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The Importance of Transformation Temperature Testing of Nickel Titanium Wire used in Medical Devices, Part 1
Nitinol alloys enjoy an ever increasing popularity as a proven biomaterial for the manufacture of medical devices, primarily due to its unique shape memory and superelastic properties. Stents (vascular and nonvascular), along with other devices, rely on Nitinol’s transformation temperatures for their performance inside the human body.
Controlling the transformation temperatures is critical to how the part will function and is particularly important when discussing implantable devices, as it plays a key role in fatigue resistance.
Transition temperatures will be affected by wire processing, and then again by the final device heat treatment. Vital to understanding and controlling the device performance is testing these transformation temperatures both at the ingot level (raw material) and in its finished device form. Device designers and manufacturers need to create specifications that contain requirements for the material at both levels.
On a Nitinol wire certification, unlike other alloys, simply checking that the chemistry requirement is met is not sufficient to ascertain its suitability for the application. Transformation temperatures, at the ingot level and, if applicable, at the wire level must also be considered critical to application.
Testing the thermal properties of Nitinol alloys requires expertise and knowledge of materials properties and handling methods, factors that are not widely known and understood in the market today.
US BioDesign is a bioengineering company focused exclusively on designing and manufacturing components for medical devices. Their technical team has extensive knowledge and experience in the field of testing and selecting the appropriate grade of Nitinol wire for braided medical applications.
Ingot Transformation Temperatures
Nickel titanium alloys present themselves in two crystal structures: Martensite and Austenite. When Nitinol wire has not been heat treated, it is in a Martensite phase, the less stable of the two and the material does not present shape memory or superelastic properties. Following a heat treatment under tension (also called a partial anneal) that brings the onset of the phase transformation, the material will be in its Austenite phase – stronger, more stable. The transition between these two phases is both instantaneous and reversible over multiple cycles.
Even though the ingot has not been through the wire drawing and heat treatment process and therefore has no supereleastic characteristics, it does have inherent transformation temperatures that stem from the melting and hot working processes. ASTM F-2004 sets forth the appropriate testing for Nitinol alloys in the ingot form, and it is important for device designers to specify raw material testing in accordance with the standard.
For Nitinol alloys, solely specifying chemical composition does not guarantee material performance. Transformation temperatures are very sensitive to the nickel/titanium ratio and the alloy formulation is difficult to measure to the degree needed to quantify these temperatures. Fig. 1, below, shows the relationship between chemistry and transformation temperatures and makes it clear that even the slightest change in nickel composition can drastically change the transformation temperature of the material.