In this SBIR Phase I project we aim to improve the key mechanical properties of highly cross-linked ultrahigh molecular weight polyethylene by forming composites with a unique new dual purpose nanoparticulate reinforcement/antioxidant. Ultrahigh molecular weight polyethylene (UHMWPE) has long been widely used in joint implants, and further improvements to its wear resistance have recently been obtained by cross-linking the polyethylene chains. The cross-links are most commonly obtained by radical formation during gamma irradiation in an inert atmosphere or electron beam irradiation, followed by melting and radical recombination. However, the cross-linking process has also been associated with decreases in mechanical properties, including toughness, fatigue and yield strengths, which have led to some premature failures of joint implants. Furthermore, some oxidative degradation of the UHMWPE results from oxidation by residual free radicals and storage in air. Incorporating our nanomaterials into cross-linked UHMWPE provides two mechanisms for improving its materials properties and decreasing the failure rate of implants utilizing cross-linked UHMWPE. First, our preliminary data shows significant improvements in toughness of the nanocomposite at ca. 1 wt.% loading, and we hypothesize that similar improvements in fatigue and yield strengths are also provided. Secondly, since the nanomaterial is a powerful antioxidant, it will reduce the residual radical content and oxidative degradation of highly cross-linked UHMWPE. We will validate these hypotheses by preparing nanocomposites at various loadings of the nanomaterial, subjecting test specimens to various processing conditions used in the industry, and then assessing their structural, mechanical and tribological properties. Should the new UHMWPE materials perform according to our hypotheses, the benefits would include longer-lasting implants that are less prone to failure, with longer lifetimes in oxidative environments.
Joint implants are an effective treatment for those with rheumatoid arthritis, osteoarthritis, osteonecrosis, and other severe destructive injuries;within the U.S. alone, roughly 500,000 joint replacements are performed each year. As life expectancy increases, patients more frequently outlive their implant. Thus, continued improvements in materials that lead to longer lifetimes for the prostheses are required (current products have an in vivo lifetime of 10 to 15 years).