Injectable, Self-Healing, and Multi-Responsive Hydrogels via Dynamic Covalent Bond Formation between Benzoxaborole and Hydroxyl Groups

Stable Antifouling and Antibacterial Coating Based on Assembly of Copper-Phenolic Networks

Biofilm formation on medical devices has become a worldwide issue arising from its resistance to bactericidal agents and presenting challenges to eradicating biofouling adhesion, especially in biological fluids. Metal-phenolic networks have been demonstrated as a versatile and efficient strategy to prevent biofilm formation by endowing medical devices with prolonged antifouling and antibacterial activities in a one-step surface modification. In this study, we report a simple and environmentally friendly method using coordination chemistry between copper ions (Cu2+) and dopamine-containing copolymer to fabricate metal-phenolic network-based coatings. The phenolic groups also imparted the adhesion of glycopolymer-containing dopamine residues to inorganic and organic substrates, resulting in dual antifouling and bactericidal surfaces. 2-gluconamidoethyl methacrylamide monomer (GAEMA) was first copolymerized with dopamine methacrylamide (DMA) using a free-radical polymerization process. The resulting copolymer (GAEMA-DMA), denoted as GADMA, was then mixed with copper ions in a one-step process to form the GADMA-Cu coating. The GADMA-Cu coating was hydrophilic and significantly reduced the water contact angle (WCA) and adsorption of bovine serum albumin protein even after incubation in a bovine serum albumin solution for 30 h. Moreover, the coating exhibited strong antibacterial activity against Escherichia coli and Staphylococcus aureus and was biocompatible with 99% cell viability toward normal human fibroblast (HDFa) cells. Thus, our coating shows great potential for application in medical devices.

Biomedical Application of Enzymatically Crosslinked Injectable Hydrogels

Hydrogels have garnered significant interest in the biomedical field owing to their tissue-like properties and capability to incorporate various fillers. Among these, injectable hydrogels have been highlighted for their unique advantages, especially their minimally invasive administration mode for implantable use. These injectable hydrogels can be utilized in their pristine forms or as composites by integrating them with therapeutic filler materials. Given their primary application in implantable platforms, enzymatically crosslinked injectable hydrogels have been actively explored due to their excellent biocompatibility and easily controllable mechanical properties for the desired use. This review introduces the crosslinking mechanisms of such hydrogels, focusing on those mediated by horseradish peroxidase (HRP), transglutaminase (TG), and tyrosinase. Furthermore, several parameters and their relationships with the intrinsic properties of hydrogels are investigated. Subsequently, the representative biomedical applications of enzymatically crosslinked-injectable hydrogels are presented, including those for wound healing, preventing post-operative adhesion (POA), and hemostasis. Furthermore, hydrogel composites containing filler materials, such as therapeutic cells, proteins, and drugs, are analyzed. In conclusion, we examine the scientific challenges and directions for future developments in the field of enzymatically crosslinked-injectable hydrogels, focusing on material selection, intrinsic properties, and filler integration.

Boronate Ester Hydrogels for Biomedical Applications: Challenges and Opportunities

Boronate ester (BE) hydrogels are increasingly used for biomedical applications. The dynamic nature of these molecular networks enables bond rearrangement, which is associated with viscoelasticity, injectability, printability, and self-healing, among other properties. BEs are also sensitive to pH, redox reactions, and the presence of sugars, which is useful for the design of stimuli-responsive materials. Together, BE hydrogels are interesting scaffolds for use in drug delivery, 3D cell culture, and biofabrication. However, designing stable BE hydrogels at physiological pH (≈7.4) remains a challenge, which is hindering their development and biomedical application. In this context, advanced chemical insights into BE chemistry are being used to design new molecular solutions for material fabrication. This review article summarizes the state of the art in BE hydrogel design for biomedical applications with a focus on the materials chemistry of this class of materials. First, we discuss updated knowledge in BE chemistry including details on the molecular mechanisms associated with BE formation and breakage. Then, we discuss BE hydrogel formation at physiological pH, with an overview of the main systems reported to date along with new perspectives. A last section covers several prominent biomedical applications of BE hydrogels, including drug delivery, 3D cell culture, and bioprinting, with critical insights on the design relevance, limitations and potential.

Cation-π Interaction-Enhanced Self-Healing Injectable Hydrogels for Gastric Perforation Repair

Surgical operations are the preferred treatment for gastric perforation (GP) but incur postoperative complications such as gastrointestinal adhesions and bacterial infections, leading to inefficient wound healing and serious complications that may even threaten the life of the patient. Developing hydrogel dressings capable of adapting to the gastric environment (acid) and decreasing visceral adhesions and bacterial infections after GP treatment is crucial. In this article, we developed an injectable, self-healing hydrogel using cation-π interactions between protonated amines and aromatic rings under acidic conditions and explored it for GP repair. The hydrogels demonstrate exceptional self-healing capabilities under acidic conditions and can be effectively tailored for the gastric environment. In addition, the hydrogel demonstrated significant efficacy in preventing gastrointestinal adhesion, reducing inflammation, promoting angiogenesis, and effectively facilitating wound healing in a rat GP model. This novel hydrogel demonstrates adaptability to the gastric environment, rendering it highly promising for potential applications in gastric trauma healing.

