Oxidoreductase-Initiated Radical Polymerizations to Design Hydrogels and Micro/Nanogels: Mechanism, Molding, and Applications

Enzymatic polymerization: Recent advances toward sustainable polymer synthesis

Enzymatic polymerization has emerged as a sustainable strategy for synthesizing biodegradable, biocompatible polymers, addressing critical environmental challenges posed by conventional petroleum-based materials. This review comprehensively explores advancements from the past five years, spotlighting six pivotal enzymes lipase, horseradish peroxidase, laccase, glucose oxidase, glucosyltransferase, and phosphorylase-alongside synergistic multi-enzymatic systems that enable complex polymerization cascades. Diverging from prior reviews focused on individual enzymes or specific polymer classes (e.g., polyesters, polyamides), our work provides a systematic classification of enzymatic polymerization mechanisms, emphasizing substrate specificity, reaction efficiency, and product diversity. Integrating advances in enzyme engineering, cascade catalysis, and green chemistry, this analysis outlines strategies to customize polymer architectures, identifies challenges in scaling enzymatic processes, and underscores opportunities for industrial applications. It advocates interdisciplinary innovation to advance sustainable polymer synthesis aligned with circular economy principles, emphasizing enzymatic methods' transformative potential for eco-friendly manufacturing paradigms.

Magnetite Nanoparticles with High Affinity Toward Target Protein for Efficient and Facile Bio-Separation

Magnetite nanoparticles (Fe3O4 NPs) with molecular recognition capabilities offer significant potential for biomedical applications, yet existing surface protein imprinting methods often suffer from low efficiency. Herein, a surface enzyme-mediated polymerization strategy is exploited for surface imprinting of bovine serum albumin (BSA) onto Fe3O4 NPs. This method, compatible with all vinyl monomers and operable under mild conditions, enables imprinting at high monomer concentrations while preventing nanoparticle agglomeration. Notably, increasing the pre-polymerization solution concentration enhances the pre-assembly of functional monomers and template molecules, thereby improving imprinting efficiency. Furthermore, replacing conventional crosslinkers with a polyglutamic acid-based peptide crosslinker introduces a pH-responsive helix-coil transition, allowing complete template removal under mild conditions and increasing the adsorption capacity and imprinting factor to 139.8 mg g⁻¹ and 10.36, respectively. The resulting BSA-imprinted Fe₃O₄ NPs exhibits high selectivity, robustness, and rapid adsorption kinetics while maintaining strong magnetic responsiveness for easy separation. These features allows for the selective extraction of BSA from bovine fetal serum, demonstrating the potential of this approach for biomedical applications, particularly in bioseparations.

A Self-Assembled Amino Acid Hydrogel for Immobilization and Protection of Enzymes

Enzymes are essential biological catalysts, which have merits such as specificity, high efficiency, and mild-acting conditions. Due to the characteristics of enzymes, problems such as poor operational stability and difficulty in reuse limit the practical application of enzymes. These problems can often be solved by immobilization of enzymes. Commonly used enzyme immobilization materials include biochar, chitosan, polymer, and metal-organic frameworks, which often do not match the nature of the enzyme. This study utilizes the self-assembled amino acid hydrogel Fmoc-Y-OMe as the immobilizing material. The hydrogelator Fomc-Y-OMe has advantages like simple synthesis, easy immobilization, environmental friendliness, and good compatibility with proteins. It is able to protect enzyme activity at high temperatures and under a wide range of acid-base conditions and has excellent versatility. In particular, immobilized polyethylene terephthalate degrading enzyme (PETase) can significantly degrade polyethylene terephthalate (PET) film at 70 °C, while free PETase completely loses its catalytic capacity at such high temperatures. The excellent performance of self-assembled hydrogels to protect the catalytic activity of enzymes at high temperatures is highlighted.

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.

A review on recent progress in polysaccharide/protein hydrogels in winter sports: Classification, synthesis routes, and application

In today's world, emerging materials play prominent roles in competitive sport applications. Among them, hydrogels gained increasing attention in winter sports applications owing to their unique advantages, such as flexibility, conductivity, and adhesion. However, traditional hydrogels prepared by synthetic routes from petroleum materials lose performance at freezing temperatures below zero degrees, limiting their direct use in winter sports. The emergence of natural polymer materials has brought new opportunities for winter sports. Polysaccharide or protein (polysaccharides/proteins) hydrogels obtained from biomass resources are renewable and abundant, especially when taking into consideration the depletion of resources and environmental pollution in contemporary society. The development and utilization of polysaccharide/protein hydrogels may contribute to solving the resource shortage problem. In this paper, the latest research dealing with natural polymer hydrogels for winter sports applications is reviewed. In the first section, recent research trends of hydrogel classification and crosslinking methods are summarized. The performance advantages and specific applications of polysaccharide/protein hydrogels in winter sports are then discussed, with the application scope covering index monitoring, event violation detection, protective equipment, rehabilitation, and venues. Finally, the practical challenges faced by polysaccharide/protein hydrogels in winter sports are prospected along with the innovation and optimization design routes, such as the introduction of natural crosslinking agents and bionic structures. These insights aim to provide a reference for the development of advanced materials for winter sports applications.

