This 0.012" Muscle Wire® (aka shape memory Nitinol) is a one foot long piece of nickel-titanium alloy that can flex and contract when a specific amount of heat or current is applied to it, thanks to shape memory and a change between the wires weaker, low temperature form (martensitic) and its stronger, high temperature form (austenite).
When the Muscle Wire® is in its martensitic form, it can be formed and bent into different shapes. However, when an electrical current of ~1.5A is applied to the wire, or is heated to ~100C it reverts to austenite form and recovers its previous shape with great force. Additionally, you can anneal at ~500C (use an industrial furnace or simply a lighter) it to whatever shape you would want, next once that current or heat is applied it will return to that new shape.
Note: The datasheet found below is for Flexinol® actuator wire, that isn’t exactly like all Muscle Wire® but it’s close. Flexinol® Actuators Wires are made specifically for linear actuators to replace small motors or solenoids.