Instant Waterproofing

In the spring, you're very likely to get caught in a downpour with no umbrella. What if you could instantly transform your coat into a raincoat whenever you need one?

As this ScienCentral News video reports, scientists say this kind of quick change could happen. They've just made the very first switchable surface.

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At Massachusetts Institute of Technology’s Institute for Soldier Nanotechnology chemical engineer Robert Langer has specialized in research where engineering meets medicine. Like other nanotechnologists, who work with materials at the molecular level, he is keenly interested in manipulating the properties of materials. For example, he has developed a shape-shifting plastic: If it’s bent or squashed into a different form, it reverts to its original shape when it’s heated. Langer’s plastic could be made into surgical thread—for stitches that tighten themselves and then melt away during healing.

Langer’s success in reversing shape led one researcher in his lab, Joerg Lahann, to wonder, “Couldn’t we switch surfaces, too?” Langer immediately recognized the enormous potential of Lahann’s idea: switchable surfaces could mean not only new kinds of useful materials, but also components for sensors or drug delivery systems. A surface that switched colors also could be made into a combat uniform that camouflages a soldier as the surrounding terrain changes.

The MIT team set out to make a surface that would toggle between attracting water and repelling it. But between Lahann’s inspiration and the first smart surface lay several challenges. The researchers began by making incredibly thin layers of gold and silicon—each only about a millionth of the width of a dime. On top, they evenly spaced rows of specially designed molecules, shaped like miniscule hairbrush bristles. Langer compares this new surface to “corn stalks in a field, all evenly spaced.” The top of each molecule was designed to attract water, while the stem repelled it. When a weak electric current passed through the layers, the researchers thought, the molecules would bend over, switching to waterproof. When the current was changed, the molecules would straighten up, reverting to water-attracting.

Langer says that the team has toggled the surface back and forth up to eight times. He is confident that the new surface will be the basis of materials that can reverse properties indefinitely. Now he and his colleagues plan to move on to more complex surfaces that could switch several properties at once.

Langer and Lahann’s research appeared in the January 23, 2003 issue of Science, and is underwritten by the National Science Foundation, the U.S. Army Research Office, and the National Institutes of Health.

By Ann Marie Cunningham