Enhancement of biocompatibility of nickel-titanium by laser surface modification technology

Abstract

Nickel Titanium shape memory alloys (SMA) is a relatively new biomaterial that has attracted immense research interest for biomedical application. The combination of good biocompatibility, good strength and ductility with specific functional properties of shape memory effect, damping capacity and superelasticity creates a smart material for medical applications. However, there are still concerns on nickel ion release of this alloy if it is going to be implanted for a long time. Nickel ion is carcinogenic and also causes allergic response and degeneration of muscle tissue. The subsequent release of Ni+ ions into the body system is fatal for the long term application of this alloy in the human body. To improve the long term biocompatibility and corrosion properties of NiTi, different surface treatment techniques have been investigated by other researches but no optimum technique has been established yet. This project will investigate the feasibility of applying laser surface alloying technique to improve the corrosion resistance and biocompatibility of NiTi in simulated body fluid condition. This thesis summarizes the result of laser surface modification of NiTi with Mo, Nb and Co using CO2 laser. The microstructure, chemical composition, surface morphology, hardness, corrosion resistance, nickel release rate, wettability, bone like apatite formation and cell adhesion behavior of the surface alloyed layer were analyzed using scanning electron microscopy (SEM), Energy Dispersive Analysis by X-rays (EDAX), X-ray diffractometry (XRD), Vicker's microhardness, polarization tests, atomic absorption spectrometry, sessile drop technique, immersion test and cell adhesion analysis. The modified layer, which is free of microcracks and pores, acts as physical barrier to reduce nickel release and enhance the surface properties. The hardness values of the Mo-alloyed NiTi, Nb-alloyed NiTi and Co-alloyed NiTi surface were found to be three to four times harder than the NiTi substrate. Corrosion polarization tests also showed that the Mo-alloyed NiTi, Nb-alloyed NiTi and Co-alloyed NiTi are significantly more resistant than the NiTi alloy. The release of Ni ions can be greatly reduced after laser surface alloying NiTi with Mo, Nb and Co. The results might be of great importance to improve the long-term biocompatibility properties of the material and to reduce sensitization to Ni and allergies. The improvement in wettability characteristics, the growth of the apatite on the specimen's surface and the adhesion of cell confirm the good biocompatibility after laser surface alloying. It is concluded that laser surface alloying is one of the potential technique not only to improve the corrosion resistance with low nickel release rate, but also retain the good biocompatibility of NiTi. The technique can be applied to bone fixation plates or implants with relatively large surface area. The results of this project are significant as they add new knowledge on the surface modification of NiTi for long term implant application.