The effect of dehydration history on PVA/PVP hydrogels for nucleus pulposus replacement

The feasibility of the use of a copolymer gel prepared from blends of polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) for endoscopic replacement of the nucleus pulposus of a lumbar intervertebral disc was examined in this study. Hydrogels were processed with the use of three freeze/thaw cycles to induce crystallinity. As-prepared samples were dehydrated to various levels: 70.4, 46.3, 25.1, and 10.3% of their as-prepared masses and subsequently rehydrated. The dehydration history controlled the dimensions upon swelling and caused distortion of the material, with major distortion occurring when the hydrogel reached around 25% of the initial hydrated mass. The dehydration history affected the mechanical behavior of the rehydrated gels. Increased dehydration resulted in increased compressive modulus for the reswollen gels. Experiments were performed to investigate the formation of a skin layer that was found on the hydrogels during the dehydration process. The skin was found to dehydrate quickly and form a barrier to further dehydration from the core. Rubber elasticity theory was used to describe the differences in the network characteristics between the skin and the core of a drying hydrogel. The dehydration/rehydration process used in this study and an implantation of a cadaver model demonstrate the feasibility of endoscopic nucleus replacement.