Modification of Hydrophobic Hydrogels into a Strongly Adhesive and Tough Hydrogel by Electrostatic Interaction

Hydrogel adhesives for tissue recovery

Hydrogel adhesives (HAs) are promising and rewarding tools for improving tissue therapy management. Such HAs had excellent properties and potential applications in biological tissues, such as suture replacement, long-term administration, and hemostatic sealing. In this review, the common designs and the latest progress of HAs based on various methodologies are systematically concluded. Thereafter, how to deal with interfacial water to form a robust wet adhesion and how to balance the adhesion and non-adhesion are underlined. This review also provides a brief description of gelation strategies and raw materials. Finally, the potentials of wound healing, hemostatic sealing, controlled drug delivery, and the current applications in dermal, dental, ocular, cardiac, stomach, and bone tissues are discussed. The comprehensive insight in this review will inspire more novel and practical HAs in the future.

High-strength bone polyurethane adhesive with rapid curing for bone tissue injury repair

Bone tissue adhesives have advantages such as preventing stress shielding and secondary surgical infections, fixing small bone fragments, easing operations, and enhancing wound adaptability. These methods can be applied for the traumatic repair of comminuted fractures. Currently, commercial tissue adhesives fail to meet the biological safety and mechanical strength requirements of bone tissue adhesives. To address this situation, we developed and screened a rapidly cured high-strength polyurethane bone adhesive. Research has shown that polyurethane bone adhesives have shorter curing times (236 s to 273 s), higher tensile moduli (425.88 MPa to 666.38 MPa), compressive moduli (214.67 MPa to 450.66 MPa), and adhesive strengths (0.92 MPa to 5.86 MPa). It can withstand cyclic stresses ranging from 0.01 MPa to 1 MPa for 1000 cycles. Polyurethane bone adhesive surpasses the inadequate adhesive performance and in vivo repair functionality of existing commercial bone cements, achieving effective repair of bone tissue injuries. Furthermore, we developed an unsaturated ester-modified secondary amine curing agent based on the Michael addition reaction, enabling rapid and safe curing of bone polyurethane adhesives and thereby providing a novel and effective repair solution for bone tissue injuries.

Hydrogel tissue adhesive: Adhesion strategy and application

Hydrogel tissue adhesives have emerged as a promising alternative to conventional wound closure methods such as sutures and staples due to their operational simplicity demonstrated biocompatibility and capacity for multifunctional integration. However, complex and variable tissue microenvironments and dynamic adhesion surfaces still challenge the actual adhesion performance of adhesives, especially natural polymer-based adhesives. In addition, to expand the application of adhesives in biomedical fields, there is an urgent need to further improve tissue adhesion performance through composition design, adhesion mechanism research and bioeffect development. This review focuses on the adhesive properties of adhesives and their applications in biomedical fields. Adhesion-cohesion equilibria, forms of adhesion failure, methods for improving cohesion and various interfacial adhesion mechanisms are presented. Moreover, practical biomedical applications of tissue adhesives are reviewed, focusing on skin, heart, stomach, liver, and cornea. Finally, this review looks ahead to a new generation of multi-functional, strong adhesion tissue adhesives, in the hope of providing inspiration to those working in the field.

Self-healing, deformable and safe integrated supercapacitor enabled by synergistic effect of multiple physical interactions in gel polymer electrolyte with dual-role Co2

With the rapid development of wearable electronic devices, flexible supercapacitors have gained strong interest. However, traditional sandwich supercapacitors have weak interfacial binding, resulting in high interface resistance and poor deformability. Herein, a self-healing integrated supercapacitor based on a polyacrylic acid-polyisodecyl methacrylate-CoSO4 gel polymer electrolyte (GPE) was developed. By incorporating ion coordination into a hydrophobic association network, a double network structure was formed, endowing the GPE with remarkable mechanical properties and self-healing abilities. Specifically, Co2+ ions functioned both as charge carrier and crosslinker, simultaneously enhancing the electrochemical (2.87 S/m) and mechanical (0.262 MPa) properties of the GPE. In situ growth of polyaniline electrode material on the GPE surface resulted in an integrated supercapacitor with a continuous morphology at the electrode/electrolyte interface, minimizing interface resistance and improving electrochemical performance. The supercapacitor exhibits high specific capacitance, exceptional cyclic stability, superior deformability and security due to the unique integrated structure. Furthermore, it demonstrates remarkable electrochemical and self-healing properties even at quite low temperature. Overall, this work offers a promising approach for reliable self-healing energy storage devices with high performance and adaptability to complex usage conditions.

Adhesive conductive wood-based hydrogel with high tensile strength as a flexible sensor

Conductive hydrogels have promising applications for flexible strain sensors. However, most hydrogels have poor tensile strength and are susceptible to damage, significantly impeding their potential for further application. Wood has been used to reinforce hydrogels, significantly enhancing their strength and dimensional stability. However, wood-based hydrogels generally lack adhesive properties or exhibit low self-adhesion. To address this issue, we introduced acryloyloxyethyltrimethyl ammonium chloride (DAC) into the hydrogel network through graft aggregation. The resulting electrostatic interactions significantly enhanced the adhesion of the wood-based hydrogel up to 270 kPa (for glass) and concurrently strengthened its cohesion. The prepared novel wood-based hydrogel (WDDH) exhibited high tensile strength (3.38 MPa), low-swelling ratio (only 2 % longitudinal), and high tensile strain (274.40 %). When WDDH was used as the wearable strain sensor, it showed a gauge factor of approximately 4.94. The device effectively captured and detected human movements, including finger and joint flexion, walking patterns, and hydration habits. The objective of this research is to develop a wood-based hydrogel with enhanced mechanical strength, adhesive properties, and flexibility for use in wearable sensors. This study provides insight into the development of flexible sensor hydrogels with improved adhesion properties using biomass materials.

