Nitinol
is an excellent material to use when creating components for minimally invasive medical devices, such as guidewires, catheters and stents. When medical professionals need to navigate in particularly tight areas, nitinol has both the flexibility to change shape as needed, and the durability to endure high amounts of strain (8 to 9 %). By comparison, stainless steel can take less than 1% strain before plastic deformation.
Nitinol is an especially valuable alloy because it is extremely elastic and can change shape depending on temperature. This means that when Nitinol is cool (martensitic), it is bendable and exceptionally soft, and when it is warm (austenitic), it is rigid and super elastic. No matter the temperature, stainless steel can be difficult to bend. Nitinol’s shapeable properties are useful for medical coils because it can “remember” its previous shape when heated.
A Brief History of Nitinol
In 1965 scientists were working to create a heat and corrosion-resistant alloy and discovered that combining nickel and titanium did the trick. They had discovered a unique alloy which remained strong and durable, no matter how often it was bent and reshaped. They named it “nitinol” to represent the atomic symbols for nickel and titanium (“ni” and “ti”) and to commemorate the name of their laboratory, Naval Ordnance Laboratory (NOL). It took manufacturers until the 1980s to get wind of the new development, but its superior qualities continue to outshine stainless steel.
Over time these properties proved to be highly valuable when applied to small medical devices for insertion into hard-to-reach places by surgeons. Soon after it started to be tested and experimented within the medical setting, a third key benefit was found, with its metallic composition offering high levels of both biocompatibility and corrosion resistance when treated, making it an ideal material for use in the human body. Since the 1990s, nitinol has proven to be a critical material within a wide range of medical devices with an ever-growing list of applications.
Nitinol vs. Stainless Steel
Thermal conductivity. Nitinol can handle heat very well. Conversely, the composition of stainless steel means heat can compromise its structural integrity.
Corrosion resistance. Once a component has been created using Nitinol, it lasts for a long time. Stainless steel’s corrosion resistance is highly dependent on what type of steel you are using; there must be a high chromium level present to make it extra resistant to corrosion.
Biocompatibility. Nitinol can be safely used in the human body, making it appealing for the medical industry. Manufacturers of medical devices must be certain that the materials they use will not only last for many years, but also not cause adverse reactions in patients. Stainless steel, on the other hand, can be used temporarily for very specific things, but generally is not preferred for implant use in the body.
Nitinol Product Applications
Given Nitinol’s unique elastic properties and biocompatibility, medical device manufacturers have been using nitinol in an ever-expanding list of product applications:
Stents: Doctors use stents to keep arteries open, so the material used to make them must be both strong and flexible. Nitinol is the perfect alloy to use because it has both those properties; additionally, it is also kink-resistant. Stainless steel would potentially make this unsafe and can be damaged. Indeed, stents are perhaps the most obvious example of why the super elasticity of nitinol can be so beneficial. When stents are inserted into the body, they can be compressed down to a minuscule size to be used in minimally invasive procedures. When the stents reach their destination, however, they expand to fill the necessary space and brace the inside arterial wall. This just could not be done with stainless steel.
Dentistry: Dentists and especially orthodontists need wires and brackets that hold braces together and perform the function of moving teeth. Nitinol’s shape memory is particularly useful with arch-wire applications. Stainless steel would be difficult to use in these situations.
Aspiration Catheters: With the superior strain characteristics, nitinol is becoming the go-to material for products used in the treatments of ischemic strokes. These catheters are built to allow physicians to remove occlusions as quickly as possible, sometimes in 10 minutes or less. The coil structure provides the open passage for removal of the clots, but the superior flexibility of nitinol allows it to reach areas of the brain previously unattainable.
Orthopedic implants: When surgeons are replacing patients’ joints, they need components that will help patients regain flexibility and range of motion—as well as easily adapt to an individual patient’s tissue. Nitinol is the preferred material for these components, as stainless steel is much less flexible.
CWT Expertise with Nitinol
Custom Wire technologies has been working with nitinol for almost 15 years, right from the beginning of the material’s boom in the medical device industry. “In 2010, we had a customer request a nitinol suture threader,” explains Bob Boldig, founder of CWT. “The threader device required the use of nitinol so it could return to the original shape after passing through the small hole in the instrument. This allowed personnel to easily load the suture through the device.”
“It all just grew from there! In the past 6-8 years, it’s really gained traction to where we’re processing a tremendous amount of nitinol. In any given month, our grinding department could be shipping in excess of 25,000 ground nitinol wires. Our coiling department routinely produces nitinol catheter coils beyond 10,000 coils per month. Furthermore, shape set nitinol from our forming department can exceed 5,000 parts monthly. I didn’t think it would ever take over stainless steel, but the demand continues to increase.”
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