Increasing Cross-Linking Efficiency of Methacrylated Chondroitin Sulfate Hydrogels by Copolymerization with Oligo(Ethylene Glycol) Diacrylates

Articulation inspired by nature: a review of biomimetic and biologically active 3D printed scaffolds for cartilage tissue engineering

In the human body, articular cartilage facilitates the frictionless movement of synovial joints. However, due to its avascular and aneural nature, it has a limited ability to self-repair when damaged due to injury or wear and tear over time. Current surgical treatment options for cartilage defects often lead to the formation of fibrous, non-durable tissue and thus a new solution is required. Nature is the best innovator and so recent advances in the field of tissue engineering have aimed to recreate the microenvironment of native articular cartilage using biomaterial scaffolds. However, the inability to mirror the complexity of native tissue has hindered the clinical translation of many products thus far. Fortunately, the advent of 3D printing has provided a potential solution. 3D printed scaffolds, fabricated using biomimetic biomaterials, can be designed to mimic the complex zonal architecture and composition of articular cartilage. The bioinks used to fabricate these scaffolds can also be further functionalised with cells and/or bioactive factors or gene therapeutics to mirror the cellular composition of the native tissue. Thus, this review investigates how the architecture and composition of native articular cartilage is inspiring the design of biomimetic bioinks for 3D printing of scaffolds for cartilage repair. Subsequently, we discuss how these 3D printed scaffolds can be further functionalised with cells and bioactive factors, as well as looking at future prospects in this field.

A Versatile Microencapsulation Platform for Hyaluronic Acid and Polyethylene Glycol

Cell microencapsulation is a rapidly expanding field with broad potential for stem cell therapies and tissue engineering research. Traditional alginate microspheres suffer from poor biocompatibility, and microencapsulation of more advanced hydrogels is challenging due to their slower gelation rates. We have developed a novel, noncytotoxic, nonemulsion-based method to produce hydrogel microspheres compatible with a wide variety of materials, called core-shell spherification (CSS). Fabrication of microspheres by CSS derived from two slow-hardening hydrogels, hyaluronic acid (HA) and polyethylene glycol diacrylate (PEGDA), was characterized. HA microspheres were manufactured with two different crosslinking methods: thiolation and methacrylation. Microspheres of methacrylated HA (MeHA) had the greatest swelling ratio, the largest average diameter, and the lowest diffusion barrier. In contrast, PEGDA microspheres had the smallest diameters, the lowest swelling ratio, and the highest diffusion barrier, while microspheres of thiolated HA had characteristics that were in between the other two groups. To test the ability of the hydrogels to protect cells, while promoting function, diabetic NOD mice received intraperitoneal injections of PEGDA or MeHA microencapsulated canine islets. PEGDA microspheres reversed diabetes for the length of the study (up to 16 weeks). In contrast, islets encapsulated in MeHA microspheres at the same dose restored normoglycemia, but only transiently (3-4 weeks). Nonencapsulated canine islet transplanted at the same dose did not restore normoglycemia for any length of time. In conclusion, CSS provides a nontoxic microencapsulation procedure compatible with various hydrogel types.

Hyaluronic acid and chondroitin sulfate (meth)acrylate-based hydrogels for tissue engineering: Synthesis, characteristics and pre-clinical evaluation

Hydrogels based on photocrosslinkable Hyaluronic Acid Methacrylate (HAMA) and Chondroitin Sulfate Methacrylate (CSMA) are presently under investigation for tissue engineering applications. HAMA and CSMA gels offer tunable characteristics such as tailorable mechanical properties, swelling characteristics, and enzymatic degradability. This review gives an overview of the scientific literature published regarding the pre-clinical development of covalently crosslinked hydrogels that (partially) are based on HAMA and/or CSMA. Throughout the review, recommendations for the next steps in clinical translation of hydrogels based on HAMA or CSMA are made and potential pitfalls are defined. Specifically, a myriad of different synthetic routes to obtain polymerizable hyaluronic acid and chondroitin sulfate derivatives are described. The effects of important parameters such as degree of (meth)acrylation and molecular weight of the synthesized polymers on the formed hydrogels are discussed and useful analytical techniques for their characterization are summarized. Furthermore, the characteristics of the formed hydrogels including their enzymatic degradability are discussed. Finally, a summary of several recent applications of these hydrogels in applied fields such as cartilage and cardiac regeneration and advanced tissue modelling is presented.

