a shape-memory alloy
A shape-memory alloy is a metal that remembers its shape: bend it while it is cold, warm it up, and it springs back to the form it was set in. Think of eyeglass frames that un-bend after you sit on them, or a stent that expands on its own once it reaches body temperature.
The mechanism is a reversible transformation between two crystal structures, martensite and austenite. When cold, the alloy is soft martensite that you can deform easily (its twinned crystals just reorient rather than permanently sliding). Heat it above a transition temperature and it snaps back to the austenite shape it remembers. The classic material is Nitinol, roughly half nickel and half titanium. The same alloy can also be superelastic: at body temperature it can absorb a huge strain (around 8 percent) and fully recover it.
This matters for stents, actuators, orthodontic wires, and eyeglass frames. The honest caveat: this is a phase transformation, not magic. The alloy fatigues with repeated cycling, its useful temperature window is narrow, and it is expensive and hard to machine.
A collapsed nitinol stent, crimped small and threaded into a blocked artery, warms to the body's 37 degrees C and springs open to its remembered wide shape, propping the vessel open. The same alloy in eyeglass frames is superelastic at room temperature: sit on them and the arms bend by 8 percent, then snap back flat instead of taking a permanent kink.
Body heat, or just room-temperature superelasticity, lets nitinol recover its programmed shape.
The effect is a reversible martensite-austenite transformation, not limitless magic: the usable temperature window is narrow, the alloy fatigues with repeated cycling, and it is expensive and hard to machine.