Nitrogen plasma-implanted nickel titanium alloys for orthopedic use

Nickel–titanium shape memory alloys (NiTi) have attracted much attention as orthopedic materials due to their shape memory effect and super-elasticity. However, this alloy consists of equal amounts of nickel and titanium and Ni is well known to cause allergy or other deleterious effects in living tissues. To improve the surface corrosion resistance and mitigate Ni leaching, we have modified the surface chemistry of this alloy with the aid of nitrogen plasma immersion ion implantation (PIII). The implanted surfaces were characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical corrosion and nano-indentation tests were conducted to assess the corrosion resistance and surface hardness. Immersion tests were carried to investigate the extent of Ni leaching under simulated human body conditions and cell cultures employing enhanced green fluorescent protein mice osteoblasts were used to evaluate the cyto-compatibility of the materials. The XPS results reveal that a thin layer of TiN with higher hardness is formed on the surface after nitrogen-PIII. The corrosion resistance of the implanted sample is also superior to that of the untreated NiTi and SS. The release of Ni ions is significantly reduced compared to the untreated NiTi and both the treated and untreated NiTi alloys favor osteoblast attachment and proliferation. The sample with surface titanium nitride exhibits the largest degree of cell proliferation whereas stainless steel fares the worst.