Weak Hydrogen Bonds Lead to Self-Healable and Bioadhesive Hybrid Polymeric Hydrogels with Mineralization-Active Functions

Antidehydration and Stable Mechanical Properties during the Phase Transition of the PNIPAM-Based Hydrogel for Body-Temperature-Monitoring Sensors

Poly(N-isopropylacrylamide) (PNIPAM) enhances the reversibility and responsiveness of wearable temperature-sensitive devices. However, an open question is whether and how the hydrogel design can prevent adhesive performance loss caused by phase-transition-induced dehydration and unstable mechanical properties between devices and human skin and reduce interfacial failure. Herein, a gelatin-mesh scaffold-based hydrogel (NAGP-Gel) is constructed to inhibit dehydration and volume change, leading to stable mechanical properties, superior adhesiveness, and thermal sensing sensitivity during the phase transition. NAGP-Gel enhances the polymer chains-water interaction and weakens the degree of aggregation of polymer chains-chains, improving antidehydration properties under 45 °C conditions that are higher than the lower critical solution temperature (LCST; i.e., ∼32 °C). The mesh scaffold greatly restricts the phase-transition-induced polymer chain movement and maintains the mechanical performance. In a 60 °C environment, the maximum water loss and volume retention ratio of NAGP-Gel are only 3.58% and 97.3%, respectively. Additionally, NAGP-Gel serves as a temperature sensor, producing a stable thermal-electrical signal within the LCST range. It also can be assembled into an electronic device enabling the transmission of information and recognition of sign language via Morse code. This work broadens the application of PNIPAM in constructing intelligent hydrogels and opens the door to exploring emerging hydrogels for temperature-monitoring applications.

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

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

Development of HEMA-Succinic Acid-PEG Bio-Based Monomers for High-Performance Hydrogels in Regenerative Medicine

In recent years, hydrogels have found a special place in regenerative medicine for tissue repair, rehabilitation, and drug delivery. To be used in regenerative medicine, hydrogels must have desirable physical, chemical, and biological properties. In this study, a new biomonomer based on hydroxyethyl methacrylate-succinic acid-polyethylene glycol 200 (HEMA-Suc-PEG) was synthesized and characterized. Then, using the synthesized monomers and different ratios of polyethylene glycol diacrylate (PEGDA) as a crosslinker, biocompatible hydrogels were synthesized through thermal and UV curing methods. The mechanical, physical, chemical, and biological properties of the hydrogels and the behavior of endothelial cells, an essential component of the cardiovascular system, were evaluated. The results showed that the hydrogel synthesized with 0.2 g of PEGDA (UV curing) has desirable mechanical and physical properties. Biological tests showed that these hydrogels are not only nontoxic to cells but also enhance cell adhesion. Therefore, the hydrogel containing the synthesized monomer HEMA-Suc-PEG and 0.2 g of PEGDA has the potential to be used in the cardiovascular system.

A Self-Powered, Skin Adhesive, and Flexible Human-Machine Interface Based on Triboelectric Nanogenerator

Human-machine interactions (HMIs) have penetrated into various academic and industrial fields, such as robotics, virtual reality, and wearable electronics. However, the practical application of most human-machine interfaces faces notable obstacles due to their complex structure and materials, high power consumption, limited effective skin adhesion, and high cost. Herein, we report a self-powered, skin adhesive, and flexible human-machine interface based on a triboelectric nanogenerator (SSFHMI). Characterized by its simple structure and low cost, the SSFHMI can easily convert touch stimuli into a stable electrical signal at the trigger pressure from a finger touch, without requiring an external power supply. A skeleton spacer has been specially designed in order to increase the stability and homogeneity of the output signals of each TENG unit and prevent crosstalk between them. Moreover, we constructed a hydrogel adhesive interface with skin-adhesive properties to adapt to easy wear on complex human body surfaces. By integrating the SSFHMI with a microcontroller, a programmable touch operation platform has been constructed that is capable of multiple interactions. These include medical calling, music media playback, security unlocking, and electronic piano playing. This self-powered, cost-effective SSFHMI holds potential relevance for the next generation of highly integrated and sustainable portable smart electronic products and applications.