Adhesive and Conductive Hydrogels for the Treatment of Myocardial Infarction

Myocardial infarction (MI) is a leading cause of mortality among cardiovascular diseases. Following MI, the damaged myocardium is progressively being replaced by fibrous scar tissue, which exhibits poor electrical conductivity, ultimately resulting in arrhythmias and adverse cardiac remodeling. Due to their extracellular matrix-like structure and excellent biocompatibility, hydrogels are emerging as a focal point in cardiac tissue engineering. However, traditional hydrogels lack the necessary conductivity to restore electrical signal transmission in the infarcted regions. Imparting conductivity to hydrogels while also enhancing their adhesive properties enables them to adhere closely to myocardial tissue, establish stable electrical connections, and facilitate synchronized contraction and myocardial tissue repair within the infarcted area. This paper reviews the strategies for constructing conductive and adhesive hydrogels, focusing on their application in MI repair. Furthermore, the challenges and future directions in developing adhesive and conductive hydrogels for MI repair are discussed.

Photocrosslinking of hyaluronic acid-based hydrogels through biotissue barriers

Photocrosslinkable hydrogels based on hyaluronic acid are promising biomaterials high in demand in tissue engineering. Typically, hydrogels are photocured under the action of UV or blue light strongly absorbed by biotissues, which limits prototyping under living organism conditions. To overcome this limitation, we propose the derivatives of well-known photosensitizers, namely chlorin p6, chlorin e6 and phthalocyanine, as those for radical polymerization in the transparency window of biotissues. Taking into account the efficiency of radical generation and dark and light cell toxicity, we evaluated water miscible pyridine phthalocyanine as a promising initiator for the intravital hydrogel photoprinting of hyaluronic acid glycidyl methacrylate (HAGM) under irradiation near 670 nm. Coinitiators (dithiothreitol or 2-mercaptoethanol) reduce the irradiation dose required for HAGM crosslinking from ∼405 J cm-2 to 80 J cm-2. Patterning by direct laser writing using a scanning 675 nm laser beam was performed to demonstrate the formation of complex shape structures. Young's moduli typical of soft tissue (∼270-460 kPa) were achieved for crosslinked hydrogels. The viability of human keratinocytes HaCaT cells within the photocrosslinking process was shown. To demonstrate scaffolding across the biotissue barrier, the subcutaneously injected photocomposition was crosslinked in BALB/c mice. The safety of the irradiation dose of 660-675 nm light (100 mW cm-2, 15 min) and the non-toxicity of the hydrogel components were confirmed by histomorphologic analysis. The intravitally photocrosslinked scaffolds maintained their shape and size for at least one month, accompanied by slow biodegradation. We conclude that the proposed technology provides a lucrative opportunity for minimally invasive scaffold formation through biotissue barriers.

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.

High-Strength Anisotropic Fluorescent Hydrogel Based on Solvent Exchange for Patterning

Aggregation-induced emission (AIE)-active fluorescent hydrogel materials have found extensive applications in soft robotics, wearable electronics, information encryption, and biomedicine. Nevertheless, it continues to be difficult to create hydrogels that are both highly luminescent and possess strong mechanical capabilities. This study introduces a combined approach of prestretching and solvent exchange to create anisotropic luminous hydrogels made of poly(methacrylic acid-methacrylamide). This method restricts the intrachain rotation of AIE molecules and adjusts the orientation of the polymer network. The increased luminescence and mechanical qualities are determined to be caused by the clustering of AIE molecules, the creation of the associated hydrophobic phase and the asymmetrical polymer network. The fluorescent hydrogels exhibit exceptional mechanical characteristics, including a high fracture stress of 5.97 MPa, an outstanding elastic modulus of 93.97 MPa, and a fracture toughness of 7.21 MJ/m3. Furthermore, the AIE fluorescent hydrogels demonstrate outstanding water retention, antiswelling capabilities, and a writing function for solvent-regulated fluorescent information. This work presents a highly efficient technique for creating anisotropic hydrogels with changeable luminescence properties, which have the potential to be used in several applications, including information encryption, flexible sensors, and soft robots.

Recent research progresses of bioengineered biliary stents

Bile duct lesion, including benign (eg. occlusion, cholelithiasis, dilatation, malformation) and malignant (cholangiocarcinoma) diseases, is a frequently encountered challenge in hepatobiliary diseases, which can be repaired by interventional or surgical procedures. A viable cure for bile duct lesions is implantation with biliary stents. Despite the placement achieved by current clinical biliary stents, the creation of functional and readily transplantable biliary stents remains a formidable obstacle. Excellent biocompatibility, stable mechanics, and absorbability are just a few benefits of using bioengineered biliary stents, which can also support and repair damaged bile ducts that drain bile. Additionally, cell sources & organoids derived from the biliary system that are loaded onto scaffolds can encourage bile duct regeneration. Therefore, the implantation of bioengineered biliary stent is considered as an ideal treatment for bile duct lesion, holding a broad potential for clinical applications in future. In this review, we look back on the development of conventional biliary stents, biodegradable biliary stents, and bioengineered biliary stents, highlighting the crucial elements of bioengineered biliary stents in promoting bile duct regeneration. After providing an overview of the various types of cell sources & organoids and fabrication methods utilized for the bioengineering process, we present the in vitro and in vivo applications of bioengineered biliary ducts, along with the latest advances in this exciting field. Finally, we also emphasize the ongoing challenges and future development of bioengineered biliary stents.

Cation-π Interactions Based Conductive Hydrogels with Slide-Ring Structure Toward Super Long-Time in-air/Underwater Linear Sensing and Communication

Conductive hydrogels (CHs) are attracted more attention in the flexible wearable sensors field, however, how to stably apply CHs underwater is still a big challenge. In order to achieve the usage of CHs in aquatic environments, the integrated properties such as water retention ability, resistance to swelling, toughness, adhesiveness, linear GF sensing, and long-term usage are necessary to consider, but rarely reported in the previous reports. This paper proposes CHs prepared using cationic and aromatic monomers along with polyrotaxanes-based crosslinkers. Due to the intermolecular cation-π interactions and topological slide-ring-based polyrotaxanes, the CHs exhibit good mechanical performance, adhesive nature, and anti-swelling properties. The presence of slide-ring-based topological architecture effectively mitigates stress concentration. Additionally, the encapsulation of PA allows CHs to maintain functionality even after 240 days of direct placement at room temperature. Notably, the designed CHs exhibit linear sensitivity in detecting land/underwater human motions, and serve as Morse code signal transmitters for information transmission. Thus, the designed CHs may have broad applications in the underwater wearable sensors field.

