Thermomechanical pre-treatment and Ni release from electropolished NiTi

A detailed understanding of how NiTi alloys release Ni ions is important when evaluating their use in medical devices. The recent update to ISO 10993-15, which extends the immersion time from one to twelve weeks, highlights the complex behavior of Ni release from NiTi. In this work, the influence of thermomechanical pre-treatment on both short- and long-term Ni release from electropolished and heat-treated NiTi wires was examined. The samples underwent a single cycle of pseudoelastic deformation and were then immersed in Ringer’s solution in accordance with ISO 10993-15. Ni concentrations were determined by inductively coupled plasma mass spectrometry with a detection limit of 0.02 µg/L. Prior to deformation, the wire surfaces appeared smooth and homogeneous. Pseudoelastic straining led to cracking of surface oxide and localized flaking, both associated with higher Ni release. All conditions showed an initial increase in release rate, whereas long-term values remained below ~30 ng·cm⁻²·d⁻¹, which is within current EU limits. The findings are discussed in terms of pre-deformation effects, oxide layer integrity, and the formation of a Ni-rich subsurface layer. Conditions showing increased Ni release correlated with metastable phases beneath the oxide, suggesting mechanisms that require further investigation in future work.