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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 7 carbides due to the melt practice in a graphite crucible. The EBR oxide inclusions have dimensions < 10 µm in the longi- tudinal direction. A recent presentation summarized the evo- lution in commercially available Nitinol and is presented in Table 1 [9] . This table illustrates that the more recent Nitinol tends to have shorter inclusions and lesser inclusion area frac- tion in the final device. The decreased particles size produced by the EBR process leads clearly to enhanced fatigue behavior at longer fatigue lifetimes. IMPLICATIONS FOR MEDICAL IMPLANT DURABILITY Saffari [10] conducted finite element analysis (FEA) with a generic Nitinol self-expanding stent model and applied cyclic axial compression and bending deformations according to de- formations considered for the SFA environment [4] . Application of an axial compression of 8.6% on this stent results in a mean strain of 3.9%with a corresponding strain amplitude of 0.69%, whereas, a 22.6 mm bending radius results in 5.6% mean strain and 1.12% strain amplitude. To illustrate how these stent strain amplitudes can be analyzed with the fatigue prob- ability, arrows to mark the strain amplitudes are shown on Figs. 1 and 2. The strain amplitude due to cyclic axial compres- sion exceeds the 10 million-cycle fatigue strain limits for Gen- eration I Nitinol, which could explain the high fracture rates for Nitinol SFA stents [4] . If we consider the 1.12% strain amplitude due to cyclic bending, fatigue fractures would be expected in both Generation I materials and the VIM Generation II Nitinol. Figure 2 shows that if these stents are instead manufactured from Generation II or Generation III Nitinol, the probability of fracture would be reduced significantly. In fact, the fatigue safety factor for these bending conditions with Generation III Nitinol is 1.9%/1.12% > 1.6, confirming the low probability of fatigue fracture under extreme in vivo conditions. This article focused on the effects of Nitinol “micropuri- ty” on the durability of SFA stents and offers a potential way to enhance durability and reduce fatigue fractures. In addition, it is expected that Generation II and especially Generation III Nitinol will be beneficial for use in even more challenging anatomies, such as heart valve repair and replacement frames. The strains in such environments may be significantly greater than those in the femoral artery and therefore, these improved Nitinol materials are expected to increase device durability. ~SMST For more information: Alan R. Pelton, chief technical officer, G. Rau Inc., 5617 Scotts Valley Dr., Scotts Valley, CA 95066, alan. pelton@g-rau.com, www.g-rau.de . References 1. M. Silva, Average Patient Walking Activity Approaches Two Million Cycles per Year, J. Anthroplasty, 17, p 693-697, 2002. 2. E.A. Hooker, et al., Respiratory Rates in Emergency Depart- ment Patients, Journal of EmergencyMedicine, 7 (2), p 129-132, 1989. 3. S.H. Duda, et al., Sirolimus-eluting Stents for the Treat- ment of Obstructive Superficial Femoral Artery Disease: Six- month Results, Circulation, 106, p 1505-1509, 2002. 4. F.Ansari, et al., Design Considerations for Studies of the Biomechanical Environment of the Femoropopliteal Arteries, J. Vasc Surg, 58 (3), p 804-13, 2013. 5. Cheng, C.P., ed., Handbook of Vascular Motion, Elsevier - Academic Press, 2019. 6. M. Launey, et al., Influence of Microstructural Purity on the Bending Fatigue Behavior of VAR-melted Superelastic Nitinol, J Mech Behav Biomed Mater, 34, p 181-186, 2014. 7. S.W. Robertson, et al., A Statistical Approach to Under- stand the Role of Inclusions on the Fatigue Resistance of Su- perelastic Nitinol Wire and Tubing, Journal of the Mechanical Behavior of Biomedical Materials, 51, p 119-131, 2015. 8. M.F. Urbano, et al., Inclusions Size-based Fatigue Life Pre- diction Model of NiTi Alloy for Biomedical Applications, Shape Memory and Superelasticity, 1 (2), p 240-251, 2015. 9. A.R. Pelton, et al., The Quest for Fatigue-Resistant Nitinol for Medical Implants, in STP1616: Fourth Symposium on Fa- tigue and Fracture of Metallic Medical Materials and Devices, M.R. Mitchell, et al., Editors, ASTM International: West Consho- hocken, PA. p 1-30, 2019. 10. P. Saffari, Best Practices for FEA Simulation of a Stent, in SIMULIA Regional User Meeting 2012, Linz, Austria, 2012. FEATURE TABLE 1 − SUMMARY OF COMMERCIALLY AVAILABLE NITINOL [9] Material Classification Year Introduced Melt Source* Conforms to ASTM F2063 MaximumDevice Inclusion Size (µm) Device Inclusion Area Fraction, % Reference Generation I ca . 1990 VAR I VIM/VAR I Yes 101 1.5 7 Generation II ca . 2005 VAR II VIM/VAR II VIM Yes 20-50 0.41-2.67 7, 9 Generation III ca . 2010 VAR/EBR III Yes <5 <0.5 9 *VAR = VacuumArc Remelt; VIM = Vacuum Induction Melt; EBR = Electron BeamRemelt 7