Bioactive citrate-based polyurethane tissue adhesive for fast sealing and promoted wound healing

As a superior alternative to sutures, tissue adhesives have been developed significantly in recent years. However, existing tissue adhesives struggle to form fast and stable adhesion between tissue interfaces, bond weakly in wet environments and lack bioactivity. In this study, a degradable and bioactive citrate-based polyurethane adhesive is constructed to achieve rapid and strong tissue adhesion. The hydrophobic layer was created with polycaprolactone to overcome the bonding failure between tissue and adhesion layer in wet environments, which can effectively improve the wet bonding strength. This citrate-based polyurethane adhesive provides rapid, non-invasive, liquid-tight and seamless closure of skin incisions, overcoming the limitations of sutures and commercial tissue adhesives. In addition, it exhibits biocompatibility, biodegradability and hemostatic properties. The degradation product citrate could promote the process of angiogenesis and accelerate wound healing. This study provides a novel approach to the development of a fast-adhering wet tissue adhesive and provides a valuable contribution to the development of polyurethane-based tissue adhesives.

Biaxial stretching of polytetrafluoroethylene in industrial scale to fabricate medical ePTFE membrane with node-fibril microstructure

Expanded polytetrafluoroethylene (ePTFE) is promising in biomedical fields such as covered stents and plastic surgery owing to its excellent biocompatibility and mechanical properties. However, ePTFE material prepared by the traditional biaxial stretching process is with thicker middle and thinner sides due to the bowing effect, which poses a major problem in industrial-scale fabrication. To solve this problem, we design an olive-shaped winding roller to provide the middle part of the ePTFE tape with a greater longitudinal stretching amplitude than the two sides, so as to make up for the excessive longitudinal retraction tendency of the middle part when it is transversely stretched. The as-fabricated ePTFE membrane has, as designed, uniform thickness and node-fibril microstructure. In addition, we examine the effects of mass ratio of lubricant to PTFE powder, biaxial stretching ratio and sintering temperature on the performance of the resultant ePTFE membranes. Particularly, the relation between the internal microstructure of the ePTFE membrane and its mechanical properties is revealed. Besides stable mechanical properties, the sintered ePTFE membrane exhibits satisfactory biological properties. We make a series of biological assessments including in vitro hemolysis, coagulation, bacterial reverse mutation and in vivo thrombosis, intracutaneous reactivity test, pyrogen test and subchronic systemic toxicity test; all of the results meet the relevant international standards. The muscle implantation of the sintered ePTFE membrane into rabbits indicates acceptable inflammatory reactions of our sintered ePTFE membrane fabricated on industrial scale. Such a medical-grade raw material with the unique physical form and condensed-state microstructure is expected to afford an inert biomaterial potentially for stent-graft membrane.

Dual responsive self-healing hydrogels with wide stability and excellent mechanical strength based on aliphatic polycarbonate

The wide development of hydrogels had been used in many filed due to the high water-containing and tough three-dimensional structure, however, the poor mechanical and multi-functional properties of hydrogel can be limited in its applications deeply. Herein, the dual responsive self-healing hydrogels with tough mechanical properties were manufactured by dual-physical cross-linking based on biodegradable aliphatic polycarbonate. Choosing the soft and hard segments to design the polymeric hydrogel not only can facilitate the dual-dynamic bonding interactions but also the resilient hydrogels possess robust and controllable mechanical strength (6.51 MPa). Furthermore, the results of swelling and stability tests of the materials indicated that the swelling ability of the biodegradable hydrogels can be regulated by the hydrophilic group, and the maximal swelling ratio in water and the equilibrium water content is 66% and 40%, respectively. It is worth mentioning that the tough hydrogels embrace dual-responsive high efficiency of self-healing ability, and the self-healing time is 2 h at 50 °C or 10 h under pH = 5, suggesting that the obtained hydrogels can respond to temperature and pH value to drive the fracture interface for fast self-healing, which will offer new opportunities for stimuli-responsive materials and wound healing.

