Effect of annealing temperature, time and cooling/heating rate on BFR curve and radial force of nitinol wire stent

Obtaining a proper hysteresis characteristic in nitinol stent is an important factor for defining fatigue life and performance of stent inside the body. In this research we demonstrate the variation in radial force and BFR (bend and free recovery) curve with changing heat treatment parameters in cold worked nitinol wire stent. Effect of heat treatment temperature and annealing time in controlling RF (radial force) has been studied in a range of 300-650°C. Radial force and BFR curve have been investigated in different cooling and heating rates at two different heat treatment temperatures of 550°C and 480 °C. result shows that the RF curve directly affected by heat treatment parameters. Different heat treatment result in different phase transformation temperatures and stress required for sliding. Heating rate had an obvious influence on RF at 480°C while its effect was insignificant at 550°C. The cooling rate was an important parameter for defining nitinol characteristic, besides annealing time and temperature.