Novel injectable biodegradable glycol chitosan-based hydrogels crosslinked by Michael-type addition reaction with oligo(acryloyl carbonate)-b-poly(ethylene glycol)-b-oligo(acryloyl carbonate) copolymers

Starch microsphere silicon-boron crosslinker for low concentration hydroxypropyl guar gum based fracturing fluid

Hydroxypropyl guar gum (HPG) is a critical thickener to increase viscosity and lubrication to improve the water-based hydraulic fracturing efficiency. However, current crosslinkers require a large amount of HPG (>0.3 wt%) to form gel with sufficient viscosity, and high concentrations of HPG may cause adverse effects to the production and the environment. In this study, a novel starch microsphere silica‑boron crosslinker (SMSB) was developed using starch microspheres as a carrier and γ-aminopropyl triethoxy silane (KH550) as a modifier with an in-house method. Both the rheology and surface reactions of the SMSB-HPG crosslinking system were studied using multiple laboratory experiments and molecular dynamics simulation. The results showed that SMSB crosslinker caused multi-site cross-linking with low concentration (only 0.2 wt%) of HPG molecules, reducing the twisting of single molecular chain in the crosslinking system, enhancing the cross-linking strength between molecular chains, and making HPG molecular chains stretcher in the aqueous solution. The apparent viscosity and viscoelasticity of the HPG system were substantially higher than the organoboron crosslinker, and the temperature resistance of the SMSB-HPG crosslinking system was up to 140 °C. This study provides an alternative green crosslinker for more sustainable industrial applications and provides theoretical basis for the modification of biomaterials.

Multi-crosslinked hydrogel built with hyaluronic acid-tyramine, thiolated glycol chitosan and copper-doped bioglass nanoparticles for expediting wound healing

The migration of fibroblasts and endothelial cells is a critical determinant of wound-healing outcomes for skin injuries. Here, hyaluronic acid-tyramine (HAT) and thiolated glycol chitosan (TGC) conjugates were combined with copper-doped bioglass (ACuBG) nanoparticles to build a novel type of multi-crosslinked hydrogel for stimulating the migration of cells, and thus, expediting wound healing. The optimally devised HAT/TGC/ACuBG gels had markedly improved strength and stiffness compared to the gels built from either HAT or TGC while showing sufficient elasticity, which contributes to stimulating the migration of fibroblasts. The sustainable release of silicon and copper ions from the gels was found to jointly induce the migration of human umbilical vein endothelial cells. The results based on mouse full-thickness skin defects demonstrated that they were able to fully restore the skin defects with formation of complete appendages within two weeks, suggesting their promising potency for use in expediting wound healing.

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.

Dextran thioparaconate - Evaluation of the multifunctional thiolactone linker for easily adaptable polysaccharide modification

For the preparation of tailored polymers based on polysaccharides, an easy adaptable modification method was developed by introduction of a multifunctional linker into the polymer backbone. Dextran was functionalized with a thiolactone compound that can be further treated with amine resulting in ring opening and formation of a thiol. The functional thiol group emerging can be used for crosslinking or introduction of a further functional compound by disulfide formation. Here, the efficient esterification of thioparaconic acid after in-situ activation and studies about reactivity of the dextran thioparaconate obtained are discussed. The derivative was converted by aminolysis with model compound hexylamin and the thiol generated therefrom was subsequently converted with an activated functional thiol to the corresponding disulfide. The thiolactone, which protects the thiol, enables efficient esterification without side reactions and allows storage of the polysaccharide derivative at ambient conditions for years. Not only the multifunctional reactivity of the derivative but also the end product with a balanced ratio of hydrophobic and cationic moiety is appealing for biomedical application.