Introductory Review of Soft Implantable Bioelectronics Using Conductive and Functional Hydrogels and Hydrogel Nanocomposites

Interfaces between implantable bioelectrodes and tissues provide critical insights into the biological and pathological conditions of targeted organs, aiding diagnosis and treatment. While conventional bioelectronics, made from rigid materials like metals and silicon, have been essential for recording signals and delivering electric stimulation, they face limitations due to the mechanical mismatch between rigid devices and soft tissues. Recently, focus has shifted toward soft conductive materials, such as conductive hydrogels and hydrogel nanocomposites, known for their tissue-like softness, biocompatibility, and potential for functionalization. This review introduces these materials and provides an overview of recent advances in soft hydrogel nanocomposites for implantable electronics. It covers material strategies for conductive hydrogels, including both intrinsically conductive hydrogels and hydrogel nanocomposites, and explores key functionalization techniques like biodegradation, bioadhesiveness, injectability, and self-healing. Practical applications of these materials in implantable electronics are also highlighted, showcasing their effectiveness in real-world scenarios. Finally, we discuss emerging technologies and future needs for chronically implantable bioelectronics, offering insights into the evolving landscape of this field.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

A novel strategy to construct hydrogels with anti-swelling and water-retention abilities by covalent surface modification

Hydrogels have been widely used in various fields due to their diverse properties and flexible preparation methods. However, limited by the open network structure, hydrogels inevitably lose water in air or absorb water in aqueous solution, resulting in the loss of intrinsic functions, which severely hinders their practical applications. To address this issue, a general strategy was developed by subsequently modifying the surface of hydrogels with branched polyethyleneimine (PEI) and (3-(methacryloxy)propyl)trimethoxysilane (MPS) to covalently construct a dense cross-linked siloxane layer on the hydrogel surface. As a proof of concept, poly(2-(dimethylamino)ethyl methacrylate)/sodium alginate (PDMAEMA/SA) hydrogels were chosen as the model hydrogels to verify the feasibility of this strategy. The hydrogels adsorbed PEI to form amino-rich surfaces through hydrogen bonding, followed by covalently grafting MPS through rapid and catalyst-free mutual chemical reactions between acrylates and amine groups, as well as the hydrolysis of MPS. After modification, robust hydrophobic surfaces were successfully fabricated on the hydrophilic hydrogels. The modified hydrogels exhibited extraordinary anti-swelling and water-retention abilities. As the most typical intrinsic properties of hydrogels, the conductivity and sensing performance were well preserved. The strategy reported here provides a new insight into the construction of hydrogels with anti-swelling and water-retention abilities.

Robust, Self-Healing, and Multi-Use Poly(Urethane-Urea-Imide) Elastomer as a Durable Adhesive for Thermal Interface Materials

Currently, research on thermal interface materials (TIMs) is primarily focused on enhancing thermal conductivity. However, strong adhesion and multifunctionality are also important characteristics for TIMs when pursing more stable interface heat conduction. Herein, a novel poly(urethane-urea-imide) (PUUI) elastomer containing abundant dynamic hydrogen bonds network and reversible disulfide linkages is successfully synthesized for application as a TIM matrix. The PUUI can self-adapt to the metal substrate surface at moderate temperatures (80 °C) and demonstrates a high adhesion strength of up to 7.39 MPa on aluminum substrates attributed its noncovalent interactions and strong intrinsic cohesion. Additionally, the PUUI displays efficient self-healing capability, which can restore 94% of its original mechanical properties after self-healing for 6 h at room temperature. Furthermore, PUUI composited with aluminum nitride and liquid metal hybrid fillers demonstrates a high thermal conductivity of 3.87 W m-1 K-1 while maintaining remarkable self-healing capability and adhesion. When used as an adhesive-type TIM, it achieves a low thermal contact resistance of 22.1 mm2 K W-1 at zero pressure, only 16.7% of that of commercial thermal pads. This study is expected to break the current research paradigm of TIMs and offers new insights for the development of advanced, reliable, and sustainable TIMs.

Ultrafast Self-Healing, Superstretchable, and Ultra-Strong Polymer Cluster-Based Adhesive Based on Aromatic Acid Cross-Linkers for Excellent Hydrogel Strain Sensors

Synthetic hydrogel strain sensors rarely exhibit a comprehensive combination of mechanical properties such as ultra-stretchability, ultrafast self-healing, and high sensitivity. Herein, seven small molecule enhanced mechanical behaviors of polymer-cluster based hydrogels are demonstrated. The oxidized polyethyleneimine/polymeric acrylic acid (ohPEI/PAA) hydrogels with aromatic formic acids as supramolecular cross-linkers are prepared by simultaneous formation of ohPEI polymer clusters and PAA upon the addition of ammonium persulfate. The optimized hydrogel adhesive exhibits comprehensive excellent properties, such as high extensibility (up to 12 298%), real-time mechanical self-healing capability (<1 s, 93% efficiency), high uniformity, underwater adhesivity, and water-sealing ability. The proper binding strength of hydrogel and skin (47 kPa) allows the hydrogel to be utilized as highly sensitive (gauge factor:16.08), highly conductive (2.58 mS cm-1), and underwater strain sensors. Specially, the adhesive strength of the adhesive to wood after dehydration is extremely high, reaching up to 29.59 MPa. Additionally, when glycerol is introduced, the obtained gel maintains the physical properties even at harsh-temperature conditions (-40 to 80 °C). It presents that multiple and hierarchical non-covalent interactions including multiple hydrogen bonding interactions, π-π stacking, electrostatic interactions, and dipole-dipole interactions of polymer clusters, allow for the energy dissipation and contribute to the excellent performance of the hydrogel.

