S. N. Rosenbloom, Corrosion Testing Laboratories, Inc., Newark, DE
Nitinol derives its corrosion resistance from a titanium-rich oxide layer that forms on its surface. If the oxide layer is properly formed, the device will exhibit excellent corrosion resistance in simulated physiological environments and would be expected to perform well in vivo. However, environment, surface defects, variations in oxide thickness and composition, and mechanical damage can all affect the ability of the oxide film to protect the metal. Corrosion testing of nitinol devices is essential for assuring that the surface is in a corrosion resistant condition. ASTM F 2129 (“Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices”) has been developed as a tool to assess the corrosion resistance of small medical implants with regards to pitting and/or crevice corrosion, two of the most damaging forms of corrosion for small implant devices. However, an acceptance criterion has not been established. The test method will be reviewed and a variety of possible results will be interpreted. An acceptance criterion, developed from testing hundreds of devices, is explained. In addition, potential difficulties with the test method will be discussed.
Summary: The human body presents an aggressive environment for metallic implants with regards to corrosion. Corrosion can lead to a host of unwanted effects including inflammation, allergic or toxic reactions, and loss of functionality of the device itself. Much research has been devoted to understanding the biocompatibility of various alloys. Some alloys are either immune to corrosion or remain passive when in the physiological environment. Others, however, can be either active or passive and the quality of a protective oxide layer on their surface determines how they will behave. Nitinol, a nickel titanium shape memory alloy, is such an alloy. It has become an increasingly popular choice for small implants, such as stents, because of its unique mechanical properties. However, as with all alloys, corrosion testing is essential in order to make predictions as to how the material will behave in vivo. ASTM F 2129 (“Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices”) is an electrochemical test method that is used to assess the resistance to pitting and crevice corrosion of biomedical implants. The method provides a reasonably straightforward and quick assessment tool and is well accepted by the USFDA as one of a battery of tests that are performed on new devices when seeking permission to market. When conducting the test, it is important to carefully specify the appropriate in vitro environment, so as to most accurately simulate the intended in vivo conditions. Proper interpretation of the results is critical to understanding the corrosion behavior of the material. While the standard does not include a pass/fail criterion, it is our opinion that pitting corrosion is wholly unacceptable in small implants, such as stents. Using published in vivo data, an understanding of the basic electrochemical reactions occurring at the material/environment interface, and experience with testing hundreds of medical devices, we have established a +600 mV threshold criterion for acceptance.
