Cytocompatible and non-fouling zwitterionic hyaluronic acid-based hydrogels using thiol-ene “click” chemistry for cell encapsulation

Click Chemistry for Biofunctional Polymers: From Observing to Steering Cell Behavior

Click chemistry has become one of the most powerful construction tools in the field of organic chemistry, materials science, and polymer science, as it offers hassle-free platforms for the high-yielding synthesis of novel materials and easy functionalization strategies. The absence of harsh reaction conditions or complicated workup procedures allowed the rapid development of novel biofunctional polymeric materials, such as biopolymers, tailor-made polymer surfaces, stimulus-responsive polymers, etc. In this review, we discuss various types of click reactions─including azide-alkyne cycloadditions, nucleophilic and radical thiol click reactions, a range of cycloadditions (Diels-Alder, tetrazole, nitrile oxide, etc.), sulfur fluoride exchange (SuFEx) click reaction, and oxime-hydrazone click reactions─and their use for the formation and study of biofunctional polymers. Following that, we discuss state-of-the-art biological applications of "click"-biofunctionalized polymers, including both passive applications (e.g., biosensing and bioimaging) and "active" ones that aim to direct changes in biosystems, e.g., for drug delivery, antiviral action, and tissue engineering. In conclusion, we have outlined future directions and existing challenges of click-based polymers for medicinal chemistry and clinical applications.

Methods for determining the structure and physicochemical properties of hyaluronic acid and its derivatives: A review

Hyaluronic acid (HA) is a linear high molecular weight polymer ubiquitously distributed in humans and animals. The D-glucuronic acid and N-acetyl-D-glucosamine repeating disaccharide backbone along with variable secondary and tertiary structures endows HA with unique rheological characteristics as well as diverse biological functions such as maintaining tissue homeostasis and mediating cell functions. Due to its excellent biocompatibility, biodegradability, viscoelasticity and moisturizing properties, natural HA and its chemically modified derivatives are widely used in medical, pharmaceutical, food and cosmetic industries. For broad application purposes, abundant HA-based biochemical products have been developed, including the methodologies for characterization of these products. This review provides an overview focusing on the methods used for determining HA structure as well as the strategies for constructing its derivatives. Apart from the analytical approaches for defining the physicochemical properties of HA (e.g., molecular weight, rheology and swelling capacity), quantitative methods for assessing the purity of HA-based materials are discussed. In addition, the biological functions and potential applications of HA and its derivatives are briefly embarked and perspectives in methodological development are discussed.

Immunomodulatory Biomaterials: Tailoring Surface Properties to Mitigate Foreign Body Reaction and Enhance Tissue Regeneration

The immune cells have demonstrated the ability to promote tissue repair by removing debris, breaking down the extracellular matrix, and regulating cytokine secretion profile. If the behavior of immune cells is not well directed, chronic inflammation and foreign body reaction (FBR) will lead to scar formation and loss of biomaterial functionality. The immunologic response toward tissue repair or chronic inflammation after injury and implantation can be modulated by manipulating the surface properties of biomaterials. Tailoring surface properties of biomaterials enables the regulation of immune cell fate such as adhesion, proliferation, recruitment, polarization, and cytokine secretion profile. This review begins with an overview of the role of immune cells in tissue healing and their interactions with biomaterials. It then discusses how the surface properties of biomaterials influence immune cell behavior. The core focus is reviewing surface modification methods to create innovative materials that reduce foreign body reactions and enhance tissue repair and regeneration by modulating immune cell activities. The review concludes with insights into future advancements in surface modification techniques and the associated challenges.

Cyanobacterial Ampholyte Hydrogels Developed by the Cationization of Sulfated Polysaccharides and Their Cell-Compatibility

Sacran is a cyanobacterial supergiant polysaccharide with carboxylate and sulfate groups that exhibits antiallergic and antiinflammatory properties. However, its high anionic functions restrict cell compatibility. Quaternary ammonium groups were substituted to form sacran ampholytes, and the cell compatibility of the cationized sacran hydrogels was evaluated. The cationization process involved the reaction of N-(3-chloro-2-hydroxypropyl)trimethylammonium chloride with the primary amine or hydroxyl groups of sacran. The degree of cationization ranged from 32 to 87% for sugar residues. Hydrogels of sacran ampholytes were prepared by annealing their dried sheets by thermal cross-linking; these hydrogels exhibited anisotropic expansion properties. The water contact angle on the hydrogels decreased from 26.5 to 15.3° with an increase in the degree of cationization, thereby enhancing hydrophilicity. The IC50 values of sacran ampholytes decreased with an increased degree of cationization, resulting in a reduction in cytotoxicity toward the L-929 mouse fibroblast cell line. This reduction was associated with an increase in the cell proliferation density after 3 days of incubation. Scanning electron microscopy images showed fibroblast intercellular connections. Therefore, the sacran ampholyte hydrogel exhibited increased hydrophilicity and cell compatibility, which is beneficial for various biomedical applications.

Droplet-based microfluidics: an efficient high-throughput portable system for cell encapsulation

One of the goals of tissue engineering and regenerative medicine is restoring primary living tissue function by manufacturing a 3D microenvironment. One of the main challenges is protecting implanted non-autologous cells or tissues from the host immune system. Cell encapsulation has emerged as a promising technique for this purpose. It involves entrapping cells in biocompatible and semi-permeable microcarriers made from natural or synthetic polymers that regulate the release of cellular secretions. In recent years, droplet-based microfluidic systems have emerged as powerful tools for cell encapsulation in tissue engineering and regenerative medicine. These systems offer precise control over droplet size, composition, and functionality, allowing for creating of microenvironments that closely mimic native tissue. Droplet-based microfluidic systems have extensive applications in biotechnology, medical diagnosis, and drug discovery. This review summarises the recent developments in droplet-based microfluidic systems and cell encapsulation techniques, as well as their applications, advantages, and challenges in biology and medicine. The integration of these technologies has the potential to revolutionise tissue engineering and regenerative medicine by providing a precise and controlled microenvironment for cell growth and differentiation. By overcoming the immune system's challenges and enabling the release of cellular secretions, these technologies hold great promise for the future of regenerative medicine.

