A comparison of friction in hydrogel and polyurethane materials for cushion-form joints

The use of a thin layer of a compliant material as a bearing surface in human replacement joints has been found to promote fluid-film lubrication in the joint during continuous movement. However, during periods of continuous loading and little or no movement the fluid film may break down and the articulating surfaces may come into direct contact. This study considers the friction of three compliant materials under conditions of continuous loading and slow sliding velocities. The friction of a non-porous polyrethane and two porous hydrogel polymers, a terpolymer and a semi-interpenetrating network, was studied. Friction was measured between a hard spherical slider and a thin layer of the compliant material. Sliding was carried out in the presence of deionised water or new-born-calf serum as the lubricant. The friction was measured at the start up of motion, after prolonged periods of static loading, and during the subsequent sliding at a constant velocity. All three materials were found to give lower friction during steady-state sliding than at start up. Friction was also higher when serum was used as a lubricant. However, the porous hydrogels gave lower friction than the non-porous polyurethane, especially at start up. The terpolymer hydrogel gave the lowest friction values of the three materials, both at start up and in the steady state.