Self-Healing Guar Gum-Based Nanocomposite Hydrogel Promotes Infected Wound Healing through Photothermal Antibacterial Therapy

Preventing bacterial infections is a crucial aspect of wound healing. There is an urgent need for multifunctional biomaterials without antibiotics to promote wound healing. In this study, we fabricated a guar gum (GG)-based nanocomposite hydrogel, termed GBTF, which exhibited photothermal antibacterial therapy for infected wound healing. The GBTF hydrogel formed a cross-linked network through dynamic borate/diol interactions between GG and borax, thereby exhibiting simultaneously self-healing, adaptable, and injectable properties. Additionally, tannic acid (TA)/Fe3+ nanocomplexes (NCs) were incorporated into the hydrogel to confer photothermal antibacterial properties. Under the irradiation of an 808 nm near-infrared laser, the TA/Fe3+ NCs in the hydrogel could rapidly generate heat, leading to the disruption of bacterial cell membranes and subsequent bacterial eradication. Furthermore, the hydrogels exhibited good cytocompatibility and hemocompatibility, making them a precandidate for preclinical and clinical applications. Finally, they could significantly promote bacteria-infected wound healing by reducing bacterial viability, accelerating collagen deposition, and promoting epithelial remodeling. Therefore, the multifunctional GBTF hydrogel, which was composed entirely of natural substances including guar gum, borax, and polyphenol/ferric ion NCs, showed great potential for regenerating infected skin wounds in clinical applications.

Triple-Responsive, Multimodal, Visual Electronic Skin toward All-in-One Health Management for Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) is one of the most common metabolic disorders during pregnancy, leading to serious complications for pregnant women and a threat to life safety of infants. Therefore, it is particularly important to establish a multipurpose monitoring pathway to important physiological indicators of pregnant women. In this work, three kinds of double network hydrogels are prepared with poly(vinyl alcohol) (PVA), borax, and cellulose ethers with varying substituents of methyl (methyl cellulose, MC), hydroxypropyl (hydroxypropyl cellulose, HPC), or both (hydroxypropyl methyl cellulose, HPMC), respectively. The corresponding toughness (143.9, 102.3, and 135.9 kJ cm-3) and conductivity (0.69, 0.45, and 0.51 S m-1) of the hydrogels demonstrate that PB-MC was endowed with the prominent performance. Molecular dynamics simulations further revealed the essence that hydrogen bond interactions between PVA and cellulose ethers play a critical role in regulating the structure and properties of hydrogels. Thermochromic capsule powders (TCPs) were subsequently doped in to achieve a composite hydrogel (TCPs@PB-MC) to indicate the change in human body temperature. Furthermore, the process of the TCPs@PB-MC response to glucose, pH, and temperature was tracked in-depth through the electrochemical window. This work provides a novel strategy for all-in-one health management of GDM.

Fe3+-induced coordination cross-linking gallic acid-carboxymethyl cellulose self-healing hydrogel

Self-healing hydrogel is a promising soft material for applications in wound dressings, drug delivery, tissue engineering, biomimetic electronic skin, and wearable electronic devices. However, it is a challenge to fabricate the self-healing hydrogels without external stimuli. Inspired by mussel, the metal-catechol complexes were introduced into the hydrogel systems to prepare the mussel-inspired hydrogels by regulating the gelation kinetics of Fe3+ crosslinkers with gallic acid (GA) in this research. The amine-functionalized carboxymethyl cellulose (CMC) was grafted with GA and then chelated with Fe3+ to form a multi-response system. The crosslinking of carboxymethyl cellulose-ethylenediamine-gallic acid (CEG) hydrogel was controlled by adjusting the pH to affect the iron coordination chemistry, which could enhance the self-healing properties and mechanical strength of hydrogels. In addition, the CEG hydrogel exhibited great antibacterial and antioxidant properties. And the CEG hydrogel could strongly adhere to the skin tissue. The adhesion strength of CEG hydrogel on pigskin was 11.44 kPa, which is higher than that of commercial wound dressings (∼5 kPa). Moreover, the thixotropy of the CEG hydrogel was confirmed with rheological test. In summary, it has great potential in the application field of wound dressing.

Injectable plant-derived polysaccharide hydrogels with intrinsic antioxidant bioactivity accelerate wound healing by promoting epithelialization and angiogenesis

Skin wound healing is a complex and dynamic process involving hemostasis, inflammatory response, cell proliferation and migration, and angiogenesis. Currently used wound dressings remain unsatisfactory in the clinic due to the lack of adjustable mechanical property for injection operation and bioactivity for accelerating wound healing. In this work, an "all-sugar" hydrogel dressing is developed based on dynamic borate bonding network between the hydroxyl groups of okra polysaccharide (OP) and xyloglucan (XG). Benefiting from the reversible crosslinking network, the resulting composite XG/OP hydrogels exhibited good shear-thinning and fast self-healing properties, which is suitable to be injected at wound beds and filled into irregular injured site. Besides, the proposed XG/OP hydrogels showed efficient antioxidant capacity by scavenging DPPH activity of 73.9 %. In vivo experiments demonstrated that XG/OP hydrogels performed hemostasis and accelerated wound healing with reduced inflammation, enhanced collagen deposition and angiogenesis. This plant-derived dynamic hydrogel offers a facile and effective approach for wound management and has great potential for clinical translation in feature.