Chemical modification of hyaluronic acid as a strategy for the development of advanced drug delivery systems

Hyaluronic acid (HA) has emerged as a promising biopolymer for various biomedical applications due to its biocompatibility, biodegradability, and intrinsic ability to interact with cell surface receptors, making it an attractive candidate for drug delivery systems and tissue engineering. Chemical modification of HA has opened up versatile possibilities to tailor its properties, enabling the development of advanced drug delivery systems and biomaterials with enhanced functionalities and targeted applications. This review analyzes the strategies and applications of chemically modified HA in the field of drug delivery and biomaterial development. The first part of the review focuses on the different methods and functional groups used for the chemical modification of HA, highlighting the impact of these modifications on its physicochemical properties, degradation behavior and interactions with drugs. The second part of the review evaluates the use of chemically modified HA in the development of advanced biomedical materials including nano- and microparticles, hydrogels and mucoadhesive materials with tailored drug release profiles, site-specific targeting and stimuli-responsive behavior. Thus, the review consolidates the current advances and future perspectives in the field of chemical modification of HA, underscoring its immense potential to drive the development of advanced drug delivery systems and biomaterials with diverse biomedical applications.

Tailor-Made Polysaccharides for Biomedical Applications

Polysaccharides (PSAs) are carbohydrate-based macromolecules widely used in the biomedical field, either in their pure form or in blends/nanocomposites with other materials. The relationship between structure, properties, and functions has inspired scientists to design multifunctional PSAs for various biomedical applications by incorporating unique molecular structures and targeted bulk properties. Multiple strategies, such as conjugation, grafting, cross-linking, and functionalization, have been explored to control their mechanical properties, electrical conductivity, hydrophilicity, degradability, rheological features, and stimuli-responsiveness. For instance, custom-made PSAs are known for their worldwide biomedical applications in tissue engineering, drug/gene delivery, and regenerative medicine. Furthermore, the remarkable advancements in supramolecular engineering and chemistry have paved the way for mission-oriented biomaterial synthesis and the fabrication of customized biomaterials. These materials can synergistically combine the benefits of biology and chemistry to tackle important biomedical questions. Herein, we categorize and summarize PSAs based on their synthesis methods, and explore the main strategies used to customize their chemical structures. We then highlight various properties of PSAs using practical examples. Lastly, we thoroughly describe the biomedical applications of tailor-made PSAs, along with their current existing challenges and potential future directions.

Exploring the Progress of Hyaluronic Acid Hydrogels: Synthesis, Characteristics, and Wide-Ranging Applications

This comprehensive review delves into the world of hyaluronic acid (HA) hydrogels, exploring their creation, characteristics, research methodologies, and uses. HA hydrogels stand out among natural polysaccharides due to their distinct features. Their exceptional biocompatibility makes them a top choice for diverse biomedical purposes, with a great ability to coexist harmoniously with living cells and tissues. Furthermore, their biodegradability permits their gradual breakdown by bodily enzymes, enabling the creation of temporary frameworks for tissue engineering endeavors. Additionally, since HA is a vital component of the extracellular matrix (ECM) in numerous tissues, HA hydrogels can replicate the ECM's structure and functions. This mimicry is pivotal in tissue engineering applications by providing an ideal setting for cellular growth and maturation. Various cross-linking techniques like chemical, physical, enzymatic, and hybrid methods impact the mechanical strength, swelling capacity, and degradation speed of the hydrogels. Assessment tools such as rheological analysis, electron microscopy, spectroscopy, swelling tests, and degradation studies are employed to examine their attributes. HA-based hydrogels feature prominently in tissue engineering, drug distribution, wound recovery, ophthalmology, and cartilage mending. Crafting HA hydrogels enables the production of biomaterials with sought-after qualities, offering avenues for advancements in the realm of biomedicine.

Large-Scale, Mechanically Robust, Solvent-Resistant, and Antioxidant MXene-Based Composites for Reliable Long-Term Infrared Stealth

MXene-based thermal camouflage materials have gained increasing attention due to their low emissivity, however, the poor anti-oxidation restricts their potential applications under complex environments. Various modification methods and strategies, e.g., the addition of antioxidant molecules and fillers have been developed to overcome this, but the realization of long-term, reliable thermal camouflage using MXene network (coating) with excellent comprehensive performance remains a great challenge. Here, a MXene-based hybrid network comodified with hyaluronic acid (HA) and hyperbranched polysiloxane (HSi) molecules is designed and fabricated. Notably, the presence of appreciated HA molecules restricts the oxidation of MXene sheets without altering infrared stealth performance, superior to other water-soluble polymers; while the HSi molecules can act as efficient cross-linking agents to generate strong interactions between MXene sheets and HA molecules. The optimized MXene/HA/HSi composites exhibit excellent mechanical flexibility (folded into crane structure), good water/solvent resistance, and long-term stable thermal camouflage capability (with low infrared emissivity of ≈0.29). The long-term thermal camouflage reliability (≈8 months) under various outdoor weathers and the scalable coating capability of the MXene-coated textile enable them to disguise the IR signal of various targets in complex environments, indicating the great promise of achieved material for thermal camouflage, IR stealth, and counter surveillance.

