Injectable dextran hydrogels fabricated by metal-free click chemistry for cartilage tissue engineering

Unveiling the Diverse Principles for Developing Sprayable Hydrogels for Biomedical Applications

Sprayable hydrogels have emerged as a transformative innovation in biomedical technology, offering a versatile, efficient, and minimally invasive platform for various clinical applications. They form gels in situ upon tissue contact, enabling seamless application on even complex surfaces. This property is especially useful in wound care, drug delivery, and tissue engineering, where localized and sustained release of therapeutics is essential. Formulations can be customized to include various bioactive compounds, such as growth factors, antibiotics, and anti-inflammatory agents, thereby enhancing targeted treatment outcomes. This review delves into the fundamental principles governing sprayable hydrogels, emphasizing critical mechanisms such as in situ cross-linking, shear-thinning properties, and thermoresponsive behavior. Furthermore, it highlights recent advancements since 2020, including the strategic incorporation of bioactive agents to augment therapeutic efficacy. By examining these core mechanisms and design strategies, this review provides a comprehensive perspective on the engineering of sprayable hydrogels for modern medical applications.

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.

Emerging Frontiers in In Situ Forming Hydrogels for Enhanced Hemostasis and Accelerated Wound Healing

With a surge in the number of accidents and chronic wounds worldwide, there is a growing need for advanced hemostatic and wound care solutions. In this regard, in situ forming hydrogels have emerged as a revolutionary biomaterial due to their inherent properties, which include biocompatibility, biodegradability, porosity, and extracellular matrix (ECM)-like mechanical strength, that render them ideal for biomedical applications. This review demonstrates the advancements of in situ forming hydrogels, tracing their evolution from injectable to more sophisticated forms, such as sprayable and 3-D printed hydrogels. These hydrogels are designed to modulate the pathophysiology of wounds, enhancing hemostasis and facilitating wound repair. The review presents different methodologies for in situ forming hydrogel synthesis, spanning a spectrum of physical and chemical cross-linking techniques. Furthermore, it showcases the adaptability of hydrogels to the dynamic requirements of wound healing processes. Through a detailed discussion, this article sheds light on the multifunctional capabilities of these hydrogels such as their antibacterial, anti-inflammatory, and antioxidant properties. This review aims to inform and inspire continued advancement in the field, ultimately contributing to the development of sophisticated wound care solutions that meet the complexity of clinical needs.

Comparative Analysis and Regeneration Strategies for Three Types of Cartilage

Cartilage tissue, encompassing hyaline cartilage, fibrocartilage, and elastic cartilage, plays a pivotal role in the human body because of its unique composition, structure, and biomechanical properties. However, the inherent avascularity and limited regenerative capacity of cartilage present significant challenges to its healing following injury. This review provides a comprehensive analysis of the current state of cartilage tissue engineering, focusing on the critical components of cell sources, scaffolds, and growth factors tailored to the regeneration of each cartilage type. We explore the similarities and differences in the composition, structure, and biomechanical properties of the three cartilage types and their implications for tissue engineering. A significant emphasis is placed on innovative strategies for cartilage regeneration, including the potential for in situ transformation of cartilage types through microenvironmental manipulation, which may offer novel avenues for repair and rehabilitation. The review underscores the necessity of a nuanced approach to cartilage tissue engineering, recognizing the distinct requirements of each cartilage type while exploring the potential of transforming one cartilage type into another as a flexible and adaptive repair strategy. Through this detailed examination, we aim to broaden the understanding of cartilage tissue engineering and inspire further research and development in this promising field.

Injectable Hydrogels in Cardiovascular Tissue Engineering

Heart problems are quite prevalent worldwide. Cardiomyocytes and stem cells are two examples of the cells and supporting matrix that are used in the integrated process of cardiac tissue regeneration. The objective is to create innovative materials that can effectively replace or repair damaged cardiac muscle. One of the most effective and appealing 3D/4D scaffolds for creating an appropriate milieu for damaged tissue growth and healing is hydrogel. In order to successfully regenerate heart tissue, bioactive and biocompatible hydrogels are required to preserve cells in the infarcted region and to bid support for the restoration of myocardial wall stress, cell survival and function. Heart tissue engineering uses a variety of hydrogels, such as natural or synthetic polymeric hydrogels. This article provides a quick overview of the various hydrogel types employed in cardiac tissue engineering. Their benefits and drawbacks are discussed. Hydrogel-based techniques for heart regeneration are also addressed, along with their clinical application and future in cardiac tissue engineering.

Recent Advances in the Degradability and Applications of Tissue Adhesives Based on Biodegradable Polymers

In clinical practice, tissue adhesives have emerged as an alternative tool for wound treatments due to their advantages in ease of use, rapid application, less pain, and minimal tissue damage. Since most tissue adhesives are designed for internal use or wound treatments, the biodegradation of adhesives is important. To endow tissue adhesives with biodegradability, in the past few decades, various biodegradable polymers, either natural polymers (such as chitosan, hyaluronic acid, gelatin, chondroitin sulfate, starch, sodium alginate, glucans, pectin, functional proteins, and peptides) or synthetic polymers (such as poly(lactic acid), polyurethanes, polycaprolactone, and poly(lactic-co-glycolic acid)), have been utilized to develop novel biodegradable tissue adhesives. Incorporated biodegradable polymers are degraded in vivo with time under specific conditions, leading to the destruction of the structure and the further degradation of tissue adhesives. In this review, we first summarize the strategies of utilizing biodegradable polymers to develop tissue adhesives. Furthermore, we provide a symmetric overview of the biodegradable polymers used for tissue adhesives, with a specific focus on the degradability and applications of these tissue adhesives. Additionally, the challenges and perspectives of biodegradable polymer-based tissue adhesives are discussed. We expect that this review can provide new inspirations for the design of novel biodegradable tissue adhesives for biomedical applications.

