Biomimetic Microstructured Hydrogels with Thermal-Triggered Switchable Underwater Adhesion and Stable Antiswelling Property

Nature-inspired surface modification strategies for implantable devices

Medical and implantable devices are essential instruments in contemporary healthcare, improving patient quality of life and meeting diverse clinical requirements. However, ongoing problems such as bacterial colonization, biofilm development, foreign body responses, and insufficient device-tissue adhesion hinder the long-term effectiveness and stability of these devices. Traditional methods to alleviate these issues frequently prove inadequate, necessitating the investigation of nature-inspired alternatives. Biomimetic surfaces, inspired by the chemical and physical principles found in biological systems, present potential opportunities to address these challenges. Recent breakthroughs in manufacturing techniques, including lithography, vapor deposition, self-assembly, and three-dimensional printing, now permit precise control of surface properties at the micro- and nanoscale. Biomimetic coatings can diminish inflammation, prevent bacterial adherence, and enhance stable tissue integration by replicating the antifouling, antibacterial, and adhesive properties observed in creatures such as geckos, mussels, and biological membranes. This review emphasizes the cutting-edge advancements in biomimetic surfaces for medical and implantable devices, outlining their design methodologies, functional results, and prospective clinical applications. Biomimetic coatings, by integrating biological inspiration with advanced surface engineering, have the potential to revolutionize implantable medical devices, providing safer, more lasting, and more effective interfaces for prolonged patient benefit.

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

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

Versatile adhesive skin enhances robotic interactions with the environment

Electronic skins endow robots with sensory functions but often lack the multifunctionality of natural skin, such as switchable adhesion. Current smart adhesives based on elastomers have limited adhesion tunability, which hinders their effective use for both carrying heavy loads and performing dexterous manipulations. Here, we report a versatile, one-size-fits-all robotic adhesive skin using shape memory polymers with tunable rubber-to-glass phase transitions. The adhesion strength of our adhesive skin can be changed from minimal (~1 kilopascal) for sensing and handling ultralightweight objects to ultrastrong (>1 megapascal) for picking up and lifting heavy objects. Our versatile adhesive skin is expected to greatly enhance the ability of intelligent robots to interact with their environment.

Liquid Metal-Based Elastomer Composite with Selective Switchable Adhesion to Solids

Selective switchable adhesion has recently attracted much attention due to its wide applications in transfer printing, information transfer, and flexible electronics. However, selective adhesive materials often have a complex adhesion or preparation process, which limits their use. To overcome this problem, this study prepares a composite of liquid metal foam and polydimethylsiloxane (PDMS) with selective photocontrolled adhesion, which can directly adhere to solids at room temperature. Utilizing the photoinduced phase transition of liquid metals, solid adhesion can be regulated by changing the backing layer modulus of the adhesive layer. Since the phase transition process is gradually completed by heat transfer from the illuminated side to the backlight side that adheres to the solid, the melting area on the backlight side can be regulated by controlling the light time, which determines the adhesion regulation area. Therefore, the accuracy of the adhesion regulation can reach less than 0.9 mm without relying on the accuracy of the infrared light. Moreover, based on the selective switchable adhesion, the selective transfer of solids with different scales can be achieved at room temperature. The findings of this study may provide strategies for the simple preparation of selective adhesive materials and the improvement of control accuracy.

Degradable and Recyclable Hydrogels for Sustainable Bioelectronics

Hydrogel bioelectronics has been widely used in wearable sensors, electronic skin, human-machine interfaces, and implantable tissue-electrode interfaces, providing great convenience for human health, safety, and education. The generation of electronic waste from bioelectronic devices jeopardizes human health and the natural environment. The development of degradable and recyclable hydrogels is recognized as a paradigm for realizing the next generation of environmentally friendly and sustainable bioelectronics. This review first summarizes the wide range of applications for bioelectronics, including wearable and implantable devices. Then, the employment of natural and synthetic polymers in hydrogel bioelectronics is discussed in terms of degradability and recyclability. Finally, this work provides constructive thoughts and perspectives on the current challenges toward hydrogel bioelectronics, providing valuable insights and guidance for the future evolution of sustainable hydrogel bioelectronics.

