Temperature dependence of nickel ion release from nitinol medical devices

Abstract

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.

Keywords: Arrhenius model; accelerated aging; exposure; nickel release; nitinol.

FIGURE 1

Results of Ni release testing of nitinol wires at four different temperatures (310, 325, 340, and 360 K) prepared using four different surface finishing methods: (a) EP = electropolish, (b) CE = chemical etch, (c) AO = amber oxide, and (d) BO = black oxide. The data points and error bars represent the mean and standard deviation of the measurements at each time point, respectively

FIGURE 2

Results of the time scaling analysis. Best-fit scalings $\left({\varphi }_i\right)$ were used to transform Ni release observations at 325, 340, and 360 K to 310 K for each surface finish (EP = electropolish, CE = chemical etch, AO = amber oxide, and BO = black oxide). The data points and error bars represent the mean and standard deviation of the measurements at each time point, respectively, for both Ni release (y-axis) and time at 310 K (x-axis). Note that the time scaling varies with material and temperature; therefore, even though the test conditions were identical, the range of scaled time will not be consistent between different nitinol materials

FIGURE 3

Arrhenius plots of the scaling ratio $\varphi$. Three of the surface finishes (i.e., EP = electropolish, CE = chemical etch, and AO = amber oxide) exhibit a linear relationship between ln $\varphi$ and 1/T. The fourth surface finish, BO = black oxide, deviates from linearity, suggesting the activation energy, E, in Equation 1, is not constant over the T range shown. The data points and error bars represent the mean and standard deviation of the best fit $\varphi$ values determined by the bootstrap method. For reference, the temperature coefficient model in Equation 2 is shown with $Q_{10}=2$

FIGURE 4

Example of using Ni ion release measurements at elevated T to predict the release behavior over longer times at 310 K. 12 measurements were made over the course of ≈ 7 days at 340 K on the BO material (light blue). Based on the mean value of $\varphi$ determined for this condition, the results were scaled to the predicted value at 310 K (dark blue). For comparison, the actual Ni release measurements made at 310 K are also shown on the plot (black). The x-axis error bars represent standard deviation in $\varphi$ determined using the bootstrap method (dark blue only). The error bars along the y-axis for Ni release are all within the size of the symbols