Surface Characterization for Ultrahigh Molecular Weight Polyethylene/Hydroxyapatite Gradient Composites Prepared by the Gelation/Crystallization Method

To establish implant longevity of hip prosthesis in orthopedics, a new approach was proposed to improve dramatically the wear resistance and to reduce the surface friction of the acetabular cup as a bearing material in the femoral head. To do so, ultrahigh molecular weight polyethylene (UHMWPE) and hydroxyapatite (HA) composites with four amounts of HA content were prepared by a sol-gel process, and the four composites were hot-molded to make a composite with HA gradient content. When the resultant UHMWPE/HA agglomerates by the sol-gel method were molded in the narrow temperature range of 145-153 degrees C, the (110) planes with highest density of atoms in the PE crystal unit were oriented predominantly parallel to the resultant film surface. Such an unusual planar orientation contributed excellent wear resistance and low friction on the surface. Polarized light-scattering patterns, SEM images, and FTIR spectra of the specimens with such unusual planar orientation supported that the narrow molding temperature range achieves good dispersion of HA particles and high crystallinity of the UHMWPE matrix on the surface layer. Negative complex Poisson's ratio reduced from complex tensile and shear moduli was attributed to spongy-like tissue formation under crystallization of UHMWPE chains on the HA particle surface. The gradient composite molded maintained the spongy-like structure, which played an important role to avoid the cracking under bending stress.