Novel Bending Fatigue Testing of Small Medical Device Cables

Neuromodulation is among one of the fastest-growing areas of the medical device industry. Neuro stimulators are being used to provide therapies to patients with chronic pain, movement disorders, overactive bladders, and psychiatric disorder symptoms. In these therapies, well-controlled stimulation current is delivered to the target nerves through implantable leads. The implantable leads must have sufficient fatigue durability to last for many years of implantation. The implantable leads are composed of insulation polymer jackets and conductor coils/cables. Conductor cables are generally very small and made from MP35N alloys (Co-35Ni-20Cr-10Mo). These conductor cables are more susceptible to fatigue failure than the insulation polymer jackets. A novel bending fatigue testing method was developed and used to test small medical device cables. 1x7 standard MP35N and low Ti MP35N cables are tested. The generalized S-N curve, in the form of curvature amplitude as a function of cycle to failure was obtained. The results show that low Ti MP35N cable has better fatigue performance than the standard MP35N cable. A SEM image of fractured surface shows that cable fatigue is dominated by crack initiation. The crack initiation is induced by shear deformation. This is a typical crack initiation mechanism under high plasticity deformation.