Fabrication Of Nanopore Layers On NiTi Alloy And The Study Of Their Corrosion Behavior And Biological Performance

Ordered Ni-Ti-O nanopore layers were prepared on the surface of nearly equiatomic NiTi alloy,which not only maintained the original superelasticity,shape memory effect,low elastic modulus and other desirable attributes,but also had a better nanobiomimetic structure,so as to improve its biocompatibility.As compared with the normally used techniques to prepare nanopores,anodization has been notable for its advantages of low cost,easy operation and environmental friendliness.In this paper,Ni-Ti-O nanopore layers were synthesized on the surface of NiTi alloy by anodization in ethylene glycol（EG）electrolyte containing Cl^-/Br^-/CO₃^2-and H₂O.SEM、EDS、TEM and XPS were employed to study the effects of the treatment on the microprocess,composition and microstructure of the nanopores,and its reaction mechanism was also discussed.The corrosion behavior and biological properties of the nanopore layers on the NiTi alloy surface were evaluated by electrochemical methods and in vitro cell and bacterial experiments.The results show that:1.In EG electrolyte containing Cl^-/Br^-/CO₃^2-and H₂O,ordered nanopore layers were constructed on the surface of the NiTi alloy by anodization.The nanopore layers were mainly composed of TiO₂ and NiO.Ni-Ti-O nanopore layers with pore diameters of 60-75 nm were produced anodically on NiTi in NaCl electrolyte.The electrolyte pH had little influence on surface morphology and composition of the nanopores,but it could significantly impact their growth rate.With the increase of the pH（pH=0.82-13.5）,a U-shaped curve could be clearly seen,namely at low and high pH values the nanopore grown fast,the minimum value was 5.7 nm/s at pH=7.0.The study also found that in EG electrolyte containing HCl,Ni-Ti-O nanopores could be prepared,with maximum length reaching up to 160μm.The nanopores with diameter concentrated at 50-60 nm could be grown in EG electrolyte containing NaBr.The nanopore layers with small diameter（25 nm）and higher Ni/Ti ratio（0.3）could be prepared in electrolytes containing Na₂CO₃.2.Ni-Ti-O nanopores layers with different length（0.55-114μm）were fabricated on the NiTi alloy in EG electrolyte containing NaCl,and the treatment could improve the corrosion resistance of the NiTi alloy.The anodized samples released more Ni²⁺than that of the bare NiTi alloy,suggesting chemical dissolution of the nanopores rather than electrochemical corrosion governed the Ni²⁺release.In addition,the Ni²⁺release amount increased with nanopore length.The nanopores with length in the range of 1-11μm showed good cytocompatibility and antibacterial ability.3.The effects of annealing on corrosion behavior,Ni²⁺release,cytocompatibility,and antibacterial ability of Ni-Ti-O nanopore layers were studied.For the nanopore layers grown in NaCl-containing electrolyte,when the annealing temperature was less than 600 ^℃,the nanopore layers could be well preserved on the substrate surface,while for them grown in NaBr-containing electrolyte,it could still fully cover the substrate when the annealing temperature was less than 800 ^℃.Annealling at 400°C led to the transformation of amorphous phase to anatase and 600 ^℃ produced rutile.Annealing at 200 ^℃ significantly enhanced the corrosion resistance and 400 ^℃ drastically reduced Ni²⁺release,but their cytocompatibility had no appreciable difference,indicating the release level of Ni²⁺was well tolerated by osteoblasts.Although the release amount of Ni²⁺was reduced after annealing especially for the sample annealed at 400 ^℃,their antibacterial ability was even better when compared with that of unannealed sample.These results suggested the nanopore layers annealed at 200-400°C were promising as coatings of biomedical NiTi alloy.4.The Ni-Ti-O nanopores layers with different lengths（0.92-10.1μm）were prepared in NaCl-containing electrolyte,biological results showed that the nanopore layers of 10.1μm in length could effectively inhibit the growth of cancer cells and kill bacteria,but it had little adverse effects to normal cells,it is supposed to be related to increased Ni²⁺release induced by acidic microenvironment generated by tumor cells.