Musculoskeletal Organs-on-Chips: An Emerging Platform for Studying the Nanotechnology-Biology Interface

Nanotechnology-based approaches are promising for the treatment of musculoskeletal (MSK) disorders, which present significant clinical burdens and challenges, but their clinical translation requires a deep understanding of the complex interplay between nanotechnology and MSK biology. Organ-on-a-chip (OoC) systems have emerged as an innovative and versatile microphysiological platform to replicate the dynamics of tissue microenvironment for studying nanotechnology-biology interactions. This review first covers recent advances and applications of MSK OoCs and their ability to mimic the biophysical and biochemical stimuli encountered by MSK tissues. Next, by integrating nanotechnology into MSK OoCs, cellular responses and tissue behaviors may be investigated by precisely controlling and manipulating the nanoscale environment. Analysis of MSK disease mechanisms, particularly bone, joint, and muscle tissue degeneration, and drug screening and development of personalized medicine may be greatly facilitated using MSK OoCs. Finally, future challenges and directions are outlined for the field, including advanced sensing technologies, integration of immune-active components, and enhancement of biomimetic functionality. By highlighting the emerging applications of MSK OoCs, this review aims to advance the understanding of the intricate nanotechnology-MSK biology interface and its significance in MSK disease management, and the development of innovative and personalized therapeutic and interventional strategies.

Insights into the impact of modification methods on the structural characteristics and health functions of pectin: A comprehensive review

Pectin is a complex polysaccharide that is widely present in plant cells and has multiple physiological functions. However, most pectin exists in the form of protopectin, which has a large molecular weight and cannot be fully absorbed and utilized in the human gut to exert its effects. The significant differences in the structure of different sources of pectin also limited their application in the food and medical fields. In order to achieve greater development and utilization of pectin functions, this paper reviewed several commonly used methods for pectin modification from physical, chemical, and biological perspectives, and elaborated on the relationship between these modification methods and the structure and functional properties of pectin. At the same time, the functional characteristics of modified pectin and its application in medical health, such as regulating intestinal health, anticancer, anti-inflammatory, and drug transport, were reviewed, so as to provide a theoretical basis for targeted modification of pectin and the development of new modified pectin products.

Polyamidoamine dendrimer-mediated hydrogel for solubility enhancement and anti-cancer drug delivery

The application of hydrogels for anti-cancer drug delivery has garnered considerable interest in the medical field. Current cancer treatment approaches, such as chemotherapy and radiation therapy, often induce severe side effects, causing significant distress and substantial health complications to patients. Hydrogels present an appealing solution as they can be precisely injected into specific sites within the body, facilitating the sustainable release of encapsulated drugs. This localized treatment approach holds great potential for reducing toxicity levels and improving drug delivery efficacy. In this study we developed a hydrogel delivery system containing polyamidoamine (PAMAM) dendrimer and polyethylene glycol (PEG) for solubility enhancement and sustained delivery of hydrophobic anti-cancer drugs. The three selected model drugs, e.g. silibinin, camptothecin, and methotrexate, possess limited aqueous solubility and thus face restricted application. In the presence of vinyl sulfone functionalized PAMAM dendrimer at 45 mg/mL concentration, drug solubility is increased by 37-fold, 4-fold, and 10-fold for silibinin, camptothecin, and methotrexate, respectively. By further crosslinking of the functionalized PAMAM dendrimer and thiolated PEG, we successfully developed a fast-crosslinking hydrogel capable of encapsulating a significant payload of solubilized cancer drugs for sustained release. In water, the drug encapsulated hydrogels release 30%-80% of their loads in 1-4 days. MTT assays of J82 and MCF7 cells with various doses of drug encapsulated hydrogels reveal that cytotoxicity is observed for all three drugs on both J82 and MCF7 cell lines after 48 h. Notably, camptothecin exhibits higher cytotoxicity to both cell lines than silibinin and methotrexate, achieving up to 95% cell death at experimental conditions, despite its lower solubility. Our experiments provide evidence that the PAMAM dendrimer-mediated hydrogel system significantly improves the solubility of hydrophobic drugs and facilitates their sustained release. These findings position the system as a promising platform for controlled delivery of hydrophobic drugs for intratumoral cancer treatment.

Advances in the Development of Nano-Engineered Mechanically Robust Hydrogels for Minimally Invasive Treatment of Bone Defects

Injectable hydrogels were discovered as attractive materials for bone tissue engineering applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties. However, traditional hydrogels suffer from weak mechanical strength, limiting their use in heavy load-bearing areas. Thus, the fabrication of mechanically robust injectable hydrogels that are suitable for load-bearing environments is of great interest. Successful material design for bone tissue engineering requires an understanding of the composition and structure of the material chosen, as well as the appropriate selection of biomimetic natural or synthetic materials. This review focuses on recent advancements in materials-design considerations and approaches to prepare mechanically robust injectable hydrogels for bone tissue engineering applications. We outline the materials-design approaches through a selection of materials and fabrication methods. Finally, we discuss unmet needs and current challenges in the development of ideal materials for bone tissue regeneration and highlight emerging strategies in the field.

A hydrogel system for drug loading toward the synergistic application of reductive/heat-sensitive drugs

The preparation of hydrogels as drug carriers via radical-mediated polymerization has significant prospects, but the strong oxidizing ability of radicals and the high temperatures generated by the vigorous reactions limits the loading for reducing/heat-sensitive drugs. Herein, an applicable hydrogel synthesized by radical-mediated polymerization is reported for the loading and synergistic application of specific drugs. First, the desired sol is obtained by polymerizing functional monomers using a radical initiator, and then tannic-acid-assisted specific drug mediates sol-branched phenylboric acid group to form the required functional hydrogel (New-gel). Compared with the conventional single-step radical-mediated drug-loading hydrogel, the New-gel not only has better chemical/physical properties but also efficiently loads and releases drugs and maintains drug activity. Particularly, the New-gel has excellent loading capacity for oxygen, and exhibits significant practical therapeutic effects for diabetic wound repair. Furthermore, owing to its high light transmittance, the New-gel synergistically promotes the antibacterial effect of photosensitive drugs. This gelation strategy for loading drugs has further promising biomedical applications.

