Shape-memory stents made of superelastic nitinol, a nickeltitanium alloy, can be compressed, fed through an artery and restored to their intended shape upon delivery to keep diseased arteries open to blood flow. And superelastic nitinol guidewires, unlike those made from stainless steel, can conform to tortuous vascular spaces without compromising their structure. Yet, despite its profound impact in some medical applications, the alloy remains too flexible for other medical applications. That is, while nitinol is universally recognized for its ability to withstand deformation from up to 8% without permanent set, it is also relatively “flimsy.” For example, 316L stainless steel has a stiffness of nearly 200 GPa versus approximately 75 GPa for binary Nitinol. The drawback of steel of course is that the metal plastically deforms at about 0.5% strain.

Demand for a stiffer version of nitinol has grown in recent years as the medical device industry has become steadily more capable of building ever smaller devices (e.g. stents, vena cava filters, embolic protection devices, guidewires, etc.). To work properly, smaller profile devices must be made from alloys that exhibit the same strength and flexibility with less material than their predecessors. In other words, currently available materials have become a limiting factor.

With a new family of stiffer alloys in hand, device makers could design smaller shape memory or superelastic devices that retain their structural integrity under stress, but that enter the body through smaller access sites. The ultimate goal: quicker ambulation of the patient and better healing during common procedures.

A stubborn problem faced by the materials industry was the limit to how stiff you can make a binary alloy. Change the amounts of nickel and titanium beyond a certain point and you compromise elasticity and shape memory. The idea of adding small amounts of a third alloy to nitinol to solve these problems was not new, but a workable solution required the capability to fine-tune ternary alloy composition at the ingot level. With these trends in mind, Memry Corp set out to develop nickel-titanium-cobalt (NiTiCo) as the first in a family of “high stiffness” shape memory alloys.

Along with the potential to improve guidewire performance, NiTiCo promises to enable the design of thinner stents that hold arteries open with greater radial force than delivered by binary nitinol equivalents. The engineering team at Memry is analyzing the performance of NiTiCo in processes used to fashion it into components (e.g. shape setting, laser cutting) like NiTiCo tubing, which may serve as the core of next-generation vascular stents.

Careful consideration

The melting of nitinol is often recognized as a challenging process because the compositional control must be strictly maintained. A shift in the nickel-weight ratio from 55.8% to 55.7%, for example, increases the austenitic start temperature from approximately -15°C to 20°C. The same holds true for the development of ternary alloys such as NiTiCo. Cobalt was known to lower the transition temperature when alloyed with binary nitinol and naturally elevate loading plateaus, which made it a good first addition candidate.

Arriving at the composition that balanced the material’s stiffness in its austenitic phase with its superelasticity required careful stoichiometric consideration built on more than 25 years of experience with melting nitinol. Loading plateau stress equals the point at which enough force (stress) has been applied to begin thansforming the austenitc microstructure to its nonsuperelosstic martenistic form. The theory behind the adding of cobalt to binary nitinol is that the cobalt atoms replace selective nickel atoms in the matrix. Given that cobalt atoms are larger than nickel atoms, the misfit creates crystallographic stresses that elevate the plateau stresses under tensile load.

Measuring an advance

To validate that NiTiCo was stiffer than nitinol, engineers completed studies that found wire processed from the NiTiCo ingot showed an approximate shift in the upper loading plateau of approximately 30% when compared to standard binary nitinol under 3% strain (Figure 1).

Researchers also looked at NiTiCo stiffness using a 3-Point Bend Test, which found the elastic modulus of NiTiCo at room temperature was more than 20% higher than the binary samples. The extra stiffness translated into small differences at lower cycles when performing fatigue testing. The Strain-N fatigue testing performed (Figure 2), using a rotary beam method, demonstrated slight differences at higher strain levels. This makes sense given that the extra stiffness of the material should exert greater stress when subjected to a given strain. For devices designed for constant stress, researchers expect identical curves to be achieved with a smaller profile (e.g. wire diameter) device. As Figure 2 shows, high cycle fatigue results for NiTiCo appear to be nearly consistent with standard nitinol.

Along with the basic mechanical characterization of NiTiCo, it was important for engineers to answer fundamental questions such as whether the cobalt addition adversely affected the biocompatibility as compared with nitinol. Nitinol is commonly understood to have outstanding corrosion resistance due to the “tenacious” TiO2 layer that forms on the material’s surface, which translates into successful biocompatibility. It was important to ensure that the extra cobalt changed neither the cytotoxicity (ISO 10993-5:2009) nor the hemolytic compatibility (ASTM F756-08) of the material, and indeed NiTiCo demonstrated equivalence to standard nitinol.

Considering that nitinol has seen little development in the past 20 years, NiTiCo represents a welcome innovation for device designers seeking to create the next generation of lower profile, higher performance devices.

Who knew? A 1968 article in Time magazine described how metallurgist William Beuhler, while working at the Naval Ordinance Laboratory, had discovered the unique properties of nitinol, an alloy of nickel and titanium. Beuhler found that if the alloy was heated then cooled, crushed, and reheated, the alloy regained its original shape. The article went on to say that Buehler envisioned using nitinol to fashion bulky tools for dismantling metallic mines, crumpling the tools into balls for easy packing into submarines, and then restoring their original shape with a little heat on the ocean floor. Instead of the deep, nitinol had its greatest impact in medical. Today, the U.S nitinol-based, medical-device market, according to Toronto-based Milennium Research Group (mrg. net) accounts for nearly $2 billion in revenue.