Highly Stretchable, Swelling-Resistant, Self-Healed, and Biocompatible Dual-Reinforced Double Polymer Network Hydrogels

Superior flexibility and toughness can be achieved in bioactive hydrogels by the use of a double polymer network with complementary properties. Inspired by this design principle, we here combine polyacrylic acid (PAA) and sodium alginate (SA) to obtain a dual-reinforced double interpenetrating network (d-DIPN) hydrogel. The dual reinforcement involves ionic cross-linking and introduction of SiO₂ nanoparticles, which leads to extraordinary improvements in strength and toughness. Compared with the standard PAA hydrogel that offers an elongation of 240% and a breakage stress of 0.03 MPa, the prepared SA(Ca²⁺)-PAA-SiO₂ hydrogel shows an elongation above 1000% and a breakage stress of 1.62 MPa. Moreover, the combination of strong covalent cross-links and weak reversible interactions provides the d-DIPN hydrogel with swelling resistance and self-healing behavior, adhesive abilities, and shape memory performance. Furthermore, we show that the biocompatibility and bone cell proliferation ability of the hydrogels can be improved through a mineralization process despite an observed reduction in breakage strain and stress. Taken as a whole, our work paves the way for the design of strong and tough hydrogels, with potential applications within biomedicine and particularly tissue engineering.

Robust hydrogel adhesives for emergency rescue and gastric perforation repair

Development of biocompatible hydrogel adhesives with robust tissue adhesion to realize instant hemorrhage control and injury sealing, especially for emergency rescue and tissue repair, is still challenging. Herein, we report a potent hydrogel adhesive by free radical polymerization of N-acryloyl aspartic acid (AASP) in a facile and straightforward way. Through delicate adjustment of steric hindrance, the synergistic effect between interface interactions and cohesion energy can be achieved in PAASP hydrogel verified by X-ray photoelectron spectroscopy (XPS) analysis and simulation calculation compared to poly (N-acryloyl glutamic acid) (PAGLU) and poly (N-acryloyl amidomalonic acid) (PAAMI) hydrogels. The adhesion strength of the PAASP hydrogel could reach 120 kPa to firmly seal the broken organs to withstand the external force with persistent stability under physiological conditions, and rapid hemostasis in different hemorrhage models on mice is achieved using PAASP hydrogel as physical barrier. Furthermore, the paper-based Fe³⁺ transfer printing method is applied to construct PAASP-based Janus hydrogel patch with both adhesive and non-adhesive surfaces, by which simultaneous wound healing and postoperative anti-adhesion can be realized in gastric perforation model on mice. This advanced hydrogel may show vast potential as bio-adhesives for emergency rescue and tissue/organ repair.

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The hydrogel with good mechanical properties and adhesiveness is designed.

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The hydrogel adhesive can act as physical barrier for emergency rescue.

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The Janus hydrogel can realize efficient gastric perforation repair on mice.

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.

Chitosan-Based Scaffolds for Facilitated Endogenous Bone Re-Generation

Facilitated endogenous tissue engineering, as a facile and effective strategy, is emerging for use in bone tissue regeneration. However, the development of bioactive scaffolds with excellent osteo-inductivity to recruit endogenous stem cells homing and differentiation towards lesion areas remains an urgent problem. Chitosan (CS), with versatile qualities including good biocompatibility, biodegradability, and tunable physicochemical and biological properties is undergoing vigorously development in the field of bone repair. Based on this, the review focus on recent advances in chitosan-based scaffolds for facilitated endogenous bone regeneration. Initially, we introduced and compared the facilitated endogenous tissue engineering with traditional tissue engineering. Subsequently, the various CS-based bone repair scaffolds and their fabrication methods were briefly explored. Furthermore, the functional design of CS-based scaffolds in bone endogenous regeneration including biomolecular loading, inorganic nanomaterials hybridization, and physical stimulation was highlighted and discussed. Finally, the major challenges and further research directions of CS-based scaffolds were also elaborated. We hope that this review will provide valuable reference for further bone repair research in the future.

Engineering Hydrogels for Modulation of Material-Cell Interactions

Hydrogels are a recurrent platform for Tissue Engineering (TE) strategies. Their versatility and the variety of available methods for tuning their properties highly contribute to hydrogels' success. As a result, the design of advanced hydrogels has been thoroughly studied, in the quest for better solutions not only for drugs- and cell-based therapies but also for more fundamental studies. The wide variety of sources, crosslinking strategies, and functionalization methods, and mostly the resemblance of hydrogels to the natural extracellular matrix, make this 3D hydrated structures an excellent tool for TE approaches. The state-of-the-art information regarding hydrogel design, processing methods, and the influence of different hydrogel formulations on the final cell-biomaterial interactions are overviewed herein. This article is protected by copyright. All rights reserved.

