WAVY MICROSTRUCTURES, induced to grow in a polymer surface by a stressful puckering process, might be useful as a diffraction grating or as a part of various microelectromechanical systems (MEMS). George Whitesides (617-495-9430, gwhitesides@gmwgroup.harvard.edu), Ned Bowden (617-495-9434), and their colleagues at Harvard begin by heating a film of the elastic polymer material PDMS (polydimethylsiloxane) attached to a glass slide. The top coating of the film expands when heated, after which it is exposed to an oxygen plasma, which makes a silica-like crust. When the whole sample is cooled, the silica layer relieves the stress by puckering (see the figure at www.aip.org/png/html/sinus.html). The waves are locally ordered but will be rather disorderly on a global level unless an extra organizational rule can be imposed, in this case in the form of a bas-relief pattern (see the second figure) on the PDMS surface. The resulting wavy structures can be made with wavelengths as small as half a micron. This might facilitate a variety of uses, such as being part of a detection system for microfluidic devices, as stamps for microcontact printing, as masks for photolithography, or as surfaces on which cells can be grown and oriented. (Bowden et al., Applied Physics Letters, 25 October 1999.)