in clinical practice. However, the complexities of various clinic stitching techniques for meniscus repair pose challenges for grassroots doctors. Hence, there is a compelling interest in innovative therapeutic strategies such as bioadhesives. An ideal bioadhesive must cure quickly in aqueous and blood environments, bind strongly, endure arthroscopic washing pressures, and degrade appropriately for tissue regeneration. Here, we present a tetra-poly (ethylene glycol) (PEG)-based hydrogel bioadhesive, boasting high biocompatibility, ultrafast gelation, facile injectable operation, and favorable mechanical strength. In view of the synergistic effects of chemical anchor and physical chain entanglement to tightly bind the meniscus, a seamless interface was formed between the surrounding meniscal tissues and hydrogels, enabling the longitudinal tear of the meniscus fused in situ to withstand large tensile force with the adhesive strength of 541.5 ± 31.4 kPa and arthroscopic washout resistance of 29.4 kPa. Superior to existing commercial adhesives, ours allows sutureless application and arthroscopic assistance, without requiring specialized clinical skills. This research is expected to significantly impact our understanding of meniscal healing and ultimately promote a simpler process for achieving functional and structural recovery in torn menisci.

Endocytosis-Inspired Zwitterionic Gel Tape for High-Efficient and Sustainable Underoil Adhesion

Marine oil exploration is important yet greatly increases the risk of oil leakage, which will result in severe environment pollution and economic losses. It is an urgent need to develop effective underoil adhesives. However, realizing underoil adhesion is even harder than those underwater, due to the stubborn attachment of a highly viscous oil layer on target surface. Here, inspired by endocytosis, a tough gel tape composed of zwitterionic polymer network and zwitterionic surfactants is developed. The amphiphilic surfactants can form micelle to capture the oil droplets and transport them from the interface to gel via electrostatic attraction of polymer backbone, mimicking the endocytosis and achieving robust underoil adhesion. Benefiting from the oil-resistance of polymer backbone, the gel further realizes a combination of i) long-term adhesion with high durability, ii) repeated adhesion in oil, and iii) renewable adhesion efficiency after exhausted use. The tape exhibits an ultra-high adhesive toughness of 2446.86 J m-2 to stainless steel in silicone oil after 30 days' oil-exposure; such value of adhesive toughness surpasses many of those achieved in underwater adhesion and is greater than underoil adhesion performance of commercial tape. The strategy illustrated here will motivate the design of sustainable and efficient adhesives for wet environments.

Carbonized Polymer Dot-Tannic Acid Nanoglue: Tissue Reinforcement with Concurrent Fluorescent Tracking, Insulin Delivery, and Reactive Oxygen Species Regulation for Normal and Diabetic Wound Healing

Nanotizing biosealant components offer a multitude of chemical functionalities for superior adhesion-cohesion, delivering unique properties for comprehensive wound healing that are otherwise impossible to achieve using commercial variants. For the first time, a two-step controlled hydrothermal pyrolysis is reported to nanotize dopamine, phloroglucinol, and glutaraldehyde into carbon dot (CD) to be subsequently converted into carbonized polymer dot (CPD) with gelatin as a co-substrate. Chemical crosslinking of CD with gelatin through Schiff base formation before the second pyrolysis step ensures a complex yet porous polymeric network. The retention of chemical functionalities indigenous to CD substrates and gelatin along with the preservation of CD photoluminescence in CPD for optical tracking is achieved. A unique nanoformulation is created with the CPD through tannic acid (TA) grafting evolving CPD-TA nanoglue demonstrating ≈1.32 MPa strength in lap shear tests conducted on porcine skin, surpassing traditional bioadhesives. CPD-TA nanoglue uploaded insulin as chosen cargo disbursal at the wound site for healing normal and in vitro diabetic wound models using HEKa cells with extraordinary biocompatibility. Most importantly, CPD-TA can generate reactive oxygen species (ROS) and scavenge simultaneously under ambient conditions (23 W white LED or dark) for on-demand sterilization or aiding wound recovery through ROS scavenging.

