Avoiding stress-induced martensitic transformation in nickel titanium alloys used in medical devices
View Patent ↗A process for assembling a medical device made from a nickel-titanium alloy for use in a mammalian body while avoiding the formation of stress-induced martensite and a medical device used in combination with a delivery system for deployment into the mammalian body are disclosed. By heating the nickel-titanium alloy of the medical device to a temperature above M d , and deforming and installing the device into a delivery system or holding capsule, it is possible to avoid the formation of stress-induced martensite in the stent, which stays in the austenitic phase throughout.
1. A process for deforming a medical device including a shape memory alloy element having a martensite deformation temperature (M d ), wherein the element includes a restrained shape and a deployed shape for use in a mammalian body, comprising:
heating the shape memory alloy element to a temperature at or above M d , wherein M d is greater than a body temperature of the mammalian body;
at least partially deforming the heated element into the restrained shape; and
applying a force to maintain the element in the restrained shape.
2. The process of claim 1 , further comprising positioning the medical device including the element at least partially inside a hollow delivery system to hold the element in the restrained shape.
3. The process of claim 2 , further including cooling the medical device inside the hollow delivery system to room temperature.
4. The process of claim 1 , wherein the shape memory alloy is in an austenitic state at room temperature of about 22 degrees C.
5. The process of claim 1 , wherein the shape memory alloy includes nickel and titanium.
6. The process of claim 1 , wherein the medical device includes a stent.
7. The process of claim 1 , wherein the step of at least partially deforming the heated element into a restrained shape does not generate stress-induced martensite.
8. The process of claim 1 , wherein the hollow delivery system includes a catheter.
9. The process for deforming a medical device of claim 1 , wherein the heating is performed by a light source.
10. The process for deforming a medical device of claim 1 , wherein the heating is performed by heated fluid.
11. The process for deforming a medical device of claim 1 , wherein the heating is performed by ultrasonic vibration.
12. The process for deforming a medical device of claim 1 , wherein the heating is performed by a current source connected to the shape memory alloy element.
13. The process for deforming a medical device of claim 1 , wherein the heating is through rolling nitinol stent on a warm plate.
14. The process for deforming a medical device of claim 1 , wherein M d is about 50 degrees C. or more than an austenite finish temperature (A f ) of the shape memory element.
15. The process for deforming a medical device of claim 1 , further comprising self-expanding the shape memory element from the restrained shape to the deployed shape within the mammalian body.