CPGAN #002 FTIR Quantification of Absorbed Radiation Dose in Polyethylene

Ultra -high molecular weight polyethylene (UHMWPE) is the current material of choice for bearing surface applications in total joint arthroplasty. In an effort to enhance the wear properties of the material many companies have recently resorted to electron beam and gamma irradiation of the components prior to the machining. It is of considerable interest to accurately determine the dose absorbed by the material, both in bulk and as a function of position, for quality control and process validation purposes. Several methods exist that yield information about the dose absorbed by the material. For instance, swell ratio testing, in which a crosslinked sample is swollen in a good solvent, will give the crosslink density and molecular weight between crosslinks assuming that the interaction parameter between the solvent and polymer is well-known. This technique will only give the volumetric -averaged crosslink density. Similarly, dynamic mechanical analysis (DMA) will give a qualitative measurement of volumetric crosslink density if a viscoelastic model is assumed. Both of these techniques can be used to infer the absorbed dose level by establishing a calibration curve between the cross-link density and the absorbed dose level. However, at high radiation dose levels (around 150kGy), the cross-link density of the polymer reaches a saturation level and the determination of dose beyond these levels becomes impossible. The advances in Fourier Transform Infrared Spectroscopy (FTIR), coupled with a greater understanding of the radiation chemistry of polyethylenes, have permitted the development of a powerful technique that indicates the radiation dose received by a sample at much greater accuracy than the swelling or the DMA techniques. The FTIR technique essentially makes use of an internal dosimeter in the polyethylene, which can be measured with a conventional FTIR or an ir-microscope to produce a radiation dose index (RDI).