Implantable electrical conductor flexure fatigue issues

Recent work calculating bending stresses in implantable conductors has been applied to cine-angiographic measurements of implanted conductor bending radii. The maximum stresses in different conductors due to flexure at the measured radii can be calculated. In evaluating the viability of this approach using the Universal Test Coil (UTC) described in the Draft European Standard for Active Implantable Medical Devices, the maximum equivalent stress at the minimum physiological bending radius was found to be substantially greater than the endurance limit of the MP35N coil material. If this maximum equivalent stress behaves cyclically, this coil conductor should experience failures. However, the UTC has been shown to have acceptable reported survival in chronic studies over three years. This apparent inconsistency shows the difficulty of interpreting static and dynamic strains and their potential contribution to the fatigue failure of implantable conductors