Resolving the Conflict between Strength and Toughness in Bioactive Silica-Polymer Hybrid Materials

Simultaneously improving the strength and toughness of materials is a major challenge. Inorganic-polymer hybrids offer the potential to combine mechanical properties of a stiff inorganic glass with a flexible organic polymer. However, the toughening mechanism at the atomic scale remains largely unknown. Based on combined experimental and molecular dynamics simulation results, we find that the deformation and fracture behavior of hybrids are governed by noncovalent intermolecular interactions between polymer and silica networks rather than the breakage of covalent bonds. We then attempt three methods to improve the balance between strength and toughness of hybrids, namely the total inorganic/organic (I/O) weight ratio, the size of silica nanoparticles, and the ratio of -C-O vs -C-C bonds in the polymer chains. Specifically, for a hybrid with matched silica size and I/O ratio, we demonstrate optimized mechanical properties in terms of strength (1.75 MPa at breakage), degree of elongation at the fracture point (31%), toughness (219 kPa), hardness (1.08 MPa), as well as Young's modulus (3.0 MPa). We also demonstrate that this hybrid material shows excellent biocompatibility and ability to support cell attachment as well as proliferation. This supports the possible application of this material as a strong yet tough bone scaffold material.

Supramolecular Adhesive Hydrogels for Tissue Engineering Applications

Tissue engineering is a promising and revolutionary strategy to treat patients who suffer the loss or failure of an organ or tissue, with the aim to restore the dysfunctional tissues and enhance life expectancy. Supramolecular adhesive hydrogels are emerging as appealing materials for tissue engineering applications owing to their favorable attributes such as tailorable structure, inherent flexibility, excellent biocompatibility, near-physiological environment, dynamic mechanical strength, and particularly attractive self-adhesiveness. In this review, the key design principles and various supramolecular strategies to construct adhesive hydrogels are comprehensively summarized. Thereafter, the recent research progress regarding their tissue engineering applications, including primarily dermal tissue repair, muscle tissue repair, bone tissue repair, neural tissue repair, vascular tissue repair, oral tissue repair, corneal tissue repair, cardiac tissue repair, fetal membrane repair, hepatic tissue repair, and gastric tissue repair, is systematically highlighted. Finally, the scientific challenges and the remaining opportunities are underlined to show a full picture of the supramolecular adhesive hydrogels. This review is expected to offer comparative views and critical insights to inspire more advanced studies on supramolecular adhesive hydrogels and pave the way for different fields even beyond tissue engineering applications.

Composite Dynamic Hydrogels Constructed on Boronic Ester Cross-Links with NIR-Enhanced Diffusivity

Dynamic hydrogels with thermosensitive cross-links are highly promising platforms for "on-demand" drug delivery systems. However, there is a problem with triggering a response in their whole volume, which reduces their efficiency. To achieve better thermoresponsiveness, a graphene oxide-filled composite hydrogel based on boronic ester cross-links, composed of hyperbranched polyglycidol, HbPGL, and poly(acrylamide-ran-2-acrylamidephenylboronic acid), poly(AM-ran-2-AAPBA), has been constructed. The homogeneous embedment of graphene oxide (GO) in the network assured near-infrared (NIR)-photothermal response in its bulk due to the rapid light-to-heat conversion. The rate and amplitude of materials response increase with graphene oxide concentration. The temperature of the hydrogel containing graphene oxide at a concentration of 13.2 mg/mL increased from 36.6 to 41 °C in 29 s upon NIR irradiation. The network diffusivity and the extent of its change with temperature can be regulated by the length of the applied boronic acid-based cross-linking agent. The hydrogel constructed on the shorter copolymer (M_n = 23 000 g/mol) displayed a significant increase in diffusivity with temperature. A diffusion ordered NMR study revealed that the diffusion coefficient determined for niacin, a model drug encapsulated in the hydrogel, increased from 6.09 × 10^-10 at 25 °C to 1.28 × 10^-9 m²/s at 41 °C. In the case of the hydrogel constructed on the longer acrylamide copolymer (M_n = 43 000 g/mol), in which physical entanglements stabilize the network, the change of encapsulated niacin diffusion coefficient was significantly smaller, i.e., from 3.83 × 10^-10 at 25 °C to 6.63 × 10^-10 m²/s at 41 °C. The possibility of on-demand NIR-regulated diffusivity of the reported boronic ester-based hydrogels makes them promising candidates for controlled drug delivery platforms.