Multihydrogen Bond Modulated Polyzwitterionic Removable Adhesive Hydrogel with Antibacterial and Hemostatic Function for Wound Healing

Wound management is a major challenge worldwide, placing a huge financial burden on the government of every nation. Wound dressings that can protect wounds, accelerate healing, prevent infection, and avoid secondary damage continue to be a major focus of research in the health care and clinical communities. Herein, a novel zwitterionic polymer (LST) hydrogel incorporated with [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA), mussel-inspired N-[tris(hydroxymethyl)methyl] acrylamide (THMA), and lithium magnesium salt was prepared for functional wound dressings. The incorporation of the THMA monomer containing three hydroxyl groups gives the hydrogel suitable adhesion properties (∼6.0 KPa). This allows the LST zwitterionic hydrogels to bind well to the skin, which not only protects the wound and ensures its therapeutic efficacy but also allows for painless removal and reduced patient pain. Zwitterionic sulfobetaine units of SBMA provide antimicrobial and mechanical properties. The chemical structure and microscopic morphology of LST zwitterionic hydrogels were systematically studied, along with their swelling ratio, adhesion, and mechanical properties. The results showed that the LST zwitterionic hydrogels had a uniform and compact porous structure with the highest swelling and mechanical strain of 1607% and 1068.74%, respectively. The antibacterial rate of LST zwitterionic hydrogels was as high as 99.49%, and the hemostatic effect was about 1.5 times that of the commercial gelatin hemostatic sponges group. In further studies, a full-thickness mouse skin model was selected to evaluate the wound healing performance. Wounds covered by LST zwitterionic hydrogels had a complete epithelial reformation and new connective tissue, and its vascular regenerative capacity was increased to about 2.4 times that of the commercial group, and the wound could completely heal within 12-13 days. This study provides significant advances in the design and construction of multifunctional zwitterionic hydrogel adhesives and wound dressings.

Fabrication of maleic anhydride-acrylamide copolymer based sodium alginate hydrogel for elimination of metals ions and dyes contaminants from polluted water

The study explores the synergy of biobased polymers and hydrogels for water purification. Polymer nanomaterial's, synthesized by combining acrylamide copolymer with maleic anhydride, were integrated into sodium alginate biopolymer using an eco-friendly approach. Crosslinking agents, calcium chloride and glutaraladehyde, facilitated seamless integration, ensuring non-toxicity, high adsorption performance, and controlled capacity. This innovative combination presents a promising solution for clean and healthy water supplies, addressing the critical need for sustainable environmental practices in water purification. In addition, the polymer sodium alginate hydrogel (MAH@AA-P/SA/H) underwent characterization via the use of several analytical procedures, such as FTIR, XPS, SEM, EDX and XRD. Adsorption studies were conducted on metals and dyes in water, and pollutant removal methods were explored. We investigated several variables (such as pH, starting concentration, duration, and absorbent quantity) affect a material's capacity to be adsorbed. Moreover, the maximum adsorption towards Cu2+ is 754 mg/g while for Cr6+ metal ions are 738 mg/g, while the adsorption towards Congo Red and Methylene Blue dye are 685 mg/g and 653 mg/g correspondingly, within 240 min. Adsorption results were further analyzed using kinetic and isothermal models, which showed that MAH@AA-P/SA/H adsorption is governed by a chemisorption process. Hence, the polymer prepared from sodium alginate hydrogel (MAH@AA-P/SA/H) has remarkable properties as a versatile material for the significantly elimination of harmful contaminants from dirty water.

Clicking in harmony: exploring the bio-orthogonal overlap in click chemistry

In the quest to scrutinize and modify biological systems, the global research community has continued to explore bio-orthogonal click reactions, a set of reactions exclusively targeting non-native molecules within biological systems. These methodologies have brought about a paradigm shift, demonstrating the feasibility of artificial chemical reactions occurring on cellular surfaces, in the cell cytosol, or within the body - an accomplishment challenging to achieve with the majority of conventional chemical reactions. This review delves into the principles of bio-orthogonal click chemistry, contrasting metal-catalyzed and metal-free reactions of bio-orthogonal nature. It comprehensively explores mechanistic details and applications, highlighting the versatility and potential of this methodology in diverse scientific contexts, from cell labelling to biosensing and polymer synthesis. Researchers globally continue to advance this powerful tool for precise and selective manipulation of biomolecules in complex biological systems.

Engineering superstable islets-laden chitosan microgels with carboxymethyl cellulose coating for long-term blood glucose regulation in vivo

Islet transplantation to restore endogenous insulin secretion is a promising therapy for type 1 diabetes in clinic. However, host immune rejection seriously limits the survival of transplanted islets. Despite of the various encapsulation strategies and materials developed so far to provide immune isolation for transplanted islets, long-term blood glucose regulation is still difficult due to the inherent defects of the encapsulation materials. Herein, a novel islet-encapsulation composite material with low immunogenicity, good biocompatibility and excellent stability is reported. Specifically, chitosan (CS) microgels (diameter: ∼302 μm) are prepared via Michael addition reaction between maleimide grafted chitosan (CS-Mal) and thiol grafted chitosan (CS-NAC) in droplet-based microfluidic device, and then zwitterionic surface layer is constructed on CS microgel surface by covalent binding between maleimide groups on CS and thiol groups on thiol modified carboxymethyl cellulose (CMC-SH). The as-formed carboxymethyl cellulose coated chitosan (CS@CMC) microgels show not only long-term stability in vivo owing to the non-biodegradability of CMC, but also fantastic anti-adsorption and antifibrosis because of the stable zwitterionic surface layer. As a result, islets encapsulated in the CS@CMC microgels exhibit high viability and good insulin secretion function in vivo, and long-term blood glucose regulation is achieved for 180 days in diabetic mice post-transplantation.

