SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 108

aluminum in their 1903 airplane.) Aluminum alloys are ideal for fly-
ing since they fulfill two of the most important requirements—they
are strong yet lightweight.
Atoms of a metal or alloy can slide around a little bit, even though
the bonds holding them together are strong. This flexibility is useful be-
cause the material can be flattened or rolled into shape. Yet the strength
of the bonds results in a hard material, capable of withstanding a lot of
force. Because an alloy is a combination of different elements, it often
tends to be harder than a pure metal—the differently sized atoms mak-
ing up an alloy tend not to slide or move against one another as easily as
the identical atoms of a pure metal. Aluminum, with an atomic number
of 13, is not massive, though in its pure form it is quite soft. But when
combined with copper, magnesium, manganese, or other elements, alu-
minum makes strong alloys. Although aluminum alloys are not gener-
ally as strong as steel, they are not all that much weaker and have about
1/3 the weight.
Most of the airplanes of today, and space ships such as the space
shuttle, are made of aluminum. But engineers are increasingly using
composites instead of aluminum. Composites, as described in chapter
1, are composed of thin fibers reinforcing a matrix of plastic or metal.
Although these materials are generally expensive and require a little
more care than aluminum alloys, they offer an excellent combination of
strength and lightness of weight. About 30 percent of military aircraft
are now made from composites.
But the most efficient material is one that adapts to changing condi-
tions. Consider the iris and pupil of the eye, for example. The iris—the
colored part of the eye—has a hole, called the pupil, which lets light
into the eye. This structure is sufficient for vision, but if it were not able
to change, it would be far from ideal. At night, the eye needs a greater
opening to let in more of the scarce light; at noon on a sunny day, the
opposite problem occurs—letting in too much light would overload the
sensitive retina, the part of the eye that converts light into electrochemi-
cal signals. For optimal functioning, a pupil should get larger at night
and smaller during the day. Thanks to muscles embedded in the iris,
this is exactly what happens.
Just as varying levels of light place different demands on the eye,
varying circumstances pose different challenges to the variety of avail-
able engineering materials. Soaring and diving during flight are best