Researchers at Pohang University of Science and Technology in South Korea have developed a film that can become wet or dry upon exposure to different wavelengths of light. Eventually, it could serve as a biosensor, a microfluidic device or an intelligent membrane.To make the material, the scientists alternately deposited layers of positively charged poly(allylamine hydrochloride) and negatively charged silicon dioxide nanoparticles on a silicon wafer, and the opposing charges enabled them to self-assemble. Then they coated the resulting material with a photoswitchable agent.For 10 minutes, the scientists focused 365-nm UV radiation onto the film through holes patterned on an aluminum mask. The UV-exposed regions of the material rapidly absorbed water in shapes like those on the mask (see figure). Shining visible light onto the film made those areas dry again, effectively erasing the patterns.Based on scanning electron microscopy and atomic force microscopy experiments, the researchers believe that UV radiation reduces the contact area between the photoswitchable agent and water, exposing the porous, underlying chemical layers to water.Journal of the American Chemical Society, published online Oct. 21, 2006, 10.1021/ja0655901.