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The aim of this investigation was to enhance the biological behavior of NiTi shape memory alloy while preserving its super-elastic behavior in order to facilitate its compatibility for application in human body. The surfaces of NiTi samples were bombarded by three different nitrogen doses. Small-angle X-ray diffraction was employed for evaluating the generated phases on the bombarded surfaces. The electrochemical behaviors of the bare and surface-modified NiTi samples were studied in simulated body fluid (SBF) using electrochemical impedance and potentio-dynamic polarization tests. Ni ion release during a 2-month period of service in the SBF environment was evaluated using atomic absorption spectrometry. The cellular behavior of nitrogen-modified samples was studied using fibroblast cells. Furthermore, the effect of surface modification on super-elasticity was investigated by tensile test. The results showed the improvement of both corrosion and biological behaviors of the modified NiTi samples. However, no significant change in the super-elasticity was observed. Samples modified at 1.4E18 ion cm(-2) showed the highest corrosion resistance and the lowest Ni ion release.

Natural hydroxyapatite (HA) was coated on NiTi alloy using electrophoretic deposition method to improve the corrosion resistance and biocompatibility. Coating process was performed at 120 s in various applied voltages of 40, 60, and 80 V. Sintering process was done at 800 8C under inert gas for 2 h. Electrochemical behavior of the coated samples was investigated in simulated body fluid by using electrochemical impedance spectroscopy and polarization tests. Furthermore, the nickel ions release from NiTi and HA coated-NiTi samples were analyzed by atomic absorption analysis. Ultimately, scanning electron microscopy micrographs and X-ray diffraction patterns were used to evaluate the morphology and phase analysis of HA coatings. The results show that the sample coated at 60 V reveals a uniform, dense coating accompanied with a higher corrosion resistance. Moreover, after 4 weeks, nickel ions release was reduced to 0.205 mg/cm 2 for coated sample at 60 V.

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