Muscle wire

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Metal strands of the Shape Memory Alloy TiNi are sometimes referred to as muscle wire. These alloy are sold under trademarks like Nitinol or Flexinol.

Muscle wire, also known as memory wire or memory shape alloy is a titanium nickel alloy that returns to a preset shape at a preset temperature. In Octofungi, the preset temperature is about 200 degrees fahrenheit. At this temperature, the wire contracts by about 3.5%. This contraction is translated over the length of the 17 inch wire into a range of motion for the legs of about 70 degrees.

The wire has a "programmed" temperature at which it has a "programmed" shape. When the wire cools, it goes back to a non-programmed shape. As the wire is heated, it tries to return to its programmed shape. Hence, the wire has two possible states. There is the cooled state (temperature) at which the wire can be stretched, and the programmed state (temperature) at which the wire returns to its programmed length. At the programmed state, the wire exhibits a crystalline structure known as austenite. As the wire cools, the structure changes to martensite, which is a herringbone shaped crystal lattice. The martensite is much more flexible than the austenite, allowing the cooled wire to expand. When the wire is heated to its transformation temperature, the structure reverts to austenite and the wire contracts. When in the austenite state, the wire is much more susceptible to stress, which can damage the wire. In addition, the resistance of the wire changes as the crystalline structure changes, making the wire difficult to control.

Muscle wire has found many application in the bio-medical field, the most notable of which is probably a method to aid in the treatment of aneurisms. It is also used as a catheter guide, an orthodontic arch shaper, and to join pipe ends together.