Multifunctional Hydrogels for the Healing of Diabetic Wounds

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

Elemental exchange: Bioisosteric replacement of phosphorus by boron in drug design

Continuous efforts are being directed toward the employment of boron in drug design due to its advantages and unique characteristics including a plethora of target engagement modes, lower metabolism, and synthetic accessibility, among others. Phosphates are components of multiple drug molecules as well as clinical candidates, since they play a vital role in various biochemical functions, being components of nucleotides, energy currency- ATP as well as several enzyme cofactors. This review discusses the unique chemistry of boron functionalities as phosphate bioisosteres - "the boron-phosphorus elemental exchange strategy" as well as the superiority of boron groups over other commonly employed phosphate bioisosteres. Boron phosphate-mimetics have been utilized for the development of enzyme inhibitors as well as novel borononucleotides. Both the boron functionalities described in this review-boronic acids and benzoxaboroles-contain a boron connected to two oxygens and one carbon atom. The boron atom of these functional groups coordinates with a water molecule in the enzyme site forming a tetrahedral molecule which mimics the phosphate structure. Although boron phosphate-mimetic molecules - FDA-approved Crisaborole and phase II/III clinical candidate Acoziborole are products of the boron-phosphorus bioisosteric elemental exchange strategy, this technique is still in its infancy. The review aims to promote the use of this strategy in future medicinal chemistry projects.

Photothermal Modulation of Dynamic Covalent Poly(ethylene glycol)/PEDOT Composite Hydrogels for On-Demand Drug Delivery

Hydrogels are cross-linked three-dimensional polymer networks that have tissue-like properties. Dynamic covalent bonds (DCB) can be utilized as hydrogel cross-links to impart injectability, self-healing ability, and stimuli responsiveness to these materials. In our research, we utilized dynamic thiol-Michael bonds as cross-links in poly(ethylene glycol) (PEG)-based hydrogels. Because the equilibrium of the reversible, exothermic thiol-Michael reaction can be modulated by temperature, we investigated the possibility of using thermal and photothermal stimuli to modulate the gel-to-sol transition of these materials with the aim of developing an on-demand pulsatile cargo release system. For this purpose, we incorporated poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles within the hydrogel to facilitate photothermal modulation using near-infrared light. PEDOT nanoparticles of 50 nm in diameter and with strong near-infrared absorption were prepared by oxidative emulsion polymerization. We then used Michael addition of thiol-ene pairs from 4-arm PEG-thiol (PEG-SH) and 4-arm PEG-benzylcyanoacetamide (PEG-BCA) to form dynamically cross-linked hydrogels. PEDOT nanoparticles were entrapped in situ to form Gel/PEDOT composites. Rheology and inverted tube test studies showed that the gel-to-sol transition occurred at 45-50 °C for 5 wt % gels and that this transition could be tailored by varying the wt % of the polymer precursors. The hydrogels were found to be capable of self-healing and being injected with a clinically relevant injection force. Bovine serum albumin-fluorescein isothiocyanate (BSA-FITC), a fluorescently labeled protein, was then loaded into the Gel/PEDOT as a therapeutic mimic. Increased release of BSA-FITC upon direct thermal stimulation and photothermal stimulation with an 808 nm laser was observed. Pulsatile release of BSA-FITC over seven cycles was demonstrated. MTS and live-dead assays demonstrated that Gel/PEDOT was cytocompatible in MDA-MB-231 breast cancer and 3T3 fibroblast cell lines. Further studies demonstrated that the encapsulation and laser-triggered release of the chemotherapeutic agent doxorubicin (DOX) could also be achieved. Altogether, this work advances our understanding of the temperature-dependent behavior of a dynamic covalent hydrogel, Gel/PEDOT, and leverages that understanding for application as a photothermally responsive biomaterial for controlled release.

Solute Stabilization Effects of Nanoparticles Containing Boronic Acids in the Absence of Binding Pairs

Boronic acids are widely used in materials science because of their ability to reversibly bind with diol and catechol moieties through dynamic covalent interactions in a pH- and oxidative-dependent manner. Considerably fewer studies focus on property modulation of boronic acid-based materials in the absence of a biding pair. Herein, we discuss the effects of the boronic acid-containing polymer block length on solute release kinetics from nanoparticles in a stimuli-responsive manner for on-demand delivery. In this study, ABC-type linear amphiphiles of poly(d,l-lactide) and poly(2-methacryloyloxyethyl phosphorylcholine) containing a middle block functionalized with 3-aminophenylboronic acid were synthesized by a combination of ring-opening and controlled free radical polymerizations. Nile red-loaded nanoparticles were self-assembled using a multi-inlet vortex mixer in a well-controlled manner. Release was evaluated at pH above and below the pKa of the boronic acid and in the presence of hydrogen peroxide. Our results show that release kinetics from nanoparticles incorporating a boronic acid-functionalized interlayer were slower than those without it, and the rate could be modulated according to pH and oxidative conditions. These effects can be attributed to several factors, including the hydrophobicity of the boronic acid block as well as hydrogen bonding interactions existing between locally confined boronic acids. While boronic acids are generally utilized as boronic/boronate esters, their stabilizing effects in the absence of appropriate binding pairs are relevant and should be considered in the design of boronic acid-based technologies.

