TECH NOTES

The International Conference on Shape Memory and Superelastic Technologies (SMST) will take place at The Bodenseeforum in Konstanz, Germany from May 13 – 17, 2019. SMST brings together a diverse group of worldwide experts active in the field of shape memory and superelastic materials. There will be a full-day Nitinol workshop on Monday, May 13th, followed by a four-day technical program that will cover a host of topics relevant to medical device manufacturers, including deep dives into fatigue behavior, Nitinol processing, corrosion, laser cutting, and many others.

Meet the Confluent Medical Team at Booth #14
Exhibits Open
Tuesday, May 14, 2019………………………………………………….12:00 p.m. to 7:00 p.m.
Wednesday, May 15, 2019……………………………………………..10:00 a.m. to 1:00 p.m.

Confluent Medical will contribute the following to the technical program:

• On the Need to Overhaul our Terminology and Specifications (T. Duerig)
  • Tuesday, May 14, 2019, 9:30 AM, K2 (Bodenseeforum Konstanz)
  • Presented by Dr. Tom Duerig, Ph.D
  • Abstract: Much of the terminology relied upon to control and certify devices traces back several decades, to times when scientists were trying to describe ideal, fully-annealed and isotropic materials. These terms and specifications are now being erroneously applied to complex engineered materials and devices where they are misleading at best. Here we explore two of the most dangerous examples.
• The Effect of Surface Condition on the Magnetic Susceptibility of Nitinol (J. Combs, S. Daly, T. Duerig)
  • Wednesday, May 15, 2019, 9:45 AM, Saal 8 (Hall 8) (Bodenseeforum Konstanz)
  • Presented by Julia W. Combs
  • Abstract: The magnetic susceptibility of fully austenitic nitinol in the electropolished condition is determined to be χg = 2.87*10-6 cgs. Susceptibility increases by more than 6% due to the formation of a nascent oxide layer, even after subsequent mechanical polishing. Thicker, thermally-produced oxide layers are shown to increase the susceptibility 100 times or more. This physically translates to nitinol wire being capable of supporting its own weight with external magnetic fields representing that of an ordinary magnet. This has significant ramifications to devices used in and near MRI fields and potentially other areas. Detailed examination of the surface shows that thermal oxidation results in five distinct layers, including a pure nickel subsurface layer determined to be the primary cause of the ferromagnetic behavior of nitinol. The susceptibility of Martensite and the R-phase are also reported.
• Recent Findings on the Effect of Aging Under Stress on Springback and Transformation Behavior of NiTi Shape Memory Alloys (H. Paranjape, B. Marsh, A. Stebner, A. Shamimi, T. Duerig)
  • Thursday, May 16, 2019, 9:15 AM, K2 (Bodenseeforum Konstanz)
  • Presented by Dr. Harshad Paranjape, Ph.D
  • Abstract: Aging under stress is known to influence the precipitate formation in NiTi shape memory alloys (SMAs) and thus alter the transformation temperatures compared to the annealed state. In this work, we present some recent results related to aging under stress of Ni-rich and Ti-rich NiTi SMAs. In Ni-rich alloys we report on the dependance of springback on the aging time and temperature. In Ti-rich alloys, we report on a peculiar differential scanning calorimetry (DSC) response that is observed on stress-free aging, but which disappears on stress-aging. The response consists of an appearance multi-step transformation in stress-free aged Ti-rich samples, which reverts to single-step transformation in stress-aged samples. We propose a plausible mechanism to explain the occurrence of multi-step transformation as seen in the DSC pattern of Ti-rich NiTi SMAs.
• Determination of the Critical Flaw Size for Crack Growth in Nitinol Material Used for Biomedical Applications Through Focused Ion Beam Notch Fatigue (L. Malito, M. Bowers, P. Briant, A. Shamimi, T. Duerig)
  • Thursday, May 16, 2019, 10:20 AM, Saal 8 (Hall 8) (Bodenseeforum Konstanz)
  • Presented by Dr. Louis G Malito, Ph.D
  • Abstract: Fatigue fracture in Nitinol medical devices can occur from near surface intrinsic or extrinsic inclusions in an area of high localized strain such as the apex of a stent strut. This has drove the industry to move towards ultra-pure extra low inclusion (ELI) material to increase the longevity of these devices. Fatigue studies of ELI Nitinol have demonstrated an improvement in the material fatigue behavior by reducing the size and amount of inclusions. Despite these improvements, these are only probabilistic determinations of inclusion size and location to aid in life prediction of Nitinol material used in biomedical devices. If the exact location, geometry, and size of a flaw could be produced, better quantification on the critical flaw size for fatigue fracture, instead of a probabilistic determination, could be found. This paper presents research on the quantification of critical flaw size produced by focused ion beam (FIB) in ELI Nitinol. Crack-like flaws were produced in 0.635mm diameter ELI Nitinol, Af =2.5°C, wire according to the Ragu solution for a semi-elliptical crack in a cylinder with a crack length, a, to half-width, c, ratio of 0.6. This ratio puts the highest stress-intensity factor at the root of the crack or flaw. Several flaws were induced with apparent crack lengths from 3µm to 14µm. Samples were tension-tension fatigue tested at a mean strain of 3% and various strain amplitudes to elicit both survival and fatigue fracture in samples to determine the critical flaw size for fatigue crack growth in biomedical Nitinol.
• Improving Fatigue: Role of the R-Phase (A. Shamimi, C. Bonsignore, T. Duerig)
  • Thursday, May 16, 2019, 12:00 PM, Saal 8 (Hall 8) (Bodenseeforum Konstanz)
  • Presented by Dr. Ali Shamimi, Ph.D
