‘Superelastic’ Properties Shape Nitinol’s Reputation as an Alternative Material for Medical Devices

Nearly four decades after its serendipitous launch at the Naval Ordnance Laboratory, nitinol has become a worthy alternative to stainless steel as a medical device material. Interest in the nickel-titanium alloy began picking up steam in the 1990s, as device manufacturers learned more about nitinol’s capabilities. Today, the material is used in guidewires, stents, and a number of other device applications

Nitinol Devices & Components Inc. (NDC) helped that effort along with its own launch in 1991. Based in Fremont, CA, the company provides nitinol materials, components, and developmental assistance to start-ups and well-established device firms alike, says Matt Boyle, NDC’s principal manufacturing engineer.

“In the grand scheme of things nitinol is a relatively new material, especially the superelastic nitinol,” Boyle says. “It found its niche back in the early 90s or so. And nitinol tubing has only been around for an even a shorter period than that, about 12 to 15 years. But it really found its legs with the superelastic phenomenon.”

Indeed, nitinol’s standing rests to a great extent on the superelastic property that makes it the flagship material for the class of shape memory alloys that includes copper-aluminum-nickel and copper-zinc-aluminum. Nitinol’s shape-retention capacity and biocompatibility are two key reasons for its appeal to device companies.

Despite the industry’s growing familiarity with the alloy, some medical company customers have an overblown idea of nitinol’s pliability, Boyle says. Some clients believe “they can get too much recoverable strain out of the material. Although it is a superelastic material, it does have its limitations.”

The initial heating of a strip of the alloy in 1961 was the “a-ha” moment that led to the discovery of nitinol’s ability to return to its original shape after being bent again and again. Depending on the type of product, the decision on the exact temperature desired in order to control the nature of this “material phase transformation” is a crucial one for clients, says Dave Niedermaier, vice president of sales and marketing.

 “It’s where the customer wants the [superelastic] transition to occur that’s important. That’s usually something that has to be determined early on in the discussion” about a new product, Niedermaier says. “It’s critical.” The phase-change temperature selection is particularly important “in a lot of component applications,” Boyle adds.

The material phases are known by the terms “martensite” and “austenite.” In its martensitic, or low temperature phase, nitinol is ductile and can be easily deformed. Upon heating, the material to the austenitic phase, the material returns to its pre-deformed state. Nitinol can switch between the martensite and austenite phases to revert to its original, or “parent,” shape over and over again.

 “Typically, stainless steel has an elastic range of 1/2% deformation,” say John DiCello, NDC’s vice president of materials. “If you go beyond that it won’t come all the way back. And that’s [being] generous. Nitinol will give you 8%. You can basically bend a piece of nitinol wire around your finger and it will come all the way back. If you did that with stainless steel wire, you’d have a U-shaped wire. That’s true with most normal metals.

“Clearly, nitinol exhibits a unique stress-versus-strain curve,” DiCello continues. The stress-versus-strain curve is a common way of testing the mechanical properties of any metal or material, he says.

For both new and returning customers, determining a new product’s function is the key first step in the design process, DiCello says. That process often begins with “a drawing, a concept, or a request for a prototype,” according to Boyle. Product requirements could involve a device that “fits into a certain envelope, triggers something, has a certain strength, or requires a certain coating or corrosion resistance,” DiCello says.

A simple product such as a guidewire will have fewer design requirements, lower developmental costs, and faster time to market than a more complex device such as a stent or a blood filter, Boyle says. “We have to know what the product is doing, where it’s going to be used, or how many cycles the customer expects [the device] to experience,” he says. A guidewire with a nitinol core will begin with the focus on the OD of the wire, which is “typically a straight wire with no additional forming or shape setting required.”

Some taper grinding could be required for a guidewire, Boyle says. For those applications, “we would be able to offer wire material right on a spool so that the customer could cut it himself.” NDC also could offer to cut wire to length or form it if the customer needs “some sort of shape set to the end of the wire.” Mechanical properties of the wire and “optimum surface finish” are other potential considerations to be worked out, he says. Coatings include Teflon, polyurethane, and proprietary designs.

Designing a nitinol-based stent involves more steps, cost, and time than a guidewire or similarly basic device application, according to Boyle and Niedermaier. Because it starts with a tube, the material manufacturing process demands control of both the OD and ID, Boyle says. Biocompatibility issues make the surface finish critical for corrosion resistance as well. “In addition, we’re interested in the amount of strain the material is going to experience and how that affects fatigue life.” A strain-modeling tool called finite element analysis plays a design role in this case, he adds.

Because of its “very intricate design features” a nitinol stent “has many processing challenges,” Niedermaier says. The process starts with laser cutting the pattern in the tube. This is followed by post-processing to remove any detrimental effects attributed to laser cutting. “With a nitinol stent you’re then going to be shaping it and expanding it, if you will—heating and setting it,” all the while retaining the intricate design. “It poses challenges with tooling, heat-setting and controlling the surface,” he says.

Given the developmental challenges posed by the more complex devices, device manufacturers need to hone their design-for-manufacturing (DFM) capabilities, Boyle says. Over the last 10 years more OEMs have become DFM-savvy as nitinol use has grown, he points out. It’s an encouraging sign that at least half of NDC’s customers walking in the door now have DFM capability, Boyle says.

Customer confidentiality is always important in medical devices. “When a customer comes to us with these various applications, we try to guide them by understanding as much of the application as he’s willing to share with us,” Niedermaier says. Disclosure of information varies with customers and runs the gamut from clients that are quite comfortable sharing information to those that hold their cards closer to the vest. Typically, most customers “want an NDA in place” before disclosing information, “and we certainly accommodate them,” says Niedermaier.

The length of time from design to finished device typically takes three months to three years, depending on the complexity of the device and market conditions, Boyle says. “Once they test the first prototype then they can understand whether their design meets their criteria, and whether the material they chose is the right one,” he says. It may take several prototype iterations to finalize their design. DiCello says testing, for instance, may reveal that fatigue life doesn’t meet goals for the product, which may lead to another design iteration.

As far as budgeting is concerned, Niedermaier says the costs associated with providing first article build-to-print prototypes for a stent product depend highly on its complexity and other customer requirements. Typically, a lot charge of $3000 to $5000 will get a customer a handful of parts to evaluate. NDC is also available to work with customers on designing components for their applications.

Sometimes nitinol is not the best fit for an application. Some customers have chosen other materials such as stainless steel or plastic over nitinol after conferring with NDC, Boyle says. ”It’s sometimes a cost issue, or they may discover that their application doesn’t require the unique properties of nitinol. And sometimes, a lack of customer experience with nitinol may push them to choose a more traditional material.”

Niedermaier says NDC faces competition “up and down the line in all aspects of the nitinol world.” Competitors in materials, testing, design, and component manufacturing “have certain strengths,” DiCello says. But he notes that NDC “is a pretty big target because we touch all facets of the nitinol journey.”

In 2008, NDC became an independent company and now says it offers the broadest range of nitinol materials, components, manufacturing, testing and development services. As Niedermaier points out: “As the new NDC, we have a scope, target and reach that sets us apart in the market.”