Ming Wu is a nitinol expert who has decades of experience with the nickel-titanium alloy. [Photo courtesy of Wu]
But we promised we’d get back to everyone with answers to their nitinol questions as soon as we could. With the on-demand replay now available for that free webinar, below are the questions we couldn’t get to and answers from Wu. (If you asked a question and don’t see it here, please check your email, because in some cases we followed up privately.)
Wu has also updated his presentation materials, and we’ve included some slides below. You can download all of the slides from the presentation here.
If you haven’t watched our webinar presentation and don’t know who Wu is, he joined the structural heart device developer and manufacturer as VP of engineering in 2006 and retired in 2023. Before Edwards, he was at Memry Corp. for two decades, where he rose from director of engineering and chief metallurgist to VP of technology. Wu has three degrees in materials science and engineering from National Tsing Hua University (bachelor of science) and the University of Illinois Urbana-Champaign (master of science degree and a Ph.D.).
Nitinol basics: What is nitinol and where is it used?
The following has been lightly edited for space and clarity.
When you decided to use nitinol from a different supplier, were there any tests that could be conducted on the nitinol itself to predict whether it would be a good fit before conducting a feasibility test on the actual product?
The first step is to ensure the alternate source meets the material specification. One could also perform testing on transformation temperatures (in both fully annealed condition and final shape-set device condition) and mechanical properties to ensure reasonable equivalency to those of the original material. Depending on the stage of the product development cycle, design verification tests may need to be repeated with change of material sourcing.
Would medtech benefit from nitinol standardization in industry, and how would you do it?
Over the last several decades, the ASTM F04 Committee has already developed several standard specifications and test methods for nitinol for medical device applications. However, these specifications tend to be fairly open to accommodate different grades of materials from all suppliers. There is room to be more specific on some attributes, such as cleanliness and inclusion, for high-risk implantable device applications. There are also opportunities to develop better guidance on fatigue testing, structural durability risk analysis and finite element analysis for nitinol implantable medical devices.
Regarding the metallurgy of nitinol, is the R-phase a stable phase or is it a metastable phase? How does it impact a medical implant?
R-phase is a stable phase below its transformation temperature. Similar to martensitic transformation, R-phase can be stress-induced at temperatures above its transformation temperature. When stress-induced R-phase occurs during the elastic deformation (before stress-induced martensite that give rise to the superelastic plateau in the stress-strain curve), the elasticity will appear to be softened, i.e., the “effective modulus” is reduced due to the presence of stress-induced R-phase transformation.
How does the precipitate phase contribute to superelasticity? Specifically, does the reduction in transformation stress due to precipitation enhance superelasticity? At the same time, since precipitates are generally considered nontransformable, do they reduce the overall recovery capability of the alloy?
In most heat-treated conditions of superelastic nitinol in medical devices, the nickel-rich precipitate is on the nanometer scale, which is much smaller than that of the martensite. Hence, the precipitate phase has minimal effects on superelasticity. However, the precipitation does eventually enrich the titanium concentration in the matrix and raises R-phase and martensitic transformation temperatures.
Is there a reliable way to remove or prevent R-phase? Is R-phase more common in certain types of ingot (VIM versus VAR)?
Adding ternary alloying elements such as copper, gold, platinum or palladium can eliminate R-phase transformation. For superelastic binary nitinol, the only way to remove R-phase is to fully anneal the material. However, fully annealing significantly compromises mechanical properties and makes the alloy less useful for medical device applications.
R-phase is not an issue in device design. Superelastic binary nitinol can be heat treated properly to impart desirable mechanical properties for the application regardless of the R-phase.
The melting process, whether vacuum induction melting (VIM) or vacuum arc melting (VAR), does not affect the R-phase transformation.
How do you view the development of single crystal nitinol and nickel-free analogues to nitinol?
In theory, a single-crystal nitinol could exhibit multiple stage superelasticity with recoverable strain going way beyond 6% when deformed along certain crystal orientations. The challenge is in the development of a viable single crystal growth and subsequent fabrication process.
