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 A cluster of actual sporangium. Picture: umich.edu
"We've shown that this idea works," said Michel Maharbiz, assistant
professor of electrical engineering and computer science and principal
investigator in the group that built the device. "If you build these
things they will move. The key is to show that you can generate
electricity from this."
As often happens, the research started while doctoral student Ruba
Borno was exploring another idea entirely. Borno was interested in
mimicking biological devices, specifically microchannels that plants
use to transport water, so Maharbiz gave her a book on plants.
But something else in the book caught her attention---the section on how ferns spread their spores.
"It's essentially a microactuator," said Maharbiz, meaning that 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 were water logged, the sporangium remained closed
like a fist, storing the spores safely inside. But when the water in
the outer wall evaporated, it caused the sporangium to unfurl and eject
the spores into the environment.
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.
"Once we saw that, we thought, 'Oh, we have to build that,'" Maharbiz said.
The method for making the material is simple enough. A wafer is
coated with silicone and the 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, Borno said, they 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 opens wide---it moves.
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,
say, a solar cell in a calculator.
The ideal application, Borno said, would be to power a remote sensor where it's impossible to change the batteries regularly.
Related Links:
A video is available at: http://www.umich.edu/news/Releases/2006/Sep06/spore.html
For more information on the Maharbiz research group, visit: http://www.eecs.umich.edu/maharbiz/
For information on electrical engineering and computer science, visit: http://www.eecs.umich.edu/index.html
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