Infant friendly adhesive film containing glucose for neonatal hypoglycemia

Neonatal hypoglycemia is a common disease in newborns, which can precipitate energy shortage and follow by irreversible brain and neurological injury. Herein, we present a novel approach for treating neonatal hypoglycemia involving an adhesive polyvinylpyrrolidone/gallic acid (PVP/GA) film loading glucose. The PVP/GA film with loose cross-linking can be obtained by mixing their ethanol solution and drying complex. When depositing this soft film onto wet tissue, it can absorb interfacial water to form a hydrogel with a rough surface, which facilitates tight contact between the hydrogel and tissue. Meanwhile, the functional groups in the hydrogels and tissues establish both covalent and non-covalent bonds, leading to robust bioadhesion. Moreover, the adhered PVP/GA hydrogel can be detached without damaging tissue as needed. Furthermore, the PVP/GA films exhibit excellent antibacterial properties and biocompatibility. Notably, these films effectively load glucose and deliver it to the sublingual tissue of newborn rabbits, showcasing a compelling therapeutic effect against neonatal hypoglycemia. The strengths of the PVP/GA film encompass excellent wet adhesion in the wet and highly dynamic environment of the oral cavity, on-demand detachment, antibacterial efficacy, biocompatibility, and straightforward preparation. Consequently, this innovative film holds promise for diverse biomedical applications, including but not limited to wearable devices, sealants, and drug delivery systems.

Hydrogel Rivet with Unidirectional Shape Morphing for Flexible Mechanical Assembly

Integrating diverse materials and functions into highly additive produce has piqued global interest due to the increasing demands of intelligent soft robotics. Nevertheless, existing assembly techniques, especially supramolecular assembly which heavily rely on precise chemical design and specific recognition, may prove inadequate when confronted with diverse external demands. Inspired by the traditional mechanical assembly, rivet connection, herein, a thermo-responsive hydrogel with unidirectional shape-morphing is fabricated and a stable mechanical assembly is constructed by emulating the rivet connection mechanism. This system employed poly(acrylamide-co-acrylic acid) [P(AAm-co-AAc)] to induce continuous swelling and hexylamine-modified polyvinyl alcohol (PVA-C6) as a molecular switch to control the swelling process. The hydrogel rivet, initially threaded through pre-fabricated hollows in two components. Subsequently, upon the disassociation of alkane chains the molecular switch would activate, inducing swelling and stable mechanical assembly via anchor structures. Moreover, to enhance the assembly strength, knots are introduced to enhance assembly strength, guiding localized stress release for programmed deformations. Additionally, the system can be remotely controlled using near-infrared light (NIR) by incorporating photo-thermal nanoparticles. This work presents a universal and efficient strategy for constructing stable mechanical assemblies without compromising overall softness, offering significant potential for the fabrication of integrated soft robots.

Mussel-inspired adhesive and anti-swelling hydrogels for underwater strain sensing

The application of hydrogels in an underwater environment is limited due to their swelling behavior and the existence of a hydration layer. In this study, a hydrogel based on poly(sulfobetaine methacrylate) (PSBMA), tannic acid (TA) and montmorillonite (MMT) was prepared with excellent anti-swelling properties and underwater self-adhesion properties. The PSBMA hydrogel has excellent anti-swelling properties due to the strong electrostatic interaction between charged groups of PSBMA chains. Inspired by marine mussels, tannic acid modified montmorillonite (TA@MMT) was introduced. Natural polyphenol tannic acid, as a catechol donor, provides a large number of catechol groups for hydrogels. Montmorillonite acts as the physical cross-linking point of PSBMA chains through electrostatic interaction to improve the cohesion of the hydrogel. By combining the adhesion mechanism of zwitterions and catechol, the hydrogel maintains adhesion in air and underwater environments. In addition, a strain sensor was prepared based on the PSBMA/TA@MMT hydrogel, which can closely fit the human skin and stably monitor different movements in air and in underwater environments. Through a Bluetooth communication system, long-distance information transmission can be achieved. Therefore, the PSBMA/TA@MMT hydrogel broadens the application prospect of wearable devices in the underwater environment.