Thermo-Responsive Hydrogels Encapsulating Targeted Core-Shell Nanoparticles as Injectable Drug Delivery Systems

As therapeutic agents that allow for minimally invasive administration, injectable biomaterials stand out as effective tools with tunable properties. Furthermore, hydrogels with responsive features present potential platforms for delivering therapeutics to desired sites in the body. Herein, temperature-responsive hydrogel scaffolds with embedded targeted nanoparticles were utilized to achieve controlled drug delivery via local drug administration. Poly(N-isopropylacrylamide) (pNIPAM) hydrogels, prepared with an ethylene-glycol-based cross-linker, demonstrated thermo-sensitive gelation ability upon injection into environments at body temperature. This hydrogel network was engineered to provide a slow and controlled drug release profile by being incorporated with curcumin-loaded nanoparticles bearing high encapsulation efficiency. A core (alginate)-shell (chitosan) nanoparticle design was preferred to ensure the stability of the drug molecules encapsulated in the core and to provide slower drug release. Nanoparticle-embedded hydrogels were shown to release curcumin at least four times slower compared to the free nanoparticle itself and to possess high water uptake capacity and more mechanically stable viscoelastic behavior. Moreover, this therapy has the potential to specifically address tumor tissues over-expressing folate receptors like ovaries, as the nanoparticles target the receptors by folic acid conjugation to the periphery. Together with its temperature-driven injectability, it can be concluded that this hydrogel scaffold with drug-loaded and embedded folate-targeting nanoparticles would provide effective therapy for tumor tissues accessible via minimally invasive routes and be beneficial for post-operative drug administration after tumor resection.

Functionalized Hydrogel-Based Wearable Gas and Humidity Sensors

Breathing is an inherent human activity; however, the composition of the air we inhale and gas exhale remains unknown to us. To address this, wearable vapor sensors can help people monitor air composition in real time to avoid underlying risks, and for the early detection and treatment of diseases for home healthcare. Hydrogels with three-dimensional polymer networks and large amounts of water molecules are naturally flexible and stretchable. Functionalized hydrogels are intrinsically conductive, self-healing, self-adhesive, biocompatible, and room-temperature sensitive. Compared with traditional rigid vapor sensors, hydrogel-based gas and humidity sensors can directly fit human skin or clothing, and are more suitable for real-time monitoring of personal health and safety. In this review, current studies on hydrogel-based vapor sensors are investigated. The required properties and optimization methods of wearable hydrogel-based sensors are introduced. Subsequently, existing reports on the response mechanisms of hydrogel-based gas and humidity sensors are summarized. Related works on hydrogel-based vapor sensors for their application in personal health and safety monitoring are presented. Moreover, the potential of hydrogels in the field of vapor sensing is elucidated. Finally, the current research status, challenges, and future trends of hydrogel gas/humidity sensing are discussed.

Improving ocular bioavailability of hydrophilic drugs through dynamic covalent complexation

The clinical benefits of diquafosol tetrasodium (DQS), a hydrophilic P2Y2 receptor agonist for dry eye, have been hindered by a demanding dosing regimen. Nevertheless, it is challenging to achieve sustained release of DQS with conventional drug delivery vehicles which are mainly designed for hydrophobic small molecule drugs. To address this, we developed an affinity hydrogel for DQS by taking advantage of borate-mediated dynamic covalent complexation between DQS and hydroxypropyl guar. The resultant formulation (3% DQS Gel) was characterized by sustained release, low corneal permeation, and extended ocular retention, which were desirable attributes for ocular surface drug delivery. Both in vitro and in vivo studies had been carried out to verify the biocompatibility of 3% DQS Gel. Using corneal fluorescein staining, the Schirmer's test, PAS staining, quantitative PCR and immunohistological analyses as outcome measures, the superior therapeutic effects of 3% DQS Gel over PBS, the hydrogel vehicle and free DQS were demonstrated in a mouse dry eye model. Our DQS delivery strategy reported herein is readily applicable to other hydrophilic small molecule drugs with cis-diol moieties, thus providing a general solution to improve clinical outcomes of numerous diseases.

Self-Healing Alginate Hydrogel Formed by Dynamic Benzoxaborolate Chemistry Protects Retinal Pigment Epithelium Cells against Oxidative Damage

Oxidative stress is considered as a major factor causing retinal pigment epithelium (RPE) dysfunction and finally leading to retinal diseases such as age-related macular degeneration (AMD). Developing hydrogels for RPE cell delivery, especially those with antioxidant feature, is emerging as a promising approach for AMD treatment. Herein, a readily prepared antioxidant alginate-based hydrogel was developed to serve as a cytoprotective agent for RPE cells against oxidative damage. Alg-BOB was synthesized via conjugation of benzoxaborole (BOB) to the polysaccharide backbone. Hydrogels were formed through self-crosslinking of Alg-BOB based on benzoxaborole-diol complexation. The resulting hydrogel showed porous micro-structure, pH dependent mechanical strength and excellent self-healing, remolding, and injectable properties. Moreover, the hydrogel exhibited excellent cytocompatibility and could efficiently scavenge reactive oxygen species (ROS) to achieve an enhanced viability of ARPE-19 cells under oxidative condition. Altogether, our study reveals that the antioxidant Alg-BOB hydrogel represents an eligible candidate for RPE delivery and AMD treatment.