Zwitterion-Modified Nanogel Responding to Temperature and Ionic Strength: A Dissipative Particle Dynamics Simulation

The self-assembly and stimuli-responsive properties of nanogel poly(n-isopropylacrylamide) (p(NIPAm)) and zwitterion-modified nanogel poly(n-isopropylacrylamide-co-sulfobetainemethacrylate) (p(NIPAm-co-SBMA)) were explored by dissipative particle dynamics simulations. Simulation results reveal that for both types of nanogel, it is beneficial to form spherical nanogels at polymer concentrations of 5-10%. When the chain length (L) elongates from 10 to 40, the sizes of the nanogels enlarge. As for the p(NIPAm) nanogel, it shows thermoresponsiveness; when it switches to the hydrophilic state, the nanogel swells, and vice versa. The zwitterion-modified nanogel p(NIPAm-co-SBMA) possesses thermoresponsiveness and ionic strength responsiveness concurrently. At 293 K, both hydrophilic p(NIPAm) and superhydrophilic polysulfobetaine methacrylate (pSBMA) could appear on the outer surface of the nanogel; however, at 318 K, superhydrophilic pSBMA is on the outer surface to cover the hydrophobic p(NIPAm) core. As the temperature rises, the nanogel shrinks and remains antifouling all through. The salt-responsive property can be reflected by the nanogel size; the volumes of the nanogels in saline systems are larger than those in salt-free systems as the ionic condition inhibits the shrinkage of the zwitterionic pSBMA. This work exhibits the temperature-responsive and salt-responsive behavior of zwitterion-modified-pNIPAm nanogels at the molecular level and provides guidance in antifouling nanogel design.

Biomedical Applications of Deformable Hydrogel Microrobots

Hydrogel, a material with outstanding biocompatibility and shape deformation ability, has recently become a hot topic for researchers studying innovative functional materials due to the growth of new biomedicine. Due to their stimulus responsiveness to external environments, hydrogels have progressively evolved into "smart" responsive (such as to pH, light, electricity, magnetism, temperature, and humidity) materials in recent years. The physical and chemical properties of hydrogels have been used to construct hydrogel micro-nano robots which have demonstrated significant promise for biomedical applications. The different responsive deformation mechanisms in hydrogels are initially discussed in this study; after which, a number of preparation techniques and a variety of structural designs are introduced. This study also highlights the most recent developments in hydrogel micro-nano robots' biological applications, such as drug delivery, stem cell treatment, and cargo manipulation. On the basis of the hydrogel micro-nano robots' current state of development, current difficulties and potential future growth paths are identified.

Cascade enzymatic preparation of carboxymethyl chitosan-based multifunctional hydrogels for promoting cutaneous wound healing

Designing wound dressings with inherent multifunctional therapeutic effects is desirable for clinical applications. Herein, a series of multifunctional carboxymethyl chitosan (CMCS)-based hydrogels were fabricated by the facile urate oxidase (UOX)-horseradish peroxidase (HRP) cascade enzymatic crosslinking system. For the first time, the cascade enzymatic crosslinking system was not only used for preparing hydrogel wound dressings but also for accelerating wound healing due to the activity retention of the self-compartmental enzymes. A CMCS derivative (HCMCS-mF) synthesized by successively grafting 4-hydroxybenzaldehyde (H) and 5-methylfurfural (mF) on CMCS and a quaternary ammonium crosslinker (QMal) with terminal grafting maleimide (Mal) groups were combined with enzymatic system for the facile preparation of hydrogels. The mild Diels-Alder (DA) crosslinking reaction between mF and Mal groups constructed the first network of hydrogels. The cascade UOX-HRP system mediated the oxidative crosslinking of phenols thus forming the second gel network. Self-entrapped UOX maintained its enzymatic activity and could continuously catalyze the oxidation of uric acid, generating therapeutic allantoin. These porous, degradable, mechanically stable hydrogels with excellent antioxidant performance and enhanced antibacterial capacity could effectively accelerate skin wound repair by simultaneously reducing oxidative stress, relieving inflammation, promoting collagen deposition and upregulating the expression level of CD31.

Tyrosine residues initiated photopolymerization in living organisms

Towards intracellular engineering of living organisms, the development of new biocompatible polymerization system applicable for an intrinsically non-natural macromolecules synthesis for modulating living organism function/behavior is a key step. Herein, we find that the tyrosine residues in the cofactor-free proteins can be employed to mediate controlled radical polymerization under 405 nm light. A proton-coupled electron transfer (PCET) mechanism between the excited-state TyrOH* residue in proteins and the monomer or the chain transfer agent is confirmed. By using Tyr-containing proteins, a wide range of well-defined polymers are successfully generated. Especially, the developed photopolymerization system shows good biocompatibility, which can achieve in-situ extracellular polymerization from the surface of yeast cells for agglutination/anti-agglutination functional manipulation or intracellular polymerization inside yeast cells, respectively. Besides providing a universal aqueous photopolymerization system, this study should contribute a new way to generate various non-natural polymers in vitro or in vivo to engineer living organism functions and behaviours.

Emerging Role of Injectable Dipeptide Hydrogels in Biomedical Applications

Owing to their properties such as biocompatibility, tunable mechanical properties, permeability toward oxygen, nutrients, and the ability to hold a significant amount of water, hydrogels have wide applications in biomedical research. They have been engaged in drug delivery systems, 3D cell culture, imaging, and extracellular matrix (ECM) mimetics. Injectable hydrogels represent a major subset of hydrogels possessing advantages of site-specific conformation with minimal invasive techniques. It preserves the inherent properties of drug/biomolecules and is devoid of any side effects associated with surgery. Various polymeric materials utilized in developing injectable hydrogels are associated with the limitations of toxicity, immunogenicity, tedious manufacturing processes, and lack of easy synthetic tunability. Peptides are an important class of biomaterials that have interesting properties such as biocompatibility, stimuli responsiveness, shear thinning, self-healing, and biosignaling. They lack immunogenicity and toxicity. Therefore, numerous peptide-based injectable hydrogels have been explored in the past, and a few of them have reached the market. In recent years, minimalistic dipeptides have shown their ability to form stable hydrogels through cooperative noncovalent interactions. In addition to inherent properties of lengthy peptide-based injectable hydrogels, dipeptides have the unique advantages of low production cost, high synthetic accessibility, and higher stability. Given the instances of expanding significance of injectable peptide hydrogels in biomedical research and an emerging recent trend of dipeptide-based injectable hydrogels, a timely review on dipeptide-based injectable hydrogels shall highlight various aspects of this interesting class of biomaterials. This concise review that focuses on the dipeptide injectable hydrogel may stimulate the current trends of research on this class of biomaterial to translate its significance as interesting products for biomedical applications.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Injectable Adhesive Hydrogels for Soft tissue Reconstruction: A Materials Chemistry Perspective