Review of polysaccharide particle-based functional drug delivery

This review investigates the significant role polysaccharide particles play in functional drug delivery. The importance of these systems is due to the wide variety of polysaccharides and their natural source meaning that they can provide biocompatible and biodegradable systems with a range of both biological and chemical functionality valuable for drug delivery. This functionality includes protection and presentation of working therapeutics through avoidance of the reticuloendothelial system, stabilization of biomacromolecules and increasing the bioavailability of incorporated small molecule drugs. Transport of the therapeutic is also key to the utility of polysaccharide particles, moving drugs from the site of administration through mucosal binding and transport and using chemistry, size and receptor mediated drug targeting to specific tissues. This review also scrutinizes the methods of synthesizing and constructing functional polysaccharide particle drug delivery systems that maintain and extend the functionality of the natural polysaccharides.

Glycosaminoglycan and Proteoglycan-Based Biomaterials: Current Trends and Future Perspectives

Proteoglycans and their glycosaminoglycans (GAG) are essential for life as they are responsible for orchestrating many essential functions in development and tissue homeostasis, including biophysical properties and roles in cell signaling and extracellular matrix assembly. In an attempt to capture these biological functions, a range of biomaterials are designed to incorporate off-the-shelf GAGs, typically isolated from animal sources, for tissue engineering, drug delivery, and regenerative medicine applications. All GAGs, with the exception of hyaluronan, are present in the body covalently coupled to the protein core of proteoglycans, yet the incorporation of proteoglycans into biomaterials remains relatively unexplored. Proteoglycan-based biomaterials are more likely to recapitulate the unique, tissue-specific GAG profiles and native GAG presentation in human tissues. The protein core offers additional biological functionality, including cell, growth factor, and extracellular matrix binding domains, as well as sites for protein immobilization chemistries. Finally, proteoglycans can be recombinantly expressed in mammalian cells and thus offer genetic manipulation and metabolic engineering opportunities for control over the protein and GAG structures and functions. This Progress Report summarizes current developments in GAG-based biomaterials and presents emerging research and future opportunities for the development of biomaterials that incorporate GAGs presented in their native proteoglycan form.

Deformable and flexible electrospun nanofiber-supported cross-linked gel polymer electrolyte membranes for high safety lithium-ion batteries

A deformable and flexible Li/CGPE/LiFePO₄ cell based on CGPE-3 exhibited a high specific capacity and superior cycling stability for lithium-ion batteries.

Synthesis and Characterization of Injectable Sulfonate-Containing Hydrogels

Sulfonate-containing hydrogels are of particular interest because of their tunable mechanical and swelling properties, as well as their biological effects. Polysulfonate copolymers were synthesized by reacting 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylamide (AM), and acrylic acid (AA). We found that the incorporation rate of sulfonate-containing monomer and the molecular weight of the copolymer were significantly enhanced by increasing the ionic strength of the solution. We introduced thiol groups by modifying the pendant carboxylates or copolymerizing along with a disulfide-containing monomer. The thiol-containing copolymers were reacted with a 4-arm acrylamide-terminated poly(ethylene glycol) via a thiol-ene click reaction, which was mediated by a photoinitiator, a redox initiator, or a base-catalyzed Michael-Addition. We were able to tailor the storage modulus (33-1800 Pa) and swelling capacity (1-91 wt %) of the hydrogel by varying the concentration of the copolymers. We determined that the injectable sulfonate-containing hydrogels were biocompatible up to 20 mg/mL, as observed by an electric cell-substrate impedance sensing (ECIS) technique, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay using three different cell lines: human retinal pigment epithelial cells (ARPE-19), fibroblasts (NIH 3T3), and Chinese hamster ovary cells (CHO).

Approaching the compressive modulus of articular cartilage with a decellularized cartilage-based hydrogel

ECM-based materials are appealing for tissue engineering strategies because they may promote stem cell recruitment, cell infiltration, and cell differentiation without the need to supplement with additional biological factors. Cartilage ECM has recently shown potential to be chondroinductive, particularly in a hydrogel-based system, which may be revolutionary in orthopedic medicine. However, hydrogels composed of natural materials are often mechanically inferior to synthetic materials, which is a major limitation for load-bearing tissue applications. The objective was therefore to create an unprecedented hydrogel derived entirely from native cartilage ECM that was both mechanically more similar to native cartilage tissue and capable of inducing chondrogenesis. Porcine cartilage was decellularized, solubilized, and then methacrylated and UV photocrosslinked to create methacrylated solubilized decellularized cartilage (MeSDCC) gels. Methacrylated gelatin (GelMA) was employed as a control for both biomechanics and bioactivity. Rat bone marrow-derived mesenchymal stem cells were encapsulated in these networks, which were cultured in vitro for 6weeks, where chondrogenic gene expression, the compressive modulus, swelling, and histology were analyzed. One day after crosslinking, the elastic compressive modulus of the 20% MeSDCC gels was 1070±150kPa. Most notably, the stress strain profile of the 20% MeSDCC gels fell within the 95% confidence interval range of native porcine cartilage. Additionally, MeSDCC gels significantly upregulated chondrogenic genes compared to GelMA as early as day 1 and supported extensive matrix synthesis as observed histologically. Given that these gels approached the mechanics of native cartilage tissue, supported matrix synthesis, and induced chondrogenic gene expression, MeSDCC hydrogels may be promising materials for cartilage tissue engineering applications. Future efforts will focus on improving fracture mechanics as well to benefit overall biomechanical performance.