Metal-Free Click-Chemistry: A Powerful Tool for Fabricating Hydrogels for Biomedical Applications

Increasing interest in the utilization of hydrogels in various areas of biomedical sciences ranging from biosensing and drug delivery to tissue engineering has necessitated the synthesis of these materials using efficient and benign chemical transformations. In this regard, the advent of "click" chemistry revolutionized the design of hydrogels and a range of efficient reactions was utilized to obtain hydrogels with increased control over their physicochemical properties. The ability to apply the "click" chemistry paradigm to both synthetic and natural polymers as hydrogel precursors further expanded the utility of this chemistry in network formation. In particular, the ability to integrate clickable handles at predetermined locations in polymeric components enables the formation of well-defined networks. Although, in the early years of "click" chemistry, the copper-catalyzed azide-alkyne cycloaddition was widely employed, recent years have focused on the use of metal-free "click" transformations, since residual metal impurities may interfere with or compromise the biological function of such materials. Furthermore, many of the non-metal-catalyzed "click" transformations enable the fabrication of injectable hydrogels, as well as the fabrication of microstructured gels using spatial and temporal control. This review article summarizes the recent advances in the fabrication of hydrogels using various metal-free "click" reactions and highlights the applications of thus obtained materials. One could envision that the use of these versatile metal-free "click" reactions would continue to revolutionize the design of functional hydrogels geared to address unmet needs in biomedical sciences.

Injectable hydrogels as promising in situ therapeutic platform for cartilage tissue engineering

Injectable hydrogels are gaining prominence as a biocompatible, minimally invasive, and adaptable platform for cartilage tissue engineering. Commencing with their synthesis, this review accentuates the tailored matrix formulations and cross-linking techniques essential for fostering three-dimensional cell culture and melding with complex tissue structures. Subsequently, it spotlights the hydrogels' enhanced properties, highlighting their augmented functionalities and broadened scope in cartilage tissue repair applications. Furthermore, future perspectives are advocated, urging continuous innovation and exploration to surmount existing challenges and harness the full clinical potential of hydrogels in regenerative medicine. Such advancements are crucial for validating the long-term efficacy and safety of hydrogels, positioning them as a promising direction in regenerative medicine to address cartilage-related ailments.

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.

Advances in injectable hydrogel: Design, functional regulation, and biomedical applications

Recently, injectable hydrogels have been considered smart materials and have been widely researched for their use as scaffolds. They resemble the extracellular matrix of native tissue and have the capability for homogeneous mixing with therapeutic agents. It can be implanted into living bodies with minimal invasiveness and usability for irregularly shaped sites. Such unique features make the injectable hydrogels as promising materials in tissue engineering, drug delivery system, and gene/protein delivery. This review article provides a comprehensive overview of the different mechanisms employed in the preparation of injectable hydrogel, as well as a detailed exploration of its applications in the biomedical field. Furthermore, the article highlights the critical importance of developing injectable hydrogels as market‐viable products, highlighting their potential impact in the field of regenerative medicine.

Injectable organo-hydrogels influenced by click chemistry as a paramount stratagem in the conveyor belt of pharmaceutical revolution

The field of injectable hydrogels has demonstrated a paramount headway in the myriad of biomedical applications and paved a path toward clinical advancements. The innate superiority of hydrogels emerging from organic constitution has exhibited dominance in overcoming the bottlenecks associated with inorganic-based hydrogels in the biological milieu. Inorganic hydrogels demonstrate various disadvantages, including limited biocompatibility, degradability, a cumbersome synthesis process, high cost, and ecotoxicity. The excellent biocompatibility, eco-friendliness, and manufacturing convenience of organo-hydrogels have demonstrated to be promising in therapizing biomedical complexities with low toxicity and augmented bioavailability. This report manifests the realization of biomimetic organo-hydrogels with the development of bioresponsive and self-healing injectable organo-hydrogels in the emerging pharmaceutical revolution. Furthermore, the influence of click chemistry in this regime as a backbone in the pharmaceutical conveyor belt has been suggested to scale up production. Moreover, we propose an avant-garde design stratagem of developing a hyaluronic acid (HA)-based injectable organo-hydrogel via click chemistry to be realized for its pharmaceutical edge. Ultimately, injectable organo-hydrogels that materialize from academia or industry are required to follow the standard set of rules established by global governing bodies, which has been delineated to comprehend their marketability. Thence, this perspective narrates the development of injectable organo-hydrogels via click chemistry as a prospective elixir to have in the arsenal of pharmaceuticals.

Past, present and future of biomedical applications of dextran-based hydrogels: A review

This review extensively surveys the biomedical applications of hydrogels containing dextran. Dextran has gained much attention as a biomaterial due to its distinctive properties such as biocompatibility, non-toxicity, water solubility and biodegradability. It has emerged as a critical constituent of hydrogels for biomedical applications including drug delivery devices, tissue engineering scaffolds and biosensor materials. The benefits, challenges and potential prospects of dextran-based hydrogels as biomaterials are highlighted in this review.

