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Highly biocompatible superelastic titanium alloys for bone implants

Высокобиосовместимые сверхупругие титановые сплавы для изготовления костных имплантов
Titanium-based alloys are widely used as biomedical materials due to their unique properties complex: high strength, low density, superior corrosion resistance and biocompatibility. One of the most important fields of titanium alloys’ application is implantology, including orthopaedic, maxillofacial and dental areas. Material for bone implants must meet the requirements of biomechanical and biochemical compatibility with bone tissue. The material’s mechanical properties must be comparable with those of living tissue, i.e. have Young’s modulus of 10 – 40 GPa and exhibit superelastic behaviour no less than 0.5%. All the alloy constituent elements must be safe and approved for biomedical application. The material must be highly stable in aggressive human body media. One of the most common materials for implants is pure titanium; its advantages are good mechanical properties, low density and high biocompatibility due to protective bioinert TiO2 film. At the same time its mechanical characteristics are poorly compatible with bone. In particular, titanium has much higher Young’s modulus (105 GPa). This leads to disturbance of mechanobiological balance in bone-implant system resulting in bone resorption and weakening of implant fixation. Titanium nickelide (Nitinol, Ti-Ni) is a perspective material for implants. It exhibits shape memory and superelastic behavior which significantly enhance its biomechanical compatibility, however it contains carcinogenic nickel; Ni ions may yield in body media in case of oxide film failure. It is also important Nitinol is a chemical compound, so its properties are strongly dependent on composition which usually cannot be guaranteed more precisely than up to 0.2%. Thus Nitionol is not used for implant production. Nowadays many works on development Ni-free titanium shape memory and superelastic alloys are carried out, which underlies the importance of the present scientific direction. Such alloys include Ti-Nb-Ta and Ti-Nb-Zr systems which exhibit the required mechanical behavior due to reversible martensitic transformation β (BCC) ↔ α'' (orthorhombic lattice). Research and development of these alloys became very intense during the past decade. However, the most published works are based on results obtained on small specimens that are not suitable for practical application. Moreover, the systematic data on their corrosion and electrochemical behaviour are lacking. So the main objective of the present study is development of massive (more than 5 kg) ingot production technology and study of their performance characteristics (biomechanical and biochemical compatibility with bone tissue).


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