Customization of an Ultrafast Thiol-Norbornene Photo-Cross-Linkable Hyaluronic Acid-Gelatin Bioink for Extrusion-Based 3D Bioprinting

Light-based three-dimensional (3D) bioprinting has been widely studied in tissue engineering. Despite the fact that free-radical chain polymerization-based bioinks like hyaluronic acid methacrylate (HAMA) and gelatin methacryloyl (GelMA) have been extensively explored in 3D bioprinting, the thiol-ene hydrogel system has attracted increasing attention for its ability in building hydrogel scaffolds in an oxygen-tolerant and cell-friendly way. Herein, we report a superfast curing thiol-ene bioink composed of norbornene-modified hyaluronic acid (NorHA) and thiolated gelatin (GelSH) for 3D bioprinting. A new facile approach was first introduced in the synthesis of NorHA, which circumvented the cumbersome steps involved in previous works. Additionally, after mixing NorHA with macro-cross-linker GelSH, the customized NorHA/GelSH bioinks exhibited fascinating superiorities over the gold standard GelMA bioinks, such as an ultrafast curing rate (1-5 s), much lowered photoinitiator concentration (0.03% w/v), and flexible physical performances. Moreover, the NorHA/GelSH hydrogel greatly avoided excess ROS generation, which is important for the survival of the encapsulated cells. Last, compared with the GelMA scaffold, the 3D-printed NorHA/GelSH scaffold not only exhibited excellent cell viability but also guaranteed cell proliferation, revealing its superior bioactivity. In conclusion, the NorHA/GelSH system is a promising candidate for 3D bioprinting and tissue engineering applications.

In Vitro Analysis of Human Cartilage Infiltrated by Hydrogels and Hydrogel-Encapsulated Chondrocytes

Osteoarthritis (OA) is a degenerative joint disease causing loss of articular cartilage and structural damage in all joint tissues. Given the limited regenerative capacity of articular cartilage, methods to support the native structural properties of articular cartilage are highly anticipated. The aim of this study was to infiltrate zwitterionic monomer solutions into human OA-cartilage explants to replace lost proteoglycans. The study included polymerization and deposition of methacryloyloxyethyl-phosphorylcholine- and a novel sulfobetaine-methacrylate-based monomer solution within ex vivo human OA-cartilage explants and the encapsulation of isolated chondrocytes within hydrogels and the corresponding effects on chondrocyte viability. The results demonstrated that zwitterionic cartilage-hydrogel networks are formed by infiltration. In general, cytotoxic effects of the monomer solutions were observed, as was a time-dependent infiltration behavior into the tissue accompanied by increasing cell death and penetration depth. The successful deposition of zwitterionic hydrogels within OA cartilage identifies the infiltration method as a potential future therapeutic option for the repair/replacement of OA-cartilage extracellular suprastructure. Due to the toxic effects of the monomer solutions, the focus should be on sealing the OA-cartilage surface, instead of complete infiltration. An alternative treatment option for focal cartilage defects could be the usage of monomer solutions, especially the novel generated sulfobetaine-methacrylate-based monomer solution, as bionic for cell-based 3D bioprintable hydrogels.

An evaluation of antibacterial properties and cytotoxicity of UV-curable biocompatible films containing hydroxyethyl cellulose and silver nanoparticles

The present study aimed to develop biocompatible film materials with antibacterial and anticancer properties that can be cured with UV rays depending on the thiol-en click reaction mechanism. The synthesized m-Ag NPs were added to formulations containing acrylate functionality HEC, pentaerythritol tetrarkis(3-mercaptopropionate), and photoinitiator at different rates (0, 20, 40, and 60 parts per hundred (phr)). The antibacterial activity of the films was evaluated against S. aureus, P. aeruginosa and E. coli by the disk diffusion test. The antibacterial effect of the films did not form an inhibition zone for the control formulation (CmAg0) against bacteria whereas the antibacterial property increased as the Ag NPs content increased in formulations containing m-Ag NPs. The strongest resistance film against the three bacterial species was observed in the CmAg60 formulation with 60 phr silver content, and the inhibition zones for S. aureus, P. aeruginosa, and E. coli were measured as 16.5 ± 0.7, 16.5 ± 2.1, and 16 ± 1.4, respectively. The cytotoxicity of the films against healthy cells and breast cancer cell (MCF-7) lines was investigated with MTT, and it was observed that all films did not cause any inhibition in the structure of the living cell but killed the cells at a high rate in the MCF-7 line. It was mainly observed that the CmAg60 formulation showed 95.576 % cell inhibition against MCF-7. According to these results, it has been predicted that the prepared films will play a vital role in the next generation of cancer treatments.

Polymers in Engineering Extracellular Vesicle Mimetics: Current Status and Prospective

The maintenance of a high delivery efficiency by traditional nanomedicines during cancer treatment is a challenging task. As a natural mediator for short-distance intercellular communication, extracellular vesicles (EVs) have garnered significant attention owing to their low immunogenicity and high targeting ability. They can load a variety of major drugs, thus offering immense potential. In order to overcome the limitations of EVs and establish them as an ideal drug delivery system, polymer-engineered extracellular vesicle mimics (EVMs) have been developed and applied in cancer therapy. In this review, we discuss the current status of polymer-based extracellular vesicle mimics in drug delivery, and analyze their structural and functional properties based on the design of an ideal drug carrier. We anticipate that this review will facilitate a deeper understanding of the extracellular vesicular mimetic drug delivery system, and stimulate the progress and advancement of this field.

Tissue engineering modalities in skeletal muscles: focus on angiogenesis and immunomodulation properties

Muscular diseases and injuries are challenging issues in human medicine, resulting in physical disability. The advent of tissue engineering approaches has paved the way for the restoration and regeneration of injured muscle tissues along with available conventional therapies. Despite recent advances in the fabrication, synthesis, and application of hydrogels in terms of muscle tissue, there is a long way to find appropriate hydrogel types in patients with congenital and/or acquired musculoskeletal injuries. Regarding specific muscular tissue microenvironments, the applied hydrogels should provide a suitable platform for the activation of endogenous reparative mechanisms and concurrently deliver transplanting cells and therapeutics into the injured sites. Here, we aimed to highlight recent advances in muscle tissue engineering with a focus on recent strategies related to the regulation of vascularization and immune system response at the site of injury.

