SMART MATERIALS—MATERIALS THAT ADAPT TO CHANGING CONDITIONS - Chemistry: Notable Research and Discoveries - page 110

Exploiting a material's adaptability usually requires a larger change
and a more effective way of controlling it. A material's I.Q.—intelligent
quotient, as whimsically used by materials scientists to describe how
“smart” a smart material is—depends on the material's responsiveness.
Smart materials should have a large response magnitude, achieved with
agility and quickness.
For example, certain automated processes—running with little or
no human intervention—require temperature regulation so that the
parts do not unduly expand or otherwise change functionality. What
kind of material could provide adequate temperature regulation for
these automated processes? Some materials such as platinum experi-
ence a rise in electrical resistance when heated; resistance refers to an
opposition to the flow of current, and an object's resistance is a property
that can be measured with a great deal of precision. Platinum is a well-
behaved material in this regard because its resistance varies smoothly
with temperature, with no jumps or dips that would throw off the mea-
surements. But the response, although smooth, is extremely small. The
response must be used to affect some mechanism in the automated pro-
cess so that any rise in temperature would be opposed or compensated,
and the small response of platinum would probably not be sufficiently
sensitive to launch an effective counteraction. But certain materials
such as barium titanate, with a small number of other elements added,
responds to even small temperature changes by increasing its resistance
thousands of times. This material can be part of an electrical circuit that
changes quickly and automatically when exposed to higher tempera-
tures, providing adequate regulation.
Other materials sense and respond to different features of the envi-
ronment. For example, compounds known as photochromic materials
change color in response to light. (The term photochromic derives from
Greek words meaning light and color.) In the 1960s, Corning Incorpo-
rated developed photochromic glass, which is now being used to make
photochromic lenses for eyeglasses. Photochromic lenses darken when
exposed to ultraviolet radiation—electromagnetic radiation of a slightly
higher frequency than the violet end of the visible light spectrum—but
remain transparent otherwise. The sun emits a lot of ultraviolet radia-
tion, which in high doses can be dangerous to the skin, resulting in sun-
burn and other damage, as well as to the eyes. The following sidebar
discusses how photochromic materials work.