A Review of the Mechanism, Properties, and Applications of Hydrogels Prepared by Enzymatic Cross-linking

Mechanisms and key factors influencing ulvan gelation

Ulvan, a sulfated polysaccharide extracted from the green seaweed Ulva, is composed primarily of rhamnose, xylose, glucuronic acid, iduronic acid, sulfate, with minor sugars such as mannose, galactose, and arabinose. Its linear structure features L-rhamnose-3-sulfate, D-glucuronic acid, L-iduronic acid, and D-xylose. Ulvan exhibits distinctive rheological properties, including shear-thinning and shear-thickening behaviors, which are crucial for its applications in biomedicine (e.g., drug delivery systems) and food science (e.g., gelling agents, functional ingredients). However, challenges persist, such as variability in mechanical stability, biocompatibility, and batch-to-batch inconsistencies resulting from extraction methods. This review comprehensively examines the gelation mechanisms of ulvan, focusing on cross-linking interactions (ionic bonding with multivalent cations) and non-cross-linking interactions (electrostatic forces and hydrogen bonding). It further reviews how both extrinsic factors (e.g., pH, concentration) and intrinsic factors (e.g., molecular weight, sulfation) influence gelation, thus advancing the potential of ulvan-based materials in biomedical and industrial applications.

Enzyme-mediated hydrogelation for biomedical applications: A review

Hydrogels possess significant potential for biomedical applications due to their flexibility and biocompatibility. However, current physical or chemical methods for their preparation often fail to balance biocompatibility and mechanical properties, limiting their application scope. Enzymatic preparation of hydrogels offers advantages including mild reaction conditions, absence of toxic substances, and superior biocompatibility. This review focuses on the enzymatic preparation systems of hydrogels and its application in the fast-growing biomedical field. Firstly, the mechanisms of enzyme-mediated hydrogel preparation can be categorized into four classes: enzyme cross-linking, enzyme polymerization, enzyme-mediated self-assembly of small molecular gelators, and enzyme-induced pH changes. Hydrogels prepared through the first two mechanisms retain the mechanical advantages of chemically cross-linked hydrogels while preserving the inherent biocompatibility. Additionally, hydrogels prepared via the latter two mechanisms exhibit rapid responses to external stimuli similar to physically crosslinked hydrogels while maintaining high biocompatibility. Furthermore, we discuss their application in biomedical scope and analyze the correlation between the mechanism of enzyme-mediated hydrogels and their respective application domains. Finally, the current challenges faced by enzymatically mediated hydrogelation are summarized; notably that enzymes incorporated and immobilized during hydrogel preparation remain active, resulting in catalytic activity exhibited by these enzymatically mediated hydrogels, which broadens their potential applications.

Protein-based nano delivery systems focusing on protein materials, fabrication strategies and applications in ischemic stroke intervention: A review

Ischemic stroke (IS), characterized by acute cerebral vascular occlusion and narrow therapeutic windows, poses formidable clinical challenges due to the blood-brain barrier (BBB) restriction, reperfusion injury risks, and limited efficacy of conventional thrombolytic therapies. These hurdles necessitate advanced delivery systems capable of precise BBB penetration, remodeled circulation, and neuroprotection. Proteins and peptides emerge as universal biomaterials for constructing nano-delivery platforms, leveraging their biocompatibility, biodegradability, low toxicity, and receptor-specific targeting. This review systematically explores protein-based nanomaterials in stroke intervention, emphasizing material selection, fabrication strategies, and therapeutic applications. Various structural proteins are analyzed for their unique advantages in carrier design, while peptide modifications are highlighted for enhancing targeted delivery. Critical fabrication techniques are discussed to balance stability and functionality. Furthermore, the applications of protein-based nanomaterials in IS therapy are summarized. Advanced preparation and application of protein-based nanomaterials, from delivery vehicles to ligand modification, potentially prolong the therapeutic window for IS and provide effective neuroprotection.