S-Protected Thiolated Chitosan versus Thiolated Chitosan as Cell Adhesive Biomaterials for Tissue Engineering

Chitosan (Ch) and different Ch derivatives have been applied in tissue engineering (TE) because of their biocompatibility, favored mechanical properties, and cost-effectiveness. Most of them, however, lack cell adhesive properties that are crucial for TE. In this study, we aimed to design an S-protected thiolated Ch derivative exhibiting high cell adhesive properties serving as a scaffold for TE. 3-((2-Acetamido-3-methoxy-3-oxopropyl)dithio) propanoic acid was covalently attached to Ch via a carbodiimide-mediated reaction. Low-, medium-, and high-modified Chs (Ch-SS-1, Ch-SS-2, and Ch-SS-3) with 54, 107 and 140 μmol of ligand per gram of polymer, respectively, were tested. In parallel, three thiolated Chs, namely Ch-SH-1, Ch-SH-2, and Ch-SH-3, were prepared by conjugating N-acetyl cysteine to Ch at the same degree of modification to compare the effectiveness of disulfide versus thiol modification on cell adhesion. Ch-SS-1 showed better cell adhesion capability than Ch-SS-2 and Ch-SS-3. This can be explained by the more lipophilic surfaces of Ch-SS as a higher modification was made. Although Ch-SH-1, Ch-SH-2, and Ch-SH-3 were shown to be good substrates for cell adhesion, growth, and proliferation, Ch-SS polymers were superior to Ch-SH polymers in the formation of 3D cell cultures. Cryogels structured by Ch-SS-1 (SSg) resulted in homogeneous scaffolds with tunable pore size and mechanical properties by changing the mass ratio between Ch-SS-1 and heparin used as a cross-linker. SSg scaffolds possessing interconnected microporous structures showed good cell migration, adhesion, and proliferation. Therefore, Ch-SS can be used to construct tunable cryogel scaffolds that are suitable for 3D cell culture and TE.

Customizing nano-chitosan for sustainable drug delivery

Chitosan is a natural polymer with acceptable biocompatibility, biodegradability, and mechanical stability; hence, it has been widely appraised for drug and gene delivery applications. However, there has been no comprehensive assessment to tailor-make chitosan cross-linkers of various types and functionalities as well as complex chitosan-based semi- and full-interpenetrating networks for drug delivery systems (DDSs). Herein, various fabrication methods developed for chitosan hydrogels are deliberated, including chitosan crosslinking with and without diverse cross-linkers. Tripolyphosphate, genipin and multi-functional aldehydes, carboxylic acids, and epoxides are common cross-linkers used in developing biomedical chitosan for DDSs. Methods deployed for modifying the properties and performance of chitosan hydrogels, via their composite production (semi- and full-interpenetrating networks), are also cogitated here. In addition, recent advances in the fabrication of advanced chitosan hydrogels for drug delivery applications such as oral drug delivery, transdermal drug delivery, and cancer therapy are discussed. Lastly, thoughts on what is needed for the chitosan field to continue to grow is also debated in this comprehensive review article.

Fast-Forming Dissolvable Redox-Responsive Hydrogels: Exploiting the Orthogonality of Thiol-Maleimide and Thiol-Disulfide Exchange Chemistry

Fast-forming yet easily dissolvable hydrogels (HGs) have potential applications in wound healing, burn incidences, and delivery of therapeutic agents. Herein, a combination of a thiol–maleimide conjugation and thiol–disulfide exchange reaction is employed to fabricate fast-forming HGs which rapidly dissolve upon exposure to dithiothreitol (DTT), a nontoxic thiol-containing hydrophilic molecule. In particular, maleimide disulfide-terminated telechelic linear poly(ethylene glycol) (PEG) polymer and PEG-based tetrathiol macromonomers are employed as gel precursors, which upon mixing yield HGs within a minute. The selectivity of the thiol–maleimide conjugation in the presence of a disulfide linkage was established through ¹H NMR spectroscopy and Ellman’s test. Rapid degradation of HGs in the presence of thiol-containing solution was evident from the reduction in storage modulus. HGs encapsulated with fluorescent dye-labeled dextran polymers and bovine serum albumin were fabricated, and their cargo release was investigated under passive and active conditions upon exposure to DTT. One can envision that the rapid gelation and fast on-demand dissolution under relatively benign conditions would make these polymeric materials attractive for a range of biomedical 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.

