Silk® NiTi: A Breakthrough in Smoothness

The smoothness of wire used in a self-expanding Nitinol stent, venous filter or other similar device is of paramount importance. Nowhere is this more critical than in applications involving braided Nitinol wire, such as stents, when wires must easily slip past each other and other components without catching. Historically, meticulous applications such as these require mechanical polishing or electropolishing to achieve a smooth surface, both of which are costly and time-consuming.. Meticulous applications such as these historically require mechanical polishing or electropolishing to achieve a smooth surface, and these processes are costly and time-consuming.

Fort Wayne Metals is making things easier for Nitinol device manufacturers with the development of Silk® NiTi, an ultra-smooth Nitinol wire. Silk® NiTi has a bright, oxide-free surface with a surface roughness of only 3 µin (0.0762 µm) RMS or less. By comparison, surface roughness in mechanically polished Nitinol usually ranges from 2-6 µin (0.051-0.15 µm) RMS and electropolished ranges from 2-9 µin (0.05 – 0.23 µm) RMS. By reducing the roughness, friction between filars of wire is reduced, which makes Silk® NiTi an ideal off-the-shelf material for braided applications without the need for additional processing.

Silk® NiTi has been shown to significantly reduce snags in braiding operations, and the exactingly finished product can help Nitinol device manufacturers streamline their processes by eliminating any requirement for further polishing.5 In addition, Silk® NiTi offers comparable or better performance than other oxide-free Nitinol finishes. Its fatigue life is higher than etched or etched and mechanically polished surfaces, and much better than pickled (chemically finished) surfaces.

Silk® NiTi can be produced in sizes from 0.0008" (0.020 mm) to 0.040" (1.01 mm), with several of Fort Wayne Metals’ Nitinol grades (#1, #2, #4 and #8) offering varied mechanical properties and transition temperatures suitable for a broad range of medical and non-medical applications.6

Fort Wayne Metals has been committed to manufacturing medical grade wire since 1971 – recently becoming the world’s first melt-to-finish source of Nitinol to the medical device industry. Now a leading authority in the production of Nitinol wire, Fort Wayne Metals manufactures Nitinol in hundreds of combinations of composition, form and finish; also offering a shape-setting service that provides Nitinol wires that readily conform to virtually any shape specified by the customer.

Understanding Nitinol

Nitinol is an alloy comprised of roughly equal parts titanium and nickel. However, its unusual temperature-dependent behavior results in a wealth of applications that are changing the shape of medical device manufacturing, robotics and other technological industries.

Nitinol is a shape memory alloy (SMA), a metal with the ability to effectively “remember” a specific configuration and revert to that shape in response to certain stimuli. In the case of Nitinol, heating the metal beyond a certain transition temperature causes it to return to whatever shape it was ‘programmed’ with during manufacture.

With careful tuning of the alloy’s composition, this transition point can occur anywhere from around -20°C to 120°C. In a controlled temperature range just above the transition temperature, Nitinol becomes superelastic, whereby it is able to spring back from huge strains of up to 8% – 16 times higher than stainless steel.1

The atoms in Nitinol are so strongly bound to their original positions that the metal is capable of exerting force to return to its initial shape when heated past its transition temperature. This combination of accurate sensing and powerful actuation in a single compound, along with Nitinol’s ability to withstand large deformations at high temperatures without kinking, has proven invaluable in applications that prioritize compact actuators and flexible components.

These properties, coupled with Nitinol’s natural corrosion resistance and biocompatibility, have made the alloy especially popular in the medical device industry. Customers have successfully used Fort Wayne Metals’ Nitinol in biomedical implants such as stents, which automatically expand after implantation in response to body heat.2 Meanwhile, dental braces sprung with Nitinol wire can withstand higher strains and require less adjustment than their stainless steel counterparts. Prosthetic heart valves based on Nitinol films offer excellent durability and efficiency as well. 3,4 With ultra-smooth Silk® NiTi, the possibilities for life-saving and life-changing devices continue to expand.

References and Further Reading

  1. Manufacturing of Nitinol Wire for the Medical Industry. Available at: https://www.azom.com/article.aspx?ArticleID=11685. (Accessed: 22nd October 2018)
  2. Markets - Fort Wayne Metals. Available at: https://www.fwmetals.com/markets/. (Accessed: 22nd October 2018)
  3. Understanding the Shape-Memory Alloys Used in Orthodontics. Fernandes, D. J., Peres, R. V, Mendes, A. M. & Elias, C. N. 2011, (2011).
  4. Fabrication and Evaluation of Nitinol Thin Film Heart Valves. Loger, K. et al. Cardiovasc. Eng. Technol. 5, 308–316 (2014).
  5. Silk Nitinol - Fort Wayne Metals. Available at: https://www.fwmetals.com/materials/nitinol/silk-nitinol/. (Accessed: 23rd October 2018)
  6. Wire - Fort Wayne Metals. Available at: https://www.fwmetals.com/materials/nitinol/wire/. (Accessed: 23rd October 2018)

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