All materials change in response to their environment. Most expand when heated, for example. Smart materials are designed by materials scientists and engineers to respond to changes in their environments – often in unusual or dramatic ways to achieve a specific purpose. NOVA’s "Making Stuff: Smarter" features scientists developing new airplane wings that will one day be able to change their shape smoothly in mid-flight, as birds do. (Nature, a master of response and change, inspires the design of many smart materials.)

A shape-memory material is a type of smart material that can be programmed to return to a previously set shape when exposed to certain change in its environment. The materials in this demonstration display their shape-shifting properties when exposed to heat. Other shape-memory materials respond to certain wavelengths of light, changes in the magnetic field, electrical currents, or chemical solutions.

Shape-memory and other smart materials are revolutionizing medicine, manufacturing, construction, and energy. Researchers are working to develop a smart fabric that senses the presence of blood and sends a signal to a handheld computer, alerting doctors that an unconscious combat soldier may be injured. Another potential application is a piezoelectric city sidewalk that senses the pressure of footsteps and converts that kinetic energy into electric current that can power streetlights and buildings.

This two-part demonstration focuses on two shape-memory materials, an alloy and a polymer. An alloy is a blend of metals. The alloy in the first part of this demonstration is a nickel (Ni) and titanium (Ti) alloy named Nitinol (pronounced “night-in-all”) whose shape-memory properties were discovered at the Naval Ordnance Laboratory (NOL) in White Oak, Maryland, in 1961 (hence the name NiTiNOL). It has a crystal structure, meaning the molecules are arranged in a rigid and regular structure, like a military marching band locked in formation. Most common materials undergo a phase change at specific transition temperatures. For example, they change from solid to liquid at their melting points, like ice to water, or from liquid to gas at their boiling points, like water to steam. Nitinol, however, when heated, undergoes a phase change while remaining solid. This causes its atoms to shift to a new arrangement, changing its outward shape, while remaining solid.

Below that transition temperature, the wire can be deformed because atoms shear past each other. It will hold that deformed shape until it is heated back above the transition temperature, at which point the molecules revert to their previous state. Training the wire to a new memorized shape requires a blast of thermal energy on the order of 500°C (about 900°F) and for the new shape to be temporarily maintained with applied force (such as pliers) until the wire sets and relaxes. Cooling the material ensures that the new shape becomes fixed.

Some other shape-memory alloys are copper-aluminum-nickel, copper-zinc-aluminum, and iron-manganese-silicon.

All plastics are polymers, which are long chains of molecules. Shape-memory polymers, however, are combinations of two polymers, each of which has a different melting point. One polymer sets the permanent memorized shape at the polymer’s melting point while the other polymer creates the temporary shape at a different, transition temperature. Heat softens this temporary shape (by breaking the crosslinks between polymer strands), and the shape-memory polymer reverts to its permanent shape. Some shape-memory polymers have up to three memorized shapes, each triggered at a different temperature.

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