Multifunctional Hydrogels for the Healing of Diabetic Wounds

The healing of diabetic wounds is hindered by various factors, including bacterial infection, macrophage dysfunction, excess proinflammatory cytokines, high levels of reactive oxygen species, and sustained hypoxia. These factors collectively impede cellular behaviors and the healing process. Consequently, this review presents intelligent hydrogels equipped with multifunctional capacities, which enable them to dynamically respond to the microenvironment and accelerate wound healing in various ways, including stimuli -responsiveness, injectable self-healing, shape -memory, and conductive and real-time monitoring properties. The relationship between the multiple functions and wound healing is also discussed. Based on the microenvironment of diabetic wounds, antibacterial, anti-inflammatory, immunomodulatory, antioxidant, and pro-angiogenic strategies are combined with multifunctional hydrogels. The application of multifunctional hydrogels in the repair of diabetic wounds is systematically discussed, aiming to provide guidelines for fabricating hydrogels for diabetic wound healing and exploring the role of intelligent hydrogels in the therapeutic processes.

Mechanically Reinforced and Injectable Universal Adhesive Based on a PEI-PAA/Alg Dual-Network Hydrogel Designed by Topological Entanglement and Catechol Chemistry

Universal adhesion of hydrogels to diverse materials is essential to their extensive applications. Unfortunately, tough adhesion of wet surfaces remains an urgent challenge so far, requiring robust cohesion strength for effective stress dissipation. In this work, a dual-network hydrogel polyethylenimine-poly(acrylic acid)/alginate (PEI-PAA/Alg) with excellent mechanical strength is realized via PEI-PAA complex and calcium alginate coordination for universal adhesion by the synergistic effort of topological entanglement and catechol chemistry. The dual networks of PEI-PAA/Alg provide mechanically reinforced cohesion strength, which is sufficient for energy dissipation during adhesion with universal materials. After the integration of mussel-inspired dopamine into PAA or Alg, the adhesive demonstrates further improved adhesion performance with a solid adherend and capability to bond cancellous bones. Notably, the dopamine-modified adhesive exhibits better instant adhesion and reversibility with wet surfaces compared with commercial fibrin. Adhesion interfaces are investigated by SEM and micro-FTIR to verify the effectiveness of strategies of topological entanglement. Furthermore, the adhesive also possesses great injectability, stability, tissue adhesion, and biocompatibility. In vivo wound healing and histological analysis indicate that the hydrogel can promote wound closure, epidermis regeneration, and tissue refunctionalization, implying its potential application for bioadhesive and wound dressing.

A Biodegradable, Adhesive, and Stretchable Hydrogel and Potential Applications for Allergic Rhinitis and Epistaxis

Bioadhesive hydrogels have attracted considerable attention as innovative materials in medical interventions and human-machine interface engineering. Despite significant advances in their application, it remains critical to develop adhesive hydrogels that meet the requirements for biocompatibility, biodegradability, long-term strong adhesion, and efficient drug delivery vehicles in moist conditions. A biocompatible, biodegradable, soft, and stretchable hydrogel made from a combination of a biopolymer (unmodified natural gelatin) and stretchable biodegradable poly(ethylene glycol) diacrylate is proposed to achieve durable and tough adhesion and explore its use for convenient and effective intranasal hemostasis and drug administration. Desirable hemostasis efficacy and enhanced therapeutic outcomes for allergic rhinitis are accomplished. Biodegradation enables the spontaneous removal of materials without causing secondary damage and minimizes medical waste. Preliminary trials on human subjects provide an essential foundation for practical applications. This work elucidates material strategies for biodegradable adhesive hydrogels, which are critical to achieving robust material interfaces and advanced drug delivery platforms for novel clinical treatments.

