Nitinol is a metal alloy with unique properties and uses. This magical metal alloy can "remember" or change shape based on temperature. Here are some things you should know before you decide whether to use Nitinol in your medical device design.
How was Nitinol developed?
Nitinol originated from a mistake in 1959. Scientists are developing an alloy that is heat-resistant and corrosion-resistant, and in the process created an alloy made of 55% nickel and 45% titanium. The name represents its elemental composition and origin. "Ni" and "Ti" are the atomic symbols for nickel and titanium, and "NOL" stands for the Naval Ordinance Laboratory, the laboratory where it was discovered.
Where is Nitinol used?
Nitinol has been around since the early 1960s, but was not commercialized until 20 years later due to the tight controls required during the manufacturing process. Since then, it has become an important material for robotics and medical devices. Nitinol is superelastic (10 times more elastic than other metals) and its thermal shape memory properties are unlike any other material available.
Nitinol is often used in applications with tight space requirements when traditional mechanisms cannot be installed. One such application requires a device or structure to be inserted into a small opening and then released to open to a larger set size. A compressed nitinol wire or structure is inserted into a small delivery tube and then placed into place. Once deployed, the nitinol structure expands in size to several times the diameter of the delivery tube.
How does NiTi work?
One of Nitinol's most valuable properties is the two-way shape memory effect. This shape memory effect occurs when a metal undergoes a reversible phase transformation between the austenite and martensite phases. Metal atoms are arranged into specific structures based on their composition, but rarely change structural shape when in the solid state.
At high temperatures, metals enter the austenite phase. At this stage it reaches maximum stiffness and bends like a spring. Metals enter the martensite phase at low temperatures. At this stage, the metal feels elastic and bends easily. When Nitinol is in its martensitic form, it can easily deform into new shapes. However, when heated to its transformation temperature, it reverts to austenite and regains its previous shape.
Slight changes in alloy composition or heat treatment can adjust the temperature at which Nitinol remembers its high-temperature form. The purpose of the equipment determines the transition temperature you choose. For example, if you are making a medical device (such as a stent), you would choose a transition temperature that is close to or equal to human body temperature.
What are the practical applications of Nitinol?
Robotics engineers often use Nitinol as an actuator. In this case, they applied an electric current (or heat) to stretched nitinol wire. The wire shrinks during charging and relaxes after charging stops. Unlike most metals, Nitinol shrinks in length when heated but maintains the same absolute volume. Additionally, its thermal movement is 100 times greater than other metals.
Bone nails are an example of thermal dimensional shrinkage of NiTi alloys. The staples are pulled apart and inserted into the two holes in the bone, then heated to restore the original shape. This technique effectively pulls the two pieces together and holds them in place during the healing process.
Another example is a stent, which is cooled and mechanically squeezed to fit a small-diameter catheter inserted into a vein. Once positioned, the stent is released from the restraining sleeve and returns to its original shape upon reaching body temperature, keeping the artery open.
To achieve elastic deployment, Nitinol wire is mechanically held in place using clamps and then heated at a defined temperature and time in a fluidized temperature bath, followed by rapid immersion in cold water. Once removed from the clamp, the wire retains its shape regardless of the angle or intensity of deformation. When released, the wire returns to its programmed shape. There are many practical examples of this application, such as the Homer Mammalok needle/thread positioner. The device threads a curved wire through the straight cannula, and upon withdrawal, it bends back into its original "J" shape. Users can repeat this process dozens of times without deforming the wires.
What are some common challenges when using Nitinol?
One challenge when working with Nitinol is determining the best way to connect one wire to another. Depending on their phase, wires can be very elastic or very stiff, making them difficult to solder or glue. One method involves using other materials such as stainless steel to mechanically crimp the wires together. These crimps can then be TIG welded to other components to create the desired end product.