Natural Polymer-Polyphenol Bioadhesive Coacervate with Stable Wet Adhesion, Antibacterial Activity, and On-Demand Detachment

Medical adhesives are emerging as an important clinical tool as adjuvants for sutures and staples in wound closure and healing and in the achievement of hemostasis. However, clinical adhesives combining cytocompatibility, as well as strong and stable adhesion in physiological conditions, are still in demand. Herein, a mussel-inspired strategy is explored to produce adhesive coacervates using tannic acid (TA) and methacrylate pullulan (PUL-MA). TA|PUL-MA coacervates mainly comprise van der Waals forces and hydrophobic interactions. The methacrylic groups in the PUL backbone increase the number of interactions in the adhesives matrix, resulting in enhanced cohesion and adhesion strength (72.7 Jm-2), compared to the non-methacrylated coacervate. The adhesive properties are kept in physiologic-mimetic solutions (72.8 Jm-2) for 72 h. The photopolymerization of TA|PUL-MA enables the on-demand detachment of the adhesive. The poor cytocompatibility associated with the use of phenolic groups is here circumvented by mixing reactive oxygen species-degrading enzyme in the adhesive coacervate. This addition does not hamper the adhesive character of the materials, nor their anti-microbial or hemostatic properties. This affordable and straightforward methodology, together with the tailorable adhesivity even in wet environments, high cytocompatibility, and anti-bacterial activity, enables foresee TA|PUL-MA as a promising ready-to-use bioadhesive for biomedical applications.

Ion-Cross-Linked Hybrid Photochromic Hydrogels with Enhanced Mechanical Properties and Shape Memory Behaviour

Shape-shifting polymers usually require not only reversible stimuli-responsive ability, but also strong mechanical properties. A novel shape-shifting photochromic hydrogel system was designed and fabricated by embedding hydrophobic spiropyran (SP) into double polymeric network (DN) through micellar copolymerisation. Here, sodium alginate (Alg) and poly acrylate-co-methyl acrylate-co-spiropyran (P(SA-co-MA-co-SPMA)) were employed as the first network and the second network, respectively, to realise high mechanical strength. After being soaked in the CaCl2 solution, the carboxyl groups in the system underwent metal complexation with Ca2+ to enhance the hydrogel. Moreover, after the hydrogel was exposed to UV-light, the closed isomer of spiropyran in the hydrogel network could be converted into an open zwitterionic isomer merocyanine (MC), which was considered to interact with Ca2+ ions. Interestingly, Ca2+ and UV-light responsive programmable shape of the copolymer hydrogel could recover to its original form via immersion in pure water. Given its excellent metal ion and UV light stimuli-responsive and mechanical properties, the hydrogel has potential applications in the field of soft actuators.

Janus Polyurethane Adhesive Patch with Antibacterial Properties for Wound Healing

Despite the rapid development of tissue adhesives, flaws including allergies, poor stability, and indiscriminate double-sided adhesive properties limit their application in the medical field. In this work, Janus polyurethane patches were spontaneously prepared by adjusting the difference in the functional group distribution between the top and bottom sides of the patch during emulsion drying. Consequently, poor adhesion was exhibited on the bottom surface, while the top surface can easily adhere to metals, polymers, glasses, and tissues. The difference in adhesive strength to pork skin between the two surfaces is more than 5 times. The quaternary ammonium salt and hydrophilic components on the surface of the polyurethane patch enable the rapid removal and absorption of water from the tissue surface to achieve wet adhesion. Animal experiments have demonstrated that this multifunctional Janus polyurethane patch can promote skin wound closure and healing of infected wounds. This facile and effective strategy to construct Janus polyurethane patch provides a promising method for the development of functional tissue-adhesives.

Dandelion-derived vesicles-laden hydrogel dressings capable of neutralizing Staphylococcus aureus exotoxins for the care of invasive wounds

Delayed wound healing caused by bacterial infection remains a major challenge in clinical treatment. Exotoxins incorporated in bacterial extracellular vesicles play a key role as the disease-causing virulence factors. Safe and specific antivirulence agents are expected to be developed as an effective anti-bacterial infection strategy, instead of single antibiotic therapy. Plant-derived extracellular vesicle-like nanoparticles have emerged as promising therapeutic agents for skin diseases, but the elucidations of specific mechanisms of action and clinical transformation still need to be advanced. Here, dandelion-derived extracellular vesicle-like nanoparticles (TH-EVNs) are isolated and exert antivirulence activity through specifically binding to Staphylococcus aureus (S. aureus) exotoxins, thereby protecting the host cell from attack. The neutralization of TH-EVNs against exotoxins has considerable binding force and stability, showing complete detoxification effect in vivo. Then gelatin methacryloyl hydrogel is developed as TH-EVNs-loaded dressing for S. aureus exotoxin-invasive wounds. Hydrogel dressings demonstrate good physical and mechanical properties, thus achieving wound retention and controlled release of TH-EVNs, in addition to promoting cell proliferation and migration. In vivo results show accelerated re-epithelialization, promotion of collagen maturity and reduction of inflammation after treatment. Collectively, the developed TH-EVNs-laden hydrogel dressings provide a potential therapeutic approach for S. aureus exotoxin- associated trauma.

Preparation of environmentally friendly, high strength, adhesion and stability hydrogel based on lignocellulose framework

Hydrogels are extensively utilized in the fields of electronic skin, environmental monitoring, biological dressings due to their excellent flexibility and conductivity. However, traditional hydrogel materials possess drawbacks such as environmental toxicity, low strength, poor stability, and water loss deactivation, which limited its frequent applications. Here, a flexible conductive hydrogel called wood-based DES hydrogel (WDH) with high strength, high adhesion, high stability, and high sensitivity was successfully synthesized by using environmentally friendly lignocellulose as skeleton and deep eutectic solvent as matrix. The strength of WDH prepared from lignocellulose framework is approximately 50 times higher than poly deep eutectic solvent hydrogel, and about 4.5 times higher than that prepared from cellulose skeleton. The WDH exhibits stable adhesion to most common materials and demonstrates exceptional dimensional stability. Its conductivity remains unaffected by water, even after prolonged exposure to air, maintaining a value of 0.0245 S/m. The anisotropy inherent in the system results in three distinct linear sensing intervals for WDH, exhibiting a maximum sensitivity of 5.45. This paper verified the advantages of lignocellulose framework in improving the strength and stability of hydrogels, which provided a new strategy for the development of sensor materials.

