Corrosion Kinetics of Laser Treated NiTi Shape Memory Alloy Biomaterials
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F. VILLERMAUX *, I. NAKATSUGAWA**, MEUNIER ****, L'H. YAHIA *
M. TABRIZIAN *, D.L. PIRON ***,
M.
Biomaterials-Biomechanics Research Group, Institute of Biomedical Engineering, Ecole Polytechnique of Montreal, C.P. 6079, Succ centre-ville, Montreal, Quebec, Canada, H3C 3A7 •* Institute of Magnesium Technology, 357 rue Franquet, Ste-Foy, Quebec, Canada, GIP 4N7 *** Department of Metallurgy, Ecole Polytechnique of Montreal, C.P. 6079, Succ centre-ville, Montreal, Quebec, Canada, H3C 3A7 **** Department of Engineering Physics, Ecole Polytechnique of Montreal, C.P. 6079, Succ centre-ville, Montreal, Quebec, Canada, H3C 3A7 *
ABSTRACT NiTi shape memory alloy presents interesting mechanical properties as surgical implants. However, due to its high amount of Ni which may dissolve and release toxic ions in human fluids, the medical use of this material is a great concern. We have developed a laser treatment which modifies the oxide layer and enhances uniform and localised corrosion resistance of NiTi alloy. In this paper we further analysed the effect of this treatment with potentiostatic and AC impedance measurements in physiological Hank's solution. We conclude that the laser treatment creates a stable passive film which results in improved corrosion resistance of this alloy.
INTRODUCTION The use of NiTi shape memory alloy (SMA) as biomaterial is steadily increasing because of its properties of superelasticity and shape memory. NiTi implants in orthopaedics can be used as a substitute to complicated traditional devices. Screws, bolts and cements are replaced by simple apparatus such as staples and nails providing, after shape recovery, compressive stresses which results in accelaration of the healing process 1'2 . In North America, medical application of such an alloy is not recommended by Canadian health organisations. However, scientists have begun to evaluate the biocompatibility of these alloys in simulating human body media and also in vivo biocompatibility by animal implantation. Some in vitro studies showed that NiTi biocompatibility was close to the one of either Ti, Co-Cr, or stainless steel 3' 4 . In addition, some authors also found that NiTi was not toxic for bone contact5. However, Assad et al. 6 and Berger-Gorbet et al. 7 revealed that NiTi plates induced in vitro a decrease of cell proliferation, and that some NiTi screws presented in vivo cytotoxic effect on bone cells as compared to the effect of stainless steel or Ti6A14V. Because of the high concentration of Ni in NiTi alloys, clinicians hesitate to use them. Ni could be involved in allergic, toxic and even carcinogenic reactions due to its release by corrosion process in the human fluids 4 '41. Even if NiTi generally presents a corrosion level between that of Ti6Al4V and stainless steel1 °, it is characterized by a very unreproducible pitting resistance1 '1 2, which could render this alloy less safe for medical applications.
477 Mat. Res. Soc. Symp. Proc. Vol. 459 01997 Materials Research Society
As the corrosion resistance, and thus the biocom
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