  • Abstract: In recent years, significant efforts have been devoted to improving the durability of NiTi for biomedical applications. Optimizing for fatigue properties is rather complex. There are various aspects that are key to fatigue life, such as: inclusion size, count, and distribution, grain size and orientation, transformation temperatures, and surface finish. The medical device community often uses the Austenite finish temperature to tune for loading and unloading stiffness. Hence during cycling, the material goes through a dominantly first-order cubic (B2) to monoclinic (B19′) martensitic transformation. This study focuses on the effect of thermomechanical treatments on fatigue behavior. Specifically, the role of R-phase. We demonstrate that by stabilizing the R-phase at body temperature and replacing the Austenite parent phase with the R-phase, fatigue life can be improved. We will show that when the duty cycle is predominantly between the R-phase and B19′ martensite, a softening in cyclic modulus will occur. We will present fatigue results in strain–controlled loading conditions over a span of 2% -5% mean strain, relevant to physiological loading conditions. Also, the mechanism responsible for the increase in fatigue life will be discussed.
• Relation Between Strain Localization Front Movement and Fatigue Lifetime in NiTi Shape Memory Alloys (H. Paranjape, A. Shamimi, C. Bonsignore, T. Duerig)
  • Thursday, May 16, 2019, 2:00 PM, K2 (Bodenseeforum Konstanz)
  • Presented by Dr. Harshad Paranjape, Ph.D
  • Abstract: Movement of transformation fronts is known to lead to transformation-induced slip and consequently structural and functional fatigue in NiTi shape memory alloys (SMAs). Thus, intuitively, larger transformation front movements should lead to larger transformation-induced slip accumulation and poorer fatigue lifetimes. In this work, we investigate the relation between the volume swept by the transformation fronts in tensile test specimens and the fatigue lifetime in NiTi SMAs. As part of this investigation, we present two standardized test specimens — one for tension and another for bending — that are designed to enable multi-modal characterization of microstructure and deformation in SMAs. Specifically, these test specimen designs are compatible with electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), micro-scale computed tomography (μCT), high-energy X-ray diffraction, and digital image correlation measurement methods. We envision an easier sharing of fatigue testing data across the industry with broader adaption of these standardized test specimens.
• Comparing Finite Element Predictions and Image-Based Measurements of Strain in a Nitinol Medical Device: A Verification, Validation, and Uncertainty Quantification (VVUQ) Study (K. Aycock, K. Senthilnathan, C. Bonsignore, R. Campbell, J. Weaver, T. Morrison, B. Craven)
  • Thursday, May 16, 2019: 2:15 PM, K2 (Bodenseeforum Konstanz)
  • Presented by Dr. Kenneth I Aycock, Ph.D
  • Abstract: Engineers typically use strain-based fatigue analyses to assess the fatigue safety of nitinol medical devices. Given the challenge of measuring strain directly on small cardiovascular devices, analysts currently rely on finite element analysis (FEA) predictions of strain coupled with surrogate force-displacement validation evidence. However, force is an integrated quantity whereas strain is inherently a local quantity. Here, we perform FEA simulations and acquire micro-scale digital image correlation (DIC) measurements of strain for a generic nitinol inferior vena cava filter subcomponent. First, tensile and compressive properties of raw SE508 nitinol are characterized using uniaxial tests. Specimen geometry is also characterized using optical methods. Speckle patterns are then applied to device coupons by coating them with finely-ground carbon powder, and a digital optical microscope is used to acquire images of the struts during fixed-free cantilever beam motion emulating physiological loading conditions. Surface strains are extracted from the images using the open-source DIC software Ncorr-C++. Stochastic simulations mimicking the DIC experiments are performed in ABAQUS (R2016x) using Latin hypercube sampling of key input parameters. Finally, experimental and computational strains, with their uncertainties, are quantitatively compared using the modified area validation metric (MAVM). Agreement between FEA predictions and DIC measurements of strain is relatively good at small tip displacements, but differences increase following austenitic-martensitic phase transformation (MAVM≈0.1% strain for peak FEA strains <1% versus MAVM≈1.5% strain for peak FEA strains ≈4%). Overall, DIC strain measurements reveal the usefulness and limitations of current nitinol modeling and enable rigorous quantification of model error.

Confluent Medical is looking forward to a highly productive and educational SMST, and we encourage your attendance! For more information about SMST 2019, please visit: https://www.asminternational.org/web/smst-2019.

For more information about Confluent Medical Technologies, please contact sales@conf.hxmbox.com.

ABOUT CONFLUENT MEDICAL TECHNOLOGIES
Improving lives by delivering world class medical devices through innovative materials science, engineering, and manufacturing. Confluent Medical Technologies is the largest and market leading contract manufacturer of specialized medical devices. With facilities in Scottsdale, Arizona, Fremont, California, Campbell, California, Laguna Niguel, California, Warwick, Rhode Island, and San Jose, Costa Rica, Confluent Medical Technologies has a proven 20-year track record of partnering with the medical device community and delivering high-quality medical devices and components in the aortic intervention, peripheral vascular, ENT, orthopedic/spine, structural heart, electrophysiology, and neurovascular spaces.