Femtosecond Laser Direct Writing of Gecko-Inspired Switchable Adhesion Interfaces on a Flexible Substrate

Biomimetic switchable adhesion interfaces (BSAIs) with dynamic adhesion states have demonstrated significant advantages in micro-manipulation and bio-detection. Among them, gecko-inspired adhesives have garnered considerable attention due to their exceptional adaptability to extreme environments. However, their high adhesion strength poses challenges in achieving flexible control. Herein, we propose an elegant and efficient approach by fabricating three-dimensional mushroom-shaped polydimethylsiloxane (PDMS) micropillars on a flexible PDMS substrate to mimic the bending and stretching of gecko footpads. The fabrication process that employs two-photon polymerization ensures high spatial resolution, resulting in micropillars with exquisite structures and ultra-smooth surfaces, even for tip/stem ratios exceeding 2 (a critical factor for maintaining adhesion strength). Furthermore, these adhesive structures display outstanding resilience, enduring 175% deformation and severe bending without collapse, ascribing to the excellent compatibility of the micropillar's composition and physical properties with the substrate. Our BSAIs can achieve highly controllable adhesion force and rapid manipulation of liquid droplets through mechanical bending and stretching of the PDMS substrate. By adjusting the spacing between the micropillars, precise control of adhesion strength is achieved. These intriguing properties make them promising candidates for various applications in the fields of microfluidics, micro-assembly, flexible electronics, and beyond.

Controllable adhesion behavior in underwater environments

Microstructure adhesive pads can effectively manipulate objects in underwater environments. Current adhesive pads can achieve adhesion and separation with rigid substrates underwater; however, challenges remain in the control of adhesion and detachment of flexible materials. Additionally, underwater object manipulation necessitates considerable pre-pressure and is sensitive to water temperature fluctuations, potentially causing object damage and complicating adhesion and detachment processes. Thus, we present a novel, controllable adhesive pad inspired by the functional attributes of microwedge adhesive pads, combined with a mussel-inspired copolymer (MAPMC). In the context of underwater applications for flexible materials, the use of a microstructure adhesion pad with microwedge characteristics (MAPMCs) is a proficient approach to adhesion and detachment operations. This innovative method relies on the precise manipulation of the microwedge structure's collapse and recovery during its operation, which serves as the foundation for its efficacy in such environments. MAPMCs exhibit self-recovering elasticity, water flow interaction, and tunable underwater adhesion and detachment. Numerical simulations elucidate the synergistic effects of MAPMCs, highlighting the advantages of the microwedge structure for controllable, non-damaging adhesion and separation processes. The integration of MAPMCs into a gripping mechanism allows for the handling of diverse objects in underwater environments. Furthermore, by merging MAPMCs and a gripper within a linked system, our approach enables automatic, non-damaging adhesion, manipulation, and release of a soft jellyfish model. The experimental results indicate the potential applicability of MACMPs in underwater operations.

Polyacrylic Acid Hydrogel Coating for Underwater Adhesion: Preparation and Characterization

