| Research Abstract |
According to the data on cytotoxicity of pure metals and representative metallic biomaterials, non-toxic elements Nb, Ta, Zr, Mo and Sn were selected as alloying elements for designing new β type titanium alloys for biomedical applications with low Young's modulus, high strength and high workability. Ti-29Nb-13Ta, Ti-29Nb-13Ta-4.6Zr, Ti-16Nb-13Ta-4Mo, Ti-29Nb-13Ta-4Mo, Ti-29Nb-13Ta-2Sn, Ti-29Nb-13Ta-4.6Sn and Ti-29Nb-12Ta-6Sn were designed using d-electron alloy design method. Small ingots of designed alloys with a weight of 45g were fabricated followed by thermomechanical treatments in the laboratory, and plastic workability and basic mechanical properties of the alloys were then evaluated. Every alloy could be expected to full fill the target performance for biomedical applications, but Ti-29Nb- 13Ta-4.6Zr was found to easily satisfy the balance of strength, elongation and Young's modulus. Furthermore, judging from the data on the cytotoxicity of alloying elements, Ti-29Nb-13Ta-4.6Zr
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was expected to be excellent in biocompatibility. Then, the cytotoxicity of Ti-29Nb-13Ta-4.6Zr was evaluated using L 929 cells. Cytotoxicity of this alloy was similar to that of pure titanium.
Therefore, the further evaluation for practical use was determined to be carried out on Ti-29Nb-13Ta-4.6Zr. For the first step, practical level ingot was fabricated. The homogeneous practical level ingot was successfully fabricated by induction melting method or levitation melting method. Thermomechanical treatments were carried on the practical level ingot, and then mechanical properties of the thermomechanical treated ingot were evaluated. Solution treatment followed by aging, or direct aging after cold rolling gave Ti-29Nb-13Ta-4.6Zr qeuivalent balance of strength and ductility to that of Ti-6Al-4V ELI with keeping low modulus. Then, wear characteristics of Ti-29Nb-13Ta-4.6Zr were evaluated in simulated body environment. The wear resistance of Ti-29Nb-13Ta-4.6Zr was greater than that of Ti-6Al-4V ELI or SUS 316 L stainless steel when zirconia was used as a mating material, but opposite trend was observed when alumina was used as a mating material. Therefore, it was concluded that the surface treatment was necessary to improve the wear resistance of Ti-29Nb-13Ta-4.6Zr. Oxidation treatment was found to be effective to improve the wear resistance of Ti-29Nb-13Ta-4.6Zr. The fatigue strength of Ti-29Nb-13ta-4.6Zr was improved very much by conducting solutionizing and aging, and was equal to that of Ti-6Al-4V ELI.The fatigue ratio of Ti-29Nb-13Ta-4.6Zr was greater than that of conventional β type titanium alloy, and was equivalent to that of conventional α+ β type biomedical titanium alloys. Fretting fatigue strength of Ti-29Nb-13Ta-4.6Zr was significantly smaller than that of plain fatigue strength, but *quivalent to that of conventional biomedical titanium alloys. The decrease in fatigue strength due to fretting was relatively smaller in Ti-29Nb-13Ta-4.6Zr.
Phosphate calcium crystallized glass could be easily formed on Ti-29Nb-13Ta-4.6Zr in air comparing with the case of pure titanium or Ti-6Al-4V.Biocompatibility of Ti-29Nb-13Ta-4.6Zr was significantly improved by this surface coating of phosphate calcium, crystallized glass.
Ti-29Nb-13Ta-4.6Zr, Ti-6Al-4V and SUS 316L stainless steel were implanted in the muscle near the spine of the rabbit, and then the muscle tissue change was examined. Each muscle tissue was nearly the same. Therefore, the biocompatibility of Ti-29Nb-13Ta-4.6Zr was found to be excellent.
From the results mentioned above, Ti-29Nb-13Ta-4.6Zr is strongly expected to be put into practical use. In order to put Ti-29Nb-13Ta-4.6Zr into practical use, further development in mechanical properties by microstructural control, establishment of surface treatment to improve the wear resistance, further evaluation of fatigue strength and fretting fatigue strength in simulated body environment, verifying the effectiveness of low modulus using living body, and biocompatibility test using bigger animals are needed. Furthermore clinical tests of the alloy is needed. Ti-29Nb-132Ta-4.6Zr is also expected to be applied for dental products. Therefore, the practical use of Ti-29Nb-13Ta-4.6Zr is highly expected in dental field. Less
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