Injectable bioadhesives offer several advantages over conventional staples and sutures in surgery to seal and close incisions or wounds. Despite the growing research in recent years few injectable bioadhesives are available for clinical use. This review summarizes the key chemical features that enable the development and improvements in the use of polymeric injectable hydrogels as bioadhesives or sealants, their design requirements, the gelation mechanism, synthesis routes, and the role of adhesion mechanisms and strategies in different biomedical applications. It is envisaged that developing a deep understanding of the underlying materials chemistry principles will enable researchers to effectively translate bioadhesive technologies into clinically-relevant products.

Enzyme-manipulated hydrogelation of small molecules for biomedical applications

Enzyme-manipulated hydrogelation based on self-assembly of small molecules is an attractive methodology for development of functional biomaterials. Upon the catalysis of enzymes, small-molecular precursors are converted into assemblable building blocks, which arrange into high-ordered nanofibers via non-covalent interactions at the molecular level, and further trap water to form hydrogels at the macroscopic level. Such approach has numerous advantages of region- and enantioselectivity, and mild reaction conditions for encapsulation of biomedications or cells that are fragile to environmental change. In addition to the common applications as drug reservoirs or cell scaffolds, the utilization of endogenous enzymes as stimuli to initiate self-assembly in the living cells and tissue is considered as an intelligent spatiotemporally controllable hydrogelation strategy for biomedical applications. The enzyme-instructed in situ self-assembly and hydrogelation can modulate the cell behavior, and even present therapeutic bioactivities, which provides a new perspective in the field of disease treatment. In this review, we categorize distinct enzymatic stimuli and elaborate substrate design, catalytic characteristics, and mechanisms of self-assembly and hydrogelation. The biomedical applications in drug delivery, tissue engineering, bioimaging, and in situ gelation-produced bioactivity are outlined. Advantages and limitations regarding the state-of-the-art enzyme-driven hydrogelation technologies and future perspectives are also discussed. STATEMENT OF SIGNIFICANCE: Hydrogel is a semi-solid soft material containing a large amount of water. Due to the features of adjustable flexibility, extremely porous architecture, and the high similarity of structure to natural extracellular matrices, the hydrogel has broad application prospects in biomedicine. In recent 20 years, enzyme-manipulated hydrogelation based on self-assembly of small molecules has developed rapidly as an attractive methodology for the construction of functional biomaterials. Upon the catalysis of enzymes, small-molecular precursors are converted into assemblable building blocks, which arrange into high-ordered nanofibers via non-covalent interactions at the molecular level, and further trap water to form hydrogels at the macroscopic level. This review summarized the characteristics of enzymatic hydrogel, as well as the traditional application and emerging prospect of enzyme-instructed self-assembly and hydrogelation.

Supramolecular Hydrogel Based on Pseudopolyrotaxanes Aggregation for Bacterial Microenvironment-Responsive Antibiotic Delivery

Due to the extensive use of antibiotics, the variety and number of drug-resistant pathogens have increased dramatically and have become a major global health problem. This imposes significant demands on the rational and effective use of antibiotics. To this end, a supramolecular hydrogel based on pseudopolyrotaxanes aggregation is proposed for antibiotic delivery. Supramolecular cross-linking strategies allow hydrogels to be obtained under mild conditions that facilitate the encapsulation of antibiotics. The presence of pH-sensitive imine bonds allows for the reversible detachment of PEG residues from the PEGylated hyaluronic acid backbone in an acidic environment, which leads to reversible changes in hydrogel crosslink density and thus controls antibiotic release behavior. Antimicrobial assessments indicated that the hydrogel exhibited good antimicrobial efficiency against both Gram-positive and negative bacteria, while responding to the bacterial microenvironment and enabling a burst release of antibiotics in severe infections. The proposed hydrogel also has excellent biocompatibility and thus possesses great potential for biomedical applications.

Enzymatic synthesis, characterization and properties of the protein-polysaccharide conjugate: A review

Poor solubility of proteins negatively affects their functional properties and greatly limits their application. Enzymatic cross-linking with polysaccharides can improve solubility and functional properties of proteins. The enzymes used include transglutaminase, laccase and peroxidase. Therefore, this work introduces the cross-linking mechanisms of these enzymes and the characterization techniques, the improved properties and the potential applications of the enzymatically-synthesized protein-polysaccharide conjugate. Transglutaminase catalyzes the formation of a new peptide bond and thus works on amino-containing polysaccharides to conjugate with proteins. However, laccase and peroxidase catalyze oxidation of various compounds with phenol and aniline structures. Therefore, these two enzymes can catalyze the conjugate reaction between proteins and feruloylated polysaccharides which are widely distributed in cereal bran. Compared with the unmodified protein, the enzymatically-synthesized protein-polysaccharide conjugate usually has higher solubility and better functional properties. Thus, it is inferred that enzymatic conjugation with polysaccharide molecules can extend the application of proteins.

