Researchers at Northwestern University in Evanston, Ill., have reported the multicolor micropatterning of a thin-film substrate using a technique called wet stamping. The development has potential applications in the production of wavelength-selective diffraction gratings and gradient-index photolithographic masks as well as in the fabrication of biological assays.In the wet stamping technique, inorganic salts react with a dopant in a thin film to produce colored precipitates. Because the diffusion and reaction rates differ for the various salts' cations, the precipitates form in spatially distinct regions of the film. Courtesy of Bartosz A. Grzybowski.In the process, an agarose stamp is soaked in an aqueous solution of two or more inorganic salts, selected for their cations' reactivity with hexacyanoferrate ions and the colors they produce upon reaction. The loaded stamp then is brought in touch with a gelatin film doped with potassium hexacyanoferrate. Over 30 minutes of contact, the inorganic salts diffuse into the doped gelatin, where they react to yield colored precipitates. Because the diffusion and reaction rates differ for the various salts' cations, the precipitates form in spatially distinct regions of the film. The phenomena involved operate at submicron scales, so the process should offer higher resolution than ink-jet printing.While the researchers continue to investigate the potential of the technique, they are working with two artists to display representations of the patterns they have produced at a local museum. An exhibition is expected in the coming months.