Cellulose nanofiber hydrogel with high conductivity electrolytes for high voltage flexible supercapacitors

Although flexible double layer capacitors based on hydrogels overcome the drawbacks of commercial double layer capacitors such as low safety and non-deformability, it is still considered as attractive challenges to achieve high conductivity for hydrogel electrolytes as well as high operating voltages for hydrogel flexible supercapacitors. In this paper, ion migration channels were engineered by immobilizing positive and negative charges on polymer skeleton and dispersing cellulose nanofibers in the polymerized polyelectrolyte network, providing ultra-high ionic conductivity (103 mS cm-1). In addition, K3[Fe(CN)6] was introduced through a soaking method, leading to redox reactions on the surface of carbon electrode during charging and discharging, supporting a relatively wide voltage window (1.8 V). Moreover, the specific capacitance at high current remained 55 % of the specific capacitance at low current, indicating excellent rate performance. In addition, the device displayed high cycling stability (80.05 % after 10,000 cycles). Notably, we successfully light up the red LED with only one device. Accordingly, this work provides a feasible design concept for the development of cellulose nanofibers (CNF) hydrogel-based solid-state electrolyte with high conductivity for flexible supercapacitors with wide potential window and high energy density.

2D nanomaterial-based 3D network hydrogels for anti-infection therapy

Two-dimensional nanomaterials (2D NMs) refer to nanomaterials that possess a planar topography with a thickness of one or several atomic layers. Due to their large specific surface areas, atomic thickness, rough edges, and electron confinement in two dimensions, they have emerged as promising antimicrobial agents over antibiotics in combating bacterial infections. However, 2D NMs encounter issues such as low bio-safety, easy aggregation, and limited tissue penetration efficiency. To address these concerns, hydrogels with three-dimensional (3D) networks have been developed to encapsulate 2D NMs, aiming to enhance their biocompatibility, biodegradability, and ability to regulate and remodel the tissue microenvironment at the infected site. This review systematically summarizes the current studies on 2D NM-based antibacterial hydrogels with 3D network structures (named 2N3Hs). Firstly, we introduce the emerging types of 2N3Hs and describe their antibacterial actions. Subsequently, we discuss the applications of 2N3Hs in three biomedical fields, including wound dressing, cancer treatment, and bone regeneration. Finally, we conclude the review with current challenges and future developments for 2N3Hs, highlighting their potential as a promising choice for next-generation biomedical devices, particularly in the field of tissue engineering and regenerative medicine. This review aims to provide a comprehensive and panoramic overview of anti-infective 2N3Hs for various biomedical applications.

Preparation of Tough and Adhesive PVA/P(AM-AMPS)/Glycerol/Laponite/Na2SO4 Organohydrogels for All-Solid-State Supercapacitors and Self-Powered Wearable Strain Sensors

Hydrogel electrolytes are ideal for flexible wearable electronic devices because of their high ionic conductivity, flexibility, and biocompatibility. However, some problems, such as poor mechanical properties, low conductivity, and lack of adhesivity, are encountered in the process of hydrogel preparation and application, which restrict the further development of hydrogel electrolytes. In this study, PVA was used as the first network, and P(AM-co-AMPS) as the second network to prepare a double-network hydrogel electrolyte. Laponite and Na2SO4 were introduced into the hydrogel during hydrogel formation as the nanofiller and salt with the salting-out effect to enhance its mechanical properties. The hydrogel electrolyte with high toughness (1663 kJ·m-3), adhesivity (77 kPa), and ionic conductivity (1.7 S·m-1) was obtained. In addition, the hydrogel electrolyte also has excellent antifatigue performance. In the 10 consecutive tensile cycles, the tensile strength does not decay. Due to the high adhesivity of the hydrogel electrolyte, a symmetrical all-solid-state supercapacitor was assembled with a tight interface between the hydrogel electrolyte and the AC/CNT composite electrode. The supercapacitor has a high specific capacitance of 186.1 mF·cm-2 at the current density of 1 mA·cm-2. In addition, the capacitor has good flexibility and can withstand bending at various angles. The hydrogel electrolyte also has excellent strain sensing performance, with an ultrafast tensile response time (0.17 s) and high sensitivity factor (GF = 10.01). Finally, the self-powered sensor system composed of a supercapacitor as the power supply device and hydrogel electrolyte as the sensing part was obtained and applied to human motion monitoring, which provides a potential application in the integrated flexible electronic system.

Fabrication of a tough, long-lasting adhesive hydrogel patch via the synergy of interfacial entanglement and adhesion group densification

Adhesive hydrogels (AHs) are considered ideal materials for flexible sensors. However, the lack of effective energy dissipation networks and sparse surface polar groups in AHs lead to poor mechanical properties and interfacial adhesion, which limit their practical application. Herein, a tough, long-lasting adhesive and highly conductive nanocomposite hydrogel (PACPH) was fabricated via the synergy of interfacial entanglement and adhesion group densification. PACPH was obtained by the in situ polymerization of highly carboxylated cellulose nanocrystals (SCNCPA, surface pre-grafted polyacrylic acid chains, C-COOH = 11.5 mmol g-1) with the acrylic acid precursor. The unique tacticity of SCNCPA provides strong interface entanglement and multiple hydrogen bonds with the PACPH network, which further increases the energy dissipated during SCNCPA displacements, and enhances the mechanical properties of PACPH (tensile strength = 1.45 MPa, modulus = 332 kPa, and fracture toughness = 13.2 MJ m-3). Meanwhile, SCNCPA increases the density of surface polar groups in PAPCH and also acts as an anchor point to improve the adhesion strength (>2-3 times) of PACPH on various substrates. The combination of excellent mechanical, adhesive, and conductive properties of the PAPCH-integrated patches enables long-term monitoring of human daily activities and electrocardiogram (ECG) signals, verifying that PAPCH is a promising material platform for the further development of flexible sensors and other health management devices.