Penetration enhancers strengthen tough hydrogel bioadhesion and modulate locoregional drug delivery

The human body possesses natural barriers, such as skin and mucosa, which limit the effective delivery of therapeutics and integration of medical devices to target tissues. Various strategies have been deployed to breach these barriers mechanically, chemically, or electronically. The development of various penetration enhancers (PEs) offers a promising solution due to their ability to increase tissue permeability using readily available reagents. However, existing PE-mediated delivery methods often rely on weak gel or liquid drug formulations, which are not ideal for sustained local delivery. Hydrogel adhesives that can seamlessly interface biological tissues with controlled drug delivery could potentially resolve these issues. Here, we demonstrate that tough adhesion between drug-laden hydrogels and biological tissue (e.g. skin and tumours) can lead to effective local delivery of drugs deep into targeted tissues by leveraging the enhanced tissue penetration mediated by PEs. The drug release profile of the hydrogel adhesives can be fine-tuned by further engineering the nanocomposite hydrogel matrix to elute chemotherapeutics from 2 weeks to 2 months. Using a 3D tumour spheroid model, we demonstrated that PEs increased the cancer-killing effectiveness of doxorubicin by facilitating its delivery into tumour microtissues. Therefore, the proposed tough bioadhesion and drug delivery strategy modulated by PEs holds promise as a platform technique to develop next-generation wearable and implantable devices for cancer management and regenerative medicine.

A tough Janus poly(vinyl alcohol)-based hydrogel for wound closure and anti postoperative adhesion

Traditional adhesive hydrogels perform well in tissue adhesion but they fail to prevent postoperative tissue adhesion. To address this challenge, a biodegradable Janus adhesive hydrogel (J-AH) was designed and fabricated by the assembly of three different functional layers including anti-adhesive layer, reinforceable layer, and wet tissue adhesive layer. Each layer of J-AH serves a specific function: the top zwitterionic polymeric anti-adhesive layer shows superior resistance to cell/protein and tissue adhesion; the middle poly(vinyl alcohol)/tannic acid reinforceable matrix layer endows the hydrogel with good mechanical toughness of ∼2.700 MJ/m3; the bottom poly(acrylic acid)/polyethyleneimine adhesive layer imparts tough adhesion (∼382.93 J/m2 of interfacial toughness) to wet tissues. In the rat liver and femoral injury models, J-AH could firmly adhere to the bleeding tissues to seal the wounds and exhibit impressive hemostatic efficiency. Moreover, in the in vivo adhesion/anti-adhesion assay of J-AH between the defected cecum and peritoneal walls, the top anti-adhesive layer can effectively inhibit undesired postoperative abdominal adhesion and inflammatory reaction. Therefore, this research may present a new strategy for the design of advanced bio-absorbable Janus adhesive hydrogels with multi-functions including tissue adhesion, anti-postoperative adhesion and biodegradation. STATEMENT OF SIGNIFICANCE: Despite many adhesive hydrogels with tough tissue adhesion capability have been reported, their proclivity for undesired postoperative adhesion remains a serious problem. The postoperative adhesion may lead to major complications and even endanger the lives of patients. The injectable hydrogels can cover the irregular wound and suppress the formation of postoperative adhesion. However, due to the lack of adhesive properties with tissue, it is difficult for the hydrogels to maintain on the wound surface, resulting in poor anti-postoperative adhesion effect. Herein, we design a Janus adhesive hydrogel (J-AH). J-AH integrates together robust wet tissue adhesion and anti-postoperative adhesion. Therefore, this research may present a new strategy for the design of advanced bio-absorbable Janus adhesive hydrogels.

Mechanically Interlocked [an]Daisy Chain Adhesives with Simultaneously Enhanced Interfacial Adhesion and Cohesion

Adhesives have been widely used to splice and repair materials to meet practical needs of humanity for thousands of years. However, developing robust adhesives with balanced adhesive and cohesive properties still remains a challenging task. Herein, we report the design and preparation of a robust mechanically interlocked [an]daisy chain network (DCMIN) adhesive by orthogonal integration of mechanical bonds and 2-ureido-4[1H]-pyrimidone (UPy) H-bonding in a single system. Specifically, the UPy moiety plays a dual role: it allows the formation of a cross-linked network and engages in multivalent interactions with the substrate for strong interfacial bonding. The mechanically interlocked [an]daisy chain, serving as the polymeric backbone of the adhesive, is able to effectively alleviate applied stress and uphold network integrity through synergistic intramolecular motions, and thus significantly improves the cohesive performance. Comparative analysis with the control made of the same quadruple H-bonding network but with non-interlocked [an]daisy chain backbones demonstrates that our DCMIN possesses superior adhesion properties over a wide temperature range. These findings not only contribute to a deep understanding of the structure-property relationship between microscopic mechanical bond motions and macroscopic adhesive properties but also provide a valuable guide for optimizing design principles of robust adhesives.

