The Ti-24Nb-4Zr-8Sn alloy (wt%, abbreviated as Ti2448) is a new kind of multi-functional -titanium alloy, which contains no toxic elements and possesses low elastic modulus and high strength. As a potential biomedical alloy, it should be biocompatible as well as elastically compatible when used as hard tissue repairing and substituting implant. The biocompatibility of titanium alloy mainly depends on its surface oxide layer. In this work, apatite forming ability of the oxide layer that naturally formed in air and that prepared by thermal oxidation with following hydrothermal treatment was investigated by in vitro soaking experiment, and its corrosion resistance was evaluated in simulated body environment. In addition, a glass-ceramic coating was fabricated on the surface of Ti2448 by a dip-coating method. The apatite formation mechanisms on the surface modified and glass-ceramic coated Ti2448 were discussed.
The apatite forming ability of the naturally formed oxide layer derives from the amount of surface Ti-OH group. When the grain size was refined from 100 μm to 100 nm, the number of surface Ti-OH group is significantly increased. Thus the apatite forming ability was enhanced by grain refinement. With further research, we found that both calcium and phosphorus on the surface layer accelerate apatite precipitation on the substrate. The surface layer of the sample subjected to thermal oxidation plus hydrothermal treatment consists of TiO2, CaTiO3, CaCO3 and Ca(OH)2. On one hand, a large number of Ti-OH formed via TiO2 hydroxylation when soaked in simulated body fluid. On the other hand, Ca2+ ions incorporated by the hydrothermal treatment in boiled Ca(OH)2 solution could act as nucleation sites for apatite and accelerate its formation. Both Ti-O-Ca bonding provided by CaTiO3 and the dissolution of the compounds such as CaCO3 and Ca(OH)2 into the aqueous solution which increases the concentrations of Ca2+ ions promote the formation of apatite. Therefore, apatite formed in 3 days when soaked in the simulated body fluid on the sample with the above treatment. When the sample with the glass-ceramic coating was soaked in the simulated body fluid, apatite formed via two ways, which is different from that on the surface modified sample. Firstly, a bit of Ti-OH and a large amount of calcium and phosphorous ions formed, which provided apatite nucleation sites. Secondly, β-Ca3(PO4)2 in the coating could transform to apatite during soaking. Thus, apatite formed in 7 days on the surface of glass-ceramic coated Ti2448 alloy.
In cell culture, the surface layer with thermal oxidation plus hydrothermal treatment has high cell attachment efficiency of osteoblast, similar to that of original surface of Ti2448 alloy. However, the glass-ceramic coating promotes osteoblast adhesion and proliferation as compared with uncoated alloy.
In vitro corrosion behaviour of Ti2448 alloy was investigated in phosphate-buffered saline by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results show that it has high corrosion potential and low corrosion current density, which is comparable to that of c.p. Ti. However, the corrosion current density is increased by thermal oxidation at 600 oC, and it decreased after further hydrothermal treatment. Protein adsorption on one hand blocked the attack of Cl- ions and on the other hand increased the dissolution of metal ions due to chelation which finally results in the increase of corrosion current density.
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