Novel associated PVA/PVP hydrogels for nucleus pulposus replacement
Authors:   , Thomas Jonathan D. , Marcolongo Michele , Lowman Anthony M. 
Published: July 2021

Degenerative disc disease in the lumbar spine is marked by a dehydration of theintervertebral disc and loss of biomechanical function of the spinal unit. Since thecurrent surgical procedures are ineffective in restoring natural biomechanical functionback to the diseased disc, researchers have looked to replace the intervertebral disc. These designs are flawed in that they either don't restore natural movement back to thespinal unit, require surgeries that are highly invasive, or they further promote discdegeneration of adjacent spinal levels. Recently, researchers have sought to only replacethe central portion of the disc called the nucleus pulposus. A potential nucleusreplacement could mimic a healthy nucleus pulposus in restoring healthy biomechanicalfunction to the spinal unit. A hydrogel, poly(vinyl alcohol) (PVA), has been investigatedto serve as a nucleus replacement. However, semicrystalline PVA suffers dissolutionunder physiological conditions. Blends of PVA and poly(vinyl pyrrolidone) (PVP) may provide a material that isa suitable nucleus pulposus replacement. Through interchain hydrogen bonding, theypossess greater stability than pure PVA hydrogels. This research is an examination of thestability of these gels under physiological conditions. Polymer dissolution and stabilitywere studied with equilibrium swelling analysis over 120 days immersion, ATR-FTIRanalysis over 56 days immersion, and mechanical tensile testing over 56 days immersion. Rubber elasticity theory was used to combine mechanical results with swelling data tocalculate network characteristics such as the molecular weight between crosslinks anddensity of crosslinks. DSC studies after polymer blend preparation were used to illustraterelative degrees of physical crosslinking. Properties were examined as a function ofPVA/PVP composition as well as PVA w M and PVP w M . Results indicate that PVA/PVP blends prepared with moderate amounts of PVP(0.5-5%) result in a polymer network stabilized through interchain hydrogen bondingbetween hydroxyl groups on PVA chains and carbonyl groups on PVP chains. Mostnotably, a significant decrease in percentage of polymer mass loss was seen for blendsprepared with 143K w M PVA. Additions of larger amounts of PVP to the blendsresulted in weaker gels that had more open network structures suffering from higheramounts of polymer dissolution. ATR-FTIR results indicate that PVP unincorporated inthe network structure through hydrogen bonding suffers significant dissolution out of thepolymer network and into solution. w M of PVA and PVP are shown to have asignificant influence on the blends' network properties. Lower w M PVA resulted in amore stable blend containing a higher density of crosslinks. However blends preparedwith a higher w M PVA showed superior polymer network stability in dissolution studies. Higher PVP w M was shown to have a better stabilizing influence on the higher w MPVA that was tested. The blend that had the best combination of network stability underphysiological conditions and a relatively tight, stable, and crosslinked network wasprepared with 99% PVA (143K) and 1% PVP (40K).