Advances in versatile anti-swelling polymer hydrogels

Swelling is ubiquitous for traditional as-prepared hydrogels, but is unfavorable in many situations, especially biomedical applications, such as tissue engineering, internal wound closure, soft actuating and bioelectronics, and so forth. As the swelling of a hydrogel usually leads to a volume expansion, which not only deteriorates the mechanical property of the hydrogel but can bring about undesirable oppression on the surrounding tissues when applied in vivo. In contrast, anti-swelling hydrogels hardly alter their volume when applied in aqueous environment, therefore reserving the original mechanical performance and size-stability and facilitating their potential application. In the past decade, with the development of advanced hydrogels, quite a number of anti-swelling hydrogels with versatile functions have been developed by researchers to meet the practical applications well, through integrating anti-swelling property with certain performance or functionality, such as high strength, self-healing, injectability, adhesiveness, antiseptics, etc. However, there has not been a general summary with regard to these hydrogels. To promote the construction of anti-swelling hydrogels with desirable functionalities in the future, this review generalizes and analyzes the tactics employed so far in the design and manufacture of anti-swelling hydrogels, starting from the viewpoint of classical swelling theories. The review will provide a relatively comprehensive understanding of anti-swelling hydrogels and clues to researchers interested in this kind of materials to develop more advanced ones suitable for practical application.

Thiolated Chitosans: A Multi-talented Class of Polymers for Various Applications

Various properties of chitosan can be customized by thiolation for very specific needs in a wide range of application areas. Since the discovery of thiolated chitosans, many studies have proven their advantageous characteristics, such as adhesion to biological surfaces, adjustable cross-linking and swelling behavior, controllable drug release, permeation as well as cellular uptake enhancement, inhibition of efflux pumps and enzymes, complexation of metal ions, antioxidative properties, and radical scavenging activity. Simultaneously, these polymers remain biodegradable without increased toxicity. Within this Review, an overview about the different possibilities to covalently attach sulfhydryl ligands to the polymeric backbone of chitosan is given, and the resulting versatile physiochemical properties are discussed in detail. Furthermore, the broad spectrum of applications for thiolated chitosans in science and industry, ranging from their most advanced use in pharmaceutical and medical science over wastewater treatment to the impregnation of textiles, is addressed.

The Application of Hydrogels Based on Natural Polymers for Tissue Engineering

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Hydrogels are known as polymer-based networks with the ability to absorb water and other body fluids. Because of this, the hydrogels are used to preserve drugs, proteins, nutrients or cells. Hydrogels possess great biocompatibility, and properties like soft tissue, and networks full of water, which allows oxygen, nutrients, and metabolites to pass. Therefore, hydrogels are extensively employed as scaffolds in tissue engineering. Specifically, hydrogels made of natural polymers are efficient structures for tissue regeneration, because they mimic natural environment which improves the expression of cellular behavior.

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Producing natural polymer-based hydrogels from collagen, hyaluronic acid (HA), fibrin, alginate, and chitosan is a significant tactic for tissue engineering because it is useful to recognize the interaction between scaffold with a tissue or cell, their cellular reactions, and potential for tissue regeneration. The present review article is focused on injectable hydrogels scaffolds made of biocompatible natural polymers with particular features, the methods that can be employed to engineer injectable hydrogels and their latest applications in tissue regeneration.

Commercial hydrogels for biomedical applications

Hydrogels are polymeric networks having the ability to absorb a large volume of water. Flexibility, versatility, stimuli-responsive, soft structure are the advantages of hydrogels. It is classified based on its source, preparation, ionic charge, response, crosslinking and physical properties. Hydrogels are used in various fields like agriculture, food industry, biosensor, biomedical, etc. Even though hydrogels are used in various industries, more researches are going in the field of biomedical applications because of its resembles to living tissue, biocompatibility, and biodegradability. Here, we are mainly focused on the commercially available hydrogels used for biomedical applications like wound dressings, contact lenses, cosmetic applications, tissue engineering, and drug delivery.

Click Chemistry-Based Injectable Hydrogels and Bioprinting Inks for Tissue Engineering Applications

BACKGROUND

The tissue engineering and regenerative medicine approach require biomaterials which are biocompatible, easily reproducible in less time, biodegradable and should be able to generate complex three-dimensional (3D) structures to mimic the native tissue structures. Click chemistry offers the much-needed multifunctional hydrogel materials which are interesting biomaterials for the tissue engineering and bioprinting inks applications owing to their excellent ability to form hydrogels with printability instantly and to retain the live cells in their 3D network without losing the mechanical integrity even under swollen state.

