Shape-Memory Materials, 4D Printing, and Bio-Active Materials

Shape-Memory Materials

Materials science is all about selecting the best material available to do a specific job. For example, when you design a jet engine, you choose strong materials that are extremely light and can cope with high temperatures. You could choose aluminum, titanium, or metal alloy. But what if you want to make an aircraft component that reacts in one way at low temperatures and in another way when it heats up? This is the kind of situation where you could use a shape-memory alloy, which can reshape itself automatically as the temperature changes.

Ordinary metal artifacts do not have a memory of their form. If you sit on a pair of aluminum eyeglass frames and bend them permanently (in scientific terms, “subject to plastic deformation”), it’s tricky to get them back exactly as they were. You have to use your own recollection of what the frames were originally like and painstakingly twist and bend; even then, there is no assurance that the frames would look as they used to, and they can even crack completely—through fatigue—if you wiggle them back and forth too often.

Shape-memory materials act differently. They are strong, lightweight alloys (usually two or two-metal mixtures) with very special properties. They can be “programmed” to remember their original shape, so if you bend or squeeze them, you can get them back to that original shape just by heating them up again. This is called the shape-memory effect (or thermal shape-memory effect, because thermal energy causes it to happen). Some shape-memory alloys remember one shape when they’re hot and another when they’re cold, so if you cool them, they’re going to jump into one shape, and if you heat them, they’re going to “forget” the shape and flexion into another. This is known as the two-way form-memory effect. Now, what if you could make a shape-memory object that bends and twists a huge amount but still returns perfectly to its original shape, even without heating? This is an element of shape- memory called pseudo-elasticity or super-elasticity and is used in those super-bendy, virtually indestructible eyeglass frames that manufacturers say are at least ten times more flexible than steel.

Although nitinol (also called nickel-titanium, Ni-Ti) is perhaps the most knowrn form-memory alloy, there is much more to it, including copper, zinc, and aluminum alloys (Cu-Zn-Al); copper, aluminum, and nickel (Cu-Al-Ni); iron, manganese, and silicone (Fe-Mn-Si); and quite a few others.