GOx/Hb Cascade Oxidized Crosslinking of Silk Fibroin for Tissue-Responsive Wound Repair

Promising wound dressings can achieve rapid soft-tissue filling while refactoring the biochemical and biophysical microenvironment to recruit endogenous cells, facilitating tissue healing, integration, and regeneration. In this study, a tissue biomolecule-responsive hydrogel matrix, employing natural silk fibroin (SF) as a functional biopolymer and haemoglobin (Hb) as a peroxidase-like biocatalyst, was fabricated through cascade enzymatic crosslinking. The hydrogels possessed mechanical tunability and displayed adjustable gelation times. A tyrosine unit on SF stabilised the structure of Hb during the cascade oxidation process; thus, the immobilized Hb in SF hydrogels exhibited higher biocatalytic efficiency than the free enzyme system, which provided a continuously antioxidative system. The regulation of the dual enzyme ratio endowed the hydrogels with favourable biocompatibility, biodegradability, and adhesion strength. These multifunctional hydrogels provided a three-dimensional porous extracellular matrix-like microenvironment for promoting cell adhesion and proliferation. A rat model with a full-thickness skin defect revealed accelerated wound regeneration via collagen deposition, re-epithelialisation and revascularisation. Enzyme-loaded hydrogels are an attractive and high-safety biofilling material with the potential for wound healing, tissue regeneration, and haemostasis.

Regulated Polyelectrolyte Nanogels for Enzyme Encapsulation and Activation

Polyelectrolyte (PE) nanogels consisting of cross-linked PE networks integrate the advanced features of both nanogels and PEs. The soft environment and abundant intrinsic charges are of special interest for enzyme immobilization. However, the crucial factors that regulate enzyme encapsulation and activation remain obscure to date. Herein, we synthesized cationic poly (dimethyl aminoethyl methacrylate), PDMAEMA, nanogels with well-defined size and cross-link degrees and fully investigated the effects of different control factors on lipase immobilization. We demonstrate that the cationic PDMAEMA nanogels indeed enable efficient and safe loading of anionic lipase without disturbing their structures. Strong charge interaction achieved by tuning pH and larger particle size are favorable for lipase loading, while the enhanced enzymatic activity demands nanogels with smaller size and a moderate cross-link degree. As such, PDMAEMA nanogels with a hydrodynamic radius of 35 nm and 30% cross-linker fraction display the optimal catalytic efficiency, which is fourfold of that of free lipase. Moreover, the immobilization endows enhanced enzymatic activity in a broad scope of pH, ionic strength, and temperature, demonstrating effective protection and activation of lipase by the designed nanogels. Our study validates the crucial controls of the size and structure of PE nanogels on enzyme encapsulation and activation, and the revealed findings shall be helpful for designing functional PE nanogels and boosting their applications for enzyme immobilization.

Bone Regeneration Using MMP-Cleavable Peptides-Based Hydrogels

Accumulating evidence has suggested the significant potential of chemically modified hydrogels in bone regeneration. Despite the progress of bioactive hydrogels with different materials, structures and loading cargoes, the desires from clinical applications have not been fully validated. Multiple biological behaviors are orchestrated precisely during the bone regeneration process, including bone marrow mesenchymal stem cells (BMSCs) recruitment, osteogenic differentiation, matrix calcification and well-organized remodeling. Since matrix metalloproteinases play critical roles in such bone metabolism processes as BMSC commitment, osteoblast survival, osteoclast activation matrix calcification and microstructure remodeling, matrix metalloproteinase (MMP) cleavable peptides-based hydrogels could respond to various MMP levels and, thus, accelerate bone regeneration. In this review, we focused on the MMP-cleavable peptides, polymers, functional modification and crosslinked reactions. Applications, perspectives and limitations of MMP-cleavable peptides-based hydrogels for bone regeneration were then discussed.

Injectable and Thermosensitive Liposomal Hydrogels for NIR-II Light-Triggered Photothermal-Chemo Therapy of Pancreatic Cancer

An injectable hydrogel sustained drug release system could be a promising technique for in situ treatment. Herein, an injectable hydrogel was prepared for photothermal-chemo therapy of cancer based on the thermosensitive liposomal hydrogel (Lip-Gel). The Lip-Gel system was fabricated by encapsulation of the NIR-II photothermal agent (DPP-BTz) and chemotherapy drugs (GEM) in thermosensitive liposomes and then combined with hydrogel precursor solution. The hydrogel precursor was used as an injectable flowing solution at room temperature and transferred into a cross-linked gel structure at physiological temperature. After being injected into the tumor, DPP-BTz in the Lip-Gel system can generate heat under irradiation of 1064 nm laser, breaking the thermosensitive liposomes and releasing GEM to kill tumor cells. From the treatment results, the Lip-Gel system showed a significant antitumor effect through chemo-/photothermal therapy combination therapy triggered by the NIR-II laser. This work provides a useful scheme for the development of drug delivery and drug treatment directions for local cancer therapy.

Amplified oxidative stress therapy by a degradable copper phosphate nanozyme coated by the in situ polymerization of PEGDA

The elimination of reactive oxygen species (ROS) caused by glutathione (GSH) is a fundamental concern in the oxidative stress therapy (OST) of tumors. This is the first report of copper phosphate nanospheres coated by poly (ethylene glycol) diacrylate (Cu₃(PO₄)₂@PEGDA) which act as nanozymes to amplify the anti-tumor effects of OST. Cu₃(PO₄)₂@PEGDA not only catalyzes the generation of ˙OH from H₂O₂ but also consumes GSH, which is counterproductive to the role of ˙OH. Moreover, the photothermal properties of Cu₃(PO₄)₂@PEGDA further enhances the outcome of the OST when exposed to an 808 nm laser. Another novelty lies in that a new PEGylation strategy of peroxidase-like nanozymes is proposed, in which the Cu₃(PO₄)₂ cores work as internal heaters and radical generators, which are necessary to initiate the radical polymerization of PEGDA. An elaborate core-shell nanostructure is obtained since the polymerization prefers to take place in the vicinity of the cores, overcoming the drawbacks of traditional PEGylation methods which include invalid polymerization far away from the cores and easy core-shell disassembly during applications.

