Characterization of the behavior of porous hydrogels in model osmotically-conditioned articular cartilage systems

The evaluation of hydrogel swelling behavior is a vital step in development of new materials for biomedical applications. Phosphate-buffered saline (PBS) is the most commonly chosen swelling medium to model hydrogel behavior in articular cartilage (AC). However, the use of PBS does not fully elucidate the osmotic pressure hydrogels will face in many tissues in vivo. Thus, there is a critical need to assess the performance of hydrogels in a model system that can better reflect the native tissues for a specified application. The aim of this study was to evaluate the mechanical properties, porosity, and swelling behavior of poly(vinyl alcohol) hydrogels with a degradable poly(lactic-co-glycolic acid) (PLGA) phase in synthetic models and in ex vivo AC model systems. The controlled degradation of the PLGA phase reflected the dynamic nature of native tissues and allowed for the assessment of hydrogel swelling characteristics under fluctuating osmotic pressures. When hydrogels were implanted ex vivo into bovine AC, their swelling ratios and water contents significantly decreased. This response was matched by hydrogels immersed in a solution of PEG having an osmotic pressure matching AC. The hydrogels were further characterized over 6 weeks in PEG and in PBS, with each system having unique affects on the hydrogel swelling behavior and material properties. The results show that a PEG solution conditioned to an osmotic pressure of AC is a strong model for the effects of the osmotic environment on hydrogels and that PBS is an ineffective predictor of swelling changes in vivo.