"Green" Cross-Linking of Poly(Vinyl Alcohol)-Based Nanostructured Biomaterials: From Eco-Friendly Approaches to Practical Applications

Recently, a growing need for sustainable materials in various industries, especially biomedical, environmental, and packaging applications, has been observed. Poly(vinyl alcohol) (PVA) is a versatile and widely used polymer, valued for its biocompatibility, water solubility, and easy processing, e.g., forming nanofibers via electrospinning. As a result of cross-linking, PVA turns into a three-dimensional structure-hydrogel with unusual sorption properties and mimicry of biological tissues. However, traditional cross-linking methods often involve toxic chemicals and harsh conditions, which can limit its eco-friendly potential and raise concerns about environmental impact. "Green" cross-linking approaches, such as the use of natural cross-linkers, freeze-thawing, enzymatic processes, irradiation, heat treatment, or immersion in alcohol, offer an environmentally friendly alternative that aligns with global trends toward sustainability. These methods not only reduce the use of harmful substances but also enhance the biodegradability and safety of the materials. By reviewing and analyzing the latest advancements in "green" PVA cross-linking approaches, this review provides a comprehensive overview of current techniques, their advantages, limitations, and potential applications. The main emphasis is placed on PVA nanostructured forms and applications of PVA-based biomaterials in areas such as wound dressings, drug delivery systems, tissue engineering, biological filters, and biosensors. Moreover, this article will contribute to the broader scientific understanding of how the materials based on PVA can be optimized both in terms of "greener" and safer production, as well as adjusting the final platform properties.

Emerging Trends and Future Prospects of Peptide-Based Hydrogels: Revolutionizing Food Technology Applications

Peptide-based hydrogels (PHs) are versatile materials with considerable potential in food technology. Advances in synthesis techniques, such as self-assembly, click chemistry, enzymatic cross-linking, and co-assembly with polymers, have improved their production efficiency and scalability. Derived from natural amino acids, PHs are biocompatible, biodegradable, and responsive to environmental factors like pH and temperature. In food technology, encapsulation and controlled release of bioactive compounds enhance nutrient stability, flavor preservation, and bioavailability. PHs serve as texture modifiers, improve product consistency, and possess antimicrobial properties for food preservation by inhibiting spoilage and pathogens. Their biodegradability supports eco-friendly practices and sustainable packaging, including edible films and coatings that extend shelf life. Adjustable properties such as ionic strength make PHs adaptable to specific needs. PHs also show potential in developing advanced food equipment, including 3D printers and encapsulation systems, promoting efficiency and sustainability. This review emphasizes that PHs offer innovative, sustainable solutions to enhance food functionality, quality, and safety, with broad applications in food processing and preservation.

Protein-Pectin Delivery Carriers for Food Bioactive Ingredients: Preparation, Release Mechanism, and Application

Food bioactive ingredients have received widespread attention due to their excellent nutritional and functional properties, regulating the organism. However, some food bioactive ingredients have the disadvantages of poor stability and low bioavailability, which limits their wider application in food. The current study has recently shown a growing interest in designing delivery systems due to their advantages in encapsulating, protecting, and controlling the release of food bioactive ingredients. This review summarizes the classification of protein-pectin delivery carriers, including emulsions, nanoparticles, microcapsules, gels, and films. Besides, the typical preparation methods and the factors affecting the stability of the carriers were presented. Moreover, the release mechanism of the protein-pectin delivery carriers was introduced. Furthermore, the applications of protein-pectin delivery carriers were also described. The protein-pectin delivery carriers have broad research prospects in the functional food and nutritional field. Protein-pectin delivery carriers can enhance the protection of food bioactive ingredient delivery due to their strong interaction force and excellent emulsification properties. Therefore, they can effectively protect food bioactive ingredients from harsh processing conditions and adverse environments in vivo, and improve their physicochemical properties, stability, and bioavailability, which have good application prospects.

Injectable Hydrogel Technologies for Bone Disease Treatment

Injectable hydrogels represent a highly promising approach for localized drug delivery systems (DDSs) in the management of bone-related conditions such as osteoporosis, osteonecrosis, osteoarthritis, osteomyelitis, and osteosarcoma. Their appeal lies in their biocompatibility, adjustable mechanical properties, and capacity to respond to external stimuli, including pH, temperature, light, redox potential, ionic strength, and enzymatic activity. These features enable enhanced targeted delivery of bioactive agents. This mini-review evaluates the synthesis of injectable hydrogels as well as recent advancements for treating a range of bone disorders, focusing on their mechanisms as localized and sustained DDSs for delivering drugs, nanoparticles, growth factors, and cells (e.g., stem cells). Moreover, it highlights their clinical studies for bone disease treatment. Additionally, it emphasizes the potential synergy between injectable hydrogels and hydrogel-based point-of-care technologies, which are anticipated to play a pivotal role in the future of bone disease therapies. Injectable hydrogels have the potential to transform bone disease treatment by facilitating precise, sustained, and minimally invasive therapeutic delivery. Nevertheless, significant challenges, including long-term biocompatibility, scalability, reproducibility, and precise regulation of drug release kinetics, must be addressed to unlock their clinical potential fully. Addressing these challenges will not only advance bone disease therapy but also open new avenues in regenerative medicine and personalized healthcare.