Underwater adhesion involves bonding substrates in aqueous environments or wet surfaces, with applications in wound dressing, underwater repairs, and underwater soft robotics. In this study, we investigate the underwater adhesion properties of a polyacrylic acid hydrogel coated substrate. The underwater adhesion is facilitated through hydrogen bonds formed at the interface. Our experimental results, obtained through probe-pull tests, demonstrate that the underwater adhesion is rapid and remains unaffected by contact pressure and pH levels ranging from 2.5 to 7.0. However, it shows a slight increase with a larger adhesion area. Additionally, we simulate the debonding process and observe that the high-stress region originates from the outermost bonding region and propagates towards the center, spanning the thickness of the target substrate. Furthermore, we showcase the potential of using the underwater adhesive hydrogel coating to achieve in-situ underwater bonding between a flexible electronic demonstration device and a hydrogel contact lens. This work highlights the advantages of employing hydrogel coatings in underwater adhesion applications and serves as inspiration for the advancement of underwater adhesive hydrogel coatings capable of interacting with a wide range of substrates through diverse chemical and physical interactions at the interface.

A sandwiched patch toward leakage-free and anti-postoperative tissue adhesion sealing of intestinal injuries

Ideal repair of intestinal injury requires a combination of leakage-free sealing and postoperative antiadhesion. However, neither conventional hand-sewn closures nor existing bioglues/patches can achieve such a combination. To this end, we develop a sandwiched patch composed of an inner adhesive and an outer antiadhesive layer that are topologically linked together through a reinforced interlayer. The inner adhesive layer tightly and instantly adheres to the wound sites via -NHS chemistry; the outer antiadhesive layer can inhibit cell and protein fouling based on the zwitterion structure; and the interlayer enhances the bulk resilience of the patch under excessive deformation. This complementary trilayer patch (TLP) possesses a unique combination of instant wet adhesion, high mechanical strength, and biological inertness. Both rat and pig models demonstrate that the sandwiched TLP can effectively seal intestinal injuries and inhibit undesired postoperative tissue adhesion. The study provides valuable insight into the design of multifunctional bioadhesives to enhance the treatment efficacy of intestinal injuries.

Bio-inspired adhesive hydrogel for biomedicine-principles and design strategies

The adhesiveness of hydrogels is urgently required in various biomedical applications such as medical patches, tissue sealants, and flexible electronic devices. However, biological tissues are often wet, soft, movable, and easily damaged. These features pose difficulties for the construction of adhesive hydrogels for medical use. In nature, organisms adhere to unique strategies, such as reversible sucker adhesion in octopuses and nontoxic and firm catechol chemistry in mussels, which provide many inspirations for medical hydrogels to overcome the above challenges. In this review, we systematically classify bioadhesion strategies into structure-related and molecular-related ones, which cover almost all known bioadhesion paradigms. We outline the principles of these strategies and summarize the corresponding designs of medical adhesive hydrogels inspired by them. Finally, conclusions and perspectives concerning the development of this field are provided. For the booming bio-inspired adhesive hydrogels, this review aims to summarize and analyze the various existing theories and provide systematic guidance for future research from an innovative perspective.

Cryopolymerized Polyampholyte Gel with Antidehydration, Self-Healing, and Shape-Memory Properties for Sustainable and Tunable Sensing Electronics

Hydrogel-based wearable flexible electronics are attracting tremendous interest for use in human healthcare. However, many of the existing hydrogel electronics are often susceptible to dehydration, leading to weakened stretchability and inaccurate signal extraction. Besides, hydrogels are desired to be much smarter for self-repairing physical damage and enabling performance manipulation. Herein, we develop a kind of cryopolymerized polyampholyte gels with the multifunctionality of antidehydration, self-healing, and shape-memory for wearable sensing electronics. The antidehydration property is enabled by the incorporation of glycerol, endowing the sensing electronics with excellent stretchability and strain-sensing performance in long-term monitoring. The ionic bonds in the polyampholyte gel possess a dynamic feature regulated by alternant NaCl(aq) and H₂O treatments, laying the foundation for self-healing and shape-memory. As a result, the sensing electronics can automatically repair physical damages without any sacrifice in sensing performance, after healing both conductivity and strain-sensing performance could return to the initial levels. The shape-memory function enables the temporal adjustment of the initial state of the sensing electronics; both the conductivity and sensing performance, for instance, signal intensity, can be manually manipulated. In all, the cryopolymerized polyampholyte gels with antidehydration, self-healing, and shape-memory properties can be an inspiration to develop sustainable and tunable gel-based electronics for human motion monitoring.

