| Biocompatibility evaluation of nickel-titanium shape memory metal alloy: | ||
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The surface of NiTi consists mainly of titanium oxides (TiO2) and smaller amounts of nickel oxides (NiO and Ni2O3) and metallic Ni, while nickel-titanium constitutes the inner layer (Hanawa 1991, Oshida et al. 1992, Endo 1995a,b, Shabalovskaya 1996, Yahia et al. 1996). The thickness of the oxide layer varies within 2-20 nm. Depending on the preparation method, the surface chemistry and the amount of Ni may vary over a wide range (Trigwell et al. 1997). The surface of untreated NiTi is mostly composed of oxygen, carbon, and titanium oxide with traces of nickel. Nickel may dissolve more easily than titanium because its oxide is not so stable. Surface layers of nickel-titanium arch wires have been found to have irregular features characterized by lengthy island-like structures, where selective dissolution of nickel may occur (Oshida et al. 1992).
Shabalovskaya (1996) found that when the surface was mechanically polished, the Ti/Ni ratio was 5.5, showing that there was five times more titanium on the surface. When the item was boiled or autoclaved in water, the concentration of Ni decreased and the Ti/Ni ratio increased up to 23.4-33.1. The findings of Hanawa (1991) were similar. The Ti/Ni ratio was 5.8 in polished samples, but increased even up to 91 when the sample was immersed for 30 days in a neutral electrolyte solution. In the above mentioned study by Hanawa, the Ti-6Al-4V surface had similar amounts of aluminum as NiTi had nickel, even though the bulk material of Ti-6Al-4V had only 6% aluminum and NiTi had 50% nickel. There was no nickel on the surface of stainless steel, but its surface had Cr and Fe.
Pure titanium and some of its alloys are considered to be among the most biocompatible materials (Albrektsson et al. 1981). The good biocompatibility is thought to be due to the stable titanium oxide layer. During implantation, the oxide layer formed on a Ti implant grows and takes up minerals and other constituents of biofluids, and these reactions, in turn, cause remodeling of the surface. Hanawa found that the surface oxide film on implants consisted two layers, calcium phosphate and titanium oxide. In other words, calcium phosphate formed on a passive oxide film. This film was thicker on pure titanium than on titanium alloys (including NiTi), and the Ca/P value of the film was close to that of hydroxyapatite. The calcium phosphates formed on NiTi or Ti-6Al-4V were less similar to hydroxyapatite. The presence of nickel in the surface film on NiTi and that of aluminum in the film on Ti-6Al-4V may have caused these results. Stainless steel also has a calcium phosphate layer of this kind. However, the formation of this layer is slower and differs in this respect from NiTi (Hanawa et al. 1991, Hanawa 1991, Hanawa et al. 1998).
The good biocompatibility of NiTi and other titanium alloys may be the cause of the calcium phosphate film, while corrosion resistance is the cause of the passive oxide film (Hanawa et al. 1991). The findings of Hanawa were also supported by a recent study by Wever et al. (1998). As far as implantation is concerned, more surface studies are certainly needed to clarify the basic surface structures in vivo.