Bio-based and bio-inspired adhesives from animals and plants for biomedical applications

With the "many-headed" slime mold Physarum polycelphalum having been voted the unicellular organism of the year 2021 by the German Society of Protozoology, we are reminded that a large part of nature's huge variety of life forms is easily overlooked - both by the general public and researchers alike. Indeed, whereas several animals such as mussels or spiders have already inspired many scientists to create novel materials with glue-like properties, there is much more to discover in the flora and fauna. Here, we provide an overview of naturally occurring slimy substances with adhesive properties and categorize them in terms of the main chemical motifs that convey their stickiness, i.e., carbohydrate-, protein-, and glycoprotein-based biological glues. Furthermore, we highlight selected recent developments in the area of material design and functionalization that aim at making use of such biological compounds for novel applications in medicine - either by conjugating adhesive motifs found in nature to biological or synthetic macromolecules or by synthetically creating (multi-)functional materials, which combine adhesive properties with additional, problem-specific (and sometimes tunable) features.

Antimicrobial Polymer-Based Hydrogels for the Intravaginal Therapies-Engineering Considerations

The review is focused on the hydrogel systems dedicated to the intravaginal delivery of antibacterial, antifungal and anti-Trichomonas vaginalis activity drugs for the treatment of gynaecological infections. The strategies for the enhancement of the hydrophobic drug solubility in the hydrogel matrix based on the formation of bigel systems and the introduction of nano- and microparticles as a drug reservoir are presented. Hydrogel carriers of natural and synthetic pharmacological substances, drug-free systems displaying antimicrobial activity thanks to the hydrogel building elements and systems combining the antimicrobial activity of both drug and polymer building components are distinguished. The design of hydrogels facilitating their administration and proper distribution in the vaginal mucosa and the vagina based on thermoresponsive systems capable of gelling at vaginal conditions and already-cross-linked injectable systems after reaching the yield stress are discussed. In addition, the mechanisms of hydrogel bioadhesion that regulate the retention time in the vagina are indicated. Finally, the prospects for the further development of hydrogel-based drug carriers in gynaecological therapies are highlighted.

Rational engineering and applications of functional bioadhesives in biomedical engineering

For the past decades, several bioadhesives have been developed to replace conventional wound closure medical tools such as sutures, staples, and clips. The bioadhesives are easy to use and can minimize tissue damage. They are designed to provide strong adhesion with stable mechanical support on tissue surfaces. However, this monofunctionality of the bioadhesives hinders their practical applications. In particular, a bioadhesive can lose its intended function under harsh tissue environments or delay tissue regeneration during wound healing. Based on several natural and synthetic biomaterials, functional bioadhesives have been developed to overcome the aforementioned limitations. The functional bioadhesives are designed to have specific characteristics such as antimicrobial, cell infiltrative, stimuli-responsive, electrically conductive, and self-healing to ensure stability under harsh tissue conditions, facilitate tissue regeneration, and effectively monitor biosignals. Herein, we thoroughly review the functional bioadhesives from their fundamental background to recent progress with their practical applications for the enhancement of tissue healing and effective biosignal sensing. Furthermore, the future perspectives on the applications of functional bioadhesives and current challenges in their commercialization are also discussed.

