Nickel-titanium (NiTi) alloys are widely used for medical implants in the human body because of their advantageous material property of pseudoelasticity. Despite their good biocompatibility, this application requires consideration regarding release of Ni ions and possible adverse reactions [1]. The requirements for the determination of metallic degradation products from medical devices are described in ISO 10993-15, which was updated in 2023. Specifically for NiTi, the previous immersion test period of 1 week was extended to 12 weeks [2]. It has already been shown that the release of Ni from NiTi depends on the pre-treatment of the material. Therefore, an accurate quantification and choice of measurement method is important to characterize the influence of pre-treatments on Ni release. The standard specifies measurement methods, including ICP-MS, to be used to measure even lowest concentrations. The aim of this study was to investigate the influence of thermomechanical pre-treatment on short and long-term Ni release. For this purpose, electropolished medical grade NiTi wires were annealed at 540°C for different periods of time. Afterwards the wires were strained pseudoelastically and immersed in Ringer's solution according to ISO 10993-15. In order to quantify small amounts of released Ni ions, the measurement was performed by ICP-MS and the detection limit of Ni was 0.02 µg/L for the developed measurement method. Microscopy results show that the surface oxide layer of annealed and deformed wires exhibited cracking and flaking, which correlated with increased nickel release. However, no consistent trend was observed with respect to annealing duration. Ni release increased in the short-term for all wires and decreased in the long-term for all thermomechanical pre-treatments. Regarding EU regulations on Ni release it is interesting to note that the limit of approx. 30 ng*cm-2*d-1 [3] was complied with in the long-term for all sample conditions, however exceeded in some short-term measurements. This research demonstrates the important role of ICP-MS in evaluating the release of Ni ions from NiTi implant materials, providing essential data for directing future research and development efforts.

References
[1] S. Nagaraja et al. Shape Memory and Superelasticity, 2022, volume 8, pages 45–63.
[2] DIN EN ISO 10993-15:2023-07. Biological evaluation of medical devices - Part 15: Identification and quantification of degradation products from metals and alloys (ISO 10993-15:2019).
[3] J. P. Thyssen et al. Contact Dermatitis, 2011, volume 65, pages 60-61.