Superelastic conductor material with enhanced fatigue durability for implantable lead service

Implantable biostimulation leads provide critical electrical conduits for cardiostimulation and neurostimulation devices. These leads are highly engineered wire constructs which must withstand many years of flexural motion and consequently require high fatigue resistance. This study proposes and

demonstrates a new wire construct which offers a step-change improvement in fatigue resistance. Wire constructs with a high conductivity silver core are characterized in a form factor that aligns with conductor subcomponents commonly used in leads. Nitinol is effectively substituted for 35N LT® (CoNiCrMo) and shown to give 50 to 100% improvements in cyclic strain-loading fatigue performance for bifilar coils or monofilament wire respectively in a lab bench test at body temperature (310K ± 2). Electrical isolation of polyimide coating of the nitinol-based conductors is visually maintained in the 2-channel coils even after high temperature (450-550°C) secondary shape setting. The ultimate strength of the nitinol-silver composite wire (NiTi-DFT®-30Ag, or 30 area % silver) measured about 1000 Mpa and was lower than 35N LT silver composite wire (35NLT-DFT-28Ag) at about 1600 MPa. Despite lower strength, the work energy to tensile fracture was 65.9 mJ/mm³ for NiTi-DFT-30Ag and 23.7 mJ/mm³ for 35NLT-DFT-28Ag, where significantly more work energy was required to overload the nitinol-based conductor. From these data, nitinol-silver composite wires could improve structural durability of next-generation implantable lead systems.