Biomimetic pHEMA Hydrogels as an Alternative Cartilage-like Model Material for Biotribological Evaluations

Poly-(vinyl alcohol) (PVA) has been widely explored as a model material for articular cartilage (AC) in biotribological evaluations. However, PVA hydrogels prepared by freeze-thawing or cast-drying methods have limitations in precisely controlling their elasticity parameters and may require reinforcement to enhance their mechanical performance and change their transparency, required in some tribological measurement setups by using fluorescence methods. To overcome these issues, poly-(hydroxyethyl methacrylate) (pHEMA) hydrogels have been introduced as alternatives. In our study, pHEMA hydrogels synthesized using free-radical polymerization with blue light under two different atmospheres (nitrogen N₂ and air) were compared with natural samples of articular bovine cartilage. The optical, mechanical, swelling, and tribological properties demonstrate the superior properties of pHEMA, which may result in the replacement of the currently used PVA-based model in future studies. Synthesis under a nitrogen atmosphere (pHEMA N ₂) resulted in the formation of smooth-surfaced hydrogels, whereas synthesis under a laboratory atmosphere (pHEMA air) resulted in the formation of wrinkled-surfaced hydrogels. The swelling of both the hydrogels and AC followed first-order kinetics. Pin-on-plate biotribology measurements showed that the coefficient of friction of the wrinkled-surface hydrogels resembled that of AC. Our results showed that pHEMA-based hydrogels are suitable biotribological AC models for a better understanding of the biological functions of bovine AC. This knowledge brings new insights into cartilage complex mechanisms and might be applied in both biomedical and engineering applications.