To spread its spores, the fern sporangium transforms one form of energy, in this case heat via the evaporation of water, into motion. When the cells in the outer wall of the sporangium are water logged, the sporangium remains closed like a fist, storing the spores safely inside. But when the water in the outer wall evaporates, it causes the sporangium to unfurl and eject the spores into the environment.
‘It's essentially a microactuator,’ said Michel Maharbiz, assistant professor of electrical engineering and computer science.
The researchers examined some fern leaves under a microscope. They found that when exposed to light or heat or any evaporation-inducing event, the sporangia opened and released the spores.
The method for making the material is simple. A wafer is coated with silicone and then hit with light, causing a pattern. The residual pattern is lifted off and that is used for the device. It resembles a curved spine with equally spaced ribs fanning outward from the spine.
To make the device move, the researchers load the space between the ribs with water, and when the water evaporates, the surface tension of the water pulls on the tips of the ribs so that the tips move toward each other, straightening out the spine of the device. In this way, the closed device moves, opening wide.
They plan to add electrical components to the device in an attempt to generate electricity. They predict that the device will be able to generate the same amount of electricity as other scavenging devices, such as a solar cell in a calculator.
One potential application would be to power a remote sensor where it is impossible to change the batteries regularly.