Friction and wear behavior of ultrahigh molecular weight polyethylene as a function of crystallinity in the presence of the phospholipid dipalmitoyl phosphatidylcholine

In this study, the friction and wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) were evaluated as a function of polymer crystallinity in the presence of the phospholipid dipalmitoyl phosphatidylcholine (DPPC) dissolved in ethanol. Samples of UHMWPE were separately heat treated to get high and low crystallinity samples. Degree of crystallinity was evaluated using differential scanning calorimetry. Quantitative friction and wear experiments were conducted using a custom-made microtribometer with commercially available spherical Si₃N₄ probes in controlled and phospholipid-dissolved lubricants. The higher crystallinity sample exhibited slightly lower friction than the lower crystallinity in the control and decreased significantly when phospholipids were present. The higher crystallinity sample showed a higher wear resistance than the lower crystallinity sample during all reciprocating wear tests. DPPC acting as a lubricant had a marginal effect on the wear resistance of high crystallinity UHMWPE, whereas the low crystallinity sample became more prone to wear. Atomic force microscopy topography images and contact angle measurements of both samples before and after phospholipid exposure indicate that the higher crystallinity sample absorbed a greater density of DPPC. Increasing crystallinity is a way of escalating adsorption of surface active phospholipids onto UHMWPE to make it a more wear-resistant load-bearing material for total joint replacements.