The smart memory metal nitinol is named after its place of discovery, the research facility: Nickel Titanium Naval Ordnance Laboratory. When the material is deformed in a cool state, it returns to its original shape after heating. It makes it ideal to use in the medical applications of the future.
The distinctive feature of nitinol is characterized by this reversible solid phase change, known as martensitic transformation. The alloy material forms a crystalline structure capable of changing from one form to another. Temperature change and/or strain induce this transformation.
Above its transformation temperature, nitinol is superelastic and thus can resist some degree of deformation when a load is applied. Once the load is removed, it returns to its original shape. Below its transformation temperature, nitinol is subject to the thermal shape memory effect. After deformation, it will remain in this state until it is heated above the transformation temperature so that it returns to its original shape.
Memory metal with superelastic features
Nitinol returns to its original shape, which means that surgeons can deform an instrument to fit the patient’s anatomy. After steam sterilization, it returns to its original shape (thermal shape memory effect). Examples of this application are dilators and suction cannulas.
Nitinol is also superelastic, allowing the material to be bent up to 10 times more than stainless steel. Thin wires and tubes made of nitinol are routed through multiple tortuous paths in the body and still remain controllable. Superelasticity allows instruments and components to maintain a wide variety of shapes even under tension.
Extreme flexible and biomechanical
Nitinol is kink-resistant and flexible, making it suitable for use in endoluminal instruments such as retrieval baskets. The baskets are extremely flexible. They allow easy access combined with high kink resistance, high set-up force and 1-to-1 motion transmission.
Nitinol’s biomechanical properties are also similar to biological material from a mechanical point of view. This makes it particularly suitable for use in implants. Materials such as stainless steel or titanium are very stiff and hardly elastic, so they do not yield even under pressure from surrounding tissue. Nitinol, on the other hand, with its biomechanical properties like human tissue, allows repeatable alternating stresses.
Processing nitinol
Nitinol requires special treatment. While the processing of other materials is usually unproblematic, with nitinol the main focus must be on maintaining the temperature-dependent properties. If the material is deformed in a cool state, it will return to its original shape after heating. Some processes can irreversibly damage both the material and the mold. For example, massive wear occurs when machining nitinol.
The use of nitinol in medical technology in the form of instruments and implants also usually requires special surface treatment. Our team has built up extensive expertise in the processing of nitinol over 20 years. Not only does the success of the process play a role, but also the cost-effectiveness in series production.
Applications
Endoscopy and soft robotics
“The super elastic properties don’t need heat. You constrain, you crimp, you pull through a tube. Then you push it into the body. The system of tubes in the body is very complicated and very long, and we don’t want to cut open the body until we get to the point of interest. We want to go endoscopically. That’s why endoscopic instruments are getting small in diameter, longer, more flexible, and softer”, says Dr Bernd Vogel, Global Technology and Innovation Manager at Alleima and an expert in processing nitinol. “
Crimping is an effective joining technique for nitinol wire, meaning it can be connected to other nitinol components or different materials, such as stainless steel.
“The next generation will work with soft endoscopic robotics, which goes deeper in the body, and there you have to use flexible instruments. This is our field, we can crimp our devices and apply them to these robotics and then they open in the point of need. This is where you need super elastic properties – stainless steel can’t do that since it is too rigid, and the elastic properties are far too low to fulfill these needs,” Dr Vogel adds.
Implants
"It’s the same with implants. You want to apply them through a very small catheter; a very small diameter application system. Then they should open up 20, to 30 times bigger when you apply it, therefore, you need extremely high superelastic properties, as well as a stable and stiff implant. These are exactly the properties nitinol offers", Dr Vogel ends.
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