Class: BE-210
Group: R2
Members: Siddharth Fernandes, Summit Gupta, Richard Kiok, Andrew James
Date: April 30, 1997
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Abstract:
The experimental goal was to determine the effects of oxygen on ultra-high molecular-weight polyethylene (UHMWPE) samples after gamma irradiation in two different environments-one inert and the other oxygenated. The material properties of two UHMWPE knee implants, one gamma irradiated in argon and the other gamma irradiated in air, were examined. Gamma radiation tends to enhance the oxidation of UHMWPE by forming free radicals via chain scission. Research has shown that oxidation is directly correlated with increased crystallinity. Researchers believe that this increase in crystallinity is responsible for the mechanical failure of the polyethylene in vivo.

Three types of mechanical tests were performed: differential scanning calorimetry (DSC), three-point bending, and hardness testing. The DSC testing provided heat of fusion and percent crystallinity data. DSC testing was performed twice, once at Day 1 and again at Day 15, in order to determine if appreciable oxidation could occur in a relatively short period of time. It was found that the percent crystallinities of the argon sample at Day 1 and Day 15 were the same within the 95% confidence limit (Day1: 82.6% + 1.02%, Day15: 82.1 + 1.35%). The same was true of the air sample (Day1: 80.7% + 1.81%, Day15: 80.2 + 1.82%). Comparing air and argon, it was found that they had similar crystallinities (on Day1 and Day 15) and heat of fusion values (argon: 128 J/g + 2.03%, air: 132 J/g + 3.05%), as well. From the hardness testing, the Brinell hardness numbers for the two sets of samples were statistically the same (argon: 48.4 MPa + 2.27%, air: 49.7 MPa + 3.38%). The three-point bending test provided the Young's modulus values (E) and the yield strength of both samples. It was found that the Young's modulus of the air samples was statistically different from that of the argon samples (argon: 1.19 GPa + 12.4%, air: 2.03 GPa + .841%). The same holds true for the yield strength of the two sets of samples (argon: 45.2 MPa + 14.4%, air: 69.8 MPa ±12.6%).