Surface Modification Of NiTi Shape Memory Alloy For Medical Applications By Chemical Methods

Surface modifications of NiTi shape memory alloy (SMA) were investigated by chemical methods including low temperature chemical oxidation and alkali bioactive treatment. And a new composite bioactive technology combining chemical oxidiaton and alkali treatment was proposed and studied. The effects of surface modifications on the surface microstructure and properties of NiTi SMA were investigated systemically by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, simulated body fluid soaking test, hemolysis experiment and cell compatibility test.It was found that the low temperature oxidation of NiTi SMA in H₂O₂ solution resulted in the formation of the oxide scale depleted in Ni and enriched with Ti-OH groups. The remnant Ni in the scale was still present as intermetallic NiTi phase. The scale was mainly composed of low crystallization TiO₂ with rutile and anatase structure, which would benefit to improve the biocompatibility of NiTi SMA. Elementary investigation of oxidation kinetics indicated that in the low-temperature oxidation process, the mass variations of NiTi SMA were dependent on both temperature and H₂O₂ concentration.A bioactive sodium titanate (Na₂TiO₃) film with low crystallization and a trait of Ni₂O₃ phase formed on chemically-polished NiTi substrate after direct NaOH treatment. Depth profiles of O, Ni, Ti and Na show the sodium titanate bioactive layer possesses a smooth graded interface structure to NiTi substrate, which benefits to increase their bonding strength. The results indicate that an ideal bioactive NiTi SMA with a graded surface structure can be obtained by alkali treatment.The composite bioactive treatment by H₂O₂ solution and subsequent NaOH solution leads to the formation of a bioactive sodium titanate/titania composite aqueous gel film. Compare to direct alkali treatment, the composite bioactive treatment can result in the direct creation of more Ti-OH groups and the decrease in the amount of Ni₂O₃, Na₂TiO₃ and remnant NiTi phases. Simulated body fluid soaking test reveals that the induction period of apatite formation was shortened by further improving the bioactivity of NaOH-treated NiTi SMA via the H₂O₂ pretreatment. As a result, an ideal apatite coating can be obtained on NiTi SMA after the composite bioactive treatment in a short term.The above chemical methods could remarkably improve the hydrophile capability of NiTi SMA. The cell compatibility experiment further proved that surface modification by the chemical methods was in favor of cell adsorption and multiplication on NiTi SMA surfaces. The toxicity of the modified sample surface reached 0 level. Hemolysis experiment indicated that the hemolysis rate of NiTi SMA was remarkably decreased after chemical modification treatment (<1%), which meets the requirement of the medical implant (<5%).