METHODS

In this review, we present the recent developments of in situ hydrogel in the field of click chemistry reported for the tissue engineering and 3D bioinks applications, by mainly covering the diverse types of click chemistry methods such as Diels-Alder reaction, strain-promoted azide-alkyne cycloaddition reactions, thiol-ene reactions, oxime reactions and other interrelated reactions, excluding enzyme-based reactions.

RESULTS

The click chemistry-based hydrogels are formed spontaneously on mixing of reactive compounds and can encapsulate live cells with high viability for a long time. The recent works reported by combining the advantages of click chemistry and 3D bioprinting technology have shown to produce 3D tissue constructs with high resolution using biocompatible hydrogels as bioinks and in situ injectable forms.

CONCLUSION

Interestingly, the emergence of click chemistry reactions in bioink synthesis for 3D bioprinting have shown the massive potential of these reaction methods in creating 3D tissue constructs. However, the limitations and challenges involved in the click chemistry reactions should be analyzed and bettered to be applied to tissue engineering and 3D bioinks. The future scope of these materials is promising, including their applications in in situ 3D bioprinting for tissue or organ regeneration.

Droplet-Assisted Microfluidic Fabrication and Characterization of Multifunctional Polysaccharide Microgels Formed by Multicomponent Reactions

Polysaccharide-based microgels have broad applications in multi-parametric cell cultures, cell-free biotechnology, and drug delivery. Multicomponent reactions like the Passerini three-component and the Ugi four-component reaction are shown in here to be versatile platforms for fabricating these polysaccharide microgels by droplet microfluidics with a narrow size distribution. While conventional microgel formation requires pre-modification of hydrogel building blocks to introduce certain functionality, in multicomponent reactions one building block can be simply exchanged by another to introduce and extend functionality in a library-like fashion. Beyond synthesizing a range of polysaccharide-based microgels utilizing hyaluronic acid, alginate and chitosan, exemplary in-depth analysis of hyaluronic acid-based Ugi four-component gels is conducted by colloidal probe atomic force microscopy, confocal Brillouin microscopy, quantitative phase imaging, and fluorescence correlation spectroscopy to elucidate the capability of microfluidic multicomponent reactions for forming defined polysaccharide microgel networks. Particularly, the impact of crosslinker amount and length is studied. A higher network density leads to higher Young's moduli accompanied by smaller pore sizes with lower diffusion coefficients of tracer molecules in the highly homogeneous network, and vice versa. Moreover, tailored building blocks allow for crosslinking the microgels and incorporating functional groups at the same time as demonstrated for biotin-functionalized, chitosan-based microgels formed by Ugi four-component reaction. To these microgels, streptavidin-labeled enzymes are easily conjugated as shown for horseradish peroxidase (HRP), which retains its activity inside the microgels.

Controlling the Release of Small, Bioactive Proteins via Dual Mechanisms with Therapeutic Potential

Injectable delivery systems that respond to biologically relevant stimuli present an attractive strategy for tailorable drug release. Here, the design and synthesis of unique polymers are reported for the creation of hydrogels that are formed in situ and degrade in response to clinically relevant endogenous and exogenous stimuli, specifically reducing microenvironments and externally applied light. Hydrogels are formed with polyethylene glycol and heparin-based polymers using a Michael-type addition reaction. The resulting hydrogels are investigated for the local controlled release of low molecular weight proteins (e.g., growth factors and cytokines), which are of interest for regulating various cellular functions and fates in vivo yet remain difficult to deliver. Incorporation of reduction-sensitive linkages and light-degradable linkages affords significant changes in the release profiles of fibroblast growth factor-2 (FGF-2) in the presence of the reducing agent glutathione or light, respectively. The bioactivity of the released FGF-2 is comparable to pristine FGF-2, indicating the ability of these hydrogels to retain the bioactivity of cargo molecules during encapsulation and release. Further, in vivo studies demonstrate degradation-mediated release of FGF-2. Overall, our studies demonstrate the potential of these unique stimuli-responsive chemistries for controlling the local release of low molecular weight proteins in response to clinically relevant stimuli.