Investigating the Effect of the Crosslinking Factor on the Properties of Hydrogel Materials Containing Tilia platyphyllos Hydrolate

The use of natural ingredients in recent years has been of great importance in many industries and medicine. In biomedical applications, hydrogel materials also play a significant role. In view of this, the aim of this study was to synthesize and characterize hydrogel materials enriched with broadleaf linden hydrolate. An important aspect was to carry out a series of syntheses with varying types and amounts of crosslinking agents so as to test the possibility of synthesizing materials with controlled properties. The obtained hydrogels were subjected to detailed physicochemical analysis. The results of the tests confirmed the relationship between the selected properties and the type of crosslinking agent used. A crosslinking agent with a lower molar mass (575 g/mol) results in a material with a compact and strongly crosslinked structure, which is characterized by high surface roughness. The use of a crosslinking agent with a molecular weight of 700 g/mol resulted in a material with a looser-packed polymer network capable of absorbing larger amounts of liquids. The work also proved that regardless of the type of crosslinking agent used, the addition of linden hydrolate provides antioxidant properties, which is particularly important in view of the target biomedical application of such materials.

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

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

All-Polymer Piezoelectric Elastomer with High Stretchability, Low Hysteresis, Self-Adhesion, and UV-Blocking as Flexible Sensor

All-polymer piezoelectric elastomers that integrate self-powered, soft, and elastic performance are attractive in the fields of flexible wearable electronics and human-machine interfaces. However, a lack of adhesion and UV-blocking performances greatly hinders the potential applications of elastomers in these emerging fields. Here, a high-performance piezoelectric elastomer with piezoelectricity, mechanical robustness, self-adhesion, and UV-resistance was developed by using poly(vinylidene fluoride) (PVDF), acrylonitrile (AN), acrylamide (AAm), and oxidized tannic acid (OTA) (named PPO). In this design, the dipole-dipole interactions between the PVDF and PAN chains promoted the content of β-PVDF, endowing high piezoelectric coefficient (d33, 58 pC/N). Besides, high stretchability (∼500%), supercompressibility (∼98%), low Young's modulus (∼0.02 MPa), and remarkable elasticity (∼13.8% hysteresis ratio) were achieved simultaneously for the elastomers. Inspired by the mussel adhesion chemistry, the OTA containing abundant catechol and quinone groups provided high adhesion (93.26 kPa to wood) and an exceptional UV-blocking property (∼99.9%). In addition, the elastomers can produce a reliable electric signal output (Vocmax = 237 mV) and show a fast response (24 ms) when subjected to external force. Furthermore, the elastomer can be easily assembled as a wearable sensor for human physiological (body pulse and speech identification) monitoring and communication.

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

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

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

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

Sustained-Drug-Release, Strong, and Anti-Swelling Water-Lipid Biphasic Hydrogels Prepared via Digital Light Processing 3D Printing for Protection against Osteoarthritis: Demonstration in a Porcine Model

Osteoarthritis is a serious disease affecting joint cartilage. Owing to poor blood supply, the meniscus and acetabular labrum of joints heal poorly after injury. However, the development of artificial alternatives to these components that have similar mechanical properties and cartilage-protection ability is challenging. In this study, a strong hydrogel with a biomimetic microstructure is prepared with an emulsion-type photosensitive resin, where both hydrophilic and hydrophobic monomers, photo-initiator, and drugs can be adopted. In this system, the hydrophobic monomer forms uniformly dispersed aggregates after curing, improving the mechanical properties of the hydrogel significantly. Furthermore, the coordination bonds between nontoxic Zr4+ cations and sulfonic acid groups prevent hydrogel swelling. In addition, the water-oil biphasic hydrogel ink enables the loading of water- and lipid-soluble drugs, yielding hydrogel scaffolds with sustained dual-drug release ability. Crucially, hydrogel scaffolds having excellent mechanical properties, low swelling, and sustained biphasic drug release ability can be prepared using digital light processing 3D printing technology, owing to the high curing rate of the hydrophobic photo-initiator. These hydrogel scaffolds are applied as meniscal and labral replacements in a porcine model and show great promise for the prevention of secondary osteoarthritis, demonstrating the broad potential clinical applications of this material.

Toughening Weak Polyampholyte Hydrogels with Weak Chain Entanglements via a Secondary Equilibrium Approach

Polyampholyte (PA) hydrogels are randomly copolymerized from anionic and cationic monomers, showing good mechanical properties owing to the existence of numerous ionic bonds in the networks. However, relatively tough PA gels can be synthesized successfully only at high monomer concentrations (C_M), where relatively strong chain entanglements exist to stabilize the primary supramolecular networks. This study aims to toughen weak PA gels with relatively weak primary topological entanglements (at relatively low C_M) via a secondary equilibrium approach. According to this approach, an as-prepared PA gel is first dialyzed in a FeCl₃ solution to reach a swelling equilibrium and then dialyzed in sufficient deionized water to remove excess free ions to achieve a new equilibrium, resulting in the modified PA gels. It is proved that the modified PA gels are eventually constructed by both ionic and metal coordination bonds, which could synergistically enhance the chain interactions and enable the network toughening. Systematic studies indicate that both C_M and FeCl₃ concentration (CFeCl3) influence the enhancement effectiveness of the modified PA gels, although all the gels could be dramatically enhanced. The mechanical properties of the modified PA gel could be optimized at C_M = 2.0 M and CFeCl3 = 0.3 M, where the Young's modulus, tensile fracture strength, and work of tension are improved by 1800%, 600%, and 820%, respectively, comparing to these of the original PA gel. By selecting a different PA gel system and diverse metal ions (i.e., Al³⁺, Mg²⁺, Ca²⁺), we further prove that the proposed approach is generally appliable. A theoretical model is used to understand the toughening mechanism. This work well extends the simple yet general approach for the toughening of weak PA gels with relatively weak chain entanglements.

Hydrogel with Robust Adhesion in Various Liquid Environments by Electrostatic-Induced Hydrophilic and Hydrophobic Polymer Chains Migration and Rearrangement

Hydrogels with wet adhesion are promising interfacial adhesive materials; however, their adhesion in water, oil, or organic solvents remains a major challenge. To address this, a pressure-sensitive P(AAm-co-C₁₈ )/PTA-Fe hydrogel is fabricated, which exhibits robust adhesion to various substrates in both aqueous solutions and oil environments. It is demonstrated that the key to wet adhesion under liquid conditions is the removal of the interfacial liquid, which can be achieved through rational molecular composition regulation. By complexing with hydrophilic polymer networks, phosphotungstic acid (PTA) is introduced into the hydrogel network as a physical cross-linker and anchor point to improve the cohesion strength and drive the migration of polymer chains. The migration and rearrangement of hydrophilic and hydrophobic polymer chains on the hydrogel surface are induced by the electrostatic interactions of Fe³⁺ , which create a surface with interfacial water- and oil-removing properties. By co-regulating the hydrophilic and hydrophobic polymer chains, the P(AAm-co-C₁₈ )/PTA-Fe hydrogel is able to act as a pressure-sensitive adhesive under water and oils with adhesion strength of 92.6 and 90.0 kPa, respectively. It is anticipated that this regulation strategy for polymer chains will promote the development of wet adhesion hydrogels, which can have a wide range of applications.

