By Sean Fenske, Editor-in-Chief
 
Metals are common materials of choice for a wide range of medical device, implant, and healthcare applications. They offer strength, durability, and reliability. While not all made the same or offering the same properties, they are often similar in many ways. There are exceptions though, and one such significant standout for medical device manufacturers is nitinol.
 
Nitinol presents with rather unique attributes, making it an desirable material for certain applications, but also challenging to manufacture with due to these same properties. Getting the right mix of nickel and titanium for the desired qualities requires an expertise unto itself. Designing a nitinol-based device can be a completely different challenge, as well.
 
Fortunately, Scott Robertson, Ph.D., Vice President—Nitinol Technology at Resonetics, has addressed a number of questions on this unusual metal in the following Q&A. He offers insights on when to use the material (as well as when not to), the types of manufacturing processes to work the metal, and considerations designers need to keep in mind.
 
Sean Fenske: What is nitinol and why is it useful for medical device manufacturing?
 
Dr. Scott Robertson: Nitinol is a metal alloy comprised of nickel and titanium. Unlike conventional metals like stainless steel, however, nitinol possesses the incredible ability to recover its shape following quite large deformations. It can be thought of as a material with the strength of a metal and the flexibility of an elastomer. Due to this unique dual nature, nitinol is particularly useful in the cardiovascular, orthopedic, and dental markets where implants and delivery systems need a minimum strength to maintain structural integrity while simultaneously allowing deformability and recoverability to accommodate varied biomechanical and physiological motions.
 
Fenske: For what types of applications is this metal being used most often?
 
Dr. Robertson: There are many examples of nitinol being used in the medical device market. An example of a commonly-used disposable application of nitinol is in guidewires where physicians require material capable of maneuvering through tortuous pathways without kinking or permanently deforming. The cardiovascular community routinely uses nitinol in so-called “self-expanding” applications such as stents, filters, and heart valves. In these applications, nitinol is used for its ability to be compacted into a very small catheter, atraumatically advanced through the arterial or venous pathways to its target anatomy, then deployed and automatically expanded into a much larger shape. Dental applications include orthodontic archwires, which can impart a uniform force on the teeth over several weeks, instead of historically used metals that required routine tightening. Lastly, orthopedic applications such as bone staples utilize the superelasticity of nitinol to provide a chronic compressive force to promote the rejoining of fractured bones.
 
Fenske: What challenges are encountered when working with this material and how do you help resolve them?
 
Dr. Robertson: The properties of nitinol “gizmos” are incredibly sensitive to variations in the material’s chemical composition and/or the thermo-mechanical history witnessed during its manufacturing into a finished component. At Resonetics, we control the complete history of the material, all the way from its initial melting (which sets its chemical makeup) into its semi-finished material form (e.g., wire, sheet, or tube), and throughout the various manufacturing steps to convert that material into a finished component. To put nitinol’s sensitivity into context, a change in the chemical composition of merely 0.1% nickel, a heat treatment temperature only 1% hotter/colder or a heat treatment time mere seconds longer or shorter can very easily change the perceived stiffness of the device by 25% or more. With subject matter experts in melting, material selection and manufacturing, laser-cutting, machining, shape-setting, chemical processing, and design, we help our customers maneuver the numerous obstacles that have befallen many engineers when trying to use this very sensitive material.
 
Fenske: What considerations should a designer keep in mind with regard to nitinol when seeking to use it for a medical device project?
 
Dr. Robertson: Unlike conventional metals, which have properties that are reasonably uniform and their availability is prolific, nitinol requires dedicated attention to precise material selection and a knowledge of state-of-the-art manufacturing techniques to derive the optimal end-product performance. One simply cannot go down to the local hardware store and buy nitinol. Despite the introduction of a handful of online resources to purchase stock nitinol, these materials may have quite a wide variability in their properties. Even when uniformity of starting material can be obtained through careful specifications, any manufacturing processes that impart heat (e.g., laser cutting, machining, or shape-setting) can steer the mechanical properties in one direction or the other. The bottom line is careful procurement of raw material and precise control of the entire thermo-mechanical history is essential to creating consistent nitinol components.
 
Fenske: What manufacturing methods are used with nitinol? Can you machine it? Mold it? Use it with additive manufacturing?
 
Dr. Robertson: Nitinol is capable of undergoing most conventional manufacturing processes, although some are more readily suited to this material. Whereas laser-cutting, EDM, and grinding are the most commonly used methodologies for creating nitinol devices (and are deep technical competencies at Resonetics), we also routinely utilize more sophisticated and specialized manufacturing modalities, such as CNC machining and micro-machining, laser-ablation, photo-chemical etching, and laser-welding, when appropriate.
 
Although still in its infancy, additive manufacturing of nitinol is growing. However, due to its sensitivity to chemical composition (which is modified by the large oxygen absorption in its powder form) and to thermal exposure (which is high in selective laser sintering), most current applications of nitinol additive manufacturing are reserved for the orthopedic space where nitinol’s soft compliance relative to conventional biometals can prevent load-shielding and the micro-porous structure can promote bone ingrowth in orthopedic applications.
 
Fenske: Are there situations where a medical device manufacturer should avoid nitinol or applications where it simply will not work?
 
Dr. Robertson: For as much as nitinol is part of our core business, we half-jokingly recommend to use nitinol only when all other conventional materials have been exhausted. There are several solutions where typical biometals—stainless steel, titanium, and cobalt-chromium—can be modified into spring-like shapes and mechanically achieve the same flexibility nitinol inherently possesses. Also, when a polymer-metal composite can be constructed that combines strength and flexibility, nitinol may not be the best option.
 
In addition, since designing with nitinol does not follow the typical stress mechanics most engineers are taught in school, partnering with a reputable supplier such as Resonetics can burst the design envelope wide open to achieving the combined strength and flexibility only nitinol can provide. Indeed, physicians now implant stents and heart valves into complex anatomies with wildly cyclic deformations and the nitinol material withstands hundreds of millions of cycles without fracturing. There are countless applications where nitinol is really the only viable solution to the problem.
 
Fenske: Do you have any additional comments you’d like to share based on any of the topics we discussed or something you’d like to tell medical device manufacturers?
 
Dr. Robertson: Designing with nitinol can be incredibly rewarding, but also frustrating. Every application differs from the next. One application might require high strength; another may need fatigue resistance. Some designers may need ultra-small profiles or high radiopacity to visualize the device using fluoroscopy. Others may not need their devices to be biocompatible at all. Ultimately, there is a nuance to designing with nitinol, but it’s not a material that should be shied away from since its benefits are so tremendous. I encourage everyone interested in the combined strength, flexibility, durability, and biocompatibility properties to explore the use of nitinol. There is significant work and discovery that can be accomplished independently. However, when you hit a roadblock on material selection, component design, or manufacturing, that’s when it’s time to seek an expert to aid in the project. Our team at Resonetics looks forward to helping solve your needs.
 
I was first introduced to nitinol over 25 years ago and it piqued my engineering interest so much I’ve made a career out of working almost exclusively with the development of nitinol medical devices. If there is anything I, or others on the Resonetics team, can do to help, we offer “Office Hours” for folks to gain access to our experts. You can directly schedule a meeting with me at https://meetings.hubspot.com/scottrobertson. We enjoy seeing all the creative ways nitinol is being used and are excited to help move the design hurdles out of your way so the path to having your device improve the quality of patients’ lives can be fulfilled.
 
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