The corrosion resistance of nanostructured nitinol (NiTi) was investigated using long-term tests in solutions simulating physiological fluids at static conditions, reflecting the material structure and metal concentration in the solutions. Mechanical polishing reduced the ion release by a factor of two to three, whereas annealing deteriorated the corrosion resistance. The depassivation and repassivation of nitinol surfaces were considered. We found that nanostructured nitinol might increase the corrosion leaching of titanium into solutions, although the nickel release decreased. Metal dissolution did not occur in the alkaline environment or artificial plasma. A Ni-free surface with a protective 25 nm-thick titanium oxide film resulted from soaking mechanically treated samples of the NiTi wire in a saline solution for two years under static conditions. Hence, the medical application of nanostructured NiTi, such as for the production of medical devices and implants such as stents, shows potential compared with microstructured NiTi.

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A technology for obtaining materials from nanostructured nitinol with titanium- or tantalum-enriched surface layers was developed. Surface layers enriched with titanium or tantalum were shown to provide a decrease in the formation of reactive oxygen species and long-lived protein radicals in comparison to untreated nitinol. It was determined that human peripheral vessel myofibroblasts and human bone marrow mesenchymal stromal cells grown on nitinol bases coated with titanium or tantalum-enriched surface layers exhibit a nearly two times higher mitotic index. Response to implantation of pure nitinol, as well as nano-structure nitinol with titanium or tantalum-enriched surface layers, was expressed though formation of a mature uniform fibrous capsule peripherally to the fragment. The thickness of this capsule in the group of animals subjected to implantation of pure nitinol was 1.5 and 3.0-fold greater than that of the capsule in the groups implanted with nitinol fragments with titanium- or tantalum-enriched layers. No signs of calcinosis in the tissues surrounding implants with coatings were observed. The nature and structure of the formed capsules testify bioinertia of the implanted samples. It was shown that the morphology and composition of the surface of metal samples does not alter following biological tests. The obtained results indicate that nano-structure nitinol with titanium or tantalum enriched surface layers is a biocompatible material potentially suitable for medical applications.

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New nanostructural shape memory alloy (55.91 wt% of Ni and 44.03 wt% of Ti) for the production of minimally invasive implantation medical devices (stents) was tested for corrosion resistance under static conditions by dipping it into solutions with various acidities (pH from 1.68 to 9.18) for 2 years, for static mechanical properties and for biocompatibility. The material for investigations was 280-μm wires before and after thermal treatment at 450°C for 15 min in air and surface mechanical treatment. The characteristic image and size of grains were determined using the transmission electron microscope (TEM), and the phase composition; surface morphology; and the layer-bylayer composition were investigated using an X-ray diffractometer; a scanning electron microscope (SEM); and an Auger spectrometer. The nickel release from the investigated nanostructural nitinol is less in comparison with data for microstructural nitinol in a solution of any acidity. Dissolution in the alkali medium is absent. A significant retardation of the nickel ion release (and insignificant concentration as a whole) and the absence of titanium ion release in the weakly acidic and neutral solutions with polished samples are observed. A simultaneous 7-11% increase in strength and plasticity in comparison with microstructural nitinol was attained. Toxicity of samples has not been revealed.

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