M, Zarrintaj P, Formela K, Saeb MR, Bencherif SA. Clickable Polysaccharides for Biomedical Applications: A Comprehensive Review

Recent advances in materials science and engineering highlight the importance of designing sophisticated biomaterials with well-defined architectures and tunable properties for emerging biomedical applications. Click chemistry, a powerful method allowing specific and controllable bioorthogonal reactions, has revolutionized our ability to make complex molecular structures with a high level of specificity, selectivity, and yield under mild conditions. These features combined with minimal byproduct formation have enabled the design of a wide range of macromolecular architectures from quick and versatile click reactions. Furthermore, copper-free click chemistry has resulted in a change of paradigm, allowing researchers to perform highly selective chemical reactions in biological environments to further understand the structure and function of cells. In living systems, introducing clickable groups into biomolecules such as polysaccharides (PSA) has been explored as a general approach to conduct medicinal chemistry and potentially help solve healthcare needs. De novo biosynthetic pathways for chemical synthesis have also been exploited and optimized to perform PSA-based bioconjugation inside living cells without interfering with their native processes or functions. This strategy obviates the need for laborious and costly chemical reactions which normally require extensive and time-consuming purification steps. Using these approaches, various PSA-based macromolecules have been manufactured as building blocks for the design of novel biomaterials. Clickable PSA provides a powerful and versatile toolbox for biomaterials scientists and will increasingly play a crucial role in the biomedical field. Specifically, bioclick reactions with PSA have been leveraged for the design of advanced drug delivery systems and minimally invasive injectable hydrogels. In this review article, we have outlined the key aspects and breadth of PSA-derived bioclick reactions as a powerful and versatile toolbox to design advanced polymeric biomaterials for biomedical applications such as molecular imaging, drug delivery, and tissue engineering. Additionally, we have also discussed the past achievements, present developments, and recent trends of clickable PSA-based biomaterials such as 3D printing, as well as their challenges, clinical translatability, and future perspectives.

Clickable and smart drug delivery vehicles accelerate the healing of infected diabetic wounds

In this study, an adipic acid dihydrazide (ADH)/ tannic acid (TA)-grafted hyaluronic acid (HA)-based multifunctional hydrogel was synthesized through a spontaneous amino-yne click reaction and used to promote the improved healing of infected diabetic wounds. This hydrogel exhibited a range of beneficial properties such as tunable gelation time, adjustable mechanical properties, pH-sensitive response characteristics, excellent injectability, the ability to readily adhere to tissue, and ultra-intimate contact capabilities. Following the encapsulation of ultrasmall Ag nanoclusters (AgNCs) and deferoxamine loaded polydopamine/ hollow mesoporous manganese dioxide (PHMD, PDA/H-mMnO₂@DFO) nanoparticles, the prepared hydrogel presented with robust antibacterial, anti-inflammatory, and pro-angiogenic properties and a desirable smart drug release profile. In this fabricated platform, PHMD was able to effectively alleviate localized oxidative stress and prolonged oxygen deprivation via the decomposition of endogenous H₂O₂ to produce O₂. Further in vivo assays revealed that this hydrogel was capable of facilitating the healing of infected wounds through the sequential engagement of antibacterial, anti-inflammatory, and pro-angiogenic activities. Together, this synthesized clickable environmentally-responsive hydrogel offers great promise as a tool that can be applied to aid in the healing of chronically infected diabetic wounds and other inflammatory conditions.

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.

Real-Time MRI Monitoring of GelMA-Based Hydrogel-Loaded Kartogenin for In Situ Cartilage Regeneration

The cartilage has poor ability to mount a sufficient healing response. Herein, kartogenin (KGN), an emerging stable non-protein compound with the ability to recruit bone marrow mesenchyme stem cells (BMSCs) to promote chondrogenic differentiation, was grafted onto dopamine-Fe(III) chelating nanoparticles, followed by involving a gelatin- and dextran-based injectable hydrogel to mimic the extracellular matrix to promote cartilage repair. The in vitro results demonstrated that KGN underwent long-term sustained release behavior and availably promoted the deep migration of BMSC cells in yielding hydrogels. Furthermore, in vivo New Zealand white rabbits’ cartilage defect model repairing results showed that cartilage defect obtained significant regeneration post operation in the 12th week, and the defect edge almost disappeared compared to adjacent normal cartilage tissue. Meanwhile, the T2-weighted magnetic resonance imaging (MRI) property resulting from dissociative Fe (III) can significantly monitor the degradation degree of the implanted hydrogels in the defect site. This integrated diagnosis and treatment system gives insight into cartilage regeneration.

Bioactive hydrogels based on polysaccharides and peptides for soft tissue wound management

Due to their inherent and tunable biomechanical and biochemical performances, bioactive hydrogels based on polysaccharides and peptides have shown attractive potential for wound management. In this review, the recent progress of bioactive hydrogels prepared by polysaccharides and peptides for soft tissue wound management is overviewed. Meanwhile, we focus on the elaboration of the relationship between chemical structures and inherent bioactive functions of polysaccharides and peptides, as well as the strategies that are taken for achieving multiple wound repairing effects including hemostasis, adhesion, wound contraction and closure, anti-bacteria, anti-oxidation, immunomodulation, molecule delivery, etc. Some innovative and important works are well introduced as well. In the end, current study limitations, clinical unmet needs, and future directions are discussed.