Mussel-inspired nanozyme catalyzed conductive and self-setting hydrogel for adhesive and antibacterial bioelectronics

Adhesive hydrogels have broad applications ranging from tissue engineering to bioelectronics; however, fabricating adhesive hydrogels with multiple functions remains a challenge. In this study, a mussel-inspired tannic acid chelated-Ag (TA-Ag) nanozyme with peroxidase (POD)-like activity was designed by the in situ reduction of ultrasmall Ag nanoparticles (NPs) with TA. The ultrasmall TA-Ag nanozyme exhibited high catalytic activity to induce hydrogel self-setting without external aid. The nanozyme retained abundant phenolic hydroxyl groups and maintained the dynamic redox balance of phenol-quinone, providing the hydrogels with long-term and repeatable adhesiveness, similar to the adhesion of mussels. The phenolic hydroxyl groups also afforded uniform distribution of the nanozyme in the hydrogel network, thereby improving its mechanical properties and conductivity. Furthermore, the nanozyme endowed the hydrogel with antibacterial activity through synergistic effects of the reactive oxygen species generated via POD-like catalytic reactions and the intrinsic bactericidal activity of Ag. Owing to these advantages, the ultrasmall TA-Ag nanozyme-catalyzed hydrogel could be effectively used as an adhesive, antibacterial, and implantable bioelectrode to detect bio-signals, and as a wound dressing to accelerate tissue regeneration while preventing infection. Therefore, this study provides a promising approach for the fabrication of adhesive hydrogel bioelectronics with multiple functions via mussel-inspired nanozyme catalysis.

In situ self-assembled peptide nanofibers for cancer theranostics

Self-assembled nanofibers hold tremendous promise for cancer theranostics owing to their in situ assembly, spatiotemporal responsiveness, and diverse bioactivity. Herein, this review summarizes the recent advances of self-assembled peptide nanofibers and their applications in biological systems, focusing on the dynamic process of capturing cancer cells from the outside-in. (1) In situ self-assembly in response to pathological or physiological changes. (2) Diverse functions at different locations of tumors, such as forming thrombus in tumor vasculature, constructing a barrier on the cancer cell membrane, and disrupting the cancer organelles. Of note, with the assembly/aggregation induced residence (AIR) effect, the nanofibers could form a drug depot in situ for sustained release of chemotherapeutic drugs to increase their local concentration and prolong the residence time. Finally, perspectives toward future directions and challenges are presented to further understand and expand this exciting field.

3D Bioprinting of Reinforced Vessels by Dual-Cross-linked Biocompatible Hydrogels

3D bioprinting offers a powerful tool to fabricate vessel channels in tissue engineering applications, but inadequate strength of the vascular walls limited the development of this strategy and reinforced channels were highly desired for vascular constructions. Herein, we demonstrated a dual cross-linking system for 3D bioprinting of tubular structures, achieved by a combination of photo-cross-linking and enzymatic cross-linking. Photo-cross-linking of gelatin methacryloyl (GelMA) was achieved with a photoactive conjugated polymer PBF under 550 nm irradiation. Enzymatic cross-linking utilized cascade reactions catalyzed by glucose peroxidase and horseradish peroxidase that can cross-link both methacrylate and tyrosine moieties of GelMA. After removing the 3D-printed sacrificial layer (Pluronic F-127), the obtained perfusable channels showed great biocompatibility that allowed endothelial cells to adhere and proliferate. Our dual cross-linking strategy has great potential in 3D bioprinting of tubular structure for biomedical applications, especially for artificial blood vessels.

In Situ Preparation of Mechanically Enhanced Hydrogel via Dispersion Polymerization in Aqueous Solution

Hydrogels with improved mechanical properties can expand to a greater range of applications. The fabrication of conventional toughened hydrogels typically requires precise modifications, multiple components, and complex steps. Here, a straightforward "one-step" polymerization method for the in situ preparation of hydrogels in aqueous solutions, is reported. Inspired by polymerization-induced self-assembly (PISA), water-miscible monomers are copolymerized during the hydrogel fabrication; the growing blocks eventually form physical bridges thus providing a mechanism for effective energy dissipation. The rheological and mechanical properties are evaluated and the results reveal that this strategy can be an effective approach to design mechanically enhanced hydrogels for a wide range of applications.

Covalently Functionalized Carbon Nano-Onions Integrated Gelatin Methacryloyl Nanocomposite Hydrogel Containing γ-Cyclodextrin as Drug Carrier for High-Performance pH-Triggered Drug Release

Herein, poly (n-(4-aminophenyl) methacrylamide)) carbon nano-onions (PAPMA-CNOs = f-CNOs) and γ-cyclodextrin/DOX-complex (CD) reinforced gelatin methacryloyl (GelMA)/f-CNOs/CD supramolecular hydrogel interfaces were fabricated using the photo-crosslinking technique. The physicochemical properties, morphology, biodegradation, and swelling properties of hydrogels were investigated. The composite hydrogels demonstrated enriched drug release under the acidic conditions (pH 4.5 = 99%, and pH 6.0 = 82%) over 18 days. Owing to the f-CNOs inclusion, GelMA/f-CNOs/CD supramolecular hydrogels presented augmented tensile strength (σ_ult = 356.1 ± 3.4 MPa), toughness (K = 51.5 ± 0.24 Jg⁻¹), and Young’s modulus (E = 41.8 ± 1.4 GPa). The strengthening of GelMA/f-CNOs/CD hydrogel systems indicates its good dispersion and the degree of polymer enveloping of f-CNOs within GelMA matrixes. Furthermore, the obtained hydrogels showed improved cell viability with human fibroblast cells. Nevertheless, the primed supramolecular hydrogels would pave the way for the controlled delivery systems for future drug delivery.