There were multiple studies on nickel-free superelastic compositions. These studies were mostly focusing on beta titanium alloys such as TiNb-X, TiV-X and TiMo-X series of compositions. These alloys require solution treatment at relatively high temperatures to impart superelastic properties. The resulting superelastic plateau strain was around 3%, far less than the 6% in nitinol. Heat treating these beta titanium alloys at intermediate temperatures (as a potential shape-setting process) could introduce omega phase precipitation which causes embrittlement.
What is a clear overview and step-by-step guide for designing a stent-like endovascular device? I am finding it difficult to create a robust framework and would appreciate a structured approach to understand and guide the design process effectively.
- Conceptualize design based on design inputs and functional requirements.
- Iterate design in finite element (FEA) simulations by applying possible deformation modes of anatomical interactions to ensure the worst-case in-vivo deformation is within the constant life of material fatigue durability.
- Collaborate with delivery system (DS) R&D to ensure the structure can be loaded and deployed with reliable DS-device interface.
- Seek manufacturing feedback to ensure the device can be manufactured with stable processes.
- Repeat FEA to finalize design.
- Run animal studies and critical pre-DV (design verification) testing including full device DFM (dynamic failure mode) tests to ensure the device meets design intent and that any failure occurs at load and locations consistent with the FEA simulation and material fatigue constant life.
- Start design verification testing.
Is it possible for the Medtronic Evolut platform and the Abbott Navitor platform to have the same radial force given the difference in valve cells/amount of scaffolding?
It is possible only if cell design and amount of scaffold are allowed to be modified. If not, the only two options are 1) adjust strut thickness and 2) modify heat treat condition. Both options have limited room to manipulate the radial force.
Any guidance on how to develop good compression data for a tube or wire sample?
Compression tests of small diameter tubes and wires are quite challenging. Large diameter specimens (> 5 mm) would be more practical. Make sure to keep the aspect ratio (height/diameter) low and deform to a small strain before buckling or barreling starts to develop.
How many cycles of loading would you consider “run-out” for nitinol?
There is no run-out in nitinol fatigue. It is critical to run sufficient tests to the required life cycle and analyze survival and failure distributions.
Which acid is good for nitinol oxide removal?
Hydrofluoric and nitric acid solution, or hydrochloric and sulfuric acid solution.
We are currently implanting a nitinol valve with 0° austenite finish (Af) temperature. In order to ease the crimp difficulty, the crimp is performed inside an ice bath. We are exploring a small increase in Af temperature in the future, but meanwhile we want to know the effect of crimping in room temperature on the recoil after deploy (clearly yes) and its implications over the material’s endurance properties.
Crimping at room temperature compared to an ice bath would likely induce a higher permanent set. In any case, the device will experience 37° C in its crimped state as it goes through the delivery procedure inside the cardiovascular system. The temperature difference between the body temperature and the Af temperature is another factor that will contribute to permanent set.
The implication on the material’s endurance properties will depend on the orientation of the residual stress. Compressive residual stress induced by tension deformation in crimped state may help in prolonging fatigue endurance. Tensile residual stress induced by compressive deformation in the crimped state may negatively influence the fatigue endurance.
Are R-phase transformations not usually introduced in the constitutive models in FE simulation?
R-phase transformation is generally not included in the FEA constitutive model (see the figures in the image below). However, a good FEA practice should include mechanical testing of specimens that are representative of the final device at body temperature. The test results should then be used to construct the FEA constitutive model.
How do you minimize burrs while milling nitinol?
Proper selection of lubricant, and optimize feeding and cutting speed.
Are there differences right now for review of nitinol devices in the U.S. versus the EU?
FDA has specific guidance for the nonclinical assessment of medical devices containing nitinol.
For high-risk implantable devices such as TAVR, FDA requires 600 million cycles structural fatigue durability assessment, while the EU requires 400 million durability analysis
In EU, regulatory review is going through notifying bodies, such as TUV, DEKRA, etc. Current EU MDR requires documentation on the RoHS (restriction of hazardous substances) compliance.
Visit the Medical Design & Outsourcing Nitinol Hub for more nitinol know-how.
