A. Undisz, F. Schrempel, W. Wesch, Friedrich-Schiller-University, Jena, Germany; M. Rettenmayr, Friedrich-Schiller-University Jena, Jena, Germany
Based on the pseudo elastic properties of nearly equi-atomic Ni-Ti alloys, a new generation of medical implants has been developed in the past decade. These implants that are commonly used for cardio vascular surgical treatments are collapsible to very small sizes because of their pseudo elastic or shape memory behavior. Thus, these implants allow for minimal invasive surgical treatments where catheters are used to insert the implant into the vascular system close to or right inside the human heart.
The effect of the high pseudo elastic deformation of such implants during the surgery on the surface oxide layer of the material has not been investigated in sufficient detail until present. As the biocompatibility of Ni-Ti alloys is based on the homogeneity and integrity of the surface oxide layer, it has to be verified that the surface oxide layer can withstand the deformation. Otherwise the contact of body fluids and tissue to the underlying material and the release of Ni ions out of the implant material can not be effectively suppressed.
In the present work the influence of pseudo elastic deformation of Ni-Ti alloys on the surface oxide layers formed by an appropriate heat treatment has been studied for different loading conditions on thin Ni-Ti wires that are commonly used for medical implants. It is shown that under certain conditions cracking as well as flaking of the surface oxide layer appears during the deformation of the wires. This behavior is assumed to reduce the biocompatibility of the material significantly. To describe the failure mechanism in detail the elongation and the accompanying transversal contraction of the wires have to be taken into consideration.
Summary: In the present work the influence of pseudo elastic deformation of Ni-Ti alloys on the surface oxide layers formed by an appropriate heat treatment has been studied for different loading conditions on thin Ni-Ti wires that are commonly used for medical implants. It is shown that under certain conditions cracking as well as flaking of the surface oxide layer appears during the deformation of the wires.
