Nitinol is one of a family of materials that are remarkable in that they exhibit a unique, unusual feature called "shape memory." Components made from these materials that have been deformed under the proper conditions return to their original, predetermined shape when heated to the proper temperature.
The remarkable properties of shape memory alloys come from the ability of the atoms in the material to change their crystal structure during the deformation process, resulting in a reversible solid-state phase transformation from an austenite to a martensite (or distorted) phase. This reversible shape transformation requires between 10,000 and 20,000 psi of mechanical stress, which enables the material to be used as an elastomer with excellent shock absorption properties.
The properties of nickel titanium alloys are sensitive to the percentage of nickel compared to the titanium, so that very small increases in the ratio can have a dramatic effect on the transformation temperatures and austenite yield strength. This sensitivity is a powerful advantage because it allows manufacturers to control the transformation temperature to produce ingots with desired properties and also gives them a means to control the material composition.
While nickel titanium alloys are widely used for a variety of devices in the medical industry, there is still much that is not known about their biocompatibility and corrosion resistance. This is especially true of the wires that are commonly used in the second stage of orthodontic treatment, which can be subject to galvanic corrosion in simulated saliva.
