Synthesis and functionalization of dendron-polymer conjugate based hydrogels via sequential thiol-ene “click” reactions

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.

A review of advanced hydrogels for cartilage tissue engineering

With the increase in weight and age of the population, the consumption of tobacco, inappropriate foods, and the reduction of sports activities in recent years, bone and joint diseases such as osteoarthritis (OA) have become more common in the world. From the past until now, various treatment strategies (e.g., microfracture treatment, Autologous Chondrocyte Implantation (ACI), and Mosaicplasty) have been investigated and studied for the prevention and treatment of this disease. However, these methods face problems such as being invasive, not fully repairing the tissue, and damaging the surrounding tissues. Tissue engineering, including cartilage tissue engineering, is one of the minimally invasive, innovative, and effective methods for the treatment and regeneration of damaged cartilage, which has attracted the attention of scientists in the fields of medicine and biomaterials engineering in the past several years. Hydrogels of different types with diverse properties have become desirable candidates for engineering and treating cartilage tissue. They can cover most of the shortcomings of other treatment methods and cause the least secondary damage to the patient. Besides using hydrogels as an ideal strategy, new drug delivery and treatment methods, such as targeted drug delivery and treatment through mechanical signaling, have been studied as interesting strategies. In this study, we review and discuss various types of hydrogels, biomaterials used for hydrogel manufacturing, cartilage-targeting drug delivery, and mechanosignaling as modern strategies for cartilage 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.

Injectable hydrogels in central nervous system: Unique and novel platforms for promoting extracellular matrix remodeling and tissue engineering

Repairing central nervous system (CNS) is difficult due to the inability of neurons to recover after damage. A clinically acceptable treatment to promote CNS functional recovery and regeneration is currently unavailable. According to recent studies, injectable hydrogels as biodegradable scaffolds for CNS tissue engineering and regeneration have exceptionally desirable attributes. Hydrogel has a biomimetic structure similar to extracellular matrix, hence has been considered a 3D scaffold for CNS regeneration. An interesting new type of hydrogel, injectable hydrogels, can be injected into target areas with little invasiveness and imitate several aspects of CNS. Injectable hydrogels are being researched as therapeutic agents because they may imitate numerous properties of CNS tissues and hence reduce subsequent injury and regenerate neural tissue. Because of their less adverse effects and cost, easier use and implantation with less pain, and faster regeneration capacity, injectable hydrogels, are more desirable than non-injectable hydrogels. This article discusses the pathophysiology of CNS and the use of several kinds of injectable hydrogels for brain and spinal cord tissue engineering, paying particular emphasis to recent experimental studies.

Thiol-Ene Click-Inspired Late-Stage Modification of Long-Chain Polyurethane Dendrimers

The construction of well-defined polyurethane dendrimers is challenging due to the high reactivity of externally added or in situ formed isocyanates leading to the formation of side products. With a primary focus of dendrimer research being the interaction of the periphery and the core, we report the synthesis of a common polyurethane dendron, which allows for the late-stage variation of both the periphery and the core. The periphery can be varied simply by installing a clickable unit in the dendron and then attaching to the core and vice-versa. Thus, a common dendron allows for varying periphery and core in the final two steps. To accomplish this, a protecting group-free, one-pot multicomponent Curtius reaction was utilized to afford a robust and versatile AB2 type polyurethane dendron employing commercially available simple molecules: 5-hydroxyisophthalic acid, 11-bromoundecanol, and 4-penten-1-ol. Subsequent late-stage modifications of either dendrons or dendrimers via a thiol-ene click reaction gave surface-functionalized alternating aromatic-aliphatic polyurethane homodendrimers to generation-three (G3). The dendrons and the dendrimers were characterized by NMR, mass spectrometry, and FT-IR analysis. A bifunctional AB2 type dendritic monomer demonstrated this approach’s versatility that can either undergo a thiol-ene click or attachment to the core. This approach enables the incorporation of functionalities at the periphery and the core that may not withstand the dendrimer growth for the synthesis of polyurethane dendrimers and other dendritic macromolecules.