Thermoresponsive and Reducible Hyperbranched Polymers Synthesized by RAFT Polymerisation

Here, we report the synthesis of new thermoresponsive hyperbranched polymers (HBPs) via one-pot reversible addition-fragmentation chain transfer (RAFT) copolymerisation of poly(ethylene glycol)methyl ether methacrylate (PEGMEMA, Mn = 475 g/mol), poly(propylene glycol)methacrylate (PPGMA, Mn = 375 g/mol), and disulfide diacrylate (DSDA) using 2-cyanoprop-2-yl dithiobenzoate as a RAFT agent. DSDA was used as the branching agent and to afford the HBPs with reducible disulfide groups. The resulting HBPs were characterised by Nuclear Magnetic Resonance Spectroscopy (NMR) and Gel Permeation Chromatography (GPC). Differential Scanning Calorimetry (DSC) was used to determine lower critical solution temperatures (LCSTs) of these copolymers, which are in the range of 17⁻57 °C. Moreover, the studies on the reducibility of HBPs and swelling behaviours of hydrogels synthesized from these HBPs were conducted. The results demonstrated that we have successfully synthesized hyperbranched polymers with desired dual responsive (thermal and reducible) and crosslinkable (via thiol-ene click chemistry) properties. In addition, these new HBPs carry the multiplicity of reactive functionalities, such as RAFT agent moieties and multivinyl functional groups, which can afford them with the capacity for further bioconjugation and structure modifications.

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.

Injectable Pore-Forming Hydrogel Scaffolds for Complex Wound Tissue Engineering: Designing and Controlling Their Porosity and Mechanical Properties

Traumatic soft tissue wounds present a significant reconstructive challenge. The adoption of closed-circuit negative pressure wound therapy (NPWT) has enabled surgeons to temporize these wounds before reconstruction. Such systems use porous synthetic foam scaffolds as wound fillers at the interface between the negative pressure system and the wound bed. The idea of using a bespoke porous biomaterial that enhances wound healing, as filler for an NPWT system, is attractive as it circumvents concerns regarding reconstructive delay and the need for dressing changes that are features of the current systems. Porous foam biomaterials are mechanically robust and able to synthesize in situ. Hence, they exhibit potential to fulfill the niche for such a functionalized injectable material. Injectable scaffolds are currently in use for minimally invasive surgery, but the design parameters for large-volume expansive foams remain unclear. Potential platforms include hydrogel systems, (particularly superabsorbent, superporous, and nanocomposite systems), polyurethane-based moisture-cured foams, and high internal phase emulsion polymer systems. The aim of this review is to discuss the design parameters for such future biomaterials and review potential candidate materials for further research into this up and coming field.

Postpolymerization Modifications of Alkene-Functional Polycarbonates for the Development of Advanced Materials Biomaterials

Functional aliphatic polycarbonates have attracted significant attention as materials for use as biomedical polymers in recent years. The incorporation of pendent functionality offers a facile method of modifying materials postpolymerization, thus enabling functionalities not compatible with ring-opening polymerization (ROP) to be introduced into the polymer. In particular, polycarbonates bearing alkene-terminated functional groups have generated considerable interest as a result of their ease of synthesis, and the wide range of materials that can be obtained by performing simple postpolymerization modifications on this functionality, for example, through radical thiol-ene addition, Michael addition, and epoxidation reactions. This review presents an in-depth appraisal of the methods used to modify alkene-functional polycarbonates postpolymerization, and the diversity of practical applications for which these materials and their derivatives have been used.

Functionalised type-I collagen as a hydrogel building block for bio-orthogonal tissue engineering applications

Modulating the hydrogel properties from injectable to implantable scaffolds using the bio-orthogonal thiol-Michael addition click reaction.