Star-Shaped Poly(furfuryl glycidyl ether)-Block-Poly(glyceryl glycerol ether) as an Efficient Agent for the Enhancement of Nifuratel Solubility and for the Formation of Injectable and Self-Healable Hydrogel Platforms for the Gynaecological Therapies

In this paper, we present novel well-defined unimolecular micelles constructed a on poly(furfuryl glycidyl ether) core and highly hydrophilic poly(glyceryl glycerol ether) shell, PFGE-b-PGGE. The copolymer was synthesized via anionic ring-opening polymerization of furfuryl glycidyl ether and (1,2-isopropylidene glyceryl) glycidyl ether, respectively. MTT assay revealed that the copolymer is non-cytotoxic against human cervical cancer endothelial (HeLa) cells. The copolymer thanks to furan moieties in its core is capable of encapsulation of nifuratel, a hydrophobic nitrofuran derivative, which is a drug applied in the gynaecology therapies that shows a broad antimicroorganism spectrum. The study shows high loading capacity of the copolymer, i.e., 146 mg of nifuratel per 1 g of copolymer. The load unimolecular micelles were characterized using DLS and TEM microscopy and compared with the reference glyceryl glycerol ether homopolymer sample. The presence of numerous 1,2-diol moieties in the shell of PFGE-b-PGG macromolecules enabled the formation of reversible cross-links with 2-acrylamidephenylboronic acid-based polyacrylamide. The obtained hydrogels were both injectable and self-healable, which was confirmed with a rheological study.

Hydroxypropyl chitosan-based dual self-healing hydrogel for adsorption of chromium ions

A facile, environmentally benign approach had been developed for the preparation of dual self-healing and adsorption hydrogel through hydroxypropyl chitosan (HPCS), polyacrylamide (PAM) and polyvinyl alcohol (PVA). The self-healing capability of the hydrogels without any external stimulus was ascribed to dynamic Schiff-base bonds, borate bonds and hydrogen bonds, while the adsorption capacity of hydrogels came from the protonated amino group effect at a specific pH. It was demonstrated that the HPP DN hydrogel had a maximum equilibrium swelling ratio of 643% and a maximum compressive strength of 267 kPa. The weight loss of HPP DN hydrogel was 14.26% lower than that of HPCS/PAM single network hydrogel, furthermore, HPP DN hydrogel could achieve self-healing within 10 h. Due to the large number of active groups, the adsorption capacity of Cr⁶⁺ reached 95.31 mg/g. It could adsorb in a wide pH range of 1 to 6, and could describe by pseudo-first-order kinetic model and Langmuir adsorption isotherm model, which would provide a new idea for the adsorption and removal of heavy metal ions. In short, the prepared HPP hydrogel had dual self-healing ability, adsorption capacity and mechanical strength, which would make it a promising candidate for long-life adsorbent.

A hybrid polyvinyl alcohol/molybdenum disulfide nanosheet hydrogel with light-triggered rapid self-healing capability

By employing the excellent photothermal conversion properties of molybdenum disulfide (MoS₂) nanosheets, a hybrid hydrogel was prepared with light-triggered rapid self-healing capability.

Fabrication of Double-Network Hydrogels with Universal Adhesion and Superior Extensibility and Cytocompatibility by One-Pot Method

Hydrogels, which demand simultaneously tailorable mechanical properties and excellent biocompatibility, act as a promoting material for biomedical applications, e.g., tissue engineering scaffolds, wound dressing materials, and cartilage substitutes. Double-network hydrogels (DN hydrogels) have attracted widespread concerns due to their extraordinary mechanical strength and toughness, while traditional DN hydrogels are limited in terms of their biofunctionality. Based on the DN hydrogels composed of agar and acrylamide (AM), we incorporate vinylphosphoric acid (VPA) into the network to obtain agar/PAM/PVPA hydrogels with universal adhesion and superior cytocompatibility. Meanwhile, the agar/PAM/PVPA hydrogel maintains its high strength and toughness. It is noted that the elongation of the agar/PAM/PVPA hydrogel (molar ratio of VPA is 2%) is up to 3418.9 ± 54.9%. The cell experiment also demonstrates that the addition of VPA in a proper concentration can promote cell adhesion and proliferation. Furthermore, the hydrogel has the potential to be used as 3D printing and injectable materials because of the thermoreversible sol-gel agar. The reported agar/PAM/PVPA hydrogel in this work with universal adhesion, excellent mechanical properties, and excellent cytocompatibility is able to be used for biomedical applications as scaffolds, wound dressing materials, or cartilage repair materials.