Dual Dynamic Covalently Crosslinked Alginate Hydrogels with Tunable Properties and Multiple Stimuli-Responsiveness

Alginate is a biopolymer that can be crosslinked with calcium ions to fabricate cytocompatible hydrogels. However, using calcium ions to crosslink alginate provides limited properties and functions to alginate hydrogels, restricting their biomedical applications. Here, phenylboronic acid-functionalized polyethyleneimine (PBA-PEI) was developed to introduce two orthogonal dynamic covalent crosslinks in the alginate hydrogels, where PBA-PEI was used to crosslink alginate dialdehyde (ADA) through imine bonds and boronate ester bonds. The grafting degree of PBA in the PEI structure was applied to fine-tune the properties of PBA-PEI/ADA hydrogels, including the rheological property, mechanical strength, swelling behavior, and antibacterial activity. In particular, the highly sensitive boronate ester bonds in the network enabled PBA-PEI/ADA hydrogels to be responsive to several stimuli, such as glucose, fructose, and hydrogen peroxide. Taken together, PBA-PEI/ADA hydrogels with tunable properties and multiple stimuli-responsiveness have been demonstrated as smart biomaterials for advanced biomedical applications.

Fabrication of lignin reinforced hybrid hydrogels with antimicrobial and self-adhesion for strain sensors

Ionic conductive hydrogels prepared from various biological macromolecules are ideal materials for the manufacture of human motion sensors from the perspective of resource regeneration and environmental sustainability. However, it is still challenging to prepare hydrogels with both high toughness and self-healing ability. In this study, lignin-based β-CD-PVA (LCP) self-healing conductive hydrogels with high tensile properties were prepared by one-step method using alkali lignin as a plasticizer. Compared with PVA hydrogel, the maximum storage modulus and elongation were increased by 2.5 and 20.0 times, respectively. Uniform distribution of lignin can increase the fluidity and distance of polymer molecular chains, thus improving the viscoelastic and tensile properties of the LCP self-healing hydrogel. LCP hydrogels can maintain self-healing ability in both high (45 °C) and low temperature (0 °C) environments, and the self-healing ability is not affected by pH. Moreover, it also has good conductivity, anti-bacterial, thermostability, and anti-UV property, which has a good application prospect in the field of 3D printing and wearable electronic devices, which expands the efficient utilization of lignin in biorefinery.

Tailor-made Glycopolymers via Reversible Deactivation Radical Polymerization: Design, Properties and Applications

Investigating the underlying mechanism of biological interactions using glycopolymer is becoming increasingly important owing to their unique recognition properties. The multivalent interactions between lectin and glycopolymer are significantly influenced by...

Dual-Responsive Material Based on Catechol-Modified Self-Immolative Poly(Disulfide) Backbones

Functional materials engineered to degrade upon triggering are in high demand due their potentially lower impact on the environment as well as their use in sensing and in medical applications. Here, stimuli-responsive polymers are prepared by decorating a self-immolative poly(dithiothreitol) backbone with pendant catechol units. The highly functional polymer is fashioned into stimuli-responsive gels, formed through pH-dependent catecholato-metal ion cross-links. The gels degrade in response to specific environmental changes, either by addressing the pH responsive, non-covalent, catecholato-metal complexes, or by addition of a thiol. The latter stimulus triggers end-to-end depolymerization of the entire self-immolative backbone through end-cap replacement via thiol-disufide exchanges. Gel degradation is visualized by release of a dye from the supramolecular gel as it itself is converted into smaller molecules.

Injectable Self-Healing Hydrogel via Biological Environment-Adaptive Supramolecular Assembly for Gastric Perforation Healing

Developing effective internal wound dressing materials is important for postoperative tissue regeneration while remains a challenge due to the poor biological environment-adaptability of conventional materials. Here, we report an example of injectable self-healing hydrogel based on gastric environment-adaptive supramolecular assembly, and have explored its application for gastric perforation healing. By leveraging the gastric environment-modulated supramolecular interactions, the self-assembled hydrogel network is orchestrated with sensitive thermo-responsibility, injectability, printability and rapid self-healing capability. The hydrogel dressing can effectively inhibit the attachment of microorganisms and demonstrates outstanding antibiofouling property. In vivo rat model further demonstrates the as-prepared hydrogel dressing simplifies the surgical procedures, reduces postoperative complications as well as enhances the healing process of gastric perforation compared with the conventional treatment. This work provides useful insights into the development of biological environment-adaptive functional materials for various biomedical applications.

Strategic conceptualization and potential of self-healing polymers in biomedical field

Smart polymeric materials and hydrogels derived from acrylate, epoxy resins, etc. mimic the healing ability of natural organisms and biological cells by showing shape memory and tissue regenerative properties wherein, the healing ability in some of the materials is triggered by external stimuli like temperature, pH and light. This article provides an overview of various conceptual strategies and chemical and mechanical interactions involved in the different types of biomimetic self-healing materials to regain the deformed structure by repairing the cracked shape which play important role in contributing to the structural properties and functional recovery. Also, different chemical bonding like π-π interaction, ligand-metal, hydrogen bonding, etc. takes place at the molecular level for replenishing the damaged structure with greater bond strength. The regeneration ability of artificial self-healing polymeric materials not only shows use in material sciences, engineering but also exhibits a wide range of applications in site-specific drug delivery, skin grafting, implantation, dentistry and bone and tissue regeneration to restore injured surfaces with better biocompatibility, healing efficiency and higher tensile strength to serve as a next-generation material for amplifying the use in biomedical field.

