Meniscus regeneration with injectable Pluronic/PMMA-reinforced fibrin hydrogels in a rabbit segmental meniscectomy model.

Injectable hydrogel systems are a facile approach to apply to the damaged meniscus in a minimally invasive way. We herein developed a clinically applicable and injectable semi-interpenetrated network (semi-IPN) hydrogel system based on fibrin (Fb), reinforced with Pluronic F127 (F127) and polymethyl methacrylate (PMMA), to improve the intrinsic weak mechanical properties. Through the dual-syringe device system, the hydrogel could form a gel state within about 50 s, and the increment of compressive modulus of Fb hydrogels was achieved by adding F127 from 3.0% (72.0 ± 4.3 kPa) to 10.0% (156.0 ± 9.8 kPa). The shear modulus was enhanced by adding PMMA microbeads (26.0 ± 1.1 kPa), which was higher than Fb (13.5 ± 0.5 kPa) and Fb/F127 (21.7 ± 0.8 kPa). Moreover, the addition of F127 and PMMA also delayed the rate of enzymatic biodegradation of Fb hydrogel. Finally, we confirmed that both Fb/F127 and Fb/F127/PMMA hydrogels showed accelerated tissue repair in the in vivo segmental defect of the rabbit meniscus model. In addition, the histological analysis showed that the quality of the regenerated tissues healed by Fb/F127 was particularly comparable to that of healthy tissue. The biomechanical strength of the regenerated tissues of Fb/F127 (3.50 ± 0.35 MPa) and Fb/F127/PMMA (3.59 ± 0.89 MPa) was much higher than that of Fb (0.82 ± 0.05 MPa) but inferior to that of healthy tissue (6.63 ± 1.12 MPa). These results suggest that the reinforcement of Fb hydrogel using FDA-approved synthetic biomaterials has great potential to be used clinically.

Injectable Fibrin/Polyethylene Oxide Semi-IPN Hydrogel for a Segmental Meniscal Defect Regeneration.

BACKGROUND: Meniscal deficiency from meniscectomy is a common situation in clinical practices. Regeneration of the deficient meniscal portion, however, is still not feasible. PURPOSE: To develop an injectable hydrogel system consisting of fibrin (Fb) and polyethylene oxide (PEO) and to estimate its clinical potential for treating a segmental defect of the meniscus in a rabbit meniscal defect model. STUDY DESIGN: Controlled laboratory study. METHODS: The Fb/PEO hydrogel was fabricated by extruding 100 mg·mL-1 of fibrinogen solution and 2,500 U·mL-1 of thrombin solution containing 100 mg·mL-1 of PEO through a dual-syringe system. The hydrogels were characterized by rheological analysis and biodegradation tests. The meniscal defects of New Zealand White male rabbits were generated by removing 60% of the medial meniscus from the anterior side. The removed portion included the central portion. The Fb/PEO hydrogel was injected into the meniscal defect of the experimental knee through the joint space between the femoral condyle and tibial plateau at the anterior knee without a skin incision. The entire medial menisci from both knees of each rabbit were collected and photographed before placement in formalin for histological processing. Hematoxylin and eosin, safranin O, and immunohistochemical staining for type II collagen was performed. The biomechanical property of the regenerated meniscus was evaluated using a universal tensile machine. RESULTS: The Fb/PEO hydrogel was fabricated by an in situ gelation process, and the hydrogel displayed a semi-interpenetrating polymer network structure. We demonstrated that the mechanical properties of Fb-based hydrogels increased in a PEO-dependent manner. Furthermore, the addition of PEO delayed the biodegradation of the hydrogel. Our in vivo data demonstrated that, as compared with Fb hydrogel, Fb/PEO hydrogel injection into the meniscectomy model showed improved tissue regeneration. The regenerated meniscal tissue by Fb/PEO hydrogel showed enhanced tissue quality, which was supported by the histological and biomechanical properties. CONCLUSION: The Fb/PEO hydrogel had an effective tissue-regenerative ability through injection into the in vivo rabbit meniscal defect model. CLINICAL RELEVANCE: This injectable hydrogel system can promote meniscal repair and be readily utilized in clinical application.

Enzyme-mediated tissue adhesive hydrogels for meniscus repair.

Meniscus tissues have limited regenerative capacity once damaged. The treatment options for the meniscus tissue regeneration have been limited to arthroscopic meniscectomy or surgical interventions. The injectable hydrogels based system would provide an alternative to the conventional meniscus therapy by providing a minimally invasive treatment alternative. Here we utilized enzyme-based approaches to fabricate tissue adhesive hydrogels for meniscus repair. Tyramine (TA) conjugated hyaluronic acid (TA-HA) and gelatin are susceptible to tyrosinase (TYR)-mediated crosslinking in vitro and in vivo. Importantly, mechanical properties and degradation kinetics are modulated by the TA substitution and TYR concentrations. In addition, TYR -mediated crosslinking displayed tissue-adhesive properties. Furthermore, fibrochondrocyte-laden and TYR-crosslinked hydrogels demonstrated strong biocompatibility and resulted in enhancement of cartilage-specific gene expression and matrix synthesis. Overall, this represents a potential application of enzyme-mediated crosslinking hydrogels for meniscus tissue engineering.

Extracellular matrix-based cryogels for cartilage tissue engineering.

