Aging behavior of PVA hydrogels for soft tissue applications after in vitro swelling using osmotic pressure solutions

The osmotic pressure of the medium used for in vitro swelling evaluation has been shown to have a significant effect on the swelling behavior of a material. In this study, the effect of osmotic pressure during swelling on poly(vinyl alcohol) hydrogel material properties was evaluated in vitro. Osmotic pressure solutions are necessary in order to mimic the swelling pressure observed in vivo for soft tissues present in load-bearing joints. Hydrogels were characterized after swelling by mechanical testing, X-ray diffraction and optical microscopy in the hydrated state. Results indicated that hydrogel mechanical properties remained tailorable with respect to initial processing parameters; however, significant aging occurred in osmotic solution. This was observed when evaluating the mechanical properties of the hydrogels, which, before swelling, ranged from 0.04 to 0.78 MPa but, after swelling in vitro using osmotic pressure solution, ranged from 0.32 to 0.93 MPa. Significant aging was also noted when evaluating crystallinity, with the relative crystallinity ranging between 0.4 and 5.0% before swelling and between 6.5 nd 8.0% after swelling. When compared to swelling in a non-osmotic pressure solution or in phosphate-buffered saline solution, the mechanical properties were more dependent upon the final swelling content. Furthermore, increases in crystallinity were not as significant after swelling. These results highlight the importance of choosing the appropriate swelling medium for in vitro characterization based on the desired application.