As the name alludes to, nitinol is a nickel-titanium alloy known for its shape memory and superelastic characteristics. Shape memory refers to nitinol’s ability to undergo deformation at one temperature, then recover its original, under formed shape upon heating above its transformation temperature.
Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the underformed shape to recover. The material exhibits enormous elasticity, some 10–30 times that of ordinary metal. It was initially discovered in 1968 in a search for mining materials. Still, it took decades for nitinol to become useful in the marketplace due to the difficulties encountered in its processing and manufacturing.
In fact, nitinol’s many uses have mostly all been realized since the mid-1990s when its usefulness in a broad range of applications derived from it being a shape memory alloys and its’ superelastic properties. It is now widely used for many medical components, including reinforcement braiding and coiling for catheter tubing and shafts, braided and formed stents, formed retrieval baskets, and many other applications. The shape memory makes it an excellent candidate for many in-body applications such as catheter reinforcement and braided stents. Nitinol is also used for these medical devices because it is biocompatible, meaning it is compatible with the human body and will, therefore, not be harmful to human tissue.
Other materials that are not biocompatible can result in issues such as blood clotting when used for similar applications. Nitinol is generally produced in large cast ingots and can be subsequently processed all the way down to ultrafine wires that are only a fraction of the size of a human hair. The final stages of the entire process are called wire drawing and require precision control in order to maintain physical and mechanical properties.
Ultra-fine wire is very small diameter (thickness) wire that is held to incredibly tight tolerances. Fine wire products are engineered with cutting-edge online gauging to deliver outstanding dimensional precision and reliability. Nitinol is produced at Ulbrich in fine and flat wire forms with micro-scale diameter and widths and is then meticulously wound to guarantee tolerance consistency and ensure downstream performance. Nitinol wire in this form is a great choice for medical devices that may be introduced into the body to perform surgical procedures or treat diseases, and their mechanical properties play a significant role in the selection of metal for their manufacture.
Applications for Nitinol Wire
Medical device manufacturers should also decide whether braid-enforced or coiled-enforced catheters are the best choice for their application. Many OEMs are now choosing to use nitinol for coiled catheters in especially sensitive surgeries or other applications where accuracy is of utmost importance, such as cardiovascular or neural surgeries. The advantage of this is the superelasticity of nitinol. It will act almost like a spring in that it can be stretched significantly and then return to its original shape. This is extremely beneficial in the medical field as the use often involves bending and twisting through arteries.
Another application for nitinol wire in the medical industry is braided stents. Nitinol stents are becoming more and more widely used in surgeries, especially to treat stenosis issues and issues below the knee. Nitinol stents would more so be used for the latter and is often a suitable choice for peripheral vascular disease, known as PAD. This is due to the force the nickel-titanium alloy can provide when it comes to holding open vessels, while still having enough elasticity to breathe. The two applications above are the primary use of nitinol round wire, but other applications include:
Resilient glass frames.
Orthodontic arc wire.
Temperature control system couplings.
Characteristics Affecting Nitinol Performance
There are multiple characteristics when it comes to producing and manufacturing this wire that will affect the final application: Physical characteristics, including size and tolerance, and mechanical properties, including tensile strength, elongation, and surface condition.
Physical Characteristics — Size and Tolerance. Size: nitinol is very abrasive on wire drawing dies and has to go through these dies multiple times to achieve the small diameters required in the medical industry. Tolerances: Tight tolerances need to be held when it comes to the diameter but just as important, they need to be consistent throughout the entire length of the wire. This is beneficial not only to the end use, but to minimize wear and tear on the drawing die.
Mechanical Properties — Tensile Strength, Elongation, and Condition. The typical two conditions for nitinol ultra-fine wire are cold worked and straight annealed. The preference depends on the processes of the medical device OEM and their manufacturing operations. Most of the wires will be heat set into superelastic condition at the next manufacturing tier.
Controlling all of these characteristics throughout the wire and ensuring uniform dimensions is essential to producing a good product and saving money in production. Precision equipment is required to keep these properties in check because even small inconsistencies result in issues, such as unusable parts or excess scrap, at the next manufacturing step in coil reinforcement.
Final Considerations of Ultra-Fine Nitinol Wire
Since nitinol also has a unique ability to adapt to extraordinary strains and is compatible with the human body, it is finding numerous applications in the medical field. What starts as ultra-fine wire ends up being used for braid products such as stents, orthodontic wire, catheters, and other surgical implant devices. With end uses like these, it is essential for medical device manufacturers to select an alloy and a supplier that provides materials with undeviating, consistent mechanical properties, ensuring that those properties are appropriate for processing medical wire. Both production methods and manufacturing sources are equally important in supply chains within the high-tech medical device industry.
This article was written by Doug Horan, Business Development/Marketing Specialist for Ulbrich Stainless Steels & Special Metals, North Haven, CT. For more information, visit here .