NiTi shape memory alloys are promising implant materials due to their shape memory effect and superelasticity. However, the high nickel content has hampered wider applications of the materials. Excessive surface corrosion and wear of NiTi raise health concerns because Ni ions released to body tissues and fluids can induce toxic and allergic responses. By choosing the suitable surface modification methods, the wear, and corrosion resistance as well as biocompatibility of NiTi shape memory alloy can be improved. As a surface treatment technique, plasma immersion ion implantation (PIII) has a number of advantages such as simplicity, low cost, efficiency, large area, batch processing, as well as non-line-of-sight processing making it an ideal technique for specimens with an irregular shape such as medical implants. Hence, we began the research by performing oxygen or nitrogen PIII to form titanium oxide or titanium nitride on the NiTi surface in order to suppress nickel release, improve biocompatibility, and increase wear and corrosion resistance. The objective is to make the materials suitable for orthopedic implants. NiTi samples were plasma-implanted with nitrogen at different implantation energies to form a TiN layer to enhance the wear resistance. According to our wearing tests, the wear mechanism of the implanted samples was adhesive-dominant under low applied loads but became abrasive-dominant at high applied loads. Moreover, we studied the effects of the oxide layers formed by oxygen plasma ion immersion implantation (O-PIII) and atmospheric-pressure oxidation on the wear properties of NiTi. The samples modified by O-PIII exhibited higher wear resistance of the oxide layer. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were carried out to investigate the surface electrochemical behavior of the control and H2O-PIII NiTi samples in simulated body fluids (SBF). Two different physical models with related equivalent electrical circuits were obtained to fit the EIS data and explain the surface electrochemical behavior of NiTi in SBF. The simulation results demonstrated that the higher resistance of the oxide layer produced by H2O-PIII was primarily responsible for the improvement in the surface corrosion resistance. The surface of NiTi was modified by nitrogen PIII at different parameters such as voltage and frequency. The near-surface Ni concentration was found to be significantly reduced after PIII and the formed TiN layer could suppress nickel release and favor osteoblast proliferation, especially for samples implanted at higher voltages. The nano-scale surface morphologies obtained by different implantation frequencies impacted the surface free energy and wettability of the NiTi surfaces, and in turn affected the osteoblast adhesion behavior. Nanophase materials are becoming potential alternative orthopedic implant materials. In the last part of the research, large-scale direct growth of nanostructured bioactive titanates on a three dimensional (3D) porous orthopedic NiTi scaffold via hydrothermal treatment was studied. The nanostructured titanates showed characteristics of 1D nanobelts / nanowires on a nanoskeleton layer. Besides resembling cancellous bone structure on the micro/macro scale, the 1D nanostructured titanate on the exposed surface was similar to the lowest level of hierarchical organization of collagen and hydroxyapatite. The resulting surface of porous NiTi displayed superhydrophilicity and favored the deposition of hydroxyapatite. In summary, we used plasma immersion ion implantation to modify the surface of NiTi shape memory alloy and significantly enhanced its wear and corrosion resistance as well as its biocompatibility. In addition, the nanostructured bioactive titanates were successfully fabricated on porous NiTi surface.
Author: Liu, Xiangmei
Source: City University of Hong Kong
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