Suppression of surface nickel concentration of nickel titanium shape memory alloys by plasma surface treatment

Abstract

Nickel-titanium shape-memory alloys (NiTi) has very good potential for surgical implantation due to two unique properties: super-elasticity and shape memory effect. These advantages cannot be seen in current biomedical metallic materials. However, nickel ion release remains a major concern particularly for large implants placed on the spine, as fretting is always expected at such implant junctions. We have invented an advanced surface treatment using plasma immersion ion implantation (PIII) technology to tackle this issue. This paper describes the efficacy of nickel suppression in NiTi alloys by using plasma surface treatment. NiTi discs with 50.8% Ni were treated by nitrogen, oxygen and carbon PIII at 40kV with the frequency of 200Hz. The treated samples were then subjected to chemical analysis by X-ray Photoelectron Spectroscopy (XPS) and electrochemical corrosion testing by potentiometer. After completing the corrosion test, the solutions were extracted for ICPMS analysis so as to investigate the amount of Ni release. Cytocompatibility of PIII treated samples using mouse osteoblasts and in-vivo compatibility were also examined. By reading the XPS results, the surfaces were enriched by titanium nitride (TiN), titanium oxide (TiO) and titanium carbide (TiC) after PIII treatments, respectively. The surface Ni concentration of all treated samples was significantly suppressed. To compare with the untreated sample, the corrosion properties of the plasma treated samples had been improved by a factor of five. The leached Ni levels from the treated samples were undetectable, whereas that from the untreated samples was 30ppm. Cells were well tolerated on plasma treated and untreated samples at Day 7 of culturing. Although there was no significant difference in cell proliferation on either surfaces, the in vivo bone formation was found to be better on the nitrogen and carbon treated surfaces at every time points. Therefore, the treated NiTi is promising orthopedic implantation without inducing harmful effects. SUMMARY: Nickel-titanium shape memory alloys (NiTi) has very good potential for surgical implantation due to two unique properties: super-elasticity and shape memory effect. These advantages cannot be seen in current biomedical metallic materials. However, nickel ion release remains a major concern particularly for large implants placed on the spine, as fretting is always expected at such implant junctions. We have invented an advanced surface treatment using plasma immersion ion implantation (PIII) technology to tackle this issue. This paper describes the efficacy of nickel suppression in NiTi alloys by using plasma surface treatment.