Biomimetic Microstructured Antifatigue Fracture Hydrogel Sensor for Human Motion Detection with Enhanced Sensing Sensitivity

Antifatigue fracture performance and high sensing sensitivity are key characteristics for hydrogel sensors used in flexible electronic applications. Herein, inspired by human muscle tissues and epidermal skin tissues, an effective and straightforward strategy is proposed to fabricate hydrogel sensors for detecting human motion with antifatigue fracture performance and high sensing sensitivity. The crystalline regions and orientation along the stretching direction of cellulose nanofiber@carbon nanotube nanohybrids in the hydrogels provide antifatigue fracture performance (the crack does not expand after 2000 stretching cycles, and the fatigue threshold was calculated to be 187 J/m2), which protects hydrogels from severe damage during long-term use. In addition, the microstructured surfaces of the hydrogels with a random height distribution increase the contact area and improve the response to weak stimuli, resulting in a sensing sensitivity of 1.11 kPa-1, 18 times higher than that of a flat hydrogel. This sensing sensitivity is higher than those of most of the hydrogel-based pressure sensors that have been reported earlier. By integrating antifatigue fracture performance and enhanced sensing sensitivity, biomimetic microstructured hydrogel sensors show great potential for use in future flexible electronic applications.

Janus membranes with asymmetric cellular adhesion behaviors for regenerating eardrum perforation

The tympanic membrane plays an important role in the human hearing system, which is easily perforated under unfavorable conditions, leading to loss of hearing and otitis media. Many autologous materials and artificial materials have been used to repair a perforated tympanic membrane, but these materials sometimes can cause severe hearing loss because of their adhesion to the ossicle during the healing process and the postoperative process. Herein, we report Janus membranes with asymmetric cellular adhesion behaviors for regenerating the eardrum. These Janus membranes are constructed by co-depositing a tannic acid (TA)/3-aminopropyltriethoxysilane (APTES) coating on one surface of the polypropylene microfiltration membrane. Cellular experiments indicate that the Janus membranes have good biocompatibility and asymmetric cellular adhesion properties. The repair of the tympanic membrane perforation experiment and laser Doppler vibrometer (LDV) measurements prove that the hydrophilic surface of Janus membranes repairs perforated eardrums, and meanwhile the hydrophobic surface can avoid adhering to the inner ear tissue for reducing hearing loss. The Janus membranes have good prospects in the treatment of tympanic membrane perforation.

Application and Development of Shape Memory Micro/Nano Patterns

Shape memory polymers (SMPs) are a class of smart materials that change shape when stimulated by environmental stimuli. Different from the shape memory effect at the macro level, the introduction of micro-patterning technology into SMPs strengthens the exploration of the shape memory effect at the micro/nano level. The emergence of shape memory micro/nano patterns provides a new direction for the future development of smart polymers, and their applications in the fields of biomedicine/textile/micro-optics/adhesives show huge potential. In this review, the authors introduce the types of shape memory micro/nano patterns, summarize the preparation methods, then explore the imminent and potential applications in various fields. In the end, their shortcomings and future development direction are also proposed.

Advances in Ultrasound-Responsive Hydrogels for Biomedical Applications

Various intelligent hydrogels have been developed for biomedical applications because they can achieve multiple, variable, controllable and reversible changes in their shape and properties in a spatial and temporal manner,...

Semicrystalline polymer networks with a swelling-enhanced water-triggered two-way shape-memory effect for programmable deformation and smart actuation

A simple, effective and universal strategy is proposed to fabricate a water-triggered two-way shape-memory polymer with the highest angle reversibility of 45.2%, which can be applied as a soft gripper and water level monitor.