STATEMENT OF SIGNIFICANCE

Extracellular matrix (ECM)-based materials are appealing for tissue engineering strategies because they may promote stem cell recruitment, cell infiltration, and cell differentiation without the need to supplement with additional biological factors. One such ECM-based material, cartilage ECM, has recently shown potential to be chondroinductive; however, hydrogels composed of natural materials are often mechanically inferior to synthetic materials, which is a major limitation for load-bearing tissue applications. Therefore, this work is significant because we were the first to create hydrogels derived entirely from cartilage ECM that had mechanical properties similar to that of native cartilage until hydrogel failure. Furthermore, these hydrogels had a compressive modulus of 1070±150kPa, they were chondroinductive, and they supported extensive matrix synthesis. In the current study, we have shown that these new hydrogels may prove to be a promising biomaterial for cartilage tissue engineering applications.

Chondroinduction from Naturally Derived Cartilage Matrix: A Comparison Between Devitalized and Decellularized Cartilage Encapsulated in Hydrogel Pastes

Hydrogel precursors are liquid solutions that are prone to leaking after surgical placement. This problem was overcome by incorporating either decellularized cartilage (DCC) or devitalized cartilage (DVC) microparticles into traditional photocrosslinkable hydrogel precursors in an effort to achieve a paste-like hydrogel precursor. DCC and DVC were selected specifically for their potential to induce chondrogenesis of stem cells, given that materials that are chondroinductive on their own without growth factors are a revolutionary goal in orthopedic medicine. We hypothesized that DVC, lacking the additional chemical processing steps in DCC to remove cell content, would lead to a more chondroinductive hydrogel with rat bone marrow-derived mesenchymal stem cells. Hydrogels composed of methacrylated hyaluronic acid (MeHA) and either DCC or DVC microparticles were tested with and without exposure to transforming growth factor (TGF)-β3 over a 6 week culture period, where swelling, mechanical analysis, and gene expression were observed. For collagen II, Sox-9, and aggrecan expression, MeHA precursors containing DVC consistently outperformed the DCC-containing groups, even when the DCC groups were exposed to TGF-β3. DVC consistently outperformed all TGF-β3-exposed groups in aggrecan and collagen II gene expression as well. In addition, when the same concentrations of MeHA with DCC or DVC microparticles were evaluated for yield stress, the yield stress with the DVC microparticles was 2.7 times greater. Furthermore, the only MeHA-containing group that exhibited shape retention was the group containing DVC microparticles. DVC appeared to be superior to DCC in both chondroinductivity and rheological performance of hydrogel precursors, and therefore DVC microparticles may hold translational potential for cartilage regeneration.

Enzyme responsive GAG-based natural-synthetic hybrid hydrogel for tunable growth factor delivery and stem cell differentiation

We describe an enzymatically formed chondroitin sulfate (CS) and poly(ethylene glycol) (PEG) based hybrid hydrogel system, which by tuning the architecture and composition of modular building blocks, allows the application-specific tailoring of growth factor delivery and cellular responses. CS, a negatively charged sulfate-rich glycosaminoglycan of the extracellular matrix (ECM), known for its growth factor binding and stem cell regulatory functions, is used as a starting material for the engineering of this biomimetic materials platform. The functionalization of CS with transglutaminase factor XIII specific substrate sequences is utilized to allow cross-linking of CS with previously described fibrin-mimetic TG-PEG hydrogel precursors. We show that the hydrogel network properties can be tuned by varying the degree of functionalization of CS as well as the ratio and concentrations of PEG and CS precursors. Taking advantage of TG-PEG hydrogel, compatible tagged bio-functional building blocks, including RGD peptides or matrix metalloproteinase sensitive domains, can be incorporated on demand allowing the three-dimensional culture and expansion of human bone marrow mesenchymal stem cells (BM-MSCs). The binding of bone morphogenetic protein-2 (BMP-2) in a CS concentration dependent manner and the BMP-2 release mediated osteogenic differentiation of BM-MSCs indicate the potential of CS-PEG hybrid hydrogels to promote regeneration of bone tissue. Their modular design allows facile incorporation of additional signaling elements, rendering CS-PEG hydrogels a highly flexible platform with potential for multiple biomedical applications.