Polymeric Hydrogels for Controlled Drug Delivery to Treat Arthritis

Rheumatoid arthritis (RA) and osteoarthritis (OA) are disabling musculoskeletal disorders that affect joints and cartilage and may lead to bone degeneration. Conventional delivery of anti-arthritic agents is limited due to short intra-articular half-life and toxicities. Innovations in polymer chemistry have led to advancements in hydrogel technology, offering a versatile drug delivery platform exhibiting tissue-like properties with tunable drug loading and high residence time properties This review discusses the advantages and drawbacks of polymeric materials along with their modifications as well as their applications for fabricating hydrogels loaded with therapeutic agents (small molecule drugs, immunotherapeutic agents, and cells). Emphasis is given to the biological potentialities of hydrogel hybrid systems/micro-and nanotechnology-integrated hydrogels as promising tools. Applications for facile tuning of therapeutic drug loading, maintaining long-term release, and consequently improving therapeutic outcome and patient compliance in arthritis are detailed. This review also suggests the advantages, challenges, and future perspectives of hydrogels loaded with anti-arthritic agents with high therapeutic potential that may alter the landscape of currently available arthritis treatment modalities.

Recent Advances in Bioorthogonal Click Chemistry for Biomedical Applications

Bioorthogonal click chemistry, first introduced in the early 2000s, has become one of the most widely used approaches for designing advanced biomaterials for applications in tissue engineering and regenerative medicine, due to the selectivity and biocompatibility of the associated reactants and reaction conditions. In this review, we present recent advances in utilizing bioorthogonal click chemistry for the development of three-dimensional, biocompatible scaffolds and cell-encapsulated biomaterials. Additionally, we highlight recent examples using these approaches for biomedical applications including drug delivery, imaging, and cell therapy and discuss their potential as next generation biomaterials.

Dual crosslinked pullulan-gelatin cryogel scaffold for chondrocyte-mediated cartilage repair: synthesis, characterization andin vitroevaluation

Cryogels, a subset of hydrogels, have recently drawn attention for cartilage tissue engineering due to its inherent microporous architecture and good mechanical properties. In this study a dual crosslinked pullulan-gelatin cryogel (PDAG) scaffold was synthesized by crosslinking gelatin with oxidized pullulan by Schiff's base reaction followed by cryogelation. Chondrocytes seeded within the PDAG scaffolds and cultured for 21 din vitrodemonstrated enhanced cell proliferation, enhanced production of cartilage-specific extracellular matrix and up-regulated sulfated glycosaminoglycan without altering the articular chondrocyte phenotype. Quantitative reverse transcription-polymerase chain reaction-based gene expression studies, immunofluorescence, and histological studies demonstrated that the PDAG scaffold significantly enhanced the expression of chondrogenic marker genes such as type II collagen, aggrecan, and SOX9. Taken together, these results demonstrated that PDAG scaffold prepared by sequential Schiff's base reaction and cryogelation would be a promising cell-responsive scaffold for cartilage tissue engineering applications.

Mechanical Integrity in a Dynamic Interpenetrating Hydrogel Network of Supramolecular Peptide-Polysaccharide Supports Enhanced Chondrogenesis

Tissue engineering demands intelligently designed scaffolds that encompass the properties of the target tissues in terms of mechanical and bioactive properties. An ideal scaffold for engineering a cartilage tissue should provide the chondrocytes with a favorable 3D microarchitecture apart from possessing optimal mechanical characteristics such as compressibility, energy dissipation, strain stiffening, etc. Herein, we used a unique design approach to develop a hydrogel having a dynamic interpenetrating network to serve as a framework to support chondrocyte growth and differentiation. An amyloid-inspired peptide amphiphile (1) was self-assembled to furnish kinetically controlled nanofibers and incorporated in a dynamic covalently cross-linked polysaccharide network of carboxymethyl cellulose dialdehyde (CMC-D) and carboxymethyl chitosan (CMCh) using Schiff base chemistry. The dynamic noncovalent interaction played a pivotal role in providing the desired modulation in the structure and mechanical properties of the double-network hydrogels that are imperative for cartilage scaffold design. The adaptable nature supported shear-induced extrusion of the hydrogel and facilitated various cellular functions while maintaining its integrity. The potential of the as-developed hydrogels to support in vitro chondrogenesis was explored using human chondrocytes. Evidence of improved cell growth and cartilage-specific ECM production confirmed the potential of the hydrogel to support cartilage tissue engineering while reaffirming the significance of mimicking the biophysical microenvironment to induce optimal tissue regeneration.

pH-Sensitive Dextran-Based Micelles from Copper-Free Click Reaction for Antitumor Drug Delivery

