NITROGEN PLASMA IMMERSION ION IMPLANTED NICKEL TITANIUM ALLOYS FOR ORTHOPEDIC USE

Stainless Steels, pure titanium and titanium alloys are the most standard metallic orthopaedic materials. Among the various materials, nickel-titanium shape memory alloys (NiTi) have attracted much attention due to their shape memory effect and super-elasticity, However, this alloy consists of equal amounts of nickel and titanium and Ni is a well known to cause allergy or other deleterious effects to riving tissue. The leaching of Ni ion is relatively high if the surface corrosion resistance is deteriorated. We have considered modifying the surface chemistry of this alloy with the aid of plasma immersion ion implantation. Nitrogen was implanted onto NiTi substrates. The surface chemistry of the implanted samples was examined by using X-ray photoelectron spectroscopy (XPS) and compared to that of the untreated NiTi alloys, stainless steel (SS) and pure titanium (Ti) sewing as controls. Electrochemical corrosion and nano-indentation testing were conducted to identify the corrosion resistance and surface hardness of all the samples. Immersion test was also carried to investigate the extent of Ni leaching under simulated human body conditions. In cyto-compatibility tests, enhanced green fluorescent protein (EGFP) mice osteoblasts were used for cell culture. Cell attachment was examined after the second day of culture, and cell proliferation examined after 4, 6 and 8 days of culture.

The XPS results reveal that a thin layer of TIN with higher hardness is formed on the surface after nitrogen Plll treatment. The corrosion resistance of the N Plll sample is also superior to that of the untreated NiTi and SS and comparable to that of Ti. The release of Ni ions is significantly reduced compared to the untreated Nili. The treated and untreated NiTi alloys favor osteoblast attachment and proliferation. The sample with surface titanium nitride exhibits the highest amount of cell proliferation whereas stainless steel fares the worst. Compared to coatings, the implanted structure does not delaminate as easily. With the improvement in the corrosion resistance nitrogen Plll is a viable way to improve the orthopedic properties of NiTi.