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.

Cyclodextrins and derivatives in drug delivery: New developments, relevant clinical trials, and advanced products

Cyclodextrins (CD) and derivatives are functional excipients that can improve the bioavailability of numerous drugs. Because of their drug solubility improving properties they are used in many pharmaceutical products. Furthermore, the stability of small molecular drugs can be improved by the incorporation in CDs and an unpleasant taste and smell can be masked. In addition to well-established CD derivatives including hydroxypropyl-β-CD, hydroxypropyl-γ-CD, methylated- β-CD and sulfobutylated- β-CD, there are promising new derivatives in development. In particular, CD-based polyrotaxanes exhibiting cellular uptake enhancing properties, CD-polymer conjugates providing sustained drug release, enhanced cellular uptake, and mucoadhesive properties, and thiolated CDs showing mucoadhesive, in situ gelling, as well as permeation and cellular uptake enhancing properties will likely result in innovative new drug delivery systems. Relevant clinical trials showed various new applications of CDs such as the formation of CD-based nanoparticles, stabilizing properties for protein drugs or the development of ready-to-use injection systems. Advanced products are making use of various benefical properties of CDs at the same time. Within this review we provide an overview on these recent developments and take an outlook on how this class of excipients will further shape the landscape of drug delivery.

Porous Microgels for Delivery of Curcumin: Microfluidics-Based Fabrication and Cytotoxicity Evaluation

Polymeric microgels, fabricated via microfluidic techniques, have garnered significant interest as versatile drug delivery carriers. Despite the advances, the loading and release of hydrophobic drugs such as curcumin from polymeric microgels is not trivial. Herein, we report that effective drug loading can be achieved by the design of porous particles and the use of supramolecular cyclodextrin-based curcumin complexes. The fabrication of porous microgels through the judicious choice of chemical precursors under flow conditions was established. The evaluation of the curcumin loading dependence on the porosity of the microgels was performed. Microgels with higher porosity exhibited better curcumin loading compared to those with lower porosity. Curcumin-loaded microgels released the drug, which, upon internalization by U87 MG human glioma cancer cells, induced cytotoxicity. The findings reported here provide valuable insights for the development of tailored drug delivery systems using a microfluidics-based platform and outline a strategy for the effective delivery of hydrophobic therapeutic agents such as curcumin through supramolecular complexation.

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.

Self-Assembly of Cyclodextrin-Coated Nanoparticles:Fabrication of Functional Nanostructures for Sensing and Delivery

In recent years, the bottom-up approach has emerged as a powerful tool in the fabrication of functional nanomaterials through the self-assembly of nanoscale building blocks. The cues embedded at the molecular level provide a handle to control and direct the assembly of nano-objects to construct higher-order structures. Molecular recognition among the building blocks can assist their precise positioning in a predetermined manner to yield nano- and microstructures that may be difficult to obtain otherwise. A well-orchestrated combination of top-down fabrication and directed self-assembly-based bottom-up approach enables the realization of functional nanomaterial-based devices. Among the various available molecular recognition-based "host-guest" combinations, cyclodextrin-mediated interactions possess an attractive attribute that the interaction is driven in aqueous environments, such as in biological systems. Over the past decade, cyclodextrin-based specific host-guest interactions have been exploited to design and construct structural and functional nanomaterials based on cyclodextrin-coated metal nanoparticles. The focus of this review is to highlight recent advances in the self-assembly of cyclodextrin-coated metal nanoparticles driven by the specific host-guest interaction.

Thiolated Polymeric Hydrogels for Biomedical Applications: A Review

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

In situ forming gelatin: Cyclodextrin hydrogels prepared by “click chemistry” to improve the sustained release of hydrophobic drugs

