Abstract
Poly (vinyl alcohol) (PVA) hydrogel is one of the anticipated materials for artificial cartilage. In our previous studies, wear of PVA hydrogel depended on content of proteins in lubricants. The secondary structures of bovine serum albumin (BSA) and human gamma-globulin (HGG) were investigated in circular dichroism spectroscopy to clarify the influence of the protein conformation on frictional properties. BSA and HGG were mainly composed of the alpha-helix and the beta-sheet, respectively. BSA containing the alpha-helix structure showed low friction compared to HGG composed of the beta-sheet structure in mixed or boundary lubrication mode. The alpha-helix structure forms a low shear layer because the alpha-helix structure is easily released from surfaces and low cohesive strength. HGG forms an uniform adsorption layer, but showed higher friction than BSA in the rubbing lubricated with single protein solution. In the repeated rubbing with changing of lubricants from HGG to BSA, however, the final friction was reduced, because an optimum layered structure of proteins was formed. Hence, a layered structure of proteins appears to play an important role to protect rubbing surfaces and to reduce friction. In heat treatment tests, heat-induced BSA showed very low friction because of reduction of the alpha-helix structure. Heat-induced HGG did not show large differences from native HGG, but could not bring low friction with heat-induced BSA. Thus it was shown that the protein conformation has effective influences on friction.
