This benchtop test measures nickel ion leaching from nitinol or other nickel-rich medical devices during fatigue testing highly elastic and shape memory mechanical properties have made it a material of choice for medical devices that are used in high deployment/cyclic strain indications such as stents, embolism devices, heart valves and more. While surface treatments can improve corrosion resistance, cyclic loading may lead to surface micro-fractures creating localized corrosion and pitting.
With an increasing awareness of the effects of nickel toxicity in medical devices, recent publications have focused on the effect of surface treatment for NiTi devices and how it affects the corrosion resistance of the device. While there are currently no standards outlining Ni ion leaching testing during fatigue loading, an FDA guidance document issued on August 18, 2015 “Select Updates for Non-Clinical Engineering Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems” recommends considering the potential for nickel ion release from devices containing nickel-rich alloys. A journal paper by Sullivan et al (Shap. Mem. Superelasticity, 2015, 1:319-327) tested nickel release from five groups of Nitinol stents manufactured by various processing methods and showed strong correlations between oxide layer thickness and cumulative Ni release. The paper also found that testing of multiple stents such as those in an overlapped condition or larger-sized stents such as those implanted into the superficial femoral artery may generate nickel release rates higher than USP’s permissible daily exposure for nickel of 0.5 µg/Kg/day. Furthermore, Sullivan et al showed that radial compression levels affected some stents but not all.
Since the FDA guidance document recommends that testing should be performed on as-manufactured devices after subjecting the device to simulated use testing, MDT has come up with a novel test setup that allows for in vitro nickel ion release measurements during fatigue testing. This type of testing closely mimics physiologic conditions and captures both the initial bolus release of nickel as well as the longer term nickel ion release profile in vitro. Nickel ions are captured in a suitable, low volume reservoir as the device is being radially fatigued. Fluid samples are regularly tested using inductively coupled plasma optical emission spectrometry (ICP-OES). This test setup can be readily modified for different types of medical devices, for which nickel ion release characterization combined with mechanical loading is desired.
Testing schedules and resource needs are determined during project kickoff meetings with MDT engineers and the customer. Services are performed in accordance with ANSI/ISO/IEC 17025-2005. In compliance with this standard, standard operating procedures and test protocols specific to the project are used.
In compliance with ANSI/ISO/IEC 17025-2005, MDT test reports include comprehensive documentation and summary of the test protocol and results. All reports are GLP compliant and can be submitted directly to regulatory agencies. Any test method deviations, additions, or exclusions are documented and reviewed with the customer prior to beginning the test. Information within the report includes the specific test conditions, statements of compliance with test protocol, and the estimated measurement uncertainty. The reported results only relate to the items tested. Any opinions and interpretations provided by MDT are clearly marked as such. Any additional information required by customer-specific methods will also be noted. At the conclusion of the test, MDT provides the customer with a Microsoft Windows formatted CD containing copies of all raw data, inspection results, and other information collected during the test (see example test reports).
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