Hyaluronic acid-based hydrogels: As an exosome delivery system in bone regeneration

Exosomes are extracellular vesicles (EVs) containing various ingredients such as DNA, RNA, lipids and proteins, which play a significant role in intercellular communication. Numerous studies have demonstrated the important role of exosomes in bone regeneration through promoting the expression of osteogenic-related genes and proteins in mesenchymal stem cells. However, the low targeting ability and short circulating half-life of exosomes limited their clinical application. In order to solve those problems, different delivery systems and biological scaffolds have been developed. Hydrogel is a kind of absorbable biological scaffold composed of three-dimensional hydrophilic polymers. It not only has excellent biocompatibility and superior mechanical strength but can also provide a suitable nutrient environment for the growth of the endogenous cells. Thus, the combination between exosomes and hydrogels can improve the stability and maintain the biological activity of exosomes while achieving the sustained release of exosomes in the bone defect sites. As an important component of the extracellular matrix (ECM), hyaluronic acid (HA) plays a critical role in various physiological and pathological processes such as cell differentiation, proliferation, migration, inflammation, angiogenesis, tissue regeneration, wound healing and cancer. In recent years, hyaluronic acid-based hydrogels have been used as an exosome delivery system for bone regeneration and have displayed positive effects. This review mainly summarized the potential mechanism of HA and exosomes in promoting bone regeneration and the application prospects and challenges of hyaluronic acid-based hydrogels as exosome delivery devices in bone regeneration.

Research Progress in Enzymatically Cross-Linked Hydrogels as Injectable Systems for Bioprinting and Tissue Engineering

Hydrogels have been developed for different biomedical applications such as in vitro culture platforms, drug delivery, bioprinting and tissue engineering. Enzymatic cross-linking has many advantages for its ability to form gels in situ while being injected into tissue, which facilitates minimally invasive surgery and adaptation to the shape of the defect. It is a highly biocompatible form of cross-linking, which permits the harmless encapsulation of cytokines and cells in contrast to chemically or photochemically induced cross-linking processes. The enzymatic cross-linking of synthetic and biogenic polymers also opens up their application as bioinks for engineering tissue and tumor models. This review first provides a general overview of the different cross-linking mechanisms, followed by a detailed survey of the enzymatic cross-linking mechanism applied to both natural and synthetic hydrogels. A detailed analysis of their specifications for bioprinting and tissue engineering applications is also included.

Salt sensitive purely zwitterionic physical hydrogel for prevention of postoperative tissue adhesion

Abdominal adhesions are a class of serious complications following abdominal surgery, resulting in a complicated and severe syndrome and sometimes leading to a Gordian knot. Traditional therapies employ hydrogels synthesized using complicated chemical formulations-often with click chemistry or thermal responsive hydrogel. The complicated synthesis process and severe conditions limit the extent of the hydrogels' applications. In this work, poly 3-[2-(methacryloyloxy)ethyl](dimethyl)-ammonio]-1-propanesulfonate (PSBMA) polymer was synthesized to self-assemble into physical hydrogels due to the inter- and intramolecular ion interactions. The strong static interaction bonding density has a substantial impact on the gelation and physicochemical properties, which is beneficial to clinical applications and offers a novel way to obtain the desired hydrogel for a specific biomedical application. Intriguingly, this PSBMA polymer can be customized into a transient network with outstanding antifouling capability depending on the ion concentration. As ion concentration increases, the PSBMA hydrogel dissociated completely, endowing it as a candidate for adhesion prevention. In the cecum-sidewall model, the PSBMA hydrogel demonstrated superior anti-adhesion properties than commercial HA hydrogel. Furthermore, we have demonstrated that this PSBMA hydrogel could inhibit the inflammatory response and encourage anti-fibrosis resulting in adhesion prevention. Most surprisingly, the recovered skins of cecum and sidewall are as smooth as the control skin without any scar and damage. In conclusion, a practical hydrogel was synthesized using a facile method based on purely zwitterionic materials, and this ion-sensitive, antifouling adjustable supramolecular hydrogel with great clinic transform potential is a promising barrier for preventing postoperative tissue adhesion. STATEMENT OF SIGNIFICANCE: The development of hydrogels with satisfactory coverage, long retention time, facile synthetic method, and good biocompatibility is vital for preventing peritoneal adhesions. Herein, we developed a salt sensitive purely zwitterionic physical hydrogel poly 3-[2-(methacryloyloxy)ethyl](dimethyl)-ammonio]-1-propanesulfonate (PSBMA) hydrogel to effectively prevent postoperative and recurrent abdominal adhesions. The hydrogel was simple to synthesize and easy to use. In the cecum-sidewall model, PSBMA hydrogel could instantaneously adhere and fix on irregular surfaces and stay in the wound for more than 10 days. The PSBMA hydrogel could inhibit the inflammatory response, encourage anti-fibrosis, and restore smoothness to damaged surfaces resulting in adhesion prevention. Overall, the PSBMA hydrogel is a promising candidate for the next generation of anti-adhesion materials to meet clinical needs.

Self-Initiated Photopolymerization of Anti-Inflammatory Zwitterionic Hydrogels with Sustained Release

Hydrogels are three-dimensional networks of hydrophilic polymers that have garnered significant attention as wound-healing materials. Many synthetic hydrogels are fabricated using a radical polymerization approach that requires an initiator molecule that is often photo- or thermosensitive. Initiator-free hydrogels are an emerging area of research that focuses on hydrogel fabrication that occurs in the absence of an initiator or cross-linker molecule, making these hydrogels highly relevant in tissue engineering and regenerative medicine due to their excellent cytocompatibility and ease of scale-up. Here we present on the development of initiator-free zwitterionic hydrogels that photopolymerize without any initiator or cross-linker while under cytocompatible conditions. The hydrogels exhibit a wide range of mechanical characteristics that are dependent on their polymer composition. They resist nonspecific protein adsorption and exhibit a sustained release of proteins and small molecules. Additionally, these self-initiated hydrogels significantly mitigate inflammatory macrophage activation in vitro. Overall, the development of self-initiated photopolymerized zwitterionic hydrogels offers significant progress in the fields of biomaterials and materials science.