Effects of Molar Ratios of Two Immiscible Monomers toward Development of an Amphiphilic, Highly Stretchable, Bioadhesive, Self-Healing Copolymeric Hydrogel and its Mineral-Active Cellular Behavior

Here, we report the striking properties such as high stretchability, self-healing, and adhesiveness of an amphiphilic copolymeric hydrogel (poly(acrylic acid)-poly(methyl methacrylate) (PAA-PMMA) gel) synthesized from two immiscible monomers-acrylic acid (AA) and methyl methacrylate (MMA)-through a simple free radical polymerization in an aqueous medium. The developed hydrogel, with a specific molar ratio of MMA and AA, is self-healable, which is attributed to the hydrophobic interaction arising from methyl groups of PMMA, as well as the breakdown and reformation of sacrificial noncovalent cross-linking through the weak hydrogen bonds between the carboxylic acid groups of PAA and methoxy groups of PMMA. The energy dissipation values in the hysteresis test signify the excellent self-recoverability of the hydrogel. The developed hydrogel showed adhesive behavior to the surfaces of polystyrene, glass, wood, metal, stone, ceramics, pork skin, and human skin. The physical and mechanical properties of the PAA-PMMA gel were fine-tuned through changes in the MMA/AA ratio and pH. Moreover, the PAA-PMMA hydrogel can serve as a template for calcium phosphate mineralization to yield a hydrogel composite, which improved MC3T3 cell adhesion and proliferation. Overall, we propose that depending on synthesis parameters and other scenarios, the synthesized PAA-PMMA hydrogel could potentially be employed in varying biomedical and industrial applications.

Dynamic covalent bonds in self-healing, shape memory, and controllable stiffness hydrogels

A review of hydrogels containing dynamic bonds that are shown to provide benefits for applications including self-healing and stimuli-induced stiffness changes.

High-performance poly(acrylic acid) hydrogels formed with a block copolymer crosslinker containing amino-acid derivatives

Two block copolymers containing two amino-acid derivatives, PEO-b-PLAA and PEO-b-PAAC, were fabricated through atom transfer radical polymerization (ATRP) or reversible addition-fragmentation chain transfer polymerization (RAFT). Then, they were employed as a macro-crosslinker to prepare high-performance poly(acrylic acid) (PAA) hydrogels named "PxAy" or "TyAz". There were numerous synergistic noncovalent interactions with hydrogen bonds between the macro-crosslinker and PAA chains, as well as entanglement of polymer chains. Hence, the hydrogels exhibited desirable mechanical properties and self-healing abilities. For PxAy hydrogels, the maximum fracture elongation and fracture strength were 9800% and 120.01 kPa, respectively. Moreover, the enhanced physical interaction enabled the hydrogels to have rapid self-healing abilities without stimulation. The hydrogels showed >80% self-healing efficiency and exhibited ∼10-3 S cm-1 electrical conductivity upon the introduction of KCl. Meanwhile, benefitting from doubling the number of carboxyl groups in the macro-crosslinker of the TyAz hydrogels compared with the PxAy hydrogels, the mechanical properties of TyAz hydrogels could be promoted further and notch-insensitivity could be observed. Tough, adhesive, self-healable, and conductive PAA hydrogels with different structures of amino-acid derivatives could aid the development of macro-crosslinkers.

Multifunctional Biomedical Adhesives

Currently available biomedical adhesives are mainly engineered to have one function (i.e., providing mechanical support for the repaired tissue). To improve the performance of existing bioadhesives and broaden their applications in medicine, numerous multifunctional bioadhesives are reported in the literature. These adhesives can be categorized as passive or active by design. Passive multifunctional bioadhesives contain inherent compositions and structural designs that can carry out additional functions without added external influences. These adhesives exhibit new functionalities such as antimicrobial properties, self-healing abilities, the ability to promote cellular ingrowth, and the ability to be reshaped. Conversely, active multifunctional bioadhesives respond to environmental changes (e.g., pH, temperature, electricity, light, and biomolecule concentration), which initiate a change in the adhesive to release encapsulated drugs or to activate or deactivate the bioadhesive for interfacial binding. This review article highlights recent advances in multifunctional bioadhesives.

