Application of Ti-Ni Alloy Wire in Medical Devices and Surgical Implant Technology
As a new type of intelligent material, with the development of its research and application, the international industrial developed countries have formulated the material and test standards of memory alloy to standardize the material production and industrial application. Table 1 shows the most influential American standard ASTM F0263 (Technical Specification for Machinable TiNi Shape Memory Alloys for Medical Devices and Surgical Implants) in the field of shape memory alloy materials. In view of the fact that there is no standardized national technical standard for memory alloy materials in China, the national "Tenth Five-Year Plan" period has begun to draft the national standard "Technical Specification for NiTi Shape Memory Alloy Processing Materials for Medical Devices and Surgical Implants" and "Medical Memory Alloy Terminology", "Technical Specification for Non-vascular NiTi Memory Alloy Interventional Stents" and other industry technical standards, and other relevant standards will be gradually incorporated into the preparation plan.
The anisotropy of TiNi binary shape memory alloy can be summarized as the following five points:
(1) Shape memory effect. The one-way memory effect of polycrystalline TiNi alloy can reach about 8%. In addition to the one-way memory effect, TiNi alloy can also produce two-way and full-way memory effect after certain heat treatment (or training), such as multiple heating-cooling cycles, constrained heating and aging, etc. The two-way shape memory effect has good stability, up to 6%. When the strain is less than 1%, the number of two-way memory cycles can reach millions.
(2) hyperelasticity. From the stress-strain curve, superelasticity can be divided into linear superelasticity and nonlinear superelasticity, nonlinear superelasticity is also called phase change pseudo-elasticity, TiNi alloy superelasticity up to 8%.
(3) damping characteristics. Damping is a measure of the material's absorption of shock energy, and shape memory alloys absorb energy due to the self-coordination of martensite transformation and the anelastic migration of various interfaces (twin planes, phase interfaces, variant interfaces) formed in martensite, thus having good damping characteristics.
(4) Corrosion resistance and biocompatibility. A large number of biochemical experiments on corrosion resistance, biocompatibility, cytotoxicity, carcinogenicity, hemolysis and sensitization at home and abroad show that TiNi alloy has better biocompatibility and corrosion resistance than stainless steel and cobalt-chromium alloy, and is similar to pure Ti.
(5) better mechanical properties. The yield strength of TiNi alloy at room temperature is about 200MPa,δ = 15%,αk = 38J/cm, and the fracture strength is 800 ~ 950MPa. At the same time, it has good plastic strain capacity and can be processed into very fine wires and thin plates (diameter or thickness <100 μm).