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

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

Asymmetric Wettability Hydrogel Surfaces for Enduring Electromyographic Monitoring

Hydrogel electrode interfaces have shown tremendous promise in the acquisition of surface electromyography (EMG) signals. However, the perspiration or moisture environments will trigger the deadhesion between hydrogel electrodes and human skin. Despite the hydrophobic/hydrophilic surfaces can perform the anti-moisture or adhesion respectively, it remains a challenge to integrally form a Janus hydrogel with homogeneous mechanical elasticity and electronic performance. Herein, a surface induction strategy is proposed to approach the hydrophobic/hydrophilic hydrogel surfaces. The hydrophobic interaction between surfactants and molds regulates the distribution of hydrophobic/hydrophilic monomers on the surface. The hydrophobic molds induce a hydrophilic hydrogel surface, while the hydrophilic molds induce a hydrophobic surface. It presents a new phenomenon of reversal wettability inducing and optional hydrogel surfaces. The integral Janus hydrogel can be easily obtained by the hydrophilic molds. Balance of adhesion, elasticity, and conductivity endows the hydrogel electrode patch with durable conformal adhesion and high-fidelity EMG signals even in the sweaty epidermis due to the asymmetric wettability surfaces. This hydrogel performs the quantitative description of muscle strength and accurate fatigue assessment. It offers a reliable candidate for future practical applications in continuous digital healthcare and intelligent human-machine interaction, even the Metaverse.

Programmable Tissue Folding Patterns in Structured Hydrogels

Folding of mucosal tissues, such as the tissue within the epithelium of the upper respiratory airways, is critical for organ function. Studying the influence of folded tissue patterns on cellular function is challenging mainly due to the lack of suitable cell culture platforms that can recreate dynamic tissue folding in vitro. Here, a bilayer hydrogel folding system, composed of alginate/polyacrylamide double-network (DN) and hyaluronic acid (HA) hydrogels, to generate static folding patterns based on mechanical instabilities, is described. By encapsulating human fibroblasts into patterned HA hydrogels, human bronchial epithelial cells form a folded pseudostratified monolayer. Using magnetic microparticles, DN hydrogels reversibly fold into pre-defined patterns and enable programmable on-demand folding of cell-laden hydrogel systems upon applying a magnetic field. This hydrogel construction provides a dynamic culture system for mimicking tissue folding in vitro, which is extendable to other cell types and organ systems.

Nature-Inspired Wet Drug Delivery Platforms

Nature has created various organisms with unique chemical components and multi-scale structures (e.g., foot proteins, toe pads, suckers, setose gill lamellae) to achieve wet adhesion functions to adapt to their complex living environments. These organisms can provide inspirations for designing wet adhesives with mediated drug release behaviors in target locations of biological surfaces. They exhibit conformal and enhanced wet adhesion, addressing the bottleneck of weaker tissue interface adhesion in the presence of body fluids. Herein, it is focused on the research progress of different wet adhesion and bioinspired fabrications, including adhesive protein-based adhesion and inspired adhesives (e.g., mussel adhesion); capillarity and Stefan adhesion and inspired adhesive surfaces (e.g., tree frog adhesion); suction-based adhesion and inspired suckers (e.g., octopus' adhesion); interlocking and friction-based adhesion and potential inspirations (e.g., mayfly larva and teleost adhesion). Other secreted protein-induced wet adhesion is also reviewed and various suckers for other organisms and their inspirations. Notably, one representative application scenario of these bioinspired wet adhesives is highlighted, where they function as efficient drug delivery platforms on target tissues and/or organs with requirements of both controllable wet adhesion and optimized drug release. Finally, the challenges of these bioinspired wet drug delivery platforms in the future is presented.

Robust and Lubricating Interface Semi-Interpenetrating Network on Inert Polymer Substrates Enabled by Subsurface-Initiated Polymerization

Lubricating hydrogel coatings on inert rubber and plastic surfaces significantly reduce friction and wear, thus enhancing material durability and lifespan. However, achieving optimal hydration lubrication typically requires a porous polymer network, which unfortunately reduces their mechanical strength and limits their applicability where robust durability and wear-resistance are essential. In the research, a hydrogel coating with remarkable wear resistance and surface stability is developed by forming a semi-interpenetrating polymer network with polymer substrate at the interface. By employing a good solvent swelling method, monomers, and photoinitiators are embedded within the substrates' subsurface, followed by in situ polymerization under ultraviolet light, creating a robust semi-interpenetrating and entangled network structure. This approach, offering a thicker energy-dissipating layer, outperforms traditional surface modifications in wear resistance while preserving anti-fatigue, hydrophilicity, oleophobicity, and other properties. Adaptable to various rubber and plastic substrates by using suitable solvents, this method provides an efficient solution for creating durable, lubricating surfaces, broadening the potential applications in multiple industries.