Formation of CAPO4 and Suppression of Ni Leaching in Nitinol Using Oxygen and Sodium Plasma Immersion Ion Implantation

Abstract

Orthopaedic applications of nitinol have been hampered by the high toxic nickel content. Our previous studies demonstrate oxygen plasma immersion ion implantation (PIII) can mitigate nickel leaching and enhance the corrosion resistance. However, the oxygen-implanted layer does not bond well to bones in vivo, thereby leading to potential mechanical failure at bone-implant interface. Sodium PIII may enhance the surface bioactivity of titanium and this study investigates the feasibility of apatite formation and enhancement of corrosion resistance of nitinol using combined Na and O PIII. Nitinol discs are implanted with oxygen plasma and some samples are subsequently treated by sodium plasma. The elemental depth profiles and chemical composition are determined by X-ray photoelectron spectroscopy, and the bioactivity and cytotoxicity are assessed by immersion tests in simulated body fluids and cell cultures respectively. The SEM and EDS spectra indicate the both treated surfaces can attract Ca and P deposition after SBF immersion. The amount of CaPO4 deposited on the Na-PIII surface is lower than that on the Na&O-PIII sample. The corrosion resistance of Na&O PIII sample increases about 3 folds as compared with the untreated nitinol. The cell attachment tests indicate the cells seeded onto both treated nitinol spread more and the number of osteoblasts attached on the Na&O-PIII sample is significantly higher than the untreated one. In our experiments, energetic Na ions are implanted, thereby changing the chemical composition and surface morphology of the substrate. In summary, it suggests these plasma treatments may contribute to the biological performance.