Jason D. Weaver

Comparisons of fatigue performance between datasets are relatively common practice, although there is not a standardized metric for conducting such analyses. In this work, two recently published nitinol fatigue datasets with relevance for cardiovascular medical devices are used to probe a range of comparative metrics. Results highlight the importance of selecting an appropriate metric as well as the importance of test planning as both can affect the final conclusion regarding the similarity of two datasets. Additionally, an unsuccessful attempt was made to predict high cycle fatigue strength based on low cycle (i.e., censored) fatigue data using a Coffin-Manson relation, which reinforces the importance of conducting fatigue tests to as many cycles as the intended device lifetime. Lessons learned from this set of analyses can inform future standards work as well as engineering practice.

Nitinol implants, especially those used in cardiovascular applications, are typically expected to remain durable beyond 10⁸ cycles, yet literature on ultra-high cycle fatigue of nitinol remains relatively scarce and its mechanisms not well understood. To investigate nitinol fatigue behavior in this domain, we conducted a multifaceted evaluation of nitinol wire subjected to rotary bend fatigue that included detailed material characterization and finite element analysis as well as post hoc analyses of the resulting fatigue life data. Below approximately 10⁵ cycles, cyclic phase transformation, as predicted by computational simulations, was associated with fatigue failure. Between 10⁵ and 10⁸ cycles, fractures were relatively infrequent. Beyond 10⁸ cycles, fatigue fractures were relatively common depending on the load level and other factors including the size of non-metallic inclusions present and the number of loading cycles. Given observations of both low cycle and ultra-high cycle fatigue fractures, a two-failure model may be more appropriate than the standard Coffin-Manson equation for characterizing nitinol fatigue life beyond 10⁸ cycles. This work provides the first documented fatigue study of medical grade nitinol to 10⁹ cycles, and the observations and insights described will be of value as design engineers seek to improve durability for future nitinol implants.

Nitinol exhibits unique (thermo)mechanical properties that make it central to the design of many medical devices. However, nitinol nominally contains 50 atomic percent nickel, which if released in sufficient quantities, can lead to adverse health effects. While nickel release from nitinol devices is typically characterized using in vitro immersion tests, these evaluations require lengthy time periods. We have explored elevated temperature as a potential method to expedite this testing. Nickel release was characterized in nitinol materials with surface oxide thickness ranging from 12 to 1564 nm at four different temperatures from 310 to 360 K. We found that for three of the materials with relatively thin oxide layers, ≤ 87 nm nickel release exhibited Arrhenius behavior over the entire temperature range with activation energies of 80 to 85 kJ/mol. Conversely, the fourth ''black–oxide'' material, with a much thicker, complex oxide layer, was not well characterized by an Arrhenius relationship. Power law release profiles were observed in all four materials; however, the exponent from the thin oxide materials was approximately 1/4 compared with 3/4 for the black–oxide material. To illustrate the potential benefit of using elevated temperature to abbreviate nickel release testing, we demonstrated that a > 50 day 310 K release profile could be accurately recovered by testing for less than 1 week at 340 K. However, because the materials explored in this study were limited, additional testing and mechanistic insight are needed to establish a protective temperature scaling that can be applied to all nitinol medical device components.

Surface area has long been thought to influence the corrosion behavior of medical devices although this phenomenon has not been systematically investigated. Because many nitinol implants are offered in a range of sizes, understanding the influence of device surface area on corrosion susceptibility is key to ensuring nitinol devices are safe. This study evaluates the corrosion susceptibility of amber oxide, mechanically polished and chemically etched nitinol wires with surface areas ranging from 0.05 to 10.0 cm² using a standard test method, ASTM F2129. The corrosion testing shows that as the surface area increased the breakdown potential decreased. Results from the amber oxide and chemically etched samples show a plateauing of breakdown potential at larger surface areas, which suggests that there may be a minimum surface area threshold that could predict the performance of larger devices. Additional testing on electropolished wire and modified laser-cut stents shows a similar decrease in breakdown potential with increased surface area thus corroborating the trend observed with nitinol wires. Overall, surface area was found to affect corrosion susceptibility as measured per ASTM F2129 for a range of nitinol samples.

Transcatheter aortic valves (TAV) are symmetrically designed, but they are often not deployed inside cylindrical conduits with circular cross-sectional areas. Many TAV patients have heavily calcified aortic valves, which often result in deformed prosthesis geometries after deployment. We investigated the effects of deformed valve annulus configurations on a surgical bioprosthetic valve as a model for TAV. We studied valve leaflet motions, stresses and strains, and analog hydrodynamic measures (using geometric methods), via finite element (FE) modeling. Two categories of annular deformations were created to approximate clinical observations: (1) noncircular annulus with valve area conserved, and (2) under-expansion (reduced area) compared to circular annulus. We found that under-expansion had more impact on increasing stenosis (with geometric orifice area metrics) than noncircularity, and that noncircularity had more impact on increasing regurgitation (with regurgitation orifice area metrics) than under-expansion. We found durability predictors (stress/strain) to be the highest in the commissure regions of noncircular configurations such as EllipMajor (noncircular and under-expansion areas). Other clinically relevant performance aspects such as leaflet kinematics and coaptation were also investigated with the noncircular configurations. This study provides a framework for choosing the most challenging TAV deformations for acute and long-term valve performance in the design and testing phase of device development.

Crevice corrosion is one of the major mechanisms that drives implant failure in orthopedic devices that have modular interfaces. Despite the prevalence of crevice corrosion in modular interfaces, very little is known with regards to the susceptibility of different material combinations to participate in crevice corrosion. In this study, we compare two electrochemical methods, ASTM F2129, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices, and a modified version of ASTM F746, Standard Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials, in their ability to induce crevice corrosion. Four commonly used metals, 316 stainless steel, commercially pure titanium (Ti grade 2), Ti-6Al-4V (Ti grade 5), and cobalt–chromium–molybdenum per ASTM F1537, Standard Specification for Wrought Cobalt-28Chromium-6Molybdenum Alloys for Surgical Implants (UNS R31537, UNS R31538, and UNS R31539), were used to form crevices with a rod and washer combination. As a control, the metal rod materials were tested alone in the absence of crevices using ASTM F2129 and the modified ASTM F746 method. As another control to determine if crevices formed with polymeric materials would influence crevice corrosion susceptibility, experiments were also conducted with metal rods and polytetrafluorethylene washers. Our results revealed more visible corrosion after ASTM F2129 than ASTM F746. Additionally, ASTM F746 was found to falsely identify crevice corrosion per the critical pitting potential when visual inspection found no evidence of crevice corrosion. Hence, ASTM F2129 was found to be more effective overall at evaluating crevice corrosion compared to ASTM F746.