Dual Cross-Linked Hydrogels with Injectable, Self-Healing, and Antibacterial Properties Based on the Chemical and Physical Cross-Linking

Injectable hydrogels have become a promising material for biomedical engineering applications, but microbial infection remains a common challenge in their application. In this study, we presented an injectable antibacterial hydrogel with self-healing property based on a dual cross-linking network structure of dynamic benzoxaborole-sugar and quadruple hydrogen bonds of the 2-ureido-4-pyrimidone (UPy) moieties at physiological pH. Dynamic rheological experiments demonstrated the gelatinous behavior of the double cross-linking network (storage modulus G' > loss modulus G″), and the modulus showed frequency-dependent behavior. The noncovalent interactions of UPy units in the polymer segment endowed the injectable hydrogels with good mechanical strength. By varying the solid contents, UPy units, as well as the pH, the mechanical properties of hydrogels could be controlled. Additionally, the hydrogels exhibited not only excellent self-healing and injectable properties but also pH and sugar dual-responsiveness. Moreover, the hydrogels could effectively inhibit the growth of both Escherichia coli and Staphylococcus aureus while exhibiting low toxicity. 3D cell encapsulation experiment results also demonstrated the potential use of these hydrogels as cell culture scaffolds. Taken together, the injectability, self-healing, and antimicrobial properties of the prepared hydrogels showed great promise for translational medicine, such as cell and tissue engineering applications.

Site-Directed Differentiation of Human Adipose-Derived Mesenchymal Stem Cells to Nucleus Pulposus Cells Using an Injectable Hydroxyl-Functional Diblock Copolymer Worm Gel

Adipose-derived mesenchymal stem cells (ASCs) have been identified for their promising therapeutic potential to regenerate and repopulate the degenerate intervertebral disk (IVD), which is a major cause of lower back pain. The optimal cell delivery system remains elusive but encapsulation of cells within scaffolds is likely to offer a decisive advantage over the delivery of cells in solution by ensuring successful retention within the tissue. Herein, we evaluate the use of a fully synthetic, thermoresponsive poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA-PHPMA) diblock copolymer worm gel that mimics the structure of hydrophilic glycosaminoglycans. The objective was to use this gel to direct differentiation of human ASCs toward a nucleus pulposus (NP) phenotype, with or without the addition of discogenic growth factors TGFβ or GDF6. Accordingly, human ASCs were incorporated into a cold, free-flowing aqueous dispersion of the diblock copolymer, gelation induced by warming to 37 °C and cell culture was conducted for 14 days with or without such growth factors to assess the expression of characteristic NP markers compared to those produced when using collagen gels. In principle, the shear-thinning nature of the biocompatible worm gel enables encapsulated human ASCs to be injected into the IVD using a 21G needle. Moreover, we find significantly higher gene expression levels of ACAN, SOX-9, KRT8, and KR18 for ASCs encapsulated within worm gels compared to collagen scaffolds, regardless of the growth factors employed. In summary, such wholly synthetic worm gels offer considerable potential as an injectable cell delivery scaffold for the treatment of degenerate disk disease by promoting the transition of ASCs toward an NP-phenotype.

Multi-responsive, injectable, and self-healing hydrogels based on benzoxaborole–tannic acid complexation

A bio-inspired, multi-responsive, injectable, and self-healing hydrogel was developed via the interaction of tannic acid (TA) and benzoxaborole-based linear copolymers.

Multi-stimuli-responsive hydrogels and their medical applications

This review highlights the medical applications of multi-stimuli-responsive hydrogels as self-healing hydrogels, antibacterial materials and drug-delivery systems.

Mussel-Inspired Adhesive Double-Network Hydrogel for Intraoral Ultrasound Imaging

Periodontal diseases could be diagnosed through intraoral ultrasound imaging with the advantages of simple operation procedures, low cost, and low safety risks. A couplant is normally placed between transducers and tissues for better ultrasound image quality. If applied intraorally, the couplants should possess good stability in water and robust mechanical properties, as well as strong adhesiveness to transducers and tissues. However, commercial couplants, such as Aquaflex (AF) cannot fulfill these requirements. In this work, inspired by the mussel adhesion mechanism, we reported a poly(vinyl alcohol)-polyacrylamide-polydopamine (PVA-PAM-PDA) hydrogel synthesized by incorporating PDA into the PAM-PVA double-network for intraoral ultrasound imaging. The hydrogel maintains good stability in water as well as exceptional mechanical properties and can adhere to different substrates (i.e., metal, glass, and porcine skin) without losing the original adhesion strength after multiple adhesion-strip cycles. Besides, when applied to porcine mandibular incisor imaging, the PVA-PAM-PDA hydrogel possesses good image quality for diagnosis as AF does. This work provides practical insights into the fabrication of multifunctional hydrogel-based interfaces between human tissues and medical devices for disease diagnosis applications.