There remains a need to develop new strategies to fabricate dextran-based biocompatible drug delivery systems for safe and effective chemotherapy. Herein, a copper-free azide-propiolate ester click reaction was introduced for dextran modification to fabricate a pH-sensitive dextran-based drug delivery system. A pH-sensitive dextran-based micelle system, self-assembled from amphiphilic dextran-graft-poly(2-(diisopropylamino)ethyl methacrylate-co-2-(2',3',5'-triiodobenzoyl)ethyl methacrylate) or dextran-g-P(DPA-co-TIBMA), is reported for effective chemotherapy. The amphiphilic dextran-g-P(DPA-co-TIBMA) was prepared via reversible addition-fragmentation chain-transfer (RAFT) polymerization and copper-free azide-propiolate ester click reaction. Doxorubicin (DOX)-loaded dextran-g-P(DPA-co-TIBMA) micelles were prepared through self-assembly of DOX and dextran-g-P(DPA-co-TIBMA) in aqueous solution, and had a mean diameter of 154 nm and a drug loading content of 9.7 wt %. The release of DOX from DOX-loaded dextran-g-P(PDPA-co-TIBMA) micelles was slow at pH 7.4, but was greatly accelerated under acidic conditions (pH 6 and 5). Confocal laser scanning microscopy and flow cytometry experiments showed that the dextran-g-P(DPA-co-TIBMA) micelles could effectively deliver and release DOX in human breast cancer cell line (MCF-7 cells). MTT assay showed that dextran-g-P(DPA-co-TIBMA) exhibited excellent biocompatibility while DOX-loaded dextran-g-P(DPA-co-TIBMA) micelles have good antitumor efficacy in vitro. The in vivo therapeutic studies indicated that the DOX-loaded dextran-g-P(PDPA-co-TIBMA) micelles could effectively reduce the growth of tumor with little body weight reduction.

Cell-laden injectable microgels: Current status and future prospects for cartilage regeneration

Injectable hydrogels have been employed extensively as versatile materials for cartilage regeneration due to their excellent biocompatibility, tunable structure, and ability to accommodate bioactive factors, as well as their ability to be locally delivered via minimally invasive injection to fill irregular defects. More recently, in vitro and in vivo studies have revealed that processing these materials to produce cell-laden microgels can enhance cell-cell and cell-matrix interactions and boost nutrient and metabolite exchange. Moreover, these studies have demonstrated gene expression profiles and matrix regeneration that are superior compared to conventional injectable bulk hydrogels. As cell-laden microgels and their application in cartilage repair are moving closer to clinical translation, this review aims to present an overview of the recent developments in this field. Here we focus on the currently used biomaterials and crosslinking strategies, the innovative fabrication techniques being used for the production of microgels, the cell sources used, the signals used for induction of chondrogenic differentiation and the resultant biological responses, and the ability to create three-dimensional, functional cartilage tissues. In addition, this review also covers the current clinical approaches for repairing cartilage as well as specific challenges faced when attempting the regeneration of damaged cartilage tissue. New findings related to the macroporous nature of the structures formed by the assembled microgel building blocks and the novel use of microgels in 3D printing for cartilage tissue engineering are also highlighted. Finally, we outline the challenges and future opportunities for employing cell-laden microgels in clinical applications.

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels

Biopolymers are natural polymers sourced from plants and animals, which include a variety of polysaccharides and polypeptides. The inclusion of biopolymers into biomedical hydrogels is of great interest because of their inherent biochemical and biophysical properties, such as cellular adhesion, degradation, and viscoelasticity. The objective of this Review is to provide a detailed overview of the design and development of biopolymer hydrogels for biomedical applications, with an emphasis on biopolymer chemical modifications and cross-linking methods. First, the fundamentals of biopolymers and chemical conjugation methods to introduce cross-linking groups are described. Cross-linking methods to form biopolymer networks are then discussed in detail, including (i) covalent cross-linking (e.g., free radical chain polymerization, click cross-linking, cross-linking due to oxidation of phenolic groups), (ii) dynamic covalent cross-linking (e.g., Schiff base formation, disulfide formation, reversible Diels-Alder reactions), and (iii) physical cross-linking (e.g., guest-host interactions, hydrogen bonding, metal-ligand coordination, grafted biopolymers). Finally, recent advances in the use of chemically modified biopolymer hydrogels for the biofabrication of tissue scaffolds, therapeutic delivery, tissue adhesives and sealants, as well as the formation of interpenetrating network biopolymer hydrogels, are highlighted.

Click-functionalized hydrogel design for mechanobiology investigations

The advancement of click-functionalized hydrogels in recent years has coincided with rapid growth in the fields of mechanobiology, tissue engineering, and regenerative medicine. Click chemistries represent a group of reactions that possess high reactivity and specificity, are cytocompatible, and generally proceed under physiologic conditions. Most notably, the high level of tunability afforded by these reactions enables the design of user-controlled and tissue-mimicking hydrogels in which the influence of important physical and biochemical cues on normal and aberrant cellular behaviors can be independently assessed. Several critical tissue properties, including stiffness, viscoelasticity, and biomolecule presentation, are known to regulate cell mechanobiology in the context of development, wound repair, and disease. However, many questions still remain about how the individual and combined effects of these instructive properties regulate the cellular and molecular mechanisms governing physiologic and pathologic processes. In this review, we discuss several click chemistries that have been adopted to design dynamic and instructive hydrogels for mechanobiology investigations. We also chart a path forward for how click hydrogels can help reveal important insights about complex tissue microenvironments.

