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 medical device and delivery system, comprising:
a shape memory alloy element having a restrained shape, a deployed shape and an associated M d temperature, the shape memory alloy element having been deformed at or above M d into an at least partially deformed configuration; and
a hollow delivery system holding the at least partially deformed shape memory alloy element in an austenitic state in its restrained shape,
wherein the shape memory element self-expands into the deployed shape when not held by the delivery system.
2. The medical device and delivery system of claim 1 , wherein the shape memory alloy includes nickel and titanium.
3. The medical device and delivery system of claim 1 , wherein the medical device includes a stent.
4. The medical device and delivery system of claim 1 , wherein the delivery system includes a catheter.
5. The medical device and delivery system of claim 1 , wherein the shape memory alloy does not generate stress-induced martensite from being at least partially deformed into a restrained shape.
6. The medical device and delivery system of claim 1 , further comprising a restraint configured to receive the shape memory alloy at a temperature at or above M d , wherein the shape memory alloy is a tubular member with proximal and distal ends and the restraint is secured around the shape memory alloy in its deformed configuration.
7. The medical device and delivery system of claim 1 , wherein the shape memory alloy is for use within a mammalian body or in close proximity to the mammalian body such that the device is at mammalian body temperature.
8. The medical device and delivery system of claim 7 , wherein the mammalian body temperature is about 35 to 40 degrees C.
9. The medical device and delivery system of claim 1 , wherein the shape memory alloy has an austenite start temperature (A s ), an austenite finish temperature (A f ), and a martensite deformation temperature (M d ).
10. The medical device and delivery system of claim 9 , wherein the shape memory alloy includes the martensite deformation temperature (M d ) of about 50 degrees C. or more above the austenite finish temperature (A f ).
11. The medical device and delivery system of claim 9 , wherein the shape memory alloy element self expands at a temperature above A f and below M d .
12. The medical device and delivery system of claim 1 , wherein the shape memory alloy is in an austenitic state at room temperature of about 22 degrees C.