Highly stretchable, injectable hydrogels with cyclic endurance and shape-stability in dynamic mechanical environments, by microunit reformation

Traditional injectable hydrogels have so far found it difficult to accommodate resistance to large deformation and shape-stability under cyclic deformation. Polyampholyte (PA) hydrogels exhibit resistance to large deformation, good fatigue-resistance and rapid self-healing under dynamic forces. The limitations of the preparation process result in non-injectability of polyampholyte (PA) hydrogels. Electrostatic interactions as a medium for resistance to large deformation and shape-stability after cyclic deformation in reformed injectable hydrogels has been explored in this study. The prepared hydrogels (as-prepared PA-N) were dried and smashed into microunits and then mixed with 0.9% NaCl solution to transform them into reformed hydrogels (as-reformed PA-N) via a needle to achieve injectability. The as-reformed PA-N could exhibit 913.6% elongation at break and showed shape-stability under cyclic deformation due to the efficient self-healing abilities of the microunits and the inherited structure of the prepared hydrogels, which are superior to those of current tough injectable hydrogels. Potential applications in elbow cyclic bending and frequent movement of mobile wounds have been proved in this study. Overall, the results showed that the as-reformed PA-N achieved convenient injectability with resistance to large deformation and shape-stability under cyclic deformation at the same time.

Tough hydrogel with high water content and ordered fibrous structures as an artificial human ligament

Natural biological tissues such as ligaments, due to their anisotropic across scale structure, have high water content, while still maintaining high strength and flexibility. Hydrogels are ideal artificial materials like human ligaments. However, conventional gel materials fail to exhibit high strength or fatigue resistance at high water content in human tissues. To address this challenge, we propose a simple integrated strategy to prepare an anisotropic hierarchical hydrogel architecture for artificial ligaments by combining freeze-casting assisted compression annealing and salting-out treatments. The hybrid polyvinyl alcohol hydrogels are of water content up to 79.5 wt%. Enhanced by the added carbon nanotubes, the hydrogels exhibit high strength of 4.5 MPa and a fatigue threshold of 1467 J m^-2, as well as excellent stress sensitivity. The outstanding durability of the artificial ligament provides an all-around solution for biomedical applications.

Deep Eutectic Solvents-based Ionogels with Ultrafast Gelation and High Adhesion in Harsh Environments

Adhesive materials have recently drawn intensive attention due to their excellent sealing ability, thereby stimulating advances in materials science and industrial usage. However, reported adhesives usually exhibit weak adhesion strength, require high pressure for strong bonding, and display severe adhesion deterioration in various harsh environments. In this work, instead of water or organic solvents, a deep eutectic solution (DES) was used as the medium for photopolymerization of zwitterionic and polarized monomers, thus generating a novel ionogel with tunable mechanical properties. Multiple hydrogen bonds and electrostatic interactions between DES and monomers facilitated ultrafast gelation and instant bonding without any external pressure, which was rarely reported previously. Furthermore, high adhesion in different harsh environments (e.g., water, acidic and basic buffers, and saline solutions) and onto hydrophilic (e.g., glass and tissues) and hydrophobic (e.g., polymethyl methacrylate, polystyrene, and polypropylene) adherends was demonstrated. Also, high stretchability of the ionogel at extreme temperatures (-80 and 80 °C) indicated its widespread applications. Furthermore, the biocompatible ionogel showed high burst pressure onto stomach and intestine tissues to prevent liquid leakage, highlighting its potential as an adhesive patch. This ionogel provides unprecedented opportunities in the fields of packaging industry, marine engineering, medical adhesives, and electronic assembly.

Facile Hydrogels of AIEgens Applied as Reusable Sensors for In Situ and Early Warning of Metallic Corrosion

Early detection of metallic corrosion is one considerable method to reduce imperceptible disasters nowadays. Fluorescent coatings with high sensitivity and long lifetimes for use in the early detection of metallic corrosion are in high demand, but they are presently difficult to prepare. Inspired by the chameleon's skin, which is capable of switching its color in different atmospheres sensitively and reversibly, we proposed herein a facile and universal all-in-one strategy of combining the fluorescent sensitivity and dynamic hydrogen bonds in a hydrogel to develop a reusable corrosion detection tape to cover metal surfaces. The fluorescent hydrogel tape was constructed using free radical copolymerization of monomers [hydroxyethyl methylacrylate (HEMA) and tetraphenylethene derivatives (TPEPy)]. Due to the aggregation-induced emission (AIE) behavior of TPEPy, the poly(HEMA-co-TPEPy) hydrogel is capable of monitoring the traces of corrosion via the release of ferric ions with a concentration as low as 10^-5 M. Moreover, due to the dynamic hydrogen bonds of hydroxyethyl groups in hydrogel networks, the fluorescent hydrogel tape exhibited good adhesion and well reusability for over 10 applications to effectively warn against early corrosion of stainless steel. This non-destructive and reversible method of early corrosion detection can provide valuable signals when maintenance is needed before the metal suffers serious damage.

Advances in Hemostatic Hydrogels That Can Adhere to Wet Surfaces

Currently, uncontrolled bleeding remains a serious problem in emergency, surgical and battlefield environments. Despite the specific properties of available hemostatic agents, sealants, and adhesives, effective hemostasis under wet and dynamic conditions remains a challenge. In recent years, polymeric hydrogels with excellent hemostatic properties have received much attention because of their adjustable mechanical properties, high porosity, and biocompatibility. In this review, to investigate the role of hydrogels in hemostasis, the mechanisms of hydrogel hemostasis and adhesion are firstly elucidated, the adhesion design strategies of hemostatic hydrogels in wet environments are briefly introduced, and then, based on a comprehensive literature review, the studies and in vivo applications of wet-adhesive hemostatic hydrogels in different environments are summarized, and the improvement directions of such hydrogels in future studies are proposed.

