When the petal of a flower is being formed, its shape is achieved by cells in one area expanding more than cells in an adjacent area. This uneven expansion causes the material to buckle, creating the desired curves and creases. Scientists from the University of Massachusetts, Amherst have taken that same principle, and applied it to flat polymer gel sheets that fold themselves into three-dimensional shapes when exposed to water. Some day, such sheets could serve a number of useful purposes.
The researchers use a photolithography process, in which parts of each sheet are masked with a thin painted-on coating, followed by an exposure to ultraviolet light. The polymer that is masked from the UV light will uniformly expand like a sponge when exposed to water. In areas that aren't masked, however, the UV light causes the molecules within the polymer to become cross-linked. This means that the material in those areas will only experience limited expansion when water is added.
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When areas with cross-linked molecules are flanked by areas of the protected polymer, buckling will occur as the one area expands more than the other. By strategically patterning the size and placement of the cross-linked areas, along with subjecting some of them to a second UV exposure, the scientists have been able to determine what 3D shapes the sheets would expand into when wetted.
So far, they've managed to create basic shapes such as spheres, saddles and cones. Down the road, however, the researchers believe that the technology could be used in fields such as biomedicine, where cultured cells could be designed to form themselves into blood vessels or specific organs. It could also find use in applications such as robotics, and tunable micro-optics.
It's reminiscent of research recently performed at North Carolina State University, in which black stripes were printed onto pre-stressed flat sheets of polymer. When subjected to infrared light, the striped areas absorbed more energy than the surrounding material. This caused the underlying polymer to contract, which in turn caused the sheets to fold themselves into 3D structures.
A paper on the University of Massachusetts research was recently published in the journal Science.