Injectable catalyst-free "click" organic-inorganic nanohybrid (click-ON) cement for minimally invasive in vivo bone repair

Injectable polymers have attracted intensive attention in tissue engineering and drug delivery applications. Current injectable polymer systems often require free-radical or heavy-metal initiators and catalysts for the crosslinking process, which may be extremely toxic to the human body. Here, we report a novel polyhedral oligomeric silsesquioxane (POSS) based strain-promoted alkyne-azide cycloaddition (SPAAC) "click" organic-inorganic nanohybrids (click-ON) system that can be click-crosslinked without any toxic initiators or catalysts. The click-ON scaffolds supported excellent adhesion, proliferation, and osteogenesis of stem cells. In vivo evaluation using a rat cranial defect model showed outstanding bone formation with minimum cytotoxicity. Essential osteogenic alkaline phosphatase (ALP) and vascular CD31 marker expression were detected on the defect site, indicating excellent support of in vivo osteogenesis and vascularization. Using salt leaching techniques, an injectable porous click-ON cement was developed to create porous structures and support better in vivo bone regeneration. Beyond defect filling, the click-ON cement also showed promising application for spinal fusion using rabbits as a model. Compared to the current clinically used poly (methyl methacrylate) (PMMA) cement, this click-ON cement showed great advantages of low heat generation, better biocompatibility and biodegradability, and thus has great potential for bone and related tissue engineering applications.

Exploiting the role of nanoparticles for use in hydrogel-based bioprinting applications: concept, design, and recent advances

Three-dimensional (3D) bioprinting is an emerging tissue engineering approach that aims to develop cell or biomolecule-laden, complex polymeric scaffolds with high precision, using hydrogel-based "bioinks". Hydrogels are water-swollen, highly crosslinked polymer networks that are soft, quasi-solid, and can support and protect biological materials. However, traditional hydrogels have weak mechanical properties and cannot retain complex structures. They must be reinforced with physical and chemical manipulations to produce a mechanically resilient bioink. Over the past few years, we have witnessed an increased use of nanoparticles and biological moiety-functionalized nanoparticles to fabricate new bioinks. Nanoparticles of varied size, shape, and surface chemistries can provide a unique solution to this problem primarily because of three reasons: (a) nanoparticles can mechanically reinforce hydrogels through physical and chemical interactions. This can favorably influence the bioink's 3D printability and structural integrity by modulating its rheological, biomechanical, and biochemical properties, allowing greater flexibility to print a wide range of structures; (b) nanoparticles can introduce new bio-functionalities to the hydrogels, which is a key metric of a bioink's performance, influencing both cell-material and cell-cell interactions within the hydrogel; (c) nanoparticles can impart "smart" features to the bioink, making the tissue constructs responsive to external stimuli. Responsiveness of the hydrogel to magnetic field, electric field, pH changes, and near-infrared light can be made possible by the incorporation of nanoparticles. Additionally, bioink polymeric networks with nanoparticles can undergo advanced chemical crosslinking, allowing greater flexibility to print structures with varied biomechanical properties. Taken together, the unique properties of various nanoparticles can help bioprint intricate constructs, bringing the process one step closer to complex tissue structure and organ printing. In this review, we explore the design principles and multifunctional properties of various nanomaterials and nanocomposite hydrogels for potential, primarily extrusion-based bioprinting applications. We illustrate the significance of biocompatibility of the designed nanocomposite hydrogel-based bioink for clinical translation and discuss the different parameters that affect cell fate after cell-nanomaterial interaction. Finally, we critically assess the current challenges of nanoengineering bioinks and provide insight into the future directions of potential hydrogel bioinks in the rapidly evolving field of bioprinting.

Recent Advancements in 3D Printing of Polysaccharide Hydrogels in Cartilage Tissue Engineering

The application of hydrogels coupled with 3-dimensional (3D) printing technologies represents a modern concept in scaffold development in cartilage tissue engineering (CTE). Hydrogels based on natural biomaterials are extensively used for this purpose. This is mainly due to their excellent biocompatibility, inherent bioactivity, and special microstructure that supports tissue regeneration. The use of natural biomaterials, especially polysaccharides and proteins, represents an attractive strategy towards scaffold formation as they mimic the structure of extracellular matrix (ECM) and guide cell growth, proliferation, and phenotype preservation. Polysaccharide-based hydrogels, such as alginate, agarose, chitosan, cellulose, hyaluronan, and dextran, are distinctive scaffold materials with advantageous properties, low cytotoxicity, and tunable functionality. These superior properties can be further complemented with various proteins (e.g., collagen, gelatin, fibroin), forming novel base formulations termed "proteo-saccharides" to improve the scaffold's physiological signaling and mechanical strength. This review highlights the significance of 3D bioprinted scaffolds of natural-based hydrogels used in CTE. Further, the printability and bioink formation of the proteo-saccharides-based hydrogels have also been discussed, including the possible clinical translation of such materials.

Modular synthesis of amphiphilic chitosan derivatives based on copper-free click reaction for drug delivery

Amphiphilic chitosan derivatives have attracted wide attention as drug carriers due to their physicochemical properties. However, obtaining a desired amphiphilic chitosan derivative by tuning the various functional groups was complex and time-consuming. Therefore, a facile and common synthesis strategy would be promising. In this study, a modular strategy based on strain-promoted azide-alkyne cycloaddition (SPAAC) click reaction was designed and applied in synthesizing deoxycholic acid- or octanoic acid-modified N-azido propionyl-N,O-sulfate chitosan through tuning the hydrophobic groups. Additionally, chitosan derivatives with the same substitute groups were prepared via amide coupling as controls. We demonstrated that these derivates via the two strategies showed no obvious difference in physicochemical properties, drug loading ability and biosafety, indicating the feasibility of modular strategy. Notably, the modular strategy exhibited advantages including high reactivity, flexibility and reproducibility. We believe that this modular strategy could provide varied chitosan derivatives in an easy and high-efficiency way for improving multifunctional drug carriers.