Chondroinductive Hydrogel Pastes Composed of Naturally Derived Devitalized Cartilage

Hydrogel precursors are liquid solutions that are prone to leaking from the defect site once implanted in vivo. Therefore, the objective of the current study was to create a hydrogel precursor that exhibited a yield stress. Additionally, devitalized cartilage extracellular matrix (DVC) was mixed with DVC that had been solubilized and methacrylated (MeSDVC) to create hydrogels that were chondroinductive. Precursors composed of 10% MeSDVC or 10% MeSDVC with 10% DVC were first evaluated rheologically, where non-Newtonian behavior was observed in all hydrogel precursors. Rat bone marrow stem cells (rBMSCs) were mixed in the precursor solutions, and the solutions were then crosslinked and cultured in vitro for 6 weeks with and without exposure to human transforming growth factor β3 (TGF-β3). The compressive modulus, gene expression, biochemical content, swelling, and histology of the gels were analyzed. The DVC-containing gels consistently outperformed the MeSDVC-only group in chondrogenic gene expression, especially at 6 weeks, where the relative collagen II expression of the DVC-containing groups with and without TGF-β3 exposure was 40- and 78-fold higher, respectively, than that of MeSDVC alone. Future work will test for chondrogenesis in vivo and overall, these two cartilage-derived components are promising materials for cartilage tissue engineering applications.

Photocrosslinkable and elastomeric hydrogels for bone regeneration

Nanocomposite biomaterials are extensively investigated for cell and tissue engineering applications due their unique physical, chemical and biological characteristics. Here, we investigated the mechanical, rheological, and degradation properties of photocrosslinkable and elastomeric nanocomposite hydrogels from nanohydroxyapatite (nHAp) and gelatin methacryloyl (GelMA). The addition of nHAp resulted in a significant increase in mechanical stiffness and physiological stability. Cells readily adhere and proliferate on the nanocomposite surfaces. Cyclic stretching of cells on the elastomeric nanocomposites revealed that nHAp elicited a stronger alignment response in the direction of strain. In vitro studies highlight enhanced bioactivity of nanocomposites as determined by alkaline phosphate (ALP) activity. Overall, the elastomeric and photocrosslinkable nanocomposite hydrogels can be used for minimally invasive therapy for bone regeneration.

Self-healable, tough, and ultrastretchable nanocomposite hydrogels based on reversible polyacrylamide/montmorillonite adsorption

Nanocomposite hydrogels with unprecedented stretchability, toughness, and self-healing have been developed by in situ polymerization of acrylamide with the presence of exfoliated montmorillonite (MMT) layers as noncovalent cross-linkers. The exfoliated MMT clay nanoplatelets with high aspect ratios, as confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) results, are well dispersed in the polyacrylamide matrix. Strong polymer/MMT interaction was confirmed by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The effective cross-link densities of these hydrogels are estimated in the range of 2.2-5.7 mol m(-3). Uniaxial tensile tests showed a very high fracture elongation up to 11 800% and a fracture toughness up to 10.1 MJ m(-3). Cyclic loading-unloading tests showed remarkable hysteresis, which indicates energy dissipation upon deformation. Residual strain after cyclic loadings could be recovered under mild conditions, with the recovery extent depending on clay content. A mechanism based on reversible desorption/adsorption of polymer chains on clay platelets surface is discussed. Finally, these nanocomposite hydrogels are demonstrated to fully heal by dry-reswell treatments.

Enabling Surgical Placement of Hydrogels Through Achieving Paste-Like Rheological Behavior in Hydrogel Precursor Solutions

Hydrogels are a promising class of materials for tissue regeneration, but they lack the ability to be molded into a defect site by a surgeon because hydrogel precursors are liquid solutions that are prone to leaking during placement. Therefore, although the main focus of hydrogel technology and developments are on hydrogels in their crosslinked form, our primary focus is on improving the fluid behavior of hydrogel precursor solutions. In this work, we introduce a method to achieve paste-like hydrogel precursor solutions by combining hyaluronic acid nanoparticles with traditional crosslinked hyaluronic acid hydrogels. Prior to crosslinking, the samples underwent rheological testing to assess yield stress and recovery using linear hyaluronic acid as a control. The experimental groups containing nanoparticles were the only solutions that exhibited a yield stress, demonstrating that the nanoparticulate rather than the linear form of hyaluronic acid was necessary to achieve paste-like behavior. The gels were also photocrosslinked and further characterized as solids, where it was demonstrated that the inclusion of nanoparticles did not adversely affect the compressive modulus and that encapsulated bone marrow-derived mesenchymal stem cells remained viable. Overall, this nanoparticle-based approach provides a platform hydrogel system that exhibits a yield stress prior to crosslinking, and can then be crosslinked into a hydrogel that is capable of encapsulating cells that remain viable. This behavior may hold significant impact for hydrogel applications where a paste-like behavior is desired in the hydrogel precursor solution.