Dual-Cross-Linked Network Hydrogels with Multiresponsive, Self-Healing, and Shear Strengthening Properties

Dual-cross-linked network (DCN) hydrogels with multiresponsive and self-healing properties are attracting intensive interests due to their enhanced mechanical strength for a wide range of applications. Herein, we developed a DCN hydrogel that combines a dynamic imine and a benzoxaboronic ester with a neutral pK_a value (∼7.2) as dual linkages and contains biocompatible zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine) [poly(MPC)] as the backbone. Oscillatory rheology result indicated shear strengthening mechanical properties compared to the single-cross-linked network (SCN) hydrogels, which use either imine bond or benzoxaboronic ester as the linkage alone. Due to the coexistence of stimuli-responsive imine and benzoxaboronic ester, the DCN hydrogels show sensitive multiple responsiveness to pH, sugar, and hydrogen peroxide. The dynamic nature of the dual linkages endows the DCN hydrogels with excellent self-healing ability after fracture. More importantly, the excellent biocompatibility and performance in three-dimensional (3D) cell encapsulation were established by a cytotoxicity Live/Dead assay, indicating DCN hydrogel's great potential as a cell culture scaffold. The biocompatible poly(MPC)-based backbone and the rapid formation of the cross-linking network make the DCN hydrogels promising candidates for future biomedical applications.

Physical and Chemical Factors Influencing the Printability of Hydrogel-based Extrusion Bioinks

Bioprinting researchers agree that "printability" is a key characteristic for bioink development, but neither the meaning of the term nor the best way to experimentally measure it has been established. Furthermore, little is known with respect to the underlying mechanisms which determine a bioink's printability. A thorough understanding of these mechanisms is key to the intentional design of new bioinks. For the purposes of this review, the domain of printability is defined as the bioink requirements which are unique to bioprinting and occur during the printing process. Within this domain, the different aspects of printability and the factors which influence them are reviewed. The extrudability, filament classification, shape fidelity, and printing accuracy of bioinks are examined in detail with respect to their rheological properties, chemical structure, and printing parameters. These relationships are discussed and areas where further research is needed, are identified. This review serves to aid the bioink development process, which will continue to play a major role in the successes and failures of bioprinting, tissue engineering, and regenerative medicine going forward.

Constructing Thermally Reversible Dynamic Hydrogels via Catalysis-Free Knoevenagel Condensation

Thermally reversible dynamic covalent bonds (TRDCBs) have attracted great interest for building polymers with self-healing and adaptable properties in bulk. However, none of the developed TRDCBs can be used in aqueous media for the fabrication of thermally reversible dynamic hydrogels due to the requirement of high temperature to initiate the retro-reaction or the susceptibility to hydrolysis. Herein, we report a thermally reversible dynamic covalent C═C double bond that was formed by catalysis-free Knoevenagel condensation (CKC) between benzaldehyde and cyanoacetate end-functionalized polymers in aqueous solution. The as-formed TRDCB shows typical thermal reversibility in the aqueous media under mild temperatures (4-70 °C). Constructing hydrogels with this TRDCB led to the formation of thermally reversible dynamic hydrogels with intriguing self-healing, injectable, thermosensitive, and thermoplastic properties. Overall, this work not only broadens the application of TRDCBs in aqueous media but also provides a thermally reversible dynamic hydrogel for potential use in various biomedical fields.

Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs)

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

NIR/Thermoresponsive Injectable Self-Healing Hydrogels Containing Polydopamine Nanoparticles for Efficient Synergistic Cancer Thermochemotherapy

Injectable and self-healing hydrogels with thermoresponsiveness as smart hydrogels displayed injectability, automatic healing, and phase and volume changes as well. Here, the thermoresponsive self-healing hydrogel was prepared via the formation of dynamic covalent enamine bonds between the amino groups in polyetherimide (PEI) and the acetoacetate groups in the four-armed star-shaped poly(2-(dimethylamino)ethyl methacrylate-co-2-hydroxyethyl methacrylate) modified with tert-butyl acetoacetate (t-BAA), SP(DMAEMA-co-HEMA-AA). After adding polydopamine nanoparticles (PDA NPs), the SP(DMAEMA-co-HEMA-AA)/PEI/PDA-NP nanocomposite hydrogel presented phase change and volume shrinkage under near-infrared (NIR) irradiation. The thermoresponsive nanocomposite hydrogel loaded with the anticancer drug doxorubicin (DOX) could be injected into the 4T1 tumor by intratumoral injection. After NIR laser irradiation, the temperature of the hydrogel increased because of the photothermal effect of PDA NPs inducing local hyperthermia. Because the hydrophilicity-hydrophobicity transition of the hydrogel occurred, DOX molecules were squeezed out from the hydrogel at temperatures higher than its lower critical solution temperature (LCST) and the tumor cells suffered from internal stress from the shrunk hydrogel. The injectable nanocomposite hydrogel not only demonstrated the synergism of highly efficient thermochemotherapy but also showed the function of improving drug utilization and precise treatment to reduce the side effects of drugs.

