This study explores the pitting susceptibility of Nitinol wires with blue oxide surfaces before processing into a vascular graft device, after processing, and explanted after in vivo exposure during a clinical trial. Only two explanted devices were made available for testing. The blue oxide surfaces of the control specimens tended to perform poorly in a commonly performed cyclic polarization test intended to screen implantable devices for pitting susceptibility, however the explanted devices performed far better under the same test conditions. Electrochemical polarization, x-ray photoelectron spectroscopy, optical and scanning electron microscopies were performed on adjacent wire samples cut from selected specimens. It must be noted that some of the details of handling and storage for the explants from the time of removal to the time that they were provided for analysis are unknown. Many screening tests are intentionally aggressive, the goal being to provide an adequate safety margin for acceptability. Nonetheless, these results raise potential questions regarding the environmental parameters of the in vitro screening test and whether it is always a reliable indicator of in vivo performance.
Nitinol, the nearly equiatomic NiTi superelastic shape memory alloy has found growing usefulness in multiple applications within the medical device industry. The application discussed here involves the use of Nitinol wire in endoluminal stent grafts for treatment of aortic abdominal aneurysms. An example of such a device may be seen in Blum et al.1 Individual Nitinol wire rings with multiple apices forming the shape of a “crown” are encased in a polymeric membrane to form the complete stent device. In portions of the devices discussed here the wires were exposed, but in most cases they were embedded between porous polymeric layers that form a tube in a complete device. With the exception of the first embedded ring, which can be in direct contact with an exposed wire segment, each embedded wire ring was electrically isolated from its neighbors. The wires had developed a relatively thick (roughly 300 nm on average based on unpublished in-house depth profile measurements), visually blue oxide film as a result of the proprietary procedures used to form the rings and create the grafts. In determining whether an implantable device is suitable for use in the in vivo environment a number of test methods are important and useful. With regard to corrosion, one such test recommended by the U.S. Food and Drug Administration (FDA) is ASTM F2129, “Standard Test Method for Conducting Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices.”2,3 This cyclic polarization test has gone through several revisions in recent years, with more likely to come. In its current form it is used to determine the susceptibility of a small implantable device to pitting and crevice corrosion with little emphasis on general surface behavior. Although acceptance criteria are not specifically defined by the FDA, those that are frequently suggested involve breakdown potentials that exceed an absolute value of 600 mV versus saturated calomel electrode (SCE).
