Study on wear reduction mechanisms of artificial cartilage by synergistic protein boundary film formation

Poly(vinyl alcohol) (PVA) hydrogel is one of the anticipated materials for artificial cartilage. PVA hydrogel has high water content and a low elastic modulus similar to natural cartilage, but its major disadvantage is its lower strength. PVA hydrogel experienced rapid wear under severe conditions such as mixed or boundary lubrication. Therefore, the existence of a protective surface film with low friction becomes important to prevent surface failure. In this study, the reciprocating frictional tests for a sliding pair of PVA hydrogel and glass plate were carried out, and fluorescent observations were performed to identify the roles of adsorbed protein film. Albumin and gamma-globulin, which are contained in natural synovial fluid, were used by mixing into the lubricant. It appears that groups of albumin molecules adsorb on the smooth gamma-globulin adsorbed layer at content of 2.1 wt% of proteins with an appropriate ratio. But in the case of a lubricant which has excessive protein at 2.8 wt%, albumin and gamma-globulin adsorbed separately. Considering the wear. reduction at 2.1 wt% content of protein, albumin and gamma-globulin constituted synergistic adsorbed film for wear reduction. It is indicated that albumin constructs a low shear layer and gamma-globulin forms a layer protecting PVA hydrogel from wear. It is considered that wear and friction of PVA hydrogel were reduced due to slip of the boundary of adsorbed albumin and gamma-globulin layer. Content of protein and ratio of albumin to gamma-globulin (AG ratio) are important to constitute the appropriate protein film.