Implanted cardiovascular devices must undergo a range of applied cyclic loads after implantation. Peripheral devices, for example, are subjected to both radial pulsatile loading and non-radial loading such as cyclic bending from limb motion. Coronary stents may also see a range of loads due to increases in heart exertion, such as during exercise. As highlighted by these examples, such variable loading commonly involves a smaller number of high amplitude cycles combined with a larger number of low amplitude cycles.

Advances is lead materials can spur advances in patient care. Rune Elmqvist’s pacemaker was delivered by Dr. Åke Senning into Arne Larsson in 1958 using stainless steel. Arne received 25 additional pacemakers until his death in 2001. In Arne’s 86 years of life, he experienced fracture of early stainless-steel leads and lived through the advent of Elema-Ericsson’s ribbon-based coiled conductors, the advance of more durable CoNiCrMo-leads, and advanced designs using silver-core composite wires in the 1980’s and 1990’s. Classic steel leads, while beneficial to patients, also came with fracture complications and multiple percentage fatality. Clinician signals for improvement were taken up by engineers developing helical conductors that reduced cyclic material stresses, brazed composites that improved fatigue damage resistance, 35N LT alloy, and composite wires that improved electromechanical performance. The aim of this study was to demonstrate the feasibility and performance of nitinol-based wire constructs for lead design.

The late John Boylan and his team in Temecula, CA, filed a patent in the 1990’s that included a variety of radiopaque ternary additions to a nitinol alloy. Several of these elements including Pt, Pd, and Au, hold potential to not only raise x-ray attenuation, but also to perfect the interface between parent and daughter phases thereby increasing certain material performance aspects. Work is needed to understand and apply specific Ni-Ti-Pt chemistries and understand their practical utility. As early as 1988, one way shape memory, superelasticity, and austenitic transformation temperatures from about 0 to 1000°C were demonstrated for the Ni-Ti-Pt system by Lindquist and colleagues at University of Illinois Urbana-Champaign. Under equilibrium, stress-free conditions, the material composition for maximum austenite-martensite interface compatibility is approximately Ti 50 Ni 42.5 Pt 7.5 . Further study is warranted since improved crystallographic compatibility may improve the structural and functional performance of critical medical and industrial subcomponents where performance gains could pay for higher initial material costs.

For producers and end-users of high-performance Nitinol, a material that functionally depends on everything that is done to it, it is critical to keep pace documenting the connection between producer-evolved grades and critical subcomponent performance. In Nitinol, the latitude of the industrial standard material specification ASTM F2063 should be continuously plied, and performance outcomes weighed against end-use demands as an aid to relevant use recommendations and requirements development. The current standard specification for wrought NiTi requires maximum oxygen and carbon levels of 400 ppm and maximum observed contiguous particle- void-assembly (PVA) lengths not exceeding 39.0 microns measured in longitudinally mounted specimens. Performance variation across impurity- and PVA length-compliant nitinol grades, especially with respect to structural fatigue, is well documented where each new study gives a snapshot in time that depends on initial chemistry and PVA distributions, processing history, test coupon geometry, fatigue test boundary conditions, and other factors. This study is a small step toward keeping pace with the connection between nitinol grades and tubing performance.