Injectable hydrogels offer a wide range of attractive benefits in drug delivery applications, such as non-invasive administration, easy drug incorporation and locally controlled release at the target sites. Herein, we designed a simple and efficient method to prepare injectable hydrogels composed of gelatin and cyclodextrin (CD) for high loading capacity of hydrophobic drugs. The hydrogels were formed by thiol-functionalized gelatin (GSH) and βCD-vinyl sulfone (βCD-VS) as cross-linker, via thiol-ene “click” chemistry. Hydrogels comprising of different cross-linker feed amount were investigated in terms of their physico-chemical properties, such as gelation time, mechanical strength, swelling ratio, porosity and degradation rates. For the use as a drug delivery vehicle, dexamethasone (DEX), a commonly anti-inflammatory, immunosuppressive but poorly water soluble drug was chosen to show the high drug loading capacity and prolonged drug release of hydrogels. The drug release was found to be depended on the concentration of βCD-VS due to the drug-CD interaction. In vitro cytotoxicity experiment also showed the cell compatibility of these hydrogels against human dermal fibroblasts. In summary, we expect this gelatin-CD “click” hydrogel will be a promising candidate for localized and long-term delivery of hydrophobic drugs.

Facile UV-induced covalent modification and crosslinking of styrene-isoprene-styrene copolymer via Paterno-Büchi [2 + 2] photocycloaddition

The chemical functionalization or modification of polymers to alter or improve the physical and mechanical properties constitutes an important field in macromolecular research. Fabrication of polymeric materials via structural tailoring of commercial or commodity polymers that are produced in vast quantities especially possess unique advantages in material applications. In the present study, we report on benign chemical modification of unsaturated styrene–isoprene–styrene (SIS) copolymer using available backbone alkene groups. Covalent attachment of aldehyde functional substrates onto reactive isoprene double bond residues was conveniently carried out using UV-induced Paterno–Büchi [2 + 2] cycloaddition. Model organic compounds with different structures were utilized in high efficiency chemical modification of parent polymer chains via oxetane ring formation. Functionalization studies were confirmed via¹H NMR, FT-IR and SEC analyses. The methodology was extended to covalent crosslinking of polymer chains to obtain organogels with tailorable crosslinking degrees and physical characteristics. Considering the outstanding elastic properties of unsaturated rubbers and their high commercial availability, abundant reactive double bonds in backbone chains of these polymers offer easy to implement structural modification via proposed Paterno–Büchi photocycloaddition.

Paterno–Büchi reaction is reported as a convenient chemical reaction tool to modify unsaturated copolymer elastomers.

Fabrication of Patterned Hydrogel Interfaces: Exploiting the Maleimide Group as a Dual Purpose Handle for Cross-Linking and Bioconjugation

Functional hydrogels that can be obtained through facile fabrication procedures and subsequently modified using straightforward reagent-free methods are indispensable materials for biomedical applications such as sensing and diagnostics. Herein a novel hydrogel platform is obtained using polymeric precursors containing the maleimide functional group as a side chain. The maleimide groups play a dual role in fabrication of functional hydrogels. They enable photochemical cross-linking of the polymers to yield bulk and patterned hydrogels. Moreover, the maleimide group can be used as a handle for efficient functionalization using the thiol-maleimide conjugation and Diels-Alder cycloaddition click reactions. Obtained hydrogels are characterized in terms of their morphology, water uptake capacity, and functionalization. Micropatterned hydrogels are obtained under UV-irradiation using a photomask to obtain reactive micropatterns, which undergo facile functionalization upon treatment with thiol-containing functional molecules such as fluorescent dyes and bioactive ligands. The maleimide group also undergoes conjugation through the Diels-Alder reaction, where the attached molecule can be released through thermal treatment via the retro Diels-Alder reaction. The antibiofouling nature of these hydrogel micropatterns enables efficient ligand-directed biomolecular immobilization, as demonstrated by attachment of streptavidin-coated quantum dots.

Cyclodextrin embedded covalently crosslinked networks: synthesis and applications of hydrogels with nano-containers

Recent advancements in the synthesis of hydrogels containing cyclodextrin (CD) units within the gel network have been reviewed.

Thiol-reactive thiosulfonate group containing copolymers: facile entry to disulfide-mediated polymer conjugation and redox-responsive functionalizable networks

Herein, we report a synthetic approach to thiol-reactive polymers containing methanethiosulfonate groups as side chains, and demonstrate their application in post-polymerization functionalization through reversible disulfide linkages.

