Processing of UHMWPE and HA/UHMWPE nanocomposite for biomedical applications

Ultrahigh molecular weight polyethylene (UHMWPE) is an implant material for orthopedic implants because of its excellent mechanical properties. However, its applications were limited by the poor processability, particularly its use as matrix materials for bone-analogue. The objective of this work is to process UHMWPE and hydroxyapatite/UHMWPE (HA/UHMWPE) nanocomposites for biomedical applications. UHMWPE was processed by extrusion in a temperature window, in which a metastable polyethylene phase transformation was induced by the elongational flow. Compared with conventional methods, the flow resistance was reduced by the mobile mesophase and the fusion of powder became faster by the improved inter-particle chain diffusion. Pin-on-disc wear tests showed that the as-extruded UHMWPE exhibited less wear loss, smaller wear debris and smoother worn surface, suggesting it has higher wear resistance than conventional sample. HA/UHMWPE nanocomposite was processed by twin-screw extrusion of HA and UHMWPE powder mixture and swelling UHMWPE in a solvent to control the shear viscosity. Microstructure showed that aggregated HA powder was broken down to nano-sized primary particles and dispersed homogeneously by the intensive shear mixing in the extruder. The HA particles and UHMWPE fibrils were intimately contacted because swelling improved the chain mobility of UHMWPE. The composite stiffness was significantly enhanced attributed to the reinforcement effect of HA nano particles to UHMWPE fibrils. Since the toughness of UHMWPE was maintained in the composite, the composite was hot drawn to further increase the strength to that of cortical bone by aligning UHMWPE fibrils along the drawing direction. Biological evaluation indicated that the composite was biocompatible and very bioactive in simulated body fluid immersion. It was shown that the composite with 30% of HA by volume had optimal mechanical and biological properties.