With medical devices approved for more demanding cardiovascular applications such as transcatheter aortic and mitral valve repair (TAVR/TMVR), the long-term structural integrity of the substrate remains of paramount concern. With these considerations in mind, much attention has been paid to factors affecting the fatigue life of nitinol in order to ensure long-term device efficacy. High-cycle fatigue life of unique material is of particular interest.25

Nitinol stents require long-term structural integrity. (Credit: Cirtec Medical)

Initial fatigue studies using diamond samples, which are meant to represent the fundamental construct of laser cut devices and are designed to capture in-vivo loading conditions, showed high-cycle fatigue strain limits on the order of 0.4–0.8 percent.69 As with wrought ductile structural metals, the role of nonmetallic inclusions act as crack nucleation sites in high-cycle fatigue. Raw material producers have improved material cleanliness, which has resulted in notable increases in fatigue life.1 However, there has been little published work exploring the effects of metal processing techniques on fatigue properties.

This article compares the effects of tube processing methodologies on the fatigue life of surrogate samples deemed representative of the fundamental closed cell comprising a self-expanding stent or heart valve. As a result of the thorough work by Robertson et al, it is possible to compare the effects of two tube processing methods (TM-1 and TM-2) on fatigue life.1 The resulting fatigue behaviors are correlated with microstructural aspects of these alloys and are discussed in terms of traditional metallurgical tenets and novel computational modeling of the potency of primary inclusions in fatigue crack nucleation. Part 1 of this article examines materials and procedures. Part 2, which will appear in a future issue, will look at fatigue behavior, including probabilistic analyses and correlation with microstructural analyses as well as a direct comparison of methods.

Materials and Experimental Procedures

Table 1. Chemical composition, transformation temperatures, and microstructural attributes as reported by the mill product suppliers for the current work and the Robertson et al. study. Values not reported are designated “NR”.

The compositions, transformation temperatures, and microstructural characteristics of the two standard grade VIM-VAR nitinol ingots used in the current research are provided in Table 1. Comparable data for the Robertson et al. work are also provided in the table. As cited by Robertson, the term standard refers to traditional melting practices used in the production of nitinol using nominal purity raw materials. Cleaner, higher-fatigue-life grades of nitinol are now available, and two of these were included in the Robertson study. A grade of exceptionally clean nitinol termed high-cycle fatigue (HCF) is now available.

Tube Manufacturing. For the study, tube hollows with an outer diameter (OD) of ~25 mm were processed to a final size of 10 × 0.457 mm (OD × wall thickness) using tube manufacturing method TM-1 at Vascotube GmbH, Birkenfeld, Germany. Two lots of tubes were manufactured from Ingot 1 material and one lot from Ingot 2. These three lots of tubing are termed 1-1, 1-2, and, 2-1, respectively. Superelastic (SE) nitinol can be a difficult material to consistently draw into tubes with sufficient quality for laser cutting of Class III cardiovascular implants. Specific manufacturing processes can vary significantly among tube redraw producers and are often held as intellectual property (IP). IP prevents disclosure of the tube manufacturing processing steps used in the current study.

However, some differences between drawing techniques are well known and can be discussed.10 Successful reduction of as-received nitinol tube hollows requires maintaining control of the outer diameter to wall thickness ratio during die drawing operations. This is typically achieved through optimization of die size, design, and materials, as well as lubricants, mandrel size, mandrel materials, and reduction schedules. All TM-1 reductions employ polycrystalline diamond (PCD) dies, straight single-draw operations using hard mandrels to preserve the high concentricity, and surface qualities of incoming hollows through final tubing.

In contrast, other nitinol tube manufacturing techniques utilize continuous operations in which the tubing is wrapped around a bull block, or take-up spool, to reduce equipment setup costs.11 While providing cost advantages, the asymmetric stress and strain gradients introduced in the tube during such nonlinear drawing operations may have a significant effect on tube uniformity, concentricity, and straightness of the final product, as well as resulting device yields. Such rotational drawing operations can also result in asymmetric microstructures and thus non-uniform mechanical properties. Many operations also use tungsten carbide drawing dies, especially during initial (large diameter) reductions (again as cost-saving measures), oxidizing environments during interpass anneals, and abrasive surface cleaning techniques such as sand or grit blasting necessary for removal of heavily oxidized surfaces. Although most nitinol tubing used for laser-cutting medical devices is centerless ground as part of final fabrication, use of these techniques even solely at large diameters introduces potent defects that may be maintained and promulgated through the tube thickness affecting final device characteristics.

Fig. 1 - Shape set diamond sample used for fatigue testing (left image). Finite element model used to compute strain-displacement relations for fatigue testing of diamond surrogate samples (right image). Diamond designed to achieve peak longitudinal strains on strut proximal to the apex similar to that during in-vivo loading of a closed cell, key-holed stent, or transcatheter valve.

Tube lots 1-1, 1-2, and 2-1 were manufactured using the methods outlined above for TM-1. Final drawing introduced ⊕30 percent cold work into the tubes. Tubes were final annealed and then laser cut into diamond-shaped samples and expanded using typical shape set parameters for a medical device to achieve microstructures representative thereof, as shown in the left image in Figure 1.

Sample Preparation

Finished diamond samples exhibited an Af = 20 °C ±3 °C as determined by the bend and free recovery method defined in ASTM F2082-16.13 Diamonds were etched and electropolished to remove surface defects and obtain a surface similar to a typical SE nitinol device implant. The process removes approximately 25 percent of the material by weight. Cross-sectional optical metallography on processed samples showed no remnant heat-affected zones from laser cutting.