Applications of bacteria and their derived biomaterials for repair and tissue regeneration

Microorganisms such as bacteria and their derived biopolymers can be used in biomaterials and tissue regeneration. Various methods have been applied to regenerate damaged tissues, but using probiotics and biomaterials derived from bacteria with improved economic-production efficiency and highly applicable properties can be a new solution in tissue regeneration. Bacteria can synthesize numerous types of biopolymers. These biopolymers possess many desirable properties such as biocompatibility and biodegradability, making them good candidates for tissue regeneration. Here, we reviewed different types of bacterial-derived biopolymers and highlight their applications for tissue regeneration.

Polysaccharides on gelatin-based hydrogels differently affect chondrogenic differentiation of human mesenchymal stromal cells

Selection of feasible hybrid-hydrogels for best chondrogenic differentiation of human mesenchymal stromal cells (hMSCs) represents an important challenge in cartilage regeneration. In this study, three-dimensional hybrid hydrogels obtained by chemical crosslinking of poly (ethylene glycol) diglycidyl ether (PEGDGE), gelatin (G) without or with chitosan (Ch) or dextran (Dx) polysaccharides were developed. The hydrogels, namely G-PEG, G-PEG-Ch and G-PEG-Dx, were prepared with an innovative, versatile and cell-friendly technique that involves two preparation steps specifically chosen to increase the degree of crosslinking and the physical-mechanical stability of the product: a first homogeneous phase reaction followed by directional freezing, freeze-drying and post-curing. Chondrogenic differentiation of human bone marrow mesenchymal stromal cells (hBM-MSC) was tested on these hydrogels to ascertain whether the presence of different polysaccharides could favor the formation of the native cartilage structure. We demonstrated that the hydrogels exhibited an open pore porous morphology with high interconnectivity and the incorporation of Ch and Dx into the G-PEG common backbone determined a slightly reduced stiffness compared to that of G-PEG hydrogels. We demonstrated that G-PEG-Dx showed a significant increase of its anisotropic characteristic and G-PEG-Ch exhibited higher and faster stress relaxation behavior than the other hydrogels. These characteristics were associated to absence of chondrogenic differentiation on G-PEG-Dx scaffold and good chondrogenic differentiation on G-PEG and G-PEG-Ch. Furthermore, G-PEG-Ch induced the minor collagen proteins and the formation of collagen fibrils with a diameter like native cartilage. This study demonstrated that both anisotropic and stress relaxation characteristics of the hybrid hydrogels were important features directly influencing the chondrogenic differentiation potentiality of hBM-MSC.

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.

Thermosensitive and pH-responsive tannin-containing hydroxypropyl chitin hydrogel with long-lasting antibacterial activity for wound healing

Polysaccharide hydrogels have been widely used as wound dressings because of their biocompatibility and ability to provide moist environment for wound healing. However, bacterial infection often delays the healing process. Herein, a novel thermosensitive and pH-sensitive hydroxypropyl chitin/tannic acid/ferric ion (HPCH/TA/Fe) composite hydrogel was fabricated by a simple assembly. The pre-cooled hydrogel precursor solution can be injected onto the irregular wound area and gel rapidly at physiological temperature. The TA not only acted as a crosslinker to enhance mechanical properties of the hydrogel, but also as an antibacterial agent which could be sustainably released in response to the acidic environment. The composite hydrogel showed excellent broad-spectrum antibacterial activity up to 7 days with negligible cytotoxicity. Moreover, the hydrogel can inhibit bacterial infection and accelerate the wound healing process without scars in the mouse experiment. These results indicate the potential application of this composite hydrogel for the infected wound healing.

Fabrication and characterization of dextran/nanocrystalline β-tricalcium phosphate nanocomposite hydrogel scaffolds

Design of bioactive three-dimensional scaffolds to support bone tissue repair and regeneration become a key area of research in tissue engineering. Herein, porous hybrid hydrogels composed of dextran incorporated with nanocrystalline β-tricalcium phosphate (β-TCP) particles were tailor made as scaffolds for bone tissue engineering. β-TCP was successfully introduced within the dextran networks crosslinked through intermolecular ionic interactions and hydrogen bonding confirmed by FTIR spectroscopy. The effect of β-TCP content on equilibrium water uptake and swelling kinetics of composite hydrogels was investigated. It was found that the homogeneous distribution of β-TCP nanoparticles through the hydrogel matrix contributes to higher porosity and swelling capacity. In depth swelling measurements revealed that while in the early stage of swelling, water diffusion follows the Fick's law, for longer time swelling behavior of hydrogels undergo the second order kinetics. XRD measurements represented the formation of apatite layer on the surface of nanocomposite hydrogels after immersion in the SBF solution, which implies their bioactivity. Cell culture assays confirmed biocompatibility of the developed hybrid hydrogels in vitro. The obtained results converge to offer dextran/β-TCP nanocomposite hydrogels as promising scaffolds for bone regeneration applications.

Fabrication of injectable hydrogels via bio-orthogonal chemistry for tissue engineering

Injectable hydrogels via bio-orthogonal chemistry.