The emerging applications of click chemistry reactions in the modification of industrial polymers

Click chemistry reactions have been applied to the modification of major industrial polymers by analysing the synthetic approaches and the resulting material properties.

Facile Fabrication of a Modular "Catch and Release" Hydrogel Interface: Harnessing Thiol-Disulfide Exchange for Reversible Protein Capture and Cell Attachment

Surfaces engineered to "specifically capture" and "release on demand" analytes ranging from biomolecules to cells find niche applications in areas such as diagnostics and detection. Utilization of a disulfide-based linker as a building block allows fabrication of a novel hydrogel-based platform that incorporates a "catch and release" attribute. Hydrogels incorporating pyridyl disulfide groups as thiol-reactive handles were prepared by photopolymerization in the presence of a poly(ethylene glycol) (PEG)-based cross-linker. A range of bulk and micropatterned hydrogels with varying amounts of the reactive group were prepared using PEG-based monomers with different chain lengths. Thiol-containing molecules were conjugated to these hydrogels through the thiol-disulfide exchange reaction under ambient conditions with high efficiencies, as determined by UV-vis spectroscopy. Facile conjugation of a thiol-containing fluorescent dye, namely 4,4-difluoro-1,3,5,7-tetramethyl-8-[(10-mercapto)]-4-bora-3 a,4 a-diaza- s-indacene, was demonstrated, followed by its effective cleavage in the presence of dithiothreitol (DTT), a thiol-containing disulfide-reducing agent. Conjugation of a biotin-containing ligand onto the hydrogels allowed specific binding of protein extravidin when exposed to a mixture of extravidin and bovine serum albumin. The bound protein could be released from the hydrogel by simple exposure to a DTT solution. Likewise, hydrogels modified with a cell-adhesive peptide unit containing the RGD sequence acted as favorable substrates for cellular attachment. Incubation of these cell-attached hydrogel surfaces in a DTT-containing solution leads to facile detachment of cells from the surfaces, while retaining a high level of cell viability. It can be envisioned that the benign nature of these hydrogels, their facile fabrication, and modular functionalization will make them attractive platforms for many applications.

Embedding Well-Defined Responsive Hydrogels with Nanocontainers: Tunable Materials from Telechelic Polymers and Cyclodextrins

Design, synthesis, and application of cyclodextrin (CD) containing thermoresponsive hydrogels fabricated from thiol-reactive telechelic polymers are reported. Hydrophilic polymers containing 2-hydroxyethyl methacrylate and/or di(ethylene glycol)methylether methacrylate monomers as side chains and thiol-reactive groups at chain ends were synthesized. A series of hydrogels was fabricated using thiol-ene conjugation of these thiol-reactive polymers with multivalent thiol-containing CDs as crosslinkers. Clear and transparent hydrogels were obtained with good conversion (79-89%) by utilizing the "nucleophilic" and "radical" thiol-ene "click" reactions. Analysis of the amount of residual thiol groups in these hydrogels using Ellman's reagent suggested that gels with a moderately well-defined network structure were obtained. Hydrogels fabricated using different telechelic polymers were examined for their properties such as morphology, equilibrium water uptake, and rheological characteristics. Cytocompatibility of these hydrogels was ascertained by a cell viability assay that demonstrated low toxicity toward fibroblast cells. Thereafter, the CD-containing hydrogels were evaluated for the loading and controlled release of puerarin, an antiglaucoma drug. Utilization of thermoresponsive polymers as the matrix for these hydrogels allows use of temperature as a stimulus to modulate the drug release. A slower and more sustained drug release was observed at physiological temperatures compared to ambient conditions. The effect of temperature on the elasticity of the hydrogel was investigated rheologically to demonstrate that the collapse of the network structure occurs near physiological temperatures. The increased hydrophobicity and compactness of the gel matrix at higher temperatures results in a slower drug release. The strategy employed here demonstrates that tuning the matrix composition of hydrogels with well-defined network structures through appropriate choice of responsive copolymers allows design of materials with control of their physical properties and drug-release behavior.

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.