Enzyme-Laden Bioactive Hydrogel for Biocatalytic Monitoring and Regulation

Enzymes, a class of highly efficient and specific catalysts in Nature, dictate a myriad of reactions that constitute various cascades in biological systems. There is growing evidence that many cellular reactions within metabolic pathways are catalyzed by matrix-associated multienzyme complexes, not via the free enzymes, verifying the vital effects of microenvironmental organization, which would reveal implications for the high efficiency, specificity, and regulation of metabolic pathways. The extracellular matrix (ECM), as the noncellular component, is composed of various proteins such as collagens, laminins, proteoglycans, and remodeling enzymes, playing the key role in tissue architecture and homeostasis. Hydrogels are defined as highly hydrated polymer materials and maintain structural integrity by physical and chemical force, which are thought of as the most suitable materials for matching the chemical, physical, and mechanical properties with natural ECM. As one specific type of soft and wet materials, hydrogels are suitable three-dimensional carriers to locally confine bioactive guests, such as enzymes, for molecular-level biological interactions. The efficient cascade catalysis can be realized by enzyme-laden hydrogels, which can potentially interact with cells and tissues by material-to-biology communication. In this Account, we present recent progress on the preparation of enzymatic bioactive hydrogels, including in situ coassembly, in situ cross-linking strategy, and in situ enzymatic radical polymerization technology, further promoting their applications on biomedical tissue engineering, biocatalytic health monitoring, and therapeutic research. First, we provide a brief introduction of the basic concept related to an enzymatic strategy in living systems and the importance of bioinspired enzyme-laden bioactive hydrogel systems. We discuss the difficulties of the fabrication of a bioactive hydrogel with a high catalytic efficiency, thereby providing the novel molecular design and regulation based on a noncovalent coassembly and in situ self-immobilization strategy to obtain the compartmentalized enzyme-laden structure. Then the applications of an enzyme-laden bioactive hydrogel for biocatalytic applications are discussed in detail. The enzyme-laden bioactive hydrogel can maintain the favorable perception and regulation behavior of enzymes with optimal enzymatic efficacy between this confined hydrogel network and a surrounding environment. A highlight to the advances in the responsively biocatalytic monitoring and regulation of bioactive hydrogel, including the enzymatic biomedical tissue engineering and health monitoring, enzymatic regulation of tumor reactive oxygen species and therapeutic research are given. Finally, the outlook of open challenges and future developments of this rapidly evolving field is provided. This Account with highlights of diverse enzyme-laden bioactive hydrogel systems not only provides interesting insights to understand the cascade enzymatic strategy of life but also inspires to broaden and enhance the molecular-level material design and bioapplications of existing enzymatic materials in chemistry, materials science, and biology.

Thermo-Triggered In Situ Chitosan-Based Gelation System for Repeated and Enhanced Sonodynamic Therapy Post a Single Injection

Sonodynamic therapy (SDT) by utilizing ultrasonic waves triggers the generation of reactive oxygen species (ROS) with the help of sonosensitizers to destruct deep-seated tumors has attracted great attention. However, the efficacy of SDT may not be robust enough due to the insufficient oxygen supply within solid tumors. Additionally, repeated injections and treatments, which are often required to achieve the optimal therapeutic responses, may cause additional side effects and patient incompliance. Herein, a thermo-triggered in situ hydrogel system is developed in which catalase (CAT) conjugated with sonosensitizer meso-tetra (4-carboxyphenyl) porphine (TCPP) is mixed into chitosan (CS) and beta-glycerol phosphate disodium (GP) to form the precursor solution. After injection of the precursor solution into tumors, the in situ sol-gel transformation will occur as triggered by the body temperature, resulting in the localized tumor retention of TCPP-CAT. The locally restrained TCPP-CAT not only produces ROS under ultrasonic treatment, but also sustainably reverses the oxygen-deficient status in solid tumors by triggering the O₂ generation from the decomposition of endogenous H₂ O₂ , further promoting the efficacy of SDT. As a result, the repeated SDT after a single dose injection of such a hydrogel can offer robust treatment effects to effectively eradicate tumors.

Combination Therapy of Killing Diseases by Injectable Hydrogels: From Concept to Medical Applications

The complexity of hard-to-treat diseases strongly undermines the therapeutic potential of available treatment options. Therefore, a paradigm shift from monotherapy toward combination therapy has been observed in clinical research to improve the efficiency of available treatment options. The advantages of combination therapy include the possibility of synchronous alteration of different biological pathways, reducing the required effective therapeutic dose, reducing drug resistance, and lowering the overall costs of treatment. The tunable physical properties, excellent biocompatibility, facile preparation, and ease of administration with minimal invasiveness of injectable hydrogels (IHs) have made them excellent candidates to solve the clinical and pharmacological limitations of present systems for multitherapy by direct delivery of therapeutic payloads and improving therapeutic responses through the formation of depots containing drugs, genes, cells, or a combination of them in the body after a single injection. In this review, currently available methods for the design and fabrication of IHs are systematically discussed in the first section. Next, as a step toward establishing IHs for future multimodal synergistic therapies, recent advances in cancer combination therapy, wound healing, and tissue engineering are addressed in detail in the following sections. Finally, opportunities and challenges associated with IHs for multitherapy are listed and further discussed.

