Nov. 26, 2007 -- In a split second, Venus flytrap leaves can snap shut, trapping an insect inside their botanical jaws. Now a microscopic lens inspired by the carnivorous plant's fast reflex can pop instantly between convex and concave when triggered.
A polymer coating made from thousands or millions of these lenses could be made into a range of applications, including paints that change color or even pop off a surface on command, or road signs that change with the light or temperature.
"This snap mechanism allows you to have a very small change in pressure lead to a very large effect," said Alfred Crosby, assistant professor of polymer science and engineering at the University of Massachusetts in Amherst. "This hasn't been previously done in terms of surface properties."
Crosby said to imagine the lens like a tennis ball that has been cut in half. If you slowly push on the half ball with both thumbs, the pressure will build to a critical point, where all of sudden, the half will turn inside out -- snapping from convex to concave.
"You won't be able to stop it," said Crosby.
That is the idea behind his team's surface. It starts with the same rubbery material used to caulk bathroom tiles and tubs. A scientist takes a flat sheet and creates an organized pattern of little depressions in the surface.
Next, the material is inflated like a balloon. During this stage, a polymer is bonded to the patterned material. When the stretching is stopped and reversed, little domes of polymer form above each depression.
Lastly, the polymer is treated with a chemical that removes a very thin layer from the surface. That changes the pressure and at a critical point, the mounds all snap into a concave shape.
"This is completely novel. There are definitely surfaces that people apply voltages to that release drugs from individual wells, but not surfaces that have the snapping effect. I've never seen anything like that before," said Jeffrey Karp, director of the Laboratory for Advanced Biomaterials and Stem Cell-Based Therapeutics at Harvard University in Cambridge, Mass.
"I don't think there are limitations in terms of what can be done. The next stage is really picking an application and moving into product development. It's a great milestone," said Karp.
Right now each depression is about the diameter of a human hair; Crosby would like to reduce the size, so that each dome would be hundreds of times smaller. Eventually Crosby and his team would like to reverse the process, too, so that the polymer starts off as a series of depressions that pop into mounds in the presence of heat, light, or voltage.
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