Bioengineering materials and conditions for obtaining low friction with PVA hydrogels

Biological materials that make up the body organs and tissues are soft, wet and visco-elastic. Hydrogels can mimic these aspects and show promise for bio-medical applications. Their tribological properties are very important for promising applications such as artificial cartilage and bio-models for endovascular surgery training. The present study investigates the friction of polyvinyl alcohol (PVA) hydrogel against nine different metallic and non-metallic bio-compatible engineering materials likely to occur as countermaterials in these two applications. All the materials exhibited a characteristic velocity-dependent peak friction coefficient. Comparing the peaks, we find that lowest peak friction coefficient is produced by ceramics and glass (μ < 0.05), followed by metal alloys (μ < 0.05-0.08) and highest for polymers (0.4 < μ < 1.5), including PTFE which typically has very low-adhesion. Our results suggest that to achieve low friction, polymers should be avoided as a counter-material to PVA-hydrogels. It is also shown that PVA surface roughness is critical for achieving early transition to elasto-hydrodynamic lubrication and low friction, as shown in our comparative calculations with two different values of surface roughness of the gel. In the mixed lubrication region, the general trend is for friction to increase with roughness of the counterbody, but internal damping properties and adhesion also play important role, as shown by a simple linear model fit.