Single Cycle Pseudo Elastic Pre-Deformation and Long-Term Ni-Release From NiTi Wire

Therapy of cardiovascular disease by minimal invasive surgery is at present reliant on NiTi exhibiting shape memory/pseudo elasticity. Although the application of NiTi implants is nowadays performed routinely, occasionally persisting Ni-release from the material is reported that may cause adverse biological response. The issue of Ni-release was addressed in a number of studies considering static or dynamic loading conditions with continuing pseudo elastic deformation of the material. However, for a number of implants like e.g. filters or occlusion devices, the material only passes through a single pseudo elastic deformation cycle during the minimal invasive application and remains essentially static once the implantation is accomplished. Considering that a single pseudo elastic deformation cycle can cause cracking and flaking of protective surface oxide layers on NiTi, it is plausible that the overall Ni-release rate would differ from that established via static undeformed or dynamic testing regimes.

In the present study, NiTi wires were annealed for emulating the common shape setting process and forming a protective surface oxide layer. Subsequently, a fraction of the wires was subjected to a single pseudo elastic deformation cycle. During the following months, pre-deformed and original wires were immersed in ultra pure water at 37°C, and the Ni-ion concentrations were determined in regular intervals using atomic absorption spectroscopy. As a result of the pre-deformation, the short-term Ni-release increased whereas the long-term Ni-release was below that of the original wire. To elucidate this behavior the integrity of the surface oxide layer in pseudo elastic deformation and it’s impact on overall Ni-release will be discussed.