Surface modification of ultra high molecular weight polyethylene with hyaluronan for total joint replacement application

Abstract

Wear debris generated from ultra high molecular weight polyethylene (UHMWPE) remains a cause of total joint replacement loosening and failure. Most efforts to improve UHMWPE wear resistance have focused on modification of the bulk material properties. Few studies have involved the enhancement of UHMWPE surface lubrication. Hyaluronan (HA), a natural lubricant molecule present in mammalian synovial fluid, was introduced into the UHMWPE surface to improve its hydrophilicity, lubricity and wear resistance for orthopedic applications. Two novel hyaluronan derivatives were created so that micro-composites of hydrophilic HA and hydrophobic UHMWPE could be made by either a solvent infiltration or melt blending process. The silylated HA was hydrophobic and soluble in organic solvents, and thus was used in the solvent infiltration process. It was not possible to silylate HA directly. Its ion-paired complex with a long aliphatic chain quaternary ammonium salt (HA-CTA) was used as the starting material in the silylation reaction. The degree of silylation (DS) was controlled by changing the reaction parameters such as temperature and time. Silylated HA-CTA with a high DS was soluble in xylenes or hexane, while that with a low DS was only soluble in acetone, THF or 1,2- dichloroethane. The HA regenerated from silylated HA-CTA via hydrolysis had the same structure as native HA. Preforms with interconnected micro-pores were used as the UHMWPE starting material to form a micro-composite with HA, rather than starting with full dense, bulk UHMWPE. A xylene solution of silylated HA-CTA quickly diffused into the connected pores of the UHMWPE performs. Following this, the silylated HA-CTA infiltrating throughout the preform was chemically crosslinked in place and hydrolyzed, and then the preform was dipped in an aqueous solution of HA which was also crosslinked. The HA-treated preform was then molded to full density. Thus the micro-composite manufacturing process was simpler and faster compared to prior efforts to create an UHMWPE-poly-L-lysine interpenetrating network. The presence of HA on the micro-composite surface was confirmed by X-ray photoelectron spectroscopy (XPS) and toluidine blue O dye assay. With appropriate process parameters, a uniform HA film layer was produced on the micro-composite surface, which quickly hydrated in water, forming a lubricious surface film that was fully wetted by water drops during contact angle measurements. The HA surface on the micro-composite was more stable and resistant to enzymatic degradation than free HA in an aqueous solution. This was attributed to the HA crosslinking and the deep rooting of HA into the UHMWPE surface. The effect of HA on the mechanical properties of UHMWPE was significant, but within ASTM guidelines for implant-grade UHMWPE. Compared with the control, the micro-composite had a decreased strength and increased elongation to failure. As the thickness of the preform surface layer increased, the composite strength decreased. There was evidence of inferior consolidation during the final molding cycle in all samples, including the non-HA treated control. Simple adjustments to the molding cycle should solve this problem and result in micro-composites with better tensile properties. The HA-UHMWPE micro-composites did have significantly lower wear and wear rates than conventional UHMWPE. The decreases were most significant when the sample treatment was appropriate. Material properties and surface chemistry are more important than surface roughness in determining the wear properties of materials. Lower molecular weight and incomplete consolidation made the wear and wear rates of the smoother porous UHMWPE control higher than that of the conventional polyethylene. Low HA content within the micro-composite, abundant HA on the surface, sufficient crosslinking and good film adhesion are necessary to achieve significant lubrication and improve wear resistance of UHMWPE. A series of HA esters that could be used to create the micro-composites via melt blending was also developed by acylating silylated HA-CTA. HA esters with an acyl chain length greater than 10 carbon atoms melted before degrading. Thus, HA caprinate and higher esters are melt-processable. The HA regenerated from HA esters via saponification had the same structure as native HA. Future work will investigate the melt blending approach to manufacture micro-composites with hot-processed HA esters and UHMWPE powder.