Introducing Glycolinkers for the Functionalization of Cytotoxic Drugs and Applications in Antibody-Drug Conjugation Chemistry

Antibody-drug conjugates (ADCs) are promising alternatives to naked antibodies for selective drug-delivery applications and treatment of diseases such as cancer. Construction of ADCs relies upon site-selective, efficient and mild conjugation technologies. The choice of a chemical linker is especially important, as it affects the overall properties of the ADC. We envisioned that hydrophilic bifunctional chemical linkers based on carbohydrates would be a useful class of derivatization agents for the construction of linker-drug conjugates and ADCs. Herein we describe the synthesis of carbohydrate-based derivatization agents, glycolinker-drug conjugates featuring the tubulin inhibitor monomethyl auristatin E and an ADC based on an anti-EGFR antibody. In addition, an initial in vitro cytotoxicity evaluation of the individual components and the ADC is provided against EGFR-positive cancer cells.

Synthesis of the copper chelator TGTA and evaluation of its ability to protect biomolecules from copper induced degradation during copper catalyzed azide-alkyne bioconjugation reactions

One of the most successful bioconjugation strategies to date is the copper(I)-catalyzed cycloaddition reaction (CuAAC), however, the typically applied reaction conditions have been found to degrade sensitive biomolecules. Herein, we present a water soluble copper chelator which can be utilized to protect biomolecules from copper induced degradation.

Design of Thiol- and Light-sensitive Degradable Hydrogels using Michael-type Addition Reactions

Injectable depots that respond to exogenous and endogenous stimuli present an attractive strategy for tunable, patient-specific drug delivery. Here, the design of injectable and multimodal degradable hydrogels that respond to externally applied light and physiological stimuli, specifically aqueous and reducing microenvironments, is reported. Rapid hydrogel formation was achieved using a thiol-maleimide click reaction between multifunctional poly(ethylene glycol) macromers. Hydrogel degradation kinetics in response to externally applied cytocompatible light, reducing conditions, and hydrolysis were characterized, and degradation of the gel was controlled over multiple time scales from seconds to days. Further, tailored release of an encapsulated model cargo, fluorescent nanobeads, was demonstrated.

Photoclick Hydrogels Prepared from Functionalized Cyclodextrin and Poly(ethylene glycol) for Drug Delivery and in Situ Cell Encapsulation

Polymers or hydrogels containing modified cyclodextrin (CD) are highly useful in drug delivery applications, as CD is a cytocompatible amphiphilic molecule that can complex with a variety of hydrophobic drugs. Here, we designed modular photoclick thiol-ene hydrogels from derivatives of βCD and poly(ethylene glycol) (PEG), including βCD-allylether (βCD-AE), βCD-thiol (βCD-SH), PEG-thiol (PEGSH), and PEG-norbornene (PEGNB). Two types of CD-PEG hybrid hydrogels were prepared using radical-mediated thiol-ene photoclick reactions. Specifically, thiol-allylether hydrogels were formed by reacting multiarm PEGSH and βCD-AE, and thiol-norbornene hydrogels were formed by cross-linking βCD-SH and multiarm PEGNB. We characterized the properties of these two types of thiol-ene hydrogels, including gelation kinetics, gel fractions, hydrolytic stability, and cytocompatibility. Compared with thiol-allylether hydrogels, thiol-norbornene photoclick reaction formed hydrogels with faster gelation kinetics at equivalent macromer contents. Using curcumin, an anti-inflammatory and anticancer hydrophobic molecule, we demonstrated that CD-cross-linked PEG-based hydrogels, when compared with pure PEG-based hydrogels, afforded higher drug loading efficiency and prolonged delivery in vitro. Cytocompatibility of these CD-cross-linked hydrogels were evaluated by in situ encapsulation of radical sensitive pancreatic MIN6 β-cells. All formulations and cross-linking conditions tested were cytocompatible for cell encapsulation. Furthermore, hydrogels cross-linked by βCD-SH showed enhanced cell proliferation and insulin secretion as compared to gels cross-linked by either dithiothreitol (DTT) or βCD-AE, suggesting the profound impact of both macromer compositions and gelation chemistry on cell fate in chemically cross-linked hydrogels.