A mechanical analysis informed fractography study on load-specific fatigue behaviors of Pt-Ir coils used in implantable medical leads.

Fatigue fracture is a major threaten to implantable medical coils such as platinum-iridium (Pt-Ir) coils used in deep brain stimulation leads. The fractography under bending and torsion fatigue was studied in comparison with mechanical analysis to grasp load-specific fatigue characteristics and understand the mechanisms. Mechanical analysis of the coil under bending and torsion was conducted with both analytical and numerical methods. Pt-Ir coils were experimentally fatigued at roughly paralleled bending and torsional load levels. The fatigue life was recorded and the fracture morphology was observed and analyzed. It is revealed that the helical structure of the coil turns bending and torsion loads into different locally distributed stresses, which mainly determine the fatigue behaviors. Features of fracture morphology, such as ratchet mark and fatigue striation, are identifiable to differentiate load types and stress levels. Both bending and torsion may play important roles in fatigue fractures of the coil. The study proposes an effective approach to study load-specific fatigue characteristics of medical coils which provides fundamental knowledge for medical lead design and clinical fracture diagnosis.