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The Importance of Transformation Temperature Testing of Nickel Titanium Wire used in Medical Devices, Part 4

Finished Device Testing

“Finished Product” is defined as material that will undergo no further processing. In the case of braided nitinol devices, final properties are achieved after final heat treatment. Active Af temperature is a very good indicator of other properties, such as lower plateau strength, and can be used as a benchmark for product confirmation.

Testing transformation temperatures in the finished product stage will ideally be representative of the functionality of the device in the body. Bend and free recovery (BFR) active Af testing is the most recommended method of assessing finished product transformation temperatures.

ASTM F-2082, “Standard Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery”, is the current standard that covers this test. Not all of the provisions of this standard apply to finish device testing, and customized tooling may be needed to perform the
test in a way that is fully compliant with the standard.

Many industry players utilize DSC testing to evaluate transformation temperatures in the device’s finished state, but there are reasons not to:

  1. Sample size is very small, and may not be representative of the entire device
  2. Does not account for dynamic shapes like a stent as it functions inside a blood vessel
  3. Material preparation for the testing is critical and requires specialized tooling

However, DSC testing can be an excellent tool for testing nitinol devices throughout various stages of product development and manufacturing, prior to the final finished product stage. Interim DSC testing can help ensure the braided nitinol component is processed in a way that ensures interim active Af temperature is on-track to achieve the desired, final active Af temperature.

A Word About Color

Color of a finished nitinol device should not be used to characterize material. A blue to purple shade of surface oxide can be expected on a heat treated Nitinol device. However, the many different shades that come from natural product variation can be misleading and should not be considered a reliable product attribute.

The key to achieving desired properties when designing braided nitinol devices is to partner with a manufacturing company that has strong materials expertise. Success starts at the beginning of the design/concept phase for every nitinol device by establishing a goal for the specific transformation temperature range of the final nitinol device. Then, active Af temperature of the braided nitinol construct is carefully monitored and controlled during each step of the various manufacturing phases.

US BioDesign can assist you in accomplishing a fully realized braided Nitinol component or device, with a repeatable process that is correctly characterized for reliable patient care.