Injectable Interpenetrating Network Hydrogels via Kinetically Orthogonal Reactive Mixing of Functionalized Polymeric Precursors

The enhanced mechanics, unique chemistries, and potential for domain formation in interpenetrating network (IPN) hydrogels have attracted significant interest in the context of biomedical applications. However, conventional IPNs are not directly injectable in a biological context, limiting their potential utility in such applications. Herein, we report a fully injectable and thermoresponsive interpenetrating polymer network formed by simultaneous reactive mixing of hydrazone cross-linked poly(N-isopropylacrylamide) (PNIPAM), and thiosuccinimide cross-linked poly(N-vinylpyrrolidone) (PVP). The resulting IPN gels rapidly (<1 min) after injection without the need for heat, UV irradiation, or small-molecule cross-linkers. The IPNs, cross-linked by kinetically orthogonal mechanisms, showed a significant synergistic enhancement in shear storage modulus compared to the individual component networks as well as distinctive pore morphology, degradation kinetics, and thermal swelling; in particular, significantly lower hysteresis was observed over the thermal phase transition relative to single-network PNIPAM hydrogels.

Nitrile Oxide-Norbornene Cycloaddition as a Bioorthogonal Crosslinking Reaction for the Preparation of Hydrogels

This communication describes the first application of cycloaddition between an in situ generated nitrile oxide with norbornene leading to a polymer crosslinking reaction for the preparation of poly(ethylene glycol) hydrogels under physiological conditions. Hydrogels with high water content and robust physical strength are readily formed within 2-5 min by a simple two-solution mixing method which allows 3D encapsulation of neuronal cells. This bioorthogonal crosslinking reaction provides a simple yet highly effective method for preparation of hydrogels to be used in bioengineering.

Injectable biodegradable hybrid hydrogels based on thiolated collagen and oligo(acryloyl carbonate)-poly(ethylene glycol)-oligo(acryloyl carbonate) copolymer for functional cardiac regeneration

Injectable biodegradable hybrid hydrogels were designed and developed based on thiolated collagen (Col-SH) and multiple acrylate containing oligo(acryloyl carbonate)-b-poly(ethylene glycol)-b-oligo(acryloyl carbonate) (OAC-PEG-OAC) copolymers for functional cardiac regeneration. Hydrogels were readily formed under physiological conditions (37°C and pH 7.4) from Col-SH and OAC-PEG-OAC via a Michael-type addition reaction, with gelation times ranging from 0.4 to 8.1 min and storage moduli from 11.4 to 55.6 kPa, depending on the polymer concentrations, solution pH and degrees of substitution of Col-SH. The collagen component in the hybrid hydrogels retained its enzymatic degradability against collagenase, and the degradation time of the hydrogels increased with increasing polymer concentration. In vitro studies showed that bone marrow mesenchymal stem cells (BMSCs) exhibited rapid cell spreading and extensive cellular network formation on these hybrid hydrogels. In a rat infarction model, the infarcted left ventricle was injected with PBS, hybrid hydrogels, BMSCs or BMSC-encapsulating hybrid hydrogels. Echocardiography demonstrated that the hybrid hydrogels and BMSC-encapsulating hydrogels could increase the ejection fraction at 28 days compared to the PBS control group, resulting in improved cardiac function. Histology revealed that the injected hybrid hydrogels significantly reduced the infarct size and increased the wall thickness, and these were further improved with the BMSC-encapsulating hybrid hydrogel treatment, probably related to the enhanced engraftment and persistence of the BMSCs when delivered within the hybrid hydrogel. Thus, these injectable hybrid hydrogels combining intrinsic bioactivity of collagen, controlled mechanical properties and enhanced stability provide a versatile platform for functional cardiac regeneration.

Injectable chitosan-based scaffolds in regenerative medicine and their clinical translatability

Injectable scaffolds (IS) are polymeric solutions that are injected in vivo and undergo gelation in response to physiological or non-physiological stimuli. Interest in using IS in regenerative medicine has been increasing this past decade. IS are administered in vivo using minimally invasive surgery, which reduces hospitalization time and risk of surgical wound infection. Here, chitosan is explored as an excellent candidate for developing IS. A literature search reveals that 27% of IS publications in the past decade investigated injectable chitosan scaffolds (ICS). This increasing interest in chitosan stems from its many desirable physicochemical properties. The first section of this Progress Report is a comprehensive study of all physical, chemical, and biological stimuli that have been explored to induce ICS gelation in vivo. Second, the use of ICS is investigated in four major regenerative medicine applications, namely bone, cartilage, cardiovascular, and neural regeneration. Finally, an overall critique of the ICS literature in light of clinical translatability is presented. Even though ICS have been widely explored in the literature, very few have progressed to clinical trials. The authors discuss the current barriers to moving ICS into the clinic and provide suggestions regarding what is needed to overcome those challenges.