A short review on polysaccharide-based nanocomposite adsorbents for separation and biomedical applications

Polysaccharides such as chitosan, alginate, cellulose, and carrageenan have emerged as promising adsorbents due to their biodegradability, abundant availability, and diverse chemical functionality. These biopolymers exhibit promising performance for adsorption of a wide range of pollutants including heavy metals (e.g., lead, cadmium), organic dyes (e.g., methylene blue, methyl orange), and even pathogenic microorganisms. However, inherent hydrophilicity and poor mechanical properties limit their broader application in environmental and biomedical fields. As an effective way to address the issues, recent advancements have focused on the incorporation of nanoparticles (e.g., metal oxides, carbon nanotubes and clays) into polysaccharides to obtain nanocomposite films. Generally, these nanocomposites offer enhanced surface area, tunable porous network, and improved chemical and mechanical resistances for adsorption and biomedical applications. The current review gives a focused overview of the recent progresses in polysaccharide-based nanocomposites, with particular attention to their fabrication methods, adsorption capacity and mechanism, and diverse applications in water purification, drug delivery, and antimicrobial treatments. Critical challenges such as the optimization of nanoparticle dispersion and the environmental impacts of nanocomposite biodegradation are also discussed to pave the road for the future research in this promising field.

Soft Actuators and Actuation: Design, Synthesis, and Applications

Soft actuators are one of the most promising technological advancements with potential solutions to diverse fields' day-to-day challenges. Soft actuators derived from hydrogel materials possess unique features such as flexibility, responsiveness to stimuli, and intricate deformations, making them ideal for soft robotics, artificial muscles, and biomedical applications. This review provides an overview of material composition and design techniques for hydrogel actuators, exploring 3D printing, photopolymerization, cross-linking, and microfabrication methods for improved actuation. It examines applications of hydrogel actuators in biomedical, soft robotics, bioinspired systems, microfluidics, lab-on-a-chip devices, and environmental, and energy systems. Finally, it discusses challenges, opportunities, advancements, and regulatory aspects related to hydrogel actuators.

Hydrogels for Cardiac Tissue Regeneration: Current and Future Developments

Myocardial infarction remains a leading cause of death worldwide due to the heart's limited regenerative capability and the current lack of viable therapeutic solutions. Therefore, there is an urgent need to develop effective treatment options to restore cardiac function after a heart attack. Stem cell-derived cardiac cells have been extensively utilised in cardiac tissue regeneration studies. However, the use of Matrigel as a substrate for the culture and maturation of these cells has been a major limitation for the translation of this research into clinical application. Hydrogels are emerging as a promising system to overcome this problem. They are biocompatible and can provide stem cells with a supportive scaffold that mimics the extracellular matrix, which is essential for repairing damaged tissue in the myocardium after an infarction. Thus, hydrogels provide an alternative and reproducible option in addressing myocardial infarction due to their unique potential therapeutic benefits. This review explores the different types of natural and synthetic polymers used to create hydrogels and their various delivery methods, the most common being via injection and cardiac patches and other applications such as bioprinting. Many challenges remain before hydrogels can be used in a clinical setting, but they hold great promise for the future of cardiac tissue regeneration.

Functional Hydrogel Interfaces for Cartilage and Bone Regeneration

Effective treatment of bone diseases is quite tricky due to the unique nature of bone tissue and the complexity of the bone repair process. In combination with biological materials, cells and biological factors can provide a highly effective and safe treatment strategy for bone repair and regeneration, especially based on these multifunctional hydrogel interface materials. However, itis still a challenge to formulate hydrogel materials with fascinating properties (e.g., biological activity, controllable biodegradability, mechanical strength, excellent cell/tissue adhesion, and controllable release properties) for their clinical applications in complex bone repair processes. In this review, we will highlight recent advances in developing functional interface hydrogels. We then discuss the barriers to  producing of functional hydrogel materials without sacrificing their inherent properties, and potential applications in cartilage and bone repair are discussed. Multifunctional hydrogel interface materials can serve as a fundamental building block for bone tissue engineering.

