1. Scope

Keywords

Rationale

F2082/F2082M is the industry standard test method for the Af temperature measurement of finished Nitinol devices as a manufacturing in-line process control, and is used to ensure that the finished products meet the specified Af temperature specifications. ASTM F2082 was originally intended to assess the transformation temperatures of semifinished products (defined as wire, tube or strip material forms) by measuring the deformation recovered during heating of a specimen in bending with an outer fiber strain of 2 to 2.5% but was not intended for finished products such as stents. However, the standard was commonly used as a guide to obtain Af temperatures of finished products as part of process control and regulatory submissions. In fact, the USFDA Nitinol guidance document recommends ASTM F2082 for "devices that are expected to be load bearing”. Most companies adapt ASTM F2082 to assess the transformation temperatures of large semi-finished and finished products. The finished device is often tested by flat plate compression of the stent in a controlled fashion to an oval shape. This leads to a local bending deformation of the surrounding struts of the stent; however, this method does not represent a pure bending load as recommended in ASTM F2082/F2082M-23. A pure (local) bending deformation mode would require extraction of a test coupon from the stent. Given the small sizes of the struts, an extraction of a single strut to apply pure bending would likely lead to erroneous and unusable results. Therefore, although the crush method does not strictly adhere to the pure bending recommendation in the latest version of the standard, the method provides recoverable bending strains during heating and might (or not) represents an acceptable assessment of the Af temperature. Furthermore, the USFDA Nitinol guidance document states that ASTM F2082 can be "modified to assess the strains the device is expected to experience in its final finished form”. In conclusion, there are currently loopholes in F2082 addressed for finished devices, and is goal of this guide is to resolve that issue. Deformation methods such as bending, radial crimping or crush might lead to different outcomes. For the work item addressing how to revise F2082 for semifinished products in excess of 3mm, see WK96773.