Implant materials represent an extreme challenge since they must satisfy many conflicting requirements. No one material can meet them all, and often one material is coated with another. The coatings represent further material challenges in that they need to bond to the substrate while maintaining their primary functional properties. This is true in the replacement of bone where metal alloys, with the structural properties needed to sustain skeletal loading, are coated with hydroxyapatite-like materials to improve osseointegration and biofixation with the remaining bone. The applicant's strategy has been to coat Ti and its alloys with bioactive glasses modified from those developed by Hench. Reliably adherent coatings have been produced that form apatite in a simulated body fluid. The glasses have been shown to promote adherence of osteosarcoma cells, which is promising for osseointegration. The proposed research will optimize the properties of bioactive coatings on Ti and similar coatings on Co-Cr alloys. Functionally graded and composite materials will be used to produce coatings with tailored properties such as bioactivity and bioresorptivity. Functionally graded materials (FGM) coatings containing glasses and hydroxyapatite (HA) and tricalcium phosphate (TCP) will be prepared. Coatings will be analyzed by SEM, TEM, and XRD to identify reactions at the interface with metal and also within the coating. The bottom layer of the FGM will be selected for adherence properties and be analyzed by Auger and XPS. The top of the FGM will be studied through aging in simulated body fluid (SBF), followed by a determination of the solubility and extent of HA formation. Tissue culture tests will be used to determine optimal surface roughness and in vivo studies will be used to examine the bone-implant interface through histomorphology, histology, SEM, and pullout tests. Studies are proposed to develop a clear mechanistic understanding of coating structural stability and adherence to metal substructures. Consequently, the mechanical properties of the coatings and their interfaces will be determined and microstructure-property relationships developed to guide clinicians in their choice of materials. Fracture mechanics models and techniques will be applied to the evaluation of strength/flaw interactions, fracture toughness, and stress corrosion crack growth. As coatings will be used in the body, nearly all these techniques will be expanded to involve cyclic fatigue conditions.
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