In this study, we investigated various highly porous extracellular matrix (ECM)-based cryogels for cartilage tissue engineering. For the fabrication of ECM-based cryogels, either methacrylated chondroitin sulfate (MeCS) or methacrylated hyaluronic acid (MeHA) were cross-linked along with poly (ethylene glycol) diacrylates (PEGDA) via free radical polymerization under freezing conditions. This procedure induces ice crystallization (used as a porogen) prior polymer crosslinking in which, after complete cryopolymerization, a thawing process transforms the ice crystals into a unique interconnected macroporous structure within ECM-cryogels. The developed ECM-cryogels exhibited an average macroporosity of 75% and supported the infiltration of chondrocyteds. When rabbit chondrocytes were cultured on ECM-cryogels, MeCS-based cryogels stimulated aggrecan gene expression and GAG accumulation, whereas MeHA-based cryogels stimulated type II collagen gene expression and collagen accumulation. These results demonstrate that design of ECM-based cryogels can play an important role in promoting specific ECM proteins secretion for cartilage tissue engineering.

Riboflavin-induced photo-crosslinking of collagen hydrogel and its application in meniscus tissue engineering.

A meniscus tear is a common knee injury, but its regeneration remains a clinical challenge. Recently, collagen-based scaffolds have been applied in meniscus tissue engineering. Despite its prevalence, application of natural collagen scaffold in clinical setting is limited due to its extremely low stiffness and rapid degradation. The purpose of the present study was to increase the mechanical properties and delay degradation rate of a collagen-based scaffold by photo-crosslinking using riboflavin (RF) and UV exposure. RF is a biocompatible vitamin B2 that showed minimal cytotoxicity compared to conventionally utilized photo-initiator. Furthermore, collagen photo-crosslinking with RF improved mechanical properties and delayed enzyme-triggered degradation of collagen scaffolds. RF-induced photo-crosslinked collagen scaffolds encapsulated with fibrochondrocytes resulted in reduced scaffold contraction and enhanced gene expression levels for the collagen II and aggrecan. Additionally, hyaluronic acid (HA) incorporation into photo-crosslinked collagen scaffold showed an increase in its retention. Based on these results, we demonstrate that photo-crosslinked collagen-HA hydrogels can be potentially applied in the scaffold-based meniscus tissue engineering.

Osteogenic priming of mesenchymal stem cells by chondrocyte-conditioned factors and mineralized matrix.

Transient cartilage and a mineralizing microenvironment play pivotal roles in mesenchymal cell ossification during bone formation. In order to recreate these microenvironmental cues, C3H10T1/2 murine mesenchymal stem cells (MSCs) were exposed to chondrocyte-conditioned medium (CM) and seeded onto three-dimensional mineralized scaffolds for bone regeneration. Expansion of C3H10T1/2 cells with CM resulted in enhanced expression levels of chondrogenic markers such as aggrecan, type II collagen, type X collagen, and Sox9, rather than of osteogenic genes. Interestingly, CM expansion led to reduced expression levels of osteogenic genes such as alkaline phosphatase (ALP), type I collagen, osteocalcin, and Runx2. However, CM-expanded C3H10T1/2 cells showed enhanced osteogenic differentiation as indicated by increased ALP and Alizarin Red S staining upon osteogenic factor exposure. In vivo, CM-expanded C3H10T1/2 mesenchymal cells were seeded onto mineralized scaffolds (fabricated with polydopamine and coated with simulated body fluids) and implanted into critical-sized calvarial-defect mouse models. After 8 weeks of implantation, mouse skulls were collected, and bone tissue regeneration was evaluated by micro-computed tumography and Masson's trichrome staining. In accordance with the in vitro analysis, CM-expanded C3H10T1/2 cells gave enhanced bone mineral deposition. Thus, chondrocyte-conditioned factors and a mineralized microenvironment stimulate the bone formation of MSCs.

Injectable multifunctional microgel encapsulating outgrowth endothelial cells and growth factors for enhanced neovascularization.

Recent cell-based therapy approaches have employed both nanotechnologies and other biomedical technologies to enhance their therapeutic potential. A combined strategy using therapeutic stem/progenitor cells and angiogenic proteins is attractive for the treatment of vascular disease. In this study, we developed an injectable multifunctional micro-sized gel system (microgel), composed of arginine-glycine-aspartic acid (RGD)-conjugated alginate, for the delivery of both cells and growth factors in vivo. The microgels encapsulated with outgrowth endothelial cells (OECs) and growth factors (vascular endothelial growth factor, VEGF, and hepatocyte growth factor, HGF) were formed via electrospraying. Cells encapsulated within the microgel exhibited a time-dependent proliferation with enhanced cell viability, and the size-controlled microgels resulted in sustained release of growth factors for enhanced new vessel formation by tube formation and rat aorta sprouting in vitro. Increased angiogenesis was also estimated in mice treated with RGD-microgel containing OECs and growth factors. Furthermore, injection of the multifunctional microgel into a hindlimb ischemia model improved blood flow perfusion and increased the capillary density by histological analysis. Compared with hydrogel system, injectable microgel system was shown to be superior with no toxicity. Overall, our injectable multifunctional microgel system can be attributed to deliver potential therapeutic agents/cells for the treatment of vascular diseases.

Application of magnetic nanoparticle for controlled tissue assembly and tissue engineering.

Magnetic nanoparticles have been subjected to extensive studies in the past few decades owing to their promising potentials in biomedical applications. The versatile intrinsic properties of magnetic nanoparticles enable their use in many biomedical applications. Recently, magnetic nanoparticles were utilized to control the cell's function. In addition, intracellular delivery of magnetic nanoparticles allowed cell's positioning by appropriate use of magnetic field and created cellular cluster. Furthermore, magnetic nanoparticles have been utilized to assemble more complex tissue structures than those that are achieved by conventional scaffold-based tissue engineering strategies. This review addresses recent work in the use magnetic nanoparticle for controlled tissue assembly and complex tissue formation.