Thiolated Polymeric Hydrogels for Biomedical Applications: A Review

Hydrogels are a three-dimensional (3D) network of hydrophilic polymers. The physical and chemical crosslinking of polymeric chains maintains the structure of the hydrogels even when they are swollen in water. They can be modified with thiol by thiol epoxy, thiol-ene, thiol-disulfide, or thiol-one reactions. Their application as a matrix for protein and drug delivery, cellular immobilization, regenerative medicine, and scaffolds for tissue engineering was initiated in the early 21st century. This review focuses on the ingredients, classification techniques, and applications of hydrogels, types of thiolation by different thiol-reducing agents, along with their mechanisms. In this study, different applications for polymers used in thiolated hydrogels, including dextran, gelatin, polyethylene glycol (PEG), cyclodextrins, chitosan, hyaluronic acid, alginate, poloxamer, polygalacturonic acid, pectin, carrageenan gum, arabinoxylan, carboxymethyl cellulose (CMC), gellan gum, and polyvinyl alcohol (PVA) are reviewed.

Zwitterionic Biomaterials

The term "zwitterionic polymers" refers to polymers that bear a pair of oppositely charged groups in their repeating units. When these oppositely charged groups are equally distributed at the molecular level, the molecules exhibit an overall neutral charge with a strong hydration effect via ionic solvation. The strong hydration effect constitutes the foundation of a series of exceptional properties of zwitterionic materials, including resistance to protein adsorption, lubrication at interfaces, promotion of protein stabilities, antifreezing in solutions, etc. As a result, zwitterionic materials have drawn great attention in biomedical and engineering applications in recent years. In this review, we give a comprehensive and panoramic overview of zwitterionic materials, covering the fundamentals of hydration and nonfouling behaviors, different types of zwitterionic surfaces and polymers, and their biomedical applications.

Prospective bacterial and fungal sources of hyaluronic acid: A review

The unique biological and rheological properties make hyaluronic acid a sought-after material for medicine and cosmetology. Due to very high purity requirements for hyaluronic acid in medical applications, the profitability of streptococcal fermentation is reduced. Production of hyaluronic acid by recombinant systems is considered a promising alternative. Variations in combinations of expressed genes and fermentation conditions alter the yield and molecular weight of produced hyaluronic acid. This review is devoted to the current state of hyaluronic acid production by recombinant bacterial and fungal organisms.

Bottlebrush inspired injectable hydrogel for rapid prevention of postoperative and recurrent adhesion

Postsurgical adhesion is a common clinic disease induced by surgical trauma, accompanying serious subsequent complications. Current non-surgical approaches of drugs treatment and biomaterial barrier administration only show limited prevention effects and couldn't effectively promote peritoneum repair. Herein, inspired by bottlebrush, a novel self-fused, antifouling, and injectable hydrogel is fabricated by the free-radical polymerization in aqueous solution between the methacrylate hyaluronic acid (HA-GMA) and N-(2-hydroxypropyl) methacrylamide (HPMA) monomer without any chemical crosslinkers, termed as H-HPMA hydrogel. The H-HPMA hydrogel can be tuned to perform excellent self-fused properties and suitable abdominal metabolism time. Intriguingly, the introduction of the ultra-hydrophilic HPMA chains to the H-HPMA hydrogel affords an unprecedented antifouling capability. The HPMA chains establish a dense hydrated layer that rapidly prevents the postsurgical adhesions and recurrent adhesions after adhesiolysis in vivo. The H-HPMA hydrogel can repair the peritoneal wound of the rat model within 5 days. Furthermore, an underlying mechanism study reveals that the H-HPMA hydrogel significantly regulated the mesothelial-to-mesenchymal transition (MMT) process dominated by the TGF-β-Smad2/3 signal pathway. Thus, we developed a simple, effective, and available approach to rapidly promote peritoneum regeneration and prevent peritoneal adhesion and adhesion recurrence after adhesiolysis, offering novel design ideas for developing biomaterials to prevent peritoneal adhesion.

Advanced strategies to thwart foreign body response to implantable devices

Mitigating the foreign body response (FBR) to implantable medical devices (IMDs) is critical for successful long-term clinical deployment. The FBR is an inevitable immunological reaction to IMDs, resulting in inflammation and subsequent fibrotic encapsulation. Excessive fibrosis may impair IMDs function, eventually necessitating retrieval or replacement for continued therapy. Therefore, understanding the implant design parameters and their degree of influence on FBR is pivotal to effective and long lasting IMDs. This review gives an overview of FBR as well as anti-FBR strategies. Furthermore, we highlight recent advances in biomimetic approaches to resist FBR, focusing on their characteristics and potential biomedical applications.

Photopolymerized Zwitterionic Hydrogels with a Sustained Delivery of Cerium Oxide Nanoparticle-miR146a Conjugate Accelerate Diabetic Wound Healing

In the United States, $87 billion per year is spent on the care of diabetic ulcers alone. Although the pathophysiology of diabetic wound healing is multifaceted, high systemic levels of inflammation and increased reactive oxygen species are often implicated in the wound healing impairment. Zwitterionic materials have been demonstrated to reduce inflammation and increase extracellular matrix deposition in wound beds, and here, we demonstrate a fabrication method for photopolymerized zwitterionic hydrogels that also enables sustained drug delivery over time. A therapeutic molecule of interest that is examined in this work is cerium oxide nanoparticle tagged with microRNA-146a (CNP-miR146a) to combat both oxidative stress and inflammation. The hydrogels are composed of zwitterionic and nonzwitterionic monomers, and the hydrogel formation occurs in the absence of a crosslinker. The hydrogels exhibit a wide range of stiffness and mechanical properties depending on their monomer content. Additionally, these hydrogels exhibit sustained release of nanoparticles and proteins. Finally, when employed in an in vivo diabetic mouse wound healing model, the zwitterionic hydrogels alone and laden with the CNP-miR146a conjugate significantly improved the rate of diabetic wound healing. Overall, these materials have excellent potential to be used as a topical treatment for chronic diabetic wounds.

