February 2000

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Draft Standards for Nitinol Alloys Present Optimum Medical Device Design Processes

ASTM Subcommittee F04.12 on Metallurgical Materials, part of Committee F04 on Medical and Surgical Materials and Devices, is developing four draft standards useful in the design and manufacture of medical devices and surgical implants.

The nickel-titanium shape memory alloys described in these standards are commonly called nitinol alloys. Nitinol is a family of alloys designated by a transformation temperature measured under controlled conditions and after a specified thermomechanical history.

Frank Sczerzenie, a material scientist and product manager for shape memory alloys at Special Metals Corporation, New Hartford, N.Y., chaired the task group that developed the standards. He explains how medical equipment specifiers would benefit if these standards were in place. "I was visiting a company that designed, and is in clinical trials on, a catheter that is using this material [nitinol]. They are just now learning how to specify the material for future products. They are on a very steep learning curve today. It would be much easier if they had these standards to work with."

Currently being reviewed within ASTM, the proposed new standards are:

Wrought Nickel-Titanium Shape Memory Alloys for Medical Devices and Surgical Implants

This specification characterizes the chemical, physical, thermomechanical, and metallurgical properties of wrought nominally 55 to 56 percent nickel-titanium bar, wire, flat rolled products, and tubing used in the manufacture of medical devices and surgical implants.

The draft specification offers sections on terminology, product classification, ordering information, manufacture, and chemical composition with tables on chemical requirements and product analysis tolerance. Comprehensive guidance is provided on transformation temperature, metallurgical structure, mechanical property requirements with a table on annealed mechanical properties, special requirements, certification, quality program maintenance, and biocompatibility.

Standard Test Method for Transformation Temperature of Nickel-Titanium Alloys by Thermal Analysis

This test method defines procedures for determining the transformation temperatures of nickel-titanium shape memory alloys using the differential scanning calorimeter. The test is useful for quality control, specification acceptance, and research.

Details are provided on the test method, interference, apparatus, sampling, calibration, procedure, graphical data reduction, reporting, and precision and bias. A graph showing a differential scanning calorimeter curve for nickel-titanium depicts heat flow and temperature.

Standard Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery

This method describes a procedure for determining the martensite-to-austenite transformation temperature by measuring the deformation recovered during the thermal transformation.

Standard Terminology for Nickel-Titanium Shape
Memory Alloys

This is a compilation of terms in ASTM standard documents relating to nickel-titanium shape memory alloys used for medical devices. Two differential scanning calorimeter graphs on single-stage and two-stage transformations are included.

“The specifications that have been developed so far are aimed primarily at the raw material,” concludes Sczerzenie. “As we go into the future, we expect to be developing additional standards for finished products. We are also planning to start an interlaboratory study to determine the precision and bias of calorimetry for determining transformation temperature. Interested qualified laboratories are invited to participate.”

Questions or comments may be directed to Frank Sczerzenie, Special Metals Corporation, Shape Memory Alloys Division, 4317 Middle Settlement Rd., New Hartford, NY 13413-9576 (315/798-2072; fax: 315/798-6860). Committee F04 on Medical and Surgical Materials and Devices meets May 24-26 in Toronto, Ontario, Canada. For meeting or membership information, contact F04 Staff Manager Teresa Cendrowska, ASTM (610/ 832-9718).