Injectable hydrogels for cartilage and bone tissue engineering

Tissue engineering has become a promising strategy for repairing damaged cartilage and bone tissue. Among the scaffolds for tissue-engineering applications, injectable hydrogels have demonstrated great potential for use as three-dimensional cell culture scaffolds in cartilage and bone tissue engineering, owing to their high water content, similarity to the natural extracellular matrix (ECM), porous framework for cell transplantation and proliferation, minimal invasive properties, and ability to match irregular defects. In this review, we describe the selection of appropriate biomaterials and fabrication methods to prepare novel injectable hydrogels for cartilage and bone tissue engineering. In addition, the biology of cartilage and the bony ECM is also summarized. Finally, future perspectives for injectable hydrogels in cartilage and bone tissue engineering are discussed.

Hooked on Cryogels: A Carbamate Linker Based Depot for Slow Drug Release

Poly(ethylene glycol) (PEG) based bulk hydrogels and cryogels containing activated carbonate groups as amine reactive handles to facilitate drug conjugations through carbamate linkages were fabricated and evaluated as slow releasing drug reservoirs. As an initial approach, photopolymerization of N-hydroxysuccinimide (NHS)-activated carbonate functional group containing monomer and PEG-methacrylate in the presence of a cross-linker was utilized to obtain bulk hydrogels with high gel conversions. The resultant hydrogels possessed moderate water uptake (170-340%) which was dependent on the monomer ratios. These hydrogels were functionalized with an anticancer drug, namely, doxorubicin. Surprisingly, while negligible drug release was observed from the bulk hydrogels under normal pH, only about 6% drug release was observed under acidic condition. Limited swelling of these hydrogels as well as lack of porous structure as deduced from scanning electron microscopy analysis might explain the poor drug release. To enhance the drug releasing capacity of these hydrogels that might stem from the increased porosity, reactive carbonate group bearing cryogels were synthesized. Compared to the bulk hydrogels, cryogels were highly porous in structure and also possessed much higher swelling capacity (1150-1500%). As a result of these distinctions, a 7-fold enhancement in drug release was observed for the cryogel system compared to the relating hydrogel. In vitro studies demonstrated that the anticancer drug doxorubicin conjugated through carbamate linkers to the cryogels was released and proved effective against MDA-MB-231 human breast cancer cells. Overall, a novel class of slow releasing nontoxic hydrogel and cryogel scaffolds with potential applications as anticancer drug reservoirs was realized.

Dendrimers and Dendrons as Versatile Building Blocks for the Fabrication of Functional Hydrogels

Hydrogels have emerged as a versatile class of polymeric materials with a wide range of applications in biomedical sciences. The judicious choice of hydrogel precursors allows one to introduce the necessary attributes to these materials that dictate their performance towards intended applications. Traditionally, hydrogels were fabricated using either polymerization of monomers or through crosslinking of polymers. In recent years, dendrimers and dendrons have been employed as well-defined building blocks in these materials. The multivalent and multifunctional nature of dendritic constructs offers advantages in either formulation or the physical and chemical properties of the obtained hydrogels. This review highlights various approaches utilized for the fabrication of hydrogels using well-defined dendrimers, dendrons and their polymeric conjugates. Examples from recent literature are chosen to illustrate the wide variety of hydrogels that have been designed using dendrimer- and dendron-based building blocks for applications, such as sensing, drug delivery and tissue engineering.

Recent Advances in Dendritic Macromonomers for Hydrogel Formation and Their Medical Applications

Hydrogels represent one of the most important classes of biomaterials and are of interest for various medical applications including wound repair, tissue engineering, and drug release. Hydrogels possess tunable mechanical properties, biocompatibility, nontoxicity, and similarity to natural soft tissues. The need for hydrogels with specific properties, based on the design requirements of the in vitro, in vivo, or clinical application, motivates researchers to develop new synthetic approaches and cross-linking methodologies to form novel hydrogels with unique properties. The use of dendritic macromonomers represents one elegant strategy for the formation of hydrogels with specific properties. Specifically, the uniformity of dendrimers combined with the control of their size, architecture, density, and surface groups make them promising cross-linkers for hydrogel formation. Over the last two decades, a large variety of dendritic-based hydrogels are reported for their potential use in the clinic. This review describes the state of the art with these different dendritic hydrogel formulations including their design requirements, the synthetic routes, the measurement and determination of their properties, the evaluation of their in vitro and in vivo performances, and future perspectives.

PEG-based hydrogels prepared by catalyst-free thiol–yne addition and their post-antibacterial modification

PEG-based hydrogels were prepared via nucleophilic thiol–yne addition and post-functionalized with an antimicrobial peptide for antibacterial applications.