Nitinol is nickel and titanium in nearly equal numbers of atoms, named for its makeup and birthplace: the Naval Ordnance Laboratory, where it was discovered in the early 1960s. The atoms settle into an ordered cubic lattice; small adjustments to ratio and heat treatment set the temperature range where it behaves superelastically.

Most metals bend by letting planes of atoms slip past one another, and that slip is permanent. Nitinol has another option. Under load its cubic lattice, austenite, shears into martensite without breaking a single bond. Remove the load and the lattice shifts back, and the part returns to its original shape.

Spring steel recovers about half a percent of strain before it bends for good. Nitinol recovers up to about 10%, roughly 20 to 30 times more. On a stress–strain curve this shows as two flat plateaus, one loading and one unloading; the area between them is energy absorbed, which damps impact and vibration.

Shape memory and superelasticity are the same transformation, triggered two ways: by heat for shape memory, by load for superelasticity. The SMART structure is tuned to work superelastically.

Air carries load at a nearly constant pressure: the tire flattens where it meets the road, conforms, and pushes back evenly. Nitinol's plateau does the same thing mechanically, holding a nearly constant force across a wide range of deflection on every rotation. Nothing to leak, puncture or reinflate, and the alloy resists fatigue well enough to be trusted in stents that flex with every heartbeat.

Titanium is the ninth most abundant element in Earth's crust; nickel is mined on six continents. Neither is a rare earth. Nitinol is already made at industrial scale for medical and aerospace parts, and both metals are routinely recycled.