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Polymers
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Ultra high molecular weight polyethylene (UHMWPE) has been used for over three decades as a bearing material in total joint replacements. It is a semicrystalline polymer (-[CH2-CH2]-n) with chains in highly ordered crystal lamellar regions and disordered amorphous regions. The percent crystallinity by mass varies between 40-60% for most UHMWPE formulations. Various formulations of UHMWPE have been, and are currently being, developed to optimize the performance of total joint replacements.
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Image of Crystalline Lamellae.
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Current research in our group involves mechanical testing, physical, and chemical characterization of clinically relevant types of UHMWPE (including first and second generation crosslinked formulations) and the information is used to evaluate the potential performance of these materials in total joint replacements. Mechanical tests include: smooth and notched specimen monotonic tensile tests; smooth and notched specimen fatigue tests; fatigue crack propagation tests; and fracture toughness and J-integral tests. Fracture micromechanisms are investigated through the use of scanning electron microscopy. Physical and chemical characterizations include: differential scanning calorimetry (to evaluate crystallinity and lamellar thickness distributions before and after mechanical testing to gain insight into morphological evolution upon loading); and density and Fourier Transform Infrared Spectroscopy (to evaluate extent of oxidative degradation of the material due to post-irradation aging).
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Clip of a Notched Specimen Tensile Test.
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The results of some of the mechanical tests are also being used to calibrate and validate a constitutive model (Hybrid Model, HM) that has been developed for UHMWPE which provides for more accurate finite element simulations of total joint replacements.
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