Multiresponsive and Self-Healing Hydrogel via Formation of Polymer-Nanogel Interfacial Dynamic Benzoxaborole Esters at Physiological pH

Nanocomposite hydrogels with multiresponsiveness and self-healing property are attracting extensive interest due to their enhanced performance for a wide range of applications. In this work, we have successfully developed novel hydrogels based on interfacial polymer-nanogel benzoxaborolate cross-linking at physiological pH. Temperature-sensitive nanogels (NG-Gal) containing galactose residues on the nanosurface were prepared and subsequently used as macro-cross-linkers to form a hydrogel network through formation of dynamic adducts with benzoxaborole groups of a hydrophilic copolymer poly(DMA-st-MAABO). Benefiting from the low pK_a value of benzoxaborole (∼7.2), hydrogels can be constructed rapidly at physiological pH, which is of great significance for biomedical applications. Changing the molar ratio between benzoxaborole and galactose was found to alter the mechanical properties of hydrogels as confirmed by rheological measurements. The dynamic nature of benzoxaborole esters endowed the hydrogel with moldability and self-healing ability after disruption. Moreover, the hydrogel showed multiresponsiveness toward pH, sugar, adenosine triphosphate (ATP), hydrogen peroxide (H₂O₂), and temperature. Therefore, the novel nanocomposite hydrogel we demonstrated here exhibits great potential for biomedical applications such as tissue engineering and controlled drug delivery.

An injectable self-healing hydrogel-cellulose nanocrystals conjugate with excellent mechanical strength and good biocompatibility

In this work, a novel strategy for the construction of injectable self-healing nanocomposite (NC) hydrogels dominated by reversible boronic ester bonds was demonstrated. Specifically, NC hydrogels were constructed by the solution-mixing of N,N-dimethylacrylamide-stat-3-acrylamidophenylboronicacid statistical copolymers (PDMA-stat-PAPBA) and poly(glycerolmonomethacrylate) (PGMA) chains grafted cellulose nanocrystals (CNC-g-PGMA). Rheology analysis indicated the as-constructed NC hydrogel displayed about 7-fold increase in the storage modulus with a low CNCs loading level of 1.43 wt% in comparison with PGMA/PDMA-stat-PAPBA hydrogel without CNCs. Furthermore, the mechanical strength of the CNC-g-PGMA/PDMA-stat-PAPBA hydrogel was far superior to that of its PGMA/PDMA-stat-PAPBA/CNCs hydrogel counterpart, in which PGMA chains were not covalently grafted on the surfaces of CNCs. Due to reversible boronic ester bonds cross-linking networks, CNC-g-PGMA/PDMA-stat-PAPBA NC hydrogel exhibited excellent self-healing and injectable properties as well as pH/glucose responsive sol-gel transitions. Good biocompatibility was also demonstrated through in vitro cytotoxicity tests.

Sustainable Production of Cellulose-Based Hydrogels with Superb Absorbing Potential in Physiological Saline

Nowadays, most of the commonly used superabsorbent polymers (SAPs) are derived from synthetic polymers, particularly acrylic acid and its copolymers made with acrylamide. Here, we describe a novel and environmentally friendly aqueous-based process for fabrication of a new, natural, cellulose-based SAP (hydrogel). In this two-step process, cellulose was first reacted with sodium monochloroacetate (MCA) to obtain carboxymethyl cellulose (CMC) and then cross-linked with epichlorohydrin (ECH). In distilled water (d-water), the water retention value (WRV) of the newly fabricated hydrogels reached 725 g d-water/g gel, which is significantly greater than any other commercially available superabsorbent cellulose-based material (WRV of 10-100 g/g) and comparable to the commercial synthetic (polyacrylate) SAP gels (WRV of up to 1000 g/g). In saline water (s-water; 0.9% NaCl), the maximum WRV attained was 118 g s-water/g gel, which exceeds more than 2-fold the WRV of commercial gels (40-50 g/g). Compositional analysis was carried out to determine the amount of carboxyl groups and average molecular mass, and the parameters for hydrogel preparation were optimized. The natural SAP was characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The hydrogels showed good re-swelling properties losing only 5-10% of their capabilities to reabsorb d-water when reused in four consecutive cycles. Because of their superior swelling properties in physiological saline, the new hydrogels can compete with their synthetic counterparts in applications such as high-value hygiene and biomedical products.

Hydroxyl-Rich PGMA-Based Cationic Glycopolymers for Intracellular siRNA Delivery: Biocompatibility and Effect of Sugar Decoration Degree

The ErbB family of proteins, structurally related to the epidermal growth factor receptor (EGFR), is found to be overexpressed in many cancers such as gliomas, a lung and cervical carcinomas. Gene therapy allows to modify the expression of genes like ErbB and has been a promising strategy to target oncogenes and tumor suppressor genes. In the current work, novel hydroxyl-rich poly(glycidyl methacrylate) (PGMA)-based cationic glycopolymers were designed for intracellular small interfering RNA (siRNA) delivery to silence the EGFR gene. The cationic polymers with different sugar decoration degrees (0, 9, and 33%) were synthesized by ring-opening reaction of PGMA with ethanolamine and a lactobionic acid-derived aminosaccharide (Lac-NH₂). Specific EGFR knockdown of the protein tyrosine kinase ErbB-overexpressing HeLa cells was achieved using these hydroxyl-rich polycation/siRNA complexes. Higher sugar content improved the biocompatibility of the polymers, but it also seems to decrease the EGFR knockdown capability, which should mainly be related to the surface charge of polyplexes. An optimum balance was observed with PGEL-1 (9% sugar content) formulation, achieving ∼52% knockdown efficiency as well as high cell viability. Considering the specific recognition between galactose residues and asialoglycoprotein receptor in hepatocytes, our novel PGMA-based cationic glycopolymers exhibited promising future to serve as a safe and targeting gene delivery vector to hepatoma cell line like HepG2.

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.