Test Methods

Longitudinal loading of struts occurring during fatigue testing require finite element analysis (FEA) methods to calculate operational stresses and strains and corresponding linear displacements. The commercial FEA solver ABAQUS (standard version vs. 2017) — which contains the user-defined material routine (UMAT) for SE materials — was used for these analyses. The displacements required to reach 6 percent prestrain and 3 percent mean strain were computed at cycle one, whereas those for the various strain amplitudes were computed at cycle three.

Measured by differential scanning calorimetry (DSC) in accordance with ASTM F2004.12

Constitutive inputs were obtained from tensile tests of tube samples that were subject to the same aging treatments used for diamond expansions. Table 2 shows results of tensile testing tube samples in accordance with ASTM F2516.14

Table 2. Tensile properties of three lots of tubing 1-1, 1-2, and 2-1, used as constitutive input for finite-element analyses.

Fatigue testing was performed on 12-station Bose Electroforce model 3200 mechanical testing units. Testing was performed in displacement control with specimens submersed in a phosphate-buffered saline (PBS) solution at 37 °C at frequencies ranging from 20 to 60 Hz. Samples were prestrained to 6 percent, then unloaded to 3 percent strain to simulate catheter loading and vascular deployment.

A minimum of six samples were cycled at each condition ranging from 0.5 to 2.9 percent strain amplitude. Samples were cycled until fracture or runout at 107 cycles. A total of 177 samples were tested with 70 fractures occurring and 107 surviving to the runout condition. All fractures occurred in the high-stress volume region of the strut.

Microstructural Analyses. Extensive statistical microanalyses were performed on highly polished longitudinal and transverse sections of diamond struts, finished tubing, and retains from the starting hollows. Two additional samples of finished tubing manufactured using TM-2 techniques were analyzed using these same methods. These samples were produced from Standard VAR and Standard VIM-VAR ingots and processed to 8 mm diameter tubes at the same tube manufacturer cited in the Robertson study.1 Although different starting ingots were used in the manufacture of these tubes than those reported in the Robertson et al. study, these samples are deemed representative of the TM-2 process and thus may be used for limited comparative microstructural analyses purposes.

Fig. 2 - Representative optical micrographs of TM-1: Std VIM-VAR; a) 1-1 longitudinal and b) 1-1 transverse, and, c) 2-1 longitudinal, and d) 2-1 transverse finished diamonds.

Scanning electron and optical microscopy) were performed on all prepared samples. No discernible differences in detecting inclusions were observed between the two imaging modalities. Images were taken with a resolution of approximately 100 nm/pixel across all imaging methods corresponding to a particle detection limit of 107.9 nm. A minimum of 40 fields of view were taken per surface corresponding to a surveyed area of about 1.5 mm2. Images were analyzed using an automated MATLAB threshold script to detect inclusions/porosity based on image contrast.

Successful reduction of as-received nitinol tube hollows requires maintaining control of the ratio of the outer diameter to wall thickness during die drawing operations. (Credit: Cirtec Medical)

Figure 2 shows representative micrographs of longitudinal and transverse sections of finished diamonds made using TM-1: Std VIM-VAR; 1-1, 1-2 (not shown) and, 2-1 tubing, respectively. Figure 3 shows representative micrographs of longitudinal and transverse sections of the TM-2: Std VIM-VAR and TM-2 VAR 8 mm tubing, respectively. Arrows indicate drawing direction.

Fig. 3 - Representative BSE micrographs of TM-2: a) Std VIM-VAR longitudinal, b) Std VIM-VAR transverse, c) Std VAR longitudinal, and d) Std VAR transverse finished 8 mm tubing.

Results of statistical analyses of non-metallic inclusions (NMI) for the three lots of tubing in the current study TM-1: Std VIM-VAR, 1-1, 1-2, and 2-1 are provided in Table 3 for both longitudinal and transverse sections and include maximum, mean, and median NMI lengths; total NMI areal fraction; NMI density (ρ); and area surveyed.

Figures 2a, 2c, 3a, and 3c show the microstructural anisotropies resulting from axisymmetric drawing operations typically revealed as stringer-like defects seen in longitudinal sections. In contrast, no microstructural anisotropies are seen in the transverse sections (Figures 2b, 2d, 3b, and 3d) as stringers are viewed end-on. NMI dimensional data for transverse sections, although listed as lengths, L, in Table 3, are more accurately described in terms of an effective diameter, d. so this term is used to differentiate transverse from longitudinal dimensional information.

Table 3. Finished diamond and retained hollow tubing optical microstructural characterization for the current study for tube lots TM-1: Std VIM-VAR 1- 1, 1-2, and 2-1.

Comparison of finished diamond microstructural data for starting hollow retains shows virtually no change in statistical lengths due to drawing operations (see Table 3). Although contrary to Robertson et al. who report up to a fivefold increase in maximum NMI length from starting hollow to final tube, the current finding is likely a result of defining an NMI as a contiguous particle void assembly (PVA).6 This definition is important because such assemblies often elongate during drawing operations, resulting in the high-aspect ratio continuous voids containing particulates spaced sporadically often seen in longitudinal sections of wire and tubing. Depending on specific processing conditions, however, surrounding material can fill elongated voids, thereby healing the matrix at these defects. This results in the often seen proximal but noncontiguous linear array of PVA, or NMI.