Influence of phospholipid and protein constituents on tribological properties of artificial hydrogel cartilage material

In this study, the influence of phospholipid and protein constituents on friction and wear behavior of artificial hydrogel cartilage was investigated. A sliding pair of an ellipsoidal specimen of poly (vinyl alcohol) (PVA) hydrogel and a flat specimen of PVA hydrogel was evaluated in simplified reciprocating friction test. Dipalmitoylphosphatidylcholine (DPPC) was selected as a phospholipid constituent and was dispersed in saline as liposome. Fluorescent-labeled albumin and γ-globulin were used as protein constituents in lubricants at concentration of 0.7 wt%. After reciprocating friction test, the boundary film formed on the surface of PVA hydrogel and the worn surface of PVA hydrogel were observed by using fluorescent microscope and confocal laser scanning microscope, respectively. When only albumin or γ-globulin was added to lubricant, adhesive wear pattern was frequently observed and large breaking-offof surface structure of PVA hydrogel occurred. Lubricants that contain both proteins and 0.01wt% DPPC showed reduction of friction and suppression of large breaking-offof surface structure of PVA hydrogel. Meanwhile, under coexistence of protein and 0.02wt% DPPC, friction increased compared to that for lubricants that contain 0.01wt% DPPC and the adhesive wear patterns became obvious. Therefore, both the concentration and the relative ratio of proteins to phospholipids are important factors to function adequately as excellent boundary lubricant for PVA hydrogel.