Wrinkle, wrinkle, little polymer

Scientists have developed a cheap and easy way to create specific patterns of tiny wrinkles on the surface of a flexible and commonly used polymer—a technique that could be used to fabricate an assortment of microdevices.

Polydimethylsiloxane (PDMS), the soft polymer that’s the main ingredient in Silly Putty, also comes in transparent, pliable sheets in which some of the material’s long-chain molecules are chemically cross-linked, says John W. Hutchinson, a mechanical engineer at Harvard University. He and his colleagues found that when they irradiated a 3-millimeter-thick sheet of PDMS with a beam of gallium ions, the material became a wrinkled, glasslike skin about 25 nanometers thick.

Bombardment by about 10 trillion ions per square centimeter formed mostly straight lines spaced about 460 nm apart. At exposures above 70 trillion ions/cm2, smaller wrinkles formed atop these lines, the researchers report in the Jan. 23 Proceedings of the National Academy of Sciences. Various combinations of repeated exposures generated similar yet distinct patterns of ridges.

Because wrinkled areas of PDMS sheets repel water more readily than unwrinkled areas do, zapping the material with gallium could create tiny channels that might steer various chemicals within “lab-on-a-chip” devices, says Hutchinson. Other possible applications include the manufacture of optical filters and sensors, the researchers suggest.

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