Recent Advances in Injectable Hydrogels for Controlled and Local Drug Delivery

Injectable hydrogels have received considerable interest in the biomedical field due to their potential applications in minimally invasive local drug delivery, more precise implantation, and site-specific drug delivery into poorly reachable tissue sites and into interface tissues, where wound healing takes a long time. Injectable hydrogels, such as in situ forming and/or shear-thinning hydrogels, can be generated using chemically and/or physically crosslinked hydrogels. Yet, for controlled and local drug delivery applications, the ideal injectable hydrogel should be able to provide controlled and sustained release of drug molecules to the target site when needed and should limit nonspecific drug molecule distribution in healthy tissues. Thus, such hydrogels should sense the environmental changes that arise in disease states and be able to release the optimal amount of drug over the necessary time period to the target region. To address this, researchers have designed stimuli-responsive injectable hydrogels. Stimuli-responsive hydrogels change their shape or volume when they sense environmental stimuli, e.g., pH, temperature, light, electrical signals, or enzymatic changes, and deliver an optimal concentration of drugs to the target site without affecting healthy tissues.

Photo-responsive supramolecular hyaluronic acid hydrogels for accelerated wound healing

Supramolecular hydrogels confer control over structural properties in a reversible, dynamic, and biomimetic fashion. The design of supramolecular hydrogels with an improved structural and functional recapitulation of damaged organs is important for clinical applications. For wound healing management, in particular, an effective healing process, through the modulation of epidermal growth factor (EGF) delivery using supramolecular polysaccharide hydrogels, has yet to be developed. In this study, photo-responsive supramolecular polysaccharide hydrogels were formed through host-guest interactions between azobenzene and β-cyclodextrin groups conjugated to hyaluronic acid chains. By exploiting the photoisomerization properties of azobenzene under different wavelengths, a supramolecular hydrogel featuring a dynamic spatial network crosslink density through the application of a light stimulus was obtained. Under ultra violet (UV) light, the loosened hydrogel can rapidly release EGF, thereby enhancing EGF delivery at the wound site. Based on an in vivo assessment of the healing process through a full-thickness skin defect model, the controlled EGF release from a supramolecular hydrogel exhibited superior wound healing efficiency with respect to granulation tissue formation, growth factor levels, and angiogenesis. Therefore, the proposed supramolecular hydrogels are potentially valuable as controlled delivery systems for future clinical wound healing applications.

Enzymatically triggered chromogenic cross-linking agents under physiological conditions

Oxidative dimerization of an indoxyl moiety, released by glycosidase action in aqueous solution, yields an indigoid dye in formats that enable bioconjugation and molecular cross-linking.

Chromogenic agents built around a multifunctional double-triazine framework for enzymatically triggered cross-linking under physiological conditions

A molecular architecture designed for bioconjugation and internal absorption ratiometry undergoes enzymatically triggered cleavage of glucosyl groups and subsequent oxidative dimerization in aqueous solution to yield indigoid-containing scaffolds.

Biocatalytic ATRP in solution and on surfaces

The promiscuity of enzymes allows for their implementation as catalysts for non-native chemical transformations. Utilizing the redox activity of metalloenzymes under activator regenerated by electron transfer (ARGET) ATRP conditions, well-controlled and defined polymers can be generated. In this chapter, we review bioATRP in solution and on surfaces and provide experimental protocols for hemoglobin-catalyzed ATRP and for surface-initiated biocatalytic ATRP. This chapter highlights the polymerization of acrylate and acrylamide monomers and provides detailed experimental protocols for the characterization of the polymers and of the polymer brushes.

Light-Triggered In Situ Gelation to Enable Robust Photodynamic-Immunotherapy by Repeated Stimulations

Photodynamic therapy (PDT) has shown the potential of triggering systemic antitumor immune responses. However, while the oxygen-deficient hypoxic tumor microenvironment is a factor that limits the PDT efficacy, the immune responses after conventional PDT usually are not strong enough to eliminate metastatic tumors. Herein, a light-triggered in situ gelation system containing photosensitizer-modified catalase together with poly(ethylene glycol) double acrylate (PEGDA) as the polymeric matrix is designed. Immune adjuvant nanoparticles are further introduced into this system to trigger robust antitumor immune responses after PDT. Following local injection of the mixed precursor solution into tumors and the subsequent light exposure, polymerization of PEGDA can be initiated to induce in situ gelation. Such hybrid hydrogel with long-term tumor retention of various agents and the ability to enable persistent tumor hypoxia relief can enable multiple rounds of PDT, which results in significantly enhanced immune responses by multiround stimulation. Further combination of such gel-based multiround PDT with anticytotoxic T-lymphocyte antigen-4 checkpoint blockade offers not only the abscopal effect to inhibit growth of distant tumors but also effective long-term immune memory protection from rechallenged tumors. Therefore, such a light-triggered in situ gelation system by a single-dose injection can enable greatly enhanced photoimmunotherapy by means of repeated stimulations.

Enzymatic crosslinking to fabricate antioxidant peptide-based supramolecular hydrogel for improving cutaneous wound healing

Peptide-based supramolecular hydrogels are promising scaffold materials and have been utilized in many fields. The mechanical properties of peptide hydrogels are usually enhanced by synthetic or natural polymers to expand their application scope. In this study, antioxidant supramolecular hydrogels based on feruloyl-modified peptide and glycol chitosan were fabricated via a mild laccase-mediated crosslinking reaction. A natural polysaccharide derivative, feruloyl glycol chitosan (GC-Fer), was used to enhance the mechanical properties of peptide hydrogels. Feruloyl groups were introduced into the gel matrix via covalent bonds, which endowed the hydrogels with inherent antioxidant properties. This was beneficial for their in vivo application via scavenging harmful free radicals existing in a cutaneous wound. Further in vivo experiments demonstrated that the feruloyl-containing antioxidant hydrogel can improve the cutaneous wound healing process. The regeneration process of mature epithelium and connective tissues was accelerated in a full-thickness skin defect model.

One-pot preparation of double network hydrogels via enzyme-mediated polymerization and post-self-assembly for wound healing

An enzymatic one-pot preparation method is used to prepare double network hydrogels for wound healing.