Advancing Gel Systems with Natural Extracts: Antioxidant, Antimicrobial Applications, and Sustainable Innovations

The integration of natural extracts into gel systems has emerged as a transformative approach to enhance functional properties, including antioxidant, antimicrobial, and therapeutic effects. This review underscores the remarkable potential of natural extract-enriched gels, which effectively combine sustainability with improved functionality. These bioactive compounds, sourced from plants and animals, encompass polyphenols, flavonoids, essential oils, chitosan, proteins, and polysaccharides. They provide an eco-friendly alternative to synthetic additives and find applications across various sectors, including pharmaceuticals, cosmetics, and food packaging. Despite their promise, challenges remain, such as the variability in natural extract composition, the stability of bioactive compounds, and scalability for industrial use. To address these issues, innovative strategies like nanoencapsulation, responsive hydrogels, and AI-driven optimization have demonstrated significant progress. Additionally, emerging technologies, such as 3D printing and adherence to circular economy principles, further enhance the versatility, efficiency, and sustainability of these systems. By integrating these advanced tools and methodologies, gel systems enriched with natural extracts are well-positioned to meet contemporary consumer and industrial demands for multifunctional and eco-friendly products. These innovations not only improve performance but also align with global sustainability goals, setting the stage for widespread adoption and continued development in various fields.

Release Profile and Antibacterial Activity of Thymus sibthorpii Essential Oil-Incorporated, Optimally Stabilized Type I Collagen Hydrogels

Antimicrobial resistance is one of the drastically increasing major global health threats due to the misuse and overuse of antibiotics as traditional antimicrobial agents, which render urgent the need for alternative and safer antimicrobial agents, such as essential oils (EOs). Although the strong antimicrobial activity of various EOs has already been studied and revealed, their characteristic high sensitivity and volatility drives the need towards a more efficient drug administration method via a biomaterial system. Herein, the potential of Thymus sibthorpii EO incorporated in functionalized antibacterial collagen hydrogels was investigated. At first, the optimally stabilized type I collagen hydrogels via six different multi-arm poly (ethylene glycol) succinimidyl glutarate (starPEG) crosslinkers were determined by assessing the free amine content and the resistance to enzymatic degradation. Subsequently, 0.5, 1, and 2% v/v of EO were incorporated into optimized collagen hydrogels, and the release profile, as well as release kinetics, were studied. Finally, biomaterial cytocompatibility tests were performed. Thymus sibthorpii EO was released from the hydrogel matrix via Fickian diffusion and showed sustained release and 0.5% v/v EO-loaded hydrogels showed adequate antibacterial activity against Staphylococcus aureus and did not show any statistically significant difference compared to penicillin (p < 0.05). Moreover, none of the fabricated composite antibacterial scaffolds displayed any cytotoxicity on NIH-3T3 fibroblasts. In conclusion, this work presents an innovative antibacterial biomaterial system for tissue engineering applications, which could serve as a promising alternative to antibiotics, contributing to coping with the issue of antimicrobial resistance.

Development of enzymatically crosslinked natural deep eutectogels: versatile gels for enhanced drug delivery

Injectable hydrogels have been extensively studied due to their minimally invasive properties, ease of application, and void-filling properties. In this work, we tested the possibility to prepare a new type of gels, so called eutectogels, where water is replaced by a natural deep eutectic system (NADES), conferring it longer stability. Eutectogels based on betaine : glycerol 1 : 2, were prepared by enzymatic mediated crosslinking, using horseradish peroxidase (HRP) as catalyst and gelatine-phenol conjugated polymer. In comparison to hydrogels, that required higher enzyme concentration (15 U mL-1) to have gelation time under 2 minutes, the eutectogels were obtained using 10 and 5 U mL-1 of HRP, with gelation times of 30 and 50 seconds, respectively. Finally, ketoprofen was loaded into the polymeric matrix, and release studies were conducted. The presence of NADES was essential for the formulation of the drug loaded gel, which was able to release up to 70% of the drug within 10 days, therefore, it was possible to conclude that these eutectogels work as matrix for the controlled delivery of ketoprofen in aqueous medium. The in vitro biological evaluation of the individual components of the eutectogel support no cytotoxic effect, an early indication of potential biocompatibility.