Elastomeric, bioadhesive and pH-responsive amphiphilic copolymers based on direct crosslinking of poly(glycerol sebacate)-co-polyethylene glycol

Poly(glycerol sebacate) (PGS), a synthetic biorubber, is characterised by its biocompatibility, high elasticity and tunable mechanical properties; however, its inherent hydrophobicity and insolubility in water make it unsuitable for use in advanced biomaterials like hydrogels fabrication. Here, we developed new hydrophilic PGS-based copolymers that enable hydrogel formation through use of two different types of polyethylene glycol (PEG), polyethylene glycol (PEG2) or glycerol ethoxylate (PEG3), combined at different ratios. A two-step polycondensation reaction was used to produce poly(glycerol sebacate)-co-polyethylene glycol (PGS-co-PEG) copolymers that were then crosslinked thermally without the use of initiators or crosslinkers, resulting in PGS-co-PEG2 and PGS-co-PEG3 amphiphilic polymers. It has been illustrated that the properties of PGS-co-PEG copolymers can be controlled by altering the type and amount of PEG. PGS-co-PEG copolymers containing PEG ≥ 40% showed high swelling, flexibility, stretching, bioadhesion and biocompatibility, and good enzymatic degradation and mechanical properties. Also, the addition of PEG created hydrogels that demonstrated pH-responsive behaviours, which can be used for bioapplications requiring responding to physicochemical dynamics. Interestingly, PGS-co-40PEG2 and PGS-co-60PEG3 had the highest shear strengths, 340.4 ± 49.7 kPa and 336.0 ± 35.1 kPa, and these are within the range of commercially available sealants or bioglues. Due to the versatile multifunctionalities of these new copolymer hydrogels, they can have great potential in soft tissue engineering and biomedicine.

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.

Strong, Tough, and Adhesive Polyampholyte/Natural Fiber Composite Hydrogels

Hydrogels with high mechanical strength, good crack resistance, and good adhesion are highly desirable in various areas, such as soft electronics and wound dressing. Yet, these properties are usually mutually exclusive, so achieving such hydrogels is difficult. Herein, we fabricate a series of strong, tough, and adhesive composite hydrogels from polyampholyte (PA) gel reinforced by nonwoven cellulose-based fiber fabric (CF) via a simple composite strategy. In this strategy, CF could form a good interface with the relatively tough PA gel matrix, providing high load-bearing capability and good crack resistance for the composite gels. The relatively soft, sticky PA gel matrix could also provide a large effective contact area to achieve good adhesion. The effect of CF content on the mechanical and adhesion properties of composite gels is systematically studied. The optimized composite gel possesses 35.2 MPa of Young's modulus, 4.3 MPa of tensile strength, 8.1 kJ m^-2 of tearing energy, 943 kPa of self-adhesive strength, and 1.4 kJ m^-2 of self-adhesive energy, which is 22.1, 2.3, 1.8, 6.0, and 4.2 times those of the gel matrix, respectively. The samples could also form good adhesion to diverse substrates. This work opens a simple route for fabricating strong, tough, and adhesive hydrogels.

Hydrogel-Tissue Interface Interactions for Implantable Flexible Bioelectronics

Hydrogels have emerged as multifunctional interface materials between implantable bioelectronic devices and biotissues. The soft and wet materials with low and alterable mechanical properties can match the mechanical, chemical, electrical, and biological properties of biotissues and thus diminish the mechanical and electrical mismatch. Interactions at the hydrogel-biotissue and hydrogel-device interfaces have attracted broad research interest. Great efforts have been devoted to establishing instant, strong, and conformal adhesion at the interface by chemical bonding, electrostatic interaction, hydrogen bonding, supramolecular recognition, hydrophobic association, and even topological entanglements at the interfaces. This Perspective provides a brief account of representative progress on the hydrogel-tissue adhesive that forms seamless and conformal interface adhesion and applications in implantable devices for physiological, cardiac, and neuronal signal collection and electrical stimulation. Major challenges such as wet adhesion and the stability of the adhesive hydrogel-tissue interface are identified for examination in future investigations.

Anti-Fouling Performance of Hydrophobic Hydrogels with Unique Surface Hydrophobicity and Nanoarchitectonics

Hydrogel is a kind of soft and wet matter, which demonstrates favorable fouling resistance owing to the hydration anti-adhesive surfaces. Different from conventional hydrogels constructed by hydrophilic or amphiphilic polymers, the recently invented “hydrophobic hydrogels” composed of hydrophobic polymers exhibit many unique properties, e.g., surface hydrophobicity and high water content, suggesting promising applications in anti-fouling. In this paper, a series of hydrophobic hydrogels were prepared with different chemical structures and water content for anti-fouling investigations. The hydrophobic hydrogels showed high static water contact angles (WCAs > 90°), indicating remarkable surface hydrophobicity, which is abnormal for conventional hydrogels. Compared with the conventional hydrogels, all the hydrophobic hydrogels exhibited less than 4% E. coli biofilm coverage, showing a contrary trend of anti-fouling ability to the water content inside the polymer. Typically, the poly(2-(2-ethoxyethoxy)ethyl acrylate) (PCBA) and poly(tetrahydrofurfuryl acrylate) (PTHFA) hydrogels with relatively high surface hydrophobicity showed as low as 5.1% and 2.4% E. coli biofilm coverage even after incubation for 7 days in bacteria suspension, which are about 0.32 and 0.15 times of that on the hydrophilic poly(N,N-dimethylacrylamide) (PDMA) hydrogels, respectively. Moreover, the hydrophobic hydrogels exhibited a similar anti-adhesion ability and trend to algae S. platensis. Based on the results, the surface hydrophobicity mainly contributes to the excellent anti-fouling ability of hydrophobic hydrogels. In the meantime, the too-high water content may be somehow detrimental to anti-fouling performance.