There are lots of good data on nitinol biocompatibility. Every medical implant commercialized in the United States is subjected to some, if not all, of the tests required by ISO 10993—the biological evaluation of medical implants. However, these tests are unique to specific designs and surface finishes and are generally proprietary.

There are also quite a bit of published data in the literature. For example, a PubMed search on nitinol biocompatibility yields 81 publications, and there are also a few biocompatibility-related publications available on the NDC Web site.

Nitinol material is commonly specified and sold for applications that demand the unique properties of superelasticity—that is, the ability of the material to experience large deformations and recover to the original shape without any permanent set. Most commercially successful medical applications of nitinol rely on superelastic properties of the material at body temperature. NDC’s designation for this material is SE508, which stands for superelastic 50.8 atomic percent nickel, and this is the most commonly sold and used nitinol composition. In some other applications, nitinol is used for its shape-memory characteristics. By adjusting the metal’s composition or how it is processed, it can be made very soft and ductile under normal operating temperatures, but it can remember its original shape when it is exposed to elevated temperatures. NDC’s designation for this material is SM495, which stands for shape-memory 49.5 atomic percent nickel.

Superelastic nitinol wire and tube are typically cold worked by a drawing process to achieve desired straightness and mechanical properties, including the ultimate strength of the material, upper plateau stress, lower plateau stress, and so on. Generally, cold working increases strength, while subsequent thermal treatments required to shape-set the material or achieve a desired transition temperature decrease strength. Generally, the increase in strength comes at a cost of fatigue durability. Consequently, when designing any nitinol component, the designer must make judgments about the relative importance of strength versus fatigue performance and select an appropriate material composition and thermal processing strategy. For applications in which strength or superelasticity are not important, a designer may choose to specify partially or fully annealed material, thus reducing strength while increasing fatigue performance. NDC can apply any custom heat treatment to our standard materials to meet customer requirements, including full or partial annealing of superelastic material.

Great question, and no magic required. For any stent design, one can choose to laser cut the geometry at the constrained diameter, the expanded diameter, or any diameter in between. For stents with a very small constrained diameter (such as intracranial stents), it is often advantageous to laser cut the geometry from a larger diameter tube. This approach requires the designer to create CAD geometry that is matched to the desired tubing diameter, a step that may be aided by finite element analysis or other techniques.

Your question about feature dimensions is also an important one. It is important to plan for material removal after laser cutting, which means designing the laser-cut geometry with features that are larger than your intended final dimensions. The amount of material removal required depends on the specific material and processes that are used. As a starting point, it’s a good idea to assume removal of 20 to 40 µm from feature widths and as much, if not more, from wall thicknesses. This allows for removal of heat-affected zones or draw lines from the inner surface of the tube and results in an optimal surface finish.

From its earliest use in orthodontic arch wires to its more-recent dominant role in cardiovascular implants such as stents, endografts, and filters, nitinol possesses unique properties that have made it the material of choice for a variety of medical applications. The vast majority of medical applications take advantage of nitinol’s unusual superelastic properties.

While conventional engineering materials typically have an elastic limit much less than 1% strain, nitinol can experience fully recoverable strains up to 8%. This capability allows a properly designed nitinol component to radically transform its shape during service, fueling the trend toward minimally invasive procedures. For example, a nitinol stent may be designed to be delivered through a 2-mm sheath and expand to support a 10-mm vessel. Similarly, an endoscopic instrument may be delivered through a 15-mm instrument, expand to 60 mm to retrieve a specimen, and then collapse to exit through a similarly sized port.

In short, if a medical component must be delivered in a compressed state and then become an expanded shape, nitinol is likely to offer design advantages unavailable with other materials.