October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 0 3 5 THE INFLUENCE OF NITINOL ‘MICROPURITY’ ON MEDICAL IMPLANT DURABILITY The effects of Nitinol micropurity on the durability of superficial femoral artery stents offers a potential way to enhance durability and reduce fatigue fractures. A.R. Pelton, FASM,* S.M. Pelton,* G. Rau Inc., Scotts Valley, California J. Ulmer*, K. Plaskonka,* and A. Keck,* G.Rau GmbH, Pforzheim, Germany M.R. Mitchell, FASM,* Mechanics and Materials Consulting LLC, Flagstaff, Arizona P. Saffari,* Engage Medical Device Services Inc., Newport Beach, California C ardiovascular implants such as stents, stent grafts, heart valve frames, and vena cava filters are designed to func- tion for the lifetime of the patient. As part of the approv- al process for these life-saving implants, the USFDA requires durability assessment of peripheral implants for a minimum of ten years that results in 380 million cycles of deformation from cardiac pulsatility, musculoskeletal motions of up to 10 million cycles [1] , and an additional 100 million respiratory cycles [2] . Over the past two decades, Nitinol stents have demon- strated superior clinical outcomes for treatment of peripheral arterial disease (PAD) comparedwith conventional balloon an- gioplasty alone [3] . Although the use of Nitinol stents has been quite successful for treatment of PAD, in vivo fractures and failures have been reported during follow-up procedures [4] . A summary overview article on superficial femoral artery (SFA) stenting conducted at the USFDA commented that stents still fracture at a measurable rate although improvements have been made to significantly improve their durability [4] . As such, recent efforts in Nitinol stenting have focused on the essential task of improved understanding of the biomechanics [5] in the diseased vessel and improved designs and material response to withstand more severe in vivo deformations, such as axial compression and bending. In parallel with these activities, there has been a con- certed effort to improve the Nitinol metallurgy, specifically the characteristics and importance of nonmetallic inclusions, to improve in vivo stent durability. The vast majority of com- mercially available SFA Nitinol stents referred to in the USFDA study [4] were manufactured from “standard-grade” Nitinol. Therefore, the purpose of this article is to highlight recent work on superior grades of “microclean” Nitinol and the effects on fatigue and durability of the devices. EFFECT OF NITINOL MICROPURITY ON DEVICE DURABILITY Several recent publications addressed the effects of non- metallic inclusions on Nitinol fatigue [6-9] . These investigations demonstrate conclusively that inclusions and the adjacent *Member of ASM International voids are internal defects and act as potent initiation sites for fatigue cracks. The investigation by Robertson et al. [7] offers the most comprehensive study to date on the effects of inclu- sion type and content on Nitinol fatigue under two modes of cyclic deformation. In the Robertson et al. study, superelastic wires (ø0.25mm) and diamond-shaped stent surrogates (pro- cessed from ø8mm tubing) were tested from five different mill product suppliers. Test specimens were processed to obtain a transformation temperature of 20˚Cand were tested in a 37˚C water bath with 6% pre-strain (equivalent stent crimp strain), 3% mean strain (equivalent stent oversizing strain), with a range of strain amplitudes (equivalent stent cyclic conditions). These test conditions simulate typical in vivo environments, whereby fatigue cycling is conducted on the unloading plateau to 10 million cycles. Figure 1 shows the fatigue fracture probability as a func- tion of strain amplitude for the diamond specimens for all five FEATURE Fig. 1 — Probability of Nitinol diamond fracture at 10M cycles ver- sus strain amplitude plots with a logit sigmoidal curve fit line for the five Generation I and II Nitinol materials. The maximum inclusion length and inclusion area fraction for the VAR I and VAR II materials measured from the finished device are shown. Reprinted from Rob- ertson et al. [7] with permission from Elsevier. 5