Long term cytocompatibility and in vivo investigation of nitrogen plasma implanted shape memory alloy

Abstract

Nickel ion release in nickel-titanium (NiTi) shape memory alloy impedes its clinical applications in particular to orthopaedic implants in which fretting is always expected at the implant junction. High level of nickel is toxic to the surrounding biological tissues. Therefore, efforts have been made to deal with this concern by applying various surface treatments. Someone has even developed nickel-free shape memory alloys. Previously, we successfully demonstrated the enhancement of biological and surface mechanical properties of NiTi alloys by using plasma immersion ion implantation (PIII) technology. However, the long-term biological effects of these plasma treatments are unknown. This paper therefore characterizes the effect of nickel release upon long-term simulated body fluid (SBF) immersion test to cytocompatibility and in-vivo behavior of PIII treated and untreated samples. NiTi discs with 50.8% Ni were treated by nitrogen PIII at 40kV with 100Hz. Long-term biological tests including SBF immersion, alkaline phosphatase (ALP) activity measured by reverse transcription – polymerase chain reaction (RT-PCR) and in-vivo animal study were performed up to 12 months. With the use of inductively coupled plasma mass spectrometry analysis, the nickel amount of nitrogen treated samples was found less than the untreated after 3 and 6 months SBF immersion. For the results of 12 months, the release of untreated was stabilized and no different as compared with nitrogen treated sample. In long-term enhanced green fluorescent protein mouse osteoblast cell culturing, ALP activity of nitrogen sample exhibited no difference in nitrogen, untreated and medical grade titanium alloy at all time points. Although no significant difference was found in such in-vitro tests, the in vivo bone formation was found to be better on the nitrogen treated surfaces at every time points. The in vivo bioactivity of nitrogen treated sample was comparable to the medical grade titanium alloy. SUMMARY: Nickel ion release in nickel-titanium (NiTi) shape memory alloy impedes its clinical applications in particular to orthopaedic implants in which fretting is always expected at the implant junction. High level of nickel is toxic to the surrounding biological tissues. This paper therefore characterizes the effect of nickel release upon long-term simulated body fluid (SBF) immersion test to cytocompatibility and in-vivo behavior of PIII treated and untreated samples.