Recent Advances in Hydrogel Technology in Delivering Mesenchymal Stem Cell for Osteoarthritis Therapy

Osteoarthritis (OA), a chronic joint disease affecting over 500 million individuals globally, is characterized by the destruction of articular cartilage and joint inflammation. Conventional treatments are insufficient for repairing damaged joint tissue, necessitating novel therapeutic approaches. Mesenchymal stem cells (MSCs), with their potential for differentiation and self-renewal, hold great promise as a treatment for OA. However, challenges such as MSC viability and apoptosis in the ischemic joint environment hinder their therapeutic effectiveness. Hydrogels with biocompatibility and degradability offer a three-dimensional scaffold that support cell viability and differentiation, making them ideal for MSC delivery in OA treatment. This review discusses the pathological features of OA, the properties of MSCs, the challenges associated with MSC therapy, and methods for hydrogel preparation and functionalization. Furthermore, it highlights the advantages of hydrogel-based MSC delivery systems while providing insights into future research directions and the clinical potential of this approach.

Functional hydrogels for hepatocellular carcinoma: therapy, imaging, and in vitro model

Hepatocellular carcinoma (HCC) is among the most common malignancies worldwide and is characterized by high rates of morbidity and mortality, posing a serious threat to human health. Interventional embolization therapy is the main treatment against middle- and late-stage liver cancer, but its efficacy is limited by the performance of embolism, hence the new embolic materials have provided hope to the inoperable patients. Especially, hydrogel materials with high embolization strength, appropriate viscosity, reliable security and multifunctionality are widely used as embolic materials, and can improve the efficacy of interventional therapy. In this review, we have described the status of research on hydrogels and challenges in the field of HCC therapy. First, various preparation methods of hydrogels through different cross-linking methods are introduced, then the functions of hydrogels related to HCC are summarized, including different HCC therapies, various imaging techniques, in vitro 3D models, and the shortcomings and prospects of the proposed applications are discussed in relation to HCC. We hope that this review is informative for readers interested in multifunctional hydrogels and will help researchers develop more novel embolic materials for interventional therapy of HCC.

Essential elements for spatiotemporal delivery of growth factors within bio-scaffolds: A comprehensive strategy for enhanced tissue regeneration

The precise delivery of growth factors (GFs) in regenerative medicine is crucial for effective tissue regeneration and wound repair. However, challenges in achieving controlled release, such as limited half-life, potential overdosing risks, and delivery control complexities, currently hinder their clinical implementation. Despite the plethora of studies endeavoring to accomplish effective loading and gradual release of GFs through diverse delivery methods, the nuanced control of spatial and temporal delivery still needs to be elucidated. In response to this pressing clinical imperative, our review predominantly focuses on explaining the prevalent strategies employed for spatiotemporal delivery of GFs over the past five years. This review will systematically summarize critical aspects of spatiotemporal GFs delivery, including judicious bio-scaffold selection, innovative loading techniques, optimization of GFs activity retention, and stimulating responsive release mechanisms. It aims to identify the persisting challenges in spatiotemporal GFs delivery strategies and offer an insightful outlook on their future development. The ultimate objective is to provide an invaluable reference for advancing regenerative medicine and tissue engineering applications.

Synthesis and Properties of Injectable Hydrogel for Tissue Filling

Hydrogels with injectability have emerged as the focal point in tissue filling, owing to their unique properties, such as minimal adverse effects, faster recovery, good results, and negligible disruption to daily activities. These hydrogels could attain their injectability through chemical covalent crosslinking, physical crosslinking, or biological crosslinking. These reactions allow for the formation of reversible bonds or delayed gelatinization, ensuring a minimally invasive approach for tissue filling. Injectable hydrogels facilitate tissue augmentation and tissue regeneration by offering slow degradation, mechanical support, and the modulation of biological functions in host cells. This review summarizes the recent advancements in synthetic strategies for injectable hydrogels and introduces their application in tissue filling. Ultimately, we discuss the prospects and prevailing challenges in developing optimal injectable hydrogels for tissue augmentation, aiming to chart a course for future investigations.