Rheological characterization of photopolymerized poly(vinyl alcohol) hydrogels for potential use in nucleus pulposus replacement

Hydrogels have been proposed as candidates for nucleus pulposus replacement because of their similarity in mechanical behavior to the native tissue when subjected to transient or static loading; however, given the viscoelastic nature of soft biological tissues, the lack of dynamic testing is a significant inadequacy in the studies performed to date. In the present work, the viscoelastic behavior of a hydrogel system obtained via photopolymerization of glycidyl methacrylate modified poly(vinyl alcohol) (PVA) was evaluated in comparison to that of the nucleus pulposus when subjected to dynamic torsional shear. The complex shear moduli and phase shift angles were modulated through the variation of PVA molecular weight and concentration of polymer prior to photopolymerization. Hydrolysis resistance was assessed by evaluation of the viscoelastic behavior of hydrogels submerged in Hank's solution for progressively longer periods of time. The phase shift angles of all hydrogels were lower than those of the nucleus pulposi; however, the complex shear moduli of the synthetic system spanned the values observed for the natural system. Over the time frame of the experiment, no changes in moduli were observed following submersion in Hank's solution. This study represents the first attempt to successfully mimic the viscoelastic nature of the nucleus pulposus exhibited under dynamic torsional loading with that of materials intended for use in tissue replacement.