ME 481: Biomedical Team

Shape Memory Alloys (SMA) are alloys that are heat treated to be able to return to their pre-deformed state when heat is applied to the material.

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SMAs, like a nickel-titanium alloy called nitinol, are known for their ability to be heat treated to bend at higher temperatures.  SMAs like nitinol must initially be led through an annealing process, with temperatures exceeding 500℃, in which the alloy will reorient its crystal-like structure to remember its current shape. After heat treatment, SMAs will cool and revert to a martensitic phase. At its base, low temperature martensitic phase, the alloy is malleable and easily bent. As heat is once again introduced to the material, there is a transition temperature phase ranging from about 70 to 130℃. During this process the SMA begins to return to a non-stress state (the shape during the annealing process) with great force. In the application of the active needle, the nitinol actuator will easily be able to bend when heat is applied. However, if the needle is inserted into human tissue, simply allowing the nitinol material to rest and cool will not provide enough force to revert the actuator to its martensitic phase.