Click hydrogels, microgels and nanogels: emerging platforms for drug delivery and tissue engineering

Hydrogels, microgels and nanogels have emerged as versatile and viable platforms for sustained protein release, targeted drug delivery, and tissue engineering due to excellent biocompatibility, a microporous structure with tunable porosity and pore size, and dimensions spanning from human organs, cells to viruses. In the past decade, remarkable advances in hydrogels, microgels and nanogels have been achieved with click chemistry. It is a most promising strategy to prepare gels with varying dimensions owing to its high reactivity, superb selectivity, and mild reaction conditions. In particular, the recent development of copper-free click chemistry such as strain-promoted azide-alkyne cycloaddition, radical mediated thiol-ene chemistry, Diels-Alder reaction, tetrazole-alkene photo-click chemistry, and oxime reaction renders it possible to form hydrogels, microgels and nanogels without the use of potentially toxic catalysts or immunogenic enzymes that are commonly required. Notably, unlike other chemical approaches, click chemistry owing to its unique bioorthogonal feature does not interfere with encapsulated bioactives such as living cells, proteins and drugs and furthermore allows versatile preparation of micropatterned biomimetic hydrogels, functional microgels and nanogels. In this review, recent exciting developments in click hydrogels, microgels and nanogels, as well as their biomedical applications such as controlled protein and drug release, tissue engineering, and regenerative medicine are presented and discussed.

Renaissance of Aliphatic Polycarbonates: New Techniques and Biomedical Applications

Aliphatic polycarbonates were discovered a long time ago, with their conventional applications mostly limited to low molecular weight oligomeric intermediates for copolymerization with other polymers. Recent developments in polymerization techniques have overcome the difficulty in preparing high molecular weight aliphatic polycarbonates. These in turn, along with new functional monomers, have enabled the preparation of a wide range of aliphatic polycarbonates with diverse chemical compositions and structures. This review summarizes the latest polymerization techniques for preparing well-defined functional aliphatic polycarbonates, as well as the new applications of those aliphatic polycarbonates, esecially in the biomedical field.

Evaluation of multifunctional polysaccharide hydrogels with varying stiffness for bone tissue engineering

The use of hydrogels for bone regeneration has been limited due to their inherent low modulus to support cell adhesion and proliferation as well as their susceptibility to bacterial infections at the wound site. To overcome these limitations, we evaluated multifunctional polysaccharide hydrogels of varying stiffness to obtain the optimum stiffness at which the gels (1) induce proliferation of human dermal fibroblasts, human umbilical vascular endothelial cells (HUVECs), and murine preosteoblasts (MC3T3-E1), (2) induce osteoblast differentiation and mineralization, and (3) exhibit an antibacterial activity. Rheological studies demonstrated that the stiffness of hydrogels made of a polysaccharide blend of methylcellulose, chitosan, and agarose was increased by crosslinking the chitosan component to different extents with increasing amounts of genipin. The gelation time decreased (from 210 to 60 min) with increasing genipin concentrations. Proliferation of HUVECs decreased by 10.7 times with increasing gel stiffness, in contrast to fibroblasts and osteoblasts, where it increased with gel stiffness by 6.37 and 7.8 times, respectively. At day 14 up to day 24, osteoblast expression of differentiation markers-osteocalcin, osteopontin-and early mineralization marker-alkaline phosphatase, were significantly enhanced in the 0.5% (w/v) crosslinked gel, which also demonstrated enhanced mineralization by day 25. The antibacterial efficacy of the hydrogels decreased with the increasing degree of crosslinking as demonstrated by biofilm formation experiments, but gels crosslinked with 0.5% (w/v) genipin still demonstrated significant bacterial inhibition. Based on these results, gels crosslinked with 0.5% (w/v) genipin, where 33% of available groups on chitosan were crosslinked, exhibited a stiffness of 502±64.5 Pa and demonstrated the optimal characteristics to support bone regeneration.