A review on cellulose nanocrystals as promising biocompounds for the synthesis of nanocomposite hydrogels

Hydrogels are hydrophilic cross-linked polymer networks formed via the simple reaction of one or more monomers with the ability to retain a significant extent of water. Owing to an increased demand for environmentally friendly, biodegradable, and biocompatible products, cellulose nanocrystals (CNCs) with high hydrophilicity have emerged as a promising sustainable material for the formation of hydrogels. The cytocompatibility, swellability, and non-toxicity make CNC hydrogels of great interest in biomedical, biosensing, and wastewater treatment applications. There has been a considerable progress in the research of CNC hydrogels, as the number of scientific publications has exponentially increased (>600%) in the last five years. In this paper, recent progress in CNC hydrogels with particular emphasis on design, materials, and fabrication techniques to control hydrogel architecture, and advanced applications are discussed.

Multiple Weak H-Bonds Lead to Highly Sensitive, Stretchable, Self-Adhesive, and Self-Healing Ionic Sensors

Herein, we demonstrate a ternary ionic hydrogel sensor consisting of tannic acid, sodium alginate, and covalent cross-linked polyacrylamide as skin-mountable and wearable sensors. Based on the multiple weak H-bonds and synergistic effects between the three components, the as-prepared hybrid hydrogel exhibits ultrastretchability with high elasticity, good self-healing, excellent conformability, and high self-adhesiveness to diverse substrates both in air and underwater. More importantly, the ternary hydrogel exhibits high strain sensitivity especially under subtle strains with a gauge factor of 2.0, which is close to the theoretical value of the ionic hydrogel sensors; an extremely large workable range of strain (0.05-2100%); and a low operating voltage 0.07 V. Consequently, the sensor demonstrates superior sensing performance for real-time monitoring of the large and subtle human motions, including limb motions, swallowing, smiling, and wrist pulse. Therefore, it is believed that the STP hydrogel has great potential applications in health monitoring, smart wearable devices, and soft robots.

Mimicking Dynamic Adhesiveness and Strain-Stiffening Behavior of Biological Tissues in Tough and Self-Healable Cellulose Nanocomposite Hydrogels

Although self-healing gels with structural resemblance to biological tissues attract great attention in biomedical fields, it remains a dilemma for combination between fast self-healing properties and high mechanical toughness. On the basis of the design of dynamic reversible cross-links, we incorporate rigid tannic acid-coated cellulose nanocrystal (TA@CNC) motifs into the poly(vinyl alcohol) (PVA)-borax dynamic networks for the fabrication of a high toughness and rapidly self-healing nanocomposite (NC) hydrogel, together with dynamically adhesive and strain-stiffening properties that are particularly indispensable for practical applications in soft tissue substitutes. The results demonstrate that the obtained NC gels present a highly interconnected network, where flexible PVA chains wrap onto the rigid TA@CNC motifs and form the dynamic TA@CNC-PVA clusters associated by hydrogen bonds, affording the critical mechanical toughness. The synergetic interactions between borate-diol bonds and hydrogen bonds impart a typical self-healing behavior into the NC gels, allowing the dynamic cross-linked networks to undergo fast rearrangement in the time scale of seconds. Moreover, the obtained NC hydrogels not only mimic the main feature of biological tissues with the unique strain-stiffening behavior but also display unique dynamic adhesiveness to nonporous and porous substrates. It is expected that this versatile approach opens up a new prospect for the rational design of multifunctional cellulosic hydrogels with remarkable performance to expand their applications.

Supramolecular and dynamic covalent hydrogel scaffolds: from gelation chemistry to enhanced cell retention and cartilage regeneration

This review highlights the most recent progress in gelation strategies of biomedical supramolecular and dynamic covalent crosslinking hydrogels and their applications for enhancing cell retention and cartilage regeneration.

Quadruple hydrogen bonds and thermo-triggered hydrophobic interactions generate dynamic hydrogels to modulate transplanted cell retention

Supramolecular dynamic hydrogels with quadruple hydrogen bonds and thermo-triggered hydrophobic interactions demonstrate a promising capability of modulating transplanted cell retention.

Engineering natural matrices with black phosphorus nanosheets to generate multi-functional therapeutic nanocomposite hydrogels

Natural matrices are engineered with black phosphorus nanosheets to generate therapeutic nanocomposite hydrogels with promising multi-functions, providing a facile and efficient therapeutic strategy for bone tissue engineering.