The Effect of Radio Frequency Plasma Nitriding on Biocompatibility of Nickel-Titanium Shape Memory Alloys

Nickel-titanium (Ni-Ti) shape memory alloys (SMAs) are attractive materials for orthopedic and dental implants, due to its two intrinsic properties including shape memory effect (SME) and superelasticity (SE), which may not be found in other commonly-used surgical metals. Possible Ni ion release, however, hampers their medical applications, particularly in orthopedic implants where fretting is always expected at the articulating surface. Inductively coupled radio frequency plasma (ICRFP) was employed to alter the surface of equiatomic Ni-Ti discs in order to create a barrier to out-diffusion of Ni ions from the bulk material. The ICRFP experiments were carried out with nitrogen, which promoted the formation of a titanium nitride layer on the surface of the Ni-Ti samples. The present paper explores the biocompatibility and performance of the ICRFP treated and untreated Ni-Ti samples. The results confirm that nitrogen plasma modified Ni-Ti alloys are potentially suitable materials for orthopedics without inducing harmful biological effects.