Characteristics of the worn surface of ultra-high molecular weight polyethylene (UHMWPE) lubricated with bovine serum were investigated after laboratory wear tests to gain understanding of the effects of the physiological environment on the wear mechanism of UHMWPE. Wear tests were carried out using both a conventional pin-on-disk test apparatus and a multidirectional sliding pin-on-plate test apparatus. The macroscopic and microscopic surface structure of the worn surfaces were examined with optical microscopy and Atomic Force Microscopy (AFM). Chemical compositions of the worn surfaces were also examined with Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray-Photoelectron Spectroscopy (XPS). As a result, the worn surface articulated in bovine serum showed the interesting morphological and chemical characteristics. The surface texture of UHMWPE worn in bovine serum was quite different from that articulated in a simple lubricant, such as distilled water or physiological saline. The AFM observation revealed that the surface articulated in bovine serum was covered with fine fibrillated structures. By comparison, such structure was not observed in the surface articulated in distilled water. It could be considered that these differences in the morphological nature indicated the difference in the wear mechanism of UHMWPE. The chemical composition of the worn surfaces was also different with lubricant. The data from FT-IR and XPS suggested that the oxygen content of the worn surface became higher in bovine serum. The existence of nitrogen and sulfur in the worn surface articulated in bovine serum was also indicated from XPS examination. These results mean that there could be a kind of tribo-chemical reaction between UHMWPE and serum constituents and the oxidative degradation of UHMWPE was increased in bovine serum. Such chemical reactions might have a critical effect on the characteristic friction and wear mechanism in bovine serum.
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