Properties of Self-Expanding Nitinol Stents Treated by Addition of 600°C Heat Setting Process

Abstract

For its two unique properties of superelasticity and shape memory, nitinol is widely used in medical applications. Traditionally, most nitinol stents are normally heat set at 485°C, which induces the formation of the Ni-rich second phases such as Ti2Ni3 and Ti3Ni4, increases the austenite finish temperature and reduces material strength and fatigue performance. The addition of 600°C heat setting was applied to the nitinol stents after 485°C heat setting process and its effect on the mechanical, dimensional and functional properties of the nitinol stents were investigated. The experimental results showed that the nitinol stent tubes with the addition of 600°C heat setting process had higher elastic modulus, ultimate tensile strength, elongation percentage, inner dimension and outer dimension than those of the nitinol stents with 485°C heat setting process. Moreover, after the addition of 600°C expansion step, the nitinol stents almost had the same average deployment force and deployment accuracy as those of the nitinol stents with 485°C heat setting process and higher average inner dimension (ID) and outer dimension than that of those without the 600°C expansion step. Either treated with the addition of 600°C heat setting process or only 485°C heat setting process, the average austenite finish temperature (Af ) of the nitinol stents was both lower than the body temperature and the average Af of the fomer was 8°C lower than that of the latter.