Self-Healing Hydrogel Embodied with Macrophage-Regulation and Responsive-Gene-Silencing Properties for Synergistic Prevention of Peritendinous Adhesion

Antiadhesion barriers such as films and hydrogels used to wrap repaired tendons are important for preventing the formation of adhesion tissue after tendon surgery. However, sliding of the tendon can compress the adjacent hydrogel barrier and cause it to rupture, which may then lead to unexpected inflammation. Here, a self-healing and deformable hyaluronic acid (HA) hydrogel is constructed as a peritendinous antiadhesion barrier. Matrix metalloproteinase-2 (MMP-2)-degradable gelatin-methacryloyl (GelMA) microspheres (MSs) encapsulated with Smad3-siRNA nanoparticles are entrapped within the HA hydrogel to inhibit fibroblast proliferation and prevent peritendinous adhesion. GelMA MSs are responsively degraded by upregulation of MMP-2, achieving on-demand release of siRNA nanoparticles. Silencing effect of Smad3-siRNA nanoparticles is around 75% toward targeted gene. Furthermore, the self-healing hydrogel shows relatively attenuated inflammation compared to non-healing hydrogel. The mean adhesion scores of composite barrier group are 1.67 ± 0.51 and 2.17 ± 0.75 by macroscopic and histological evaluation, respectively. The proposed self-healing hydrogel antiadhesion barrier with MMP-2-responsive drug release behavior is highly effective for decreasing inflammation and inhibiting tendon adhesion. Therefore, this research provides a new strategy for the development of safe and effective antiadhesion barriers.

Recent Advances in Zwitterionic Hydrogels: Preparation, Property, and Biomedical Application

Nonspecific protein adsorption impedes the sustainability of materials in biologically related applications. Such adsorption activates the immune system by quick identification of allogeneic materials and triggers a rejection, resulting in the rapid failure of implant materials and drugs. Antifouling materials have been rapidly developed in the past 20 years, from natural polysaccharides (such as dextran) to synthetic polymers (such as polyethylene glycol, PEG). However, recent studies have shown that traditional antifouling materials, including PEG, still fail to overcome the challenges of a complex human environment. Zwitterionic materials are a class of materials that contain both cationic and anionic groups, with their overall charge being neutral. Compared with PEG materials, zwitterionic materials have much stronger hydration, which is considered the most important factor for antifouling. Among zwitterionic materials, zwitterionic hydrogels have excellent structural stability and controllable regulation capabilities for various biomedical scenarios. Here, we first describe the mechanism and structure of zwitterionic materials. Following the preparation and property of zwitterionic hydrogels, recent advances in zwitterionic hydrogels in various biomedical applications are reviewed.

Click Chemistry Hydrogels for Extrusion Bioprinting: Progress, Challenges, and Opportunities

The emergence of 3D bioprinting has allowed a variety of hydrogel-based "bioinks" to be printed in the presence of cells to create precisely defined cell-loaded 3D scaffolds in a single step for advancing tissue engineering and/or regenerative medicine. While existing bioinks based primarily on ionic cross-linking, photo-cross-linking, or thermogelation have significantly advanced the field, they offer technical limitations in terms of the mechanics, degradation rates, and the cell viabilities achievable with the printed scaffolds, particularly in terms of aiming to match the wide range of mechanics and cellular microenvironments. Click chemistry offers an appealing solution to this challenge given that proper selection of the chemistry can enable precise tuning of both the gelation rate and the degradation rate, both key to successful tissue regeneration; simultaneously, the often bio-orthogonal nature of click chemistry is beneficial to maintain high cell viabilities within the scaffolds. However, to date, relatively few examples of 3D-printed click chemistry hydrogels have been reported, mostly due to the technical challenges of controlling mixing during the printing process to generate high-fidelity prints without clogging the printer. This review aims to showcase existing cross-linking modalities, characterize the advantages and disadvantages of different click chemistries reported, highlight current examples of click chemistry hydrogel bioinks, and discuss the design of mixing strategies to enable effective 3D extrusion bioprinting of click hydrogels.

Recent developments in hydrogels containing copper and palladium for the catalytic reduction/degradation of organic pollutants

The elimination of toxic and hazardous contaminants from different environmental media has become a global challenge, causing researchers to focus on the treatment of pollutants. Accordingly, the elimination of inorganic and organic pollutants using sustainable, effective, and low-cost heterogeneous catalysts is considered as one of the most essential routes for this aim. Thus, many efforts have been devoted to the synthesis of novel compounds and improving their catalytic performance. Recently, palladium- and copper-based hydrogels have been used as catalysts for reduction, degradation, and decomposition reactions because they have significant features such as high mechanical strength, thermal stability, and high surface area. Herein, we summarize the progress achieved in this field, including the various methods for the synthesis of copper- and palladium-based hydrogel catalysts and their applications for environmental remediation. Moreover, palladium- and copper-based hydrogel catalysts, which have certain advantages, including high catalytic ability, reusability, easy work-up, and simple synthesis, are proposed as a new group of effective catalysts.

The elimination of toxic and hazardous contaminants from different environmental media has become a global challenge, causing researchers to focus on the treatment of pollutants.

Highly antifouling, biocompatible and tough double network hydrogel based on carboxybetaine-type zwitterionic polymer and alginate

Because of resistance to bio-macromolecular adhesion, antifouling hydrogels have attracted great attention in biomedical field. But traditional antifouling hydrogels made by hydrophilic polymers are always poor of mechanical properties. Herein, a new hybrid ionic-covalent cross-linked double network (DN) hydrogel was prepared by a simple one-pot method based on sodium alginate and the zwitterionic material carboxybetaine acrylamide (CBAA). The DN hydrogel has good mechanical properties, including high elastic modulus (0.28 MPa), high tensile strength (0.69 MPa), as well as good self-recovery capability. More importantly, the DN hydrogel is highly resistance to the adsorption of non-specific protein, cells, bacteria and algae, exhibiting an outstanding antifouling property. The in vitro and in vivo experiments prove that the DN hydrogel is highly biocompatible. This study provides a new strategy for the preparation of antifouling DN hydrogels with good mechanical properties for different needs, such as tissue scaffolds, wound dressings, implantable devices, and other fields.

