Custom shapes from swell gels

The curves and folds of a flower, the wrinkling of our skin, and the wavy edge of torn plastic are among the countless examples of patterns that emerge from the physics of thin elastic sheets. As cells grow nonuniformly, such as in a leaf or petal, stresses build up. To accommodate the deformation and relieve that stress the material has to either compress internally or bend. Both distortions cost energy, but which one Nature chooses depends on thickness: The thinner the sheet, the cheaper it is to buckle out of the plane. For the past decade, researchers have been striving to translate that competition into practical strategies for controlling the three-dimensional shape of a thin film. In a recent effort, researchers led by polymer scientist Ryan Hayward and physicist Christian Santangelo, both at the University of Massachusetts, Amherst, use a thin film of photoactive polymer gel that, when cooled in water, can swell up. The extent of the swelling depends on its prior exposure to UV light. The researchers exploit that tunability by using the tools of differential geometry to calculate the varied local exposures required for a film to spontaneously adopt a desired shape—for instance, a saddle, cap, cone, or other surface, as pictured here. They then imprint the calculated exposure pattern (the red dots) through a photomask and dunk the film in water. The method can produce more complex shapes, and applications in soft robotics, tunable optics, biomedicine, and elsewhere may come eventually. But for now, says Santangelo, “this is a toolkit for doing math with experiments.” (J. Kim et al., Science 335, 1201, 2012.)—R. Mark Wilson