Injectable Chitosan Hydrogels with Enhanced Mechanical Properties for Nucleus Pulposus Regeneration

Intervertebral disc (IVD) degeneration has been implicated as a major component of spine pathology. As the IVD degenerates, the tissue becomes dehydrated, fibrotic, fissured, acellular, and calcified. These changes can lead to disc bulging, herniation, Schmorl's nodes, inflammation, and hyperinnervation. Injectable hydrogels have received much attention in recent years as scaffold for seeding cells to replenish disc cellularity and restore disc properties and function. However, they generally present poor mechanical properties. In this study, we investigate several novel thermosensitive chitosan hydrogels for their ability to mimic the mechanical properties of the nucleus pulposus (NP) tissue, while being injectable, able to entrap and maintain viability of NP cells, and retain matrix proteins. These new hydrogels were prepared by mixing chitosan (CH) with various combinations of three gelling agents: sodium hydrogen carbonate (SHC) and/or beta-glycerophosphate (BGP) and/or phosphate buffer (PB). The kinetics of gelation was studied at room and body temperature by rheology. Mechanical properties of the hydrogels were characterized under compression and torsion, and compared with human NP tissue. NP cells were seeded in the hydrogel when still liquid at room temperature, before its gelation at 37 degrees C. Hydrogel cytocompatibility and functionality were assessed by measuring cell viability, metabolism, and proteoglycan synthesis. Although all the proposed hydrogels exhibited enhanced strength compared to CH-BGP thermosensitive hydrogels, and suitable cytocompatibility and rheological properties, one formulation (containing 2% chitosan, 7.5mM of SHC, and 0.1M of BGP) showed mechanical properties similar to human NP tissue, and stimulated better the synthesis and retention of proteoglycans from NP cells. Thus, this novel thermosensitive CH hydrogel shows promise for IVD regeneration.