Click chemistry-based biopolymeric hydrogels for regenerative medicine

Click chemistry is not a single specific reaction, but describes ways of generating products which emulate examples in nature. Click reactions occur in one pot, are not disturbed by water, generate minimal and inoffensive byproducts, and are characterized by a high thermodynamic driving force, driving the reaction quickly and irreversibly towards a high yield of a single reaction product. As a result, over the past 15 years it has become a very useful bio-orthogonal method for the preparation of chemical cross-linked biopolymer-based hydrogel, in the presence of e.g. growth factors and live cells, or in-vivo. Biopolymers are renewable and non-toxic, providing a myriad of potential backbone toolboxes for hydrogel design. The goal of this review is to summarize recent advances in the development of click chemistry-based biopolymeric hydrogels, and their applications in regenerative medicine. In particular, various click chemistry approaches, including copper-catalyzed azide-alkyne cycloaddition reactions, copper-free click reactions (e.g. the Diels-Alder reactions, the strain-promoted azide-alkyne cycloaddition reactions, the radical mediated thiol-ene reactions, and the oxime-forming reactions), and pseudo-click reactions (e.g. the thiol-Michael addition reactions and the Schiff base reactions) are highlighted in the first section. In addition, numerous biopolymers, including proteins (e.g. collagen, gelatin, silk, and mucin), polysaccharides (e.g. hyaluronic acid, alginate, dextran, and chitosan) and polynucleotides (e.g. deoxyribonucleic acid), are discussed. Finally, we discuss biopolymeric hydrogels, cross-linked by click chemistry, intended for the regeneration of skin, bone, spinal cord, cartilage, and cornea. This article provides new insights for readers in terms of the design of regenerative medicine, and the use of biopolymeric hydrogels based on click chemistry reactions.

Droplet nucleation in miniemulsion thiol–ene step photopolymerization

Reaction parameters, such as droplet size, initiator solubility and monomer solubility, which are important in favouring droplet nucleation in a miniemulsion thiol–ene step polymerization are reviewed.

Recent advances in bio-orthogonal and dynamic crosslinking of biomimetic hydrogels

In recent years, dynamic, 'click' hydrogels have been applied in numerous biomedical applications. Owing to the mild, cytocompatible, and highly specific reaction kinetics, a multitude of orthogonal handles have been developed for fabricating dynamic hydrogels to facilitate '4D' cell culture. The high degree of tunability in crosslinking reactions of orthogonal 'click' chemistry has enabled a bottom-up approach to install specific biomimicry in an artificial extracellular matrix. In addition to click chemistry, highly specific enzymatic reactions are also increasingly used for network crosslinking and for spatiotemporal control of hydrogel properties. On the other hand, covalent adaptable chemistry has been used to recapitulate the viscoelastic component of biological tissues and for formulating self-healing and shear-thinning hydrogels. The common feature of these three classes of chemistry (i.e., orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry) is that they can be carried out under ambient and aqueous conditions, a prerequisite for maintaining cell viability for in situ cell encapsulation and post-gelation modification of network properties. Due to their orthogonality, different chemistries can also be applied sequentially to provide additional biochemical and mechanical control to guide cell behavior. Herein, we review recent advances in the use of orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry for the development of dynamically tunable and biomimetic hydrogels.

Click-Chemistry Approach toward Antibacterial and Degradable Hybrid Hydrogels Based on Octa-Betaine Ester Polyhedral Oligomeric Silsesquioxane

An efficient process for the synthesis of degradable hydrogels containing octa-betaine ester polyhedral oligomeric silsesquioxane (POSS) through efficient thiol-ene and Menschutkin click reactions was developed. The hydrogels exhibited a yield strength of 0.36 MPa and a compressive modulus of 4.38 MPa and displayed excellent flexibility as well as torsion resistance. Antibacterial efficacy of hydrogels (and degradation products) was evaluated using Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). Efficacy was found to increase with the concentration of cetyl chloroacetate (CCA) in the hydrogel network, reaching 93% and 99% for Escherichia coli and Staphylococcus aureus, respectively. Degradation of hydrogels was observed in weak alkali conditions (pH = 8) and at physiological conditions (pH = 7.4). The degradation time of the hydrogels could be finely tuned by variation of the CCA content in the hydrogel and environmental stimulus. The tunable degradation behavior under physiological conditions combined with high antibacterial efficacy could render the presented materials interesting for tissue engineering applications.

Simultaneous Interpenetrating Polymer Network of Collagen and Hyaluronic Acid as an In Situ-Forming Corneal Defect Filler

Timely treatment of corneal injuries injury can help to prevent corneal scarring, blindness, and the need for corneal transplantation. This work describes a novel hydrogel that can fill corneal defects and assist in corneal regeneration. This hydrogel is a simultaneous interpenetrating polymer network (IPN) composed of collagen cross-linked via strain-promoted azide-alkyne cycloaddition reaction and hyaluronic acid cross-linked via thiol-ene Michael click reaction. The formation of the IPN gel was confirmed via FTIR spectra, UV-vis spectra, and morphological changes. We compared the gelation time, mechanical properties, transmittance, and refractive index of the IPN gel to the collagen gel, hyaluronic acid gel, and semi-IPN gel. The IPN combined the advantages of collagen and hyaluronic acid gels and supported corneal epithelial cell growth on its surface. When applied to corneal stromal defects in vivo, the IPN avoided epithelial hyperplasia, decreased stromal myofibroblast formation, and increased tight junction formation in the regenerated epithelium.

Functional zwitterionic biomaterials for administration of insulin

This review summarizes the structures and biomedical applications of zwitterionic biomaterials in the administration of insulin.

Adjustable and ultrafast light-cured hyaluronic acid hydrogel: promoting biocompatibility and cell growth

Bio-sourced hydrogels are attractive materials for diagnosing, repairing and improving the function of human tissues and organs.