An injectable vehicle for nucleus pulposus cell-based therapy

An injectable hydrogel, acting as a reservoir for cell delivery and mimicking the native environment, offers promise for nucleus pulposus (NP) repair and regeneration. Herein, the potential of a stabilised type II collagen hydrogel using poly(ethylene glycol) ether tetrasuccinimidyl glutarate (4S-StarPEG) cross-linker, enriched with hyaluronic acid (HA) was investigated. The optimally stabilised type II collagen hydrogel was determined by assessing free amine groups, resistance to enzymatic degradation, gel point. The potential toxicity of the cross-linker was initially assessed against adipose-derived stem cells (ADSCs). After addition of HA (molar ratio type II collagen:HA 9:0, 9:1, 9:4.5, 9:9) within the hydrogel, the behaviour of the encapsulated NP cells was evaluated using cell proliferation assay, gene expression analysis, cell distribution and cell morphology. A significant decrease (p < 0.05) in the free amine groups of collagen was observed, confirming successful cross-linking. Gelation was independent of the concentration of 4S-StarPEG (8 min at 37 °C). The 1 mm cross-linked hydrogel yielded the most stable after enzymatic degradation (p < 0.05). No toxicity of the 4S-StarPEG was noted for the ADSCs. NP cell viability was high regardless of the concentration of HA (>80%). A cell proliferation was not seen after 14 days in its presence. At a gene expression level, HA did not influence NP cells phenotype after seven days in culture. After seven days in culture, the type I collagen mRNA expression was maintained (p > 0.05). The optimally stabilised and functionalised type II collagen/HA hydrogel system developed in this study shows promise as an injectable reservoir system for intervertebral disc regeneration.