The future application of nanomedicine and biomimicry in plastic and reconstructive surgery

Plastic surgery encompasses a broad spectrum of reconstructive challenges and prides itself upon developing and adopting new innovations. Practice has transitioned from microsurgery to supermicrosurgery with a possible future role in even smaller surgical frontiers. Exploiting materials on a nanoscale has enabled better visualization and enhancement of biological processes toward better wound healing, tumor identification and viability of tissues, all cornerstones of plastic surgery practice. Recent advances in nanomedicine and biomimicry herald further reconstructive progress facilitating soft and hard tissue, nerve and vascular engineering. These lay the foundation for improved biocompatibility and tissue integration by the optimization of engineered implants or tissues. This review will broadly examine each of these technologies, highlighting areas of progress that reconstructive surgeons may not be familiar with, which could see adoption into our armamentarium in the not-so-distant future.

Functionalization of Novel Theranostic Hydrogels with Kartogenin-Grafted USPIO Nanoparticles To Enhance Cartilage Regeneration

Here, kartogenin (KGN), an emerging stable nonprotein compound with the ability to promote differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) into chondrocytes, was grafted onto the surface of modified ultrasmall superparamagnetic iron-oxide (USPIO) and then integrated into cellulose nanocrystal/dextran hydrogels. The hydrogels served as a carrier for the USPIO-KGN and a matrix for cartilage repair. We carried out in vitro and in vivo studies, the results of which demonstrated that KGN undergoes long-term stable sustained release, recruits endogenous host cells, and induces BMSCs to differentiate into chondrocytes, thus enabling in situ cartilage regeneration. Meanwhile, the USPIO-incorporated theranostic hydrogels exhibited a distinct magnetic resonance contrast enhancement and maintained a stable relaxation rate, with almost no loss, both in vivo and in vitro. According to noninvasive in vivo observation results and immunohistochemistry analyses, the regenerated cartilage tissue was very similar to natural hyaline cartilage. This innovative diagnosis and treatment system increases the convenience and effectiveness of chondrogenesis.

[Development of cartilage extracellular matrix in cartilage tissue engineering]

Cartilage tissue engineering, an effective way to repair cartilage defects, requires an ideal scaffold to promote the regeneration performance of stem cells. Cartilage extracellular matrix (CECM) can imitate the living environment of cartilage cells to the greatest extent. CECM not only exhibits good biocompatibility with chondrocytes and stem cells, which can meet the basic requirements of scaffolds, but also promotes chondrocytes to secrete matrix and induce stem cells to differentiate into chondrocytes; as such, this matrix is a better scaffold and has more advantages than existing ones. The promotion and induction effects could be related to various cartilage-related proteins inside. However, the practical application of this technique is hindered by problems, such as poor mechanical properties and insufficient cell penetration of CECM. Association with other materials can compensate for these inadequacies to a certain degree, and finding a combination mode with optimized performance is the application trend of CECM. This review focuses on research of CECM materials in cartilage tissue engineering.

Injectable PNIPAM/Hyaluronic acid hydrogels containing multipurpose modified particles for cartilage tissue engineering: Synthesis, characterization, drug release and cell culture study

Novel injectable thermosensitive PNIPAM/hyaluronic acid hydrogels containing various amounts of chitosan-g-acrylic acid coated PLGA (ACH-PLGA) micro/nanoparticles were synthesized and designed to facilitate the regeneration of cartilage tissue. The ACH-PLGA particles were used in the hydrogels to play a triple role: first, the allyl groups on the chitosan-g-acrylic acid shell act as crosslinkers for PNIPAM and improved the mechanical properties of the hydrogel to mimic the natural cartilage tissue. Second, PLGA core acts as a carrier for the controlled release of chondrogenic small molecule melatonin. Third, they could reduce the syneresis of the thermosensitive hydrogel during gelation. The optimum hydrogel with the minimum syneresis and the maximum compression modulus was chosen for further evaluations. This hydrogel showed a great integration with the natural cartilage during the adhesion test, and also, presented an interconnected porous structure in scanning electron microscopy images. Eventually, to evaluate the cytotoxicity, mesenchymal stem cells were encapsulated inside the hydrogel. MTT and Live/Dead assay showed that the hydrogel improved the cells growth and proliferation as compared to the tissue culture polystyrene. Histological study of glycosaminoglycan (GAG) showed that melatonin treatment has the ability to increase the GAG synthesis. Overall, due to the improved mechanical properties, low syneresis, the ability of sustained drug release and also high bioactivity, this injectable hydrogel is a promising material system for cartilage tissue engineering.

Advances of injectable hydrogel-based scaffolds for cartilage regeneration

Articular cartilage is an important load-bearing tissue distributed on the surface of diarthrodial joints. Due to its avascular, aneural and non-lymphatic features, cartilage has limited self-regenerative properties. To date, the utilization of biomaterials to aid in cartilage regeneration, especially through the use of injectable scaffolds, has attracted considerable attention. Various materials, therapeutics and fabrication approaches have emerged with a focus on manipulating the cartilage microenvironment to induce the formation of cartilaginous structures that have similar properties to the native tissues. In particular, the design and fabrication of injectable hydrogel-based scaffolds have advanced in recent years with the aim of enhancing its therapeutic efficacy and improving its ease of administration. This review summarizes recent progress in these efforts, including the structural improvement of scaffolds, network cross-linking techniques and strategies for controlled release, which present new opportunities for the development of injectable scaffolds for cartilage regeneration.