Preprint for: "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 castdrying 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 using fluorescence methods. To overcome these issues, poly(hydroxyethyl methacrylate) (pHEMA) hydrogels have been introduced as alternatives. In our study, pHEMA hydrogels synthesised using free-radical polymerisation with blue light under two different atmospheres (N2 and air) were compared with natural samples of articular bovine cartilage. Its optical, mechanical, swelling and tribological properties demonstrate the superior properties of pHEMA, which may result in the replacement of the currently used PVAbased model in future studies. Synthesis under a nitrogen atmosphere (pHEMA N2) resulted in the formation of smooth-surfaced hydrogels, whereas synthesis under a laboratory atmosphere (pHEMA air) resulted in the formation of corrugated-surfaced hydrogels, which were more similar to the AC surface. The swelling of both hydrogels and AC followed first-order kinetics. Pin-onplate biotribology measurements showed that the coefficient of friction (COF) of the corrugatedsurface hydrogels resembled that of AC. In terms of permeability, pHEMA air is a faithful model of healthy human cartilage, whereas pHEMA N2 can be a model of cartilage with incipient osteoarthritis. Our results showed that pHEMA-based hydrogels are suitable biotribological articular cartilage models for a better understanding of the biological function of articular cartilage. Knowledge brings new insights into the cartilage complex mechanisms and might be imitated in both biomedical as well as engineering applications.