U.S. scientists say bacterial spores harnessing the untapped power of evaporating water could lead to generators turning humidity changes into electricity.

Researchers at Harvard University report experiments with a sheet of rubber coated on one side with spores that cause the sheet to bend when it dries out, in the same way a pine cone opens as it dries or a freshly fallen leaf curls, and then straightens when humidity rises.

Such bending back and forth means spore-coated sheets or tiny planks could act as actuators to create movement, and that movement can be harvested to generate electricity, a university release reported Monday.

"If this technology is developed fully, it has a very promising endgame," said Ozgur Sahin, who led the study first at Harvard and now at Columbia University.

Water evaporation is the single largest power source in nature, he said.

"Sunlight hits the ocean, heats it up, and energy has to leave the ocean through evaporation," he explained. "If you think about all the ice on top of Mount Everest -- who took this huge amount of material up there? There's energy in evaporation, but it's so subtle we don't see it."

Sahin and research colleagues studied a soil bacterium called Bacillus subtilis that wrinkles as it dries out like a grape becoming a raisin, then is restored to its original shape when water is present.

The researchers realized the reversible shrinkage meant they had to be storing energy.

"Since changing moisture levels deform these spores, it followed that devices containing these materials should be able to move in response to changing humidity levels," Harvard researcher L. Mahadevan said.

Increasing the humidity from that of a dry, sunny day to a humid, misty one enabled a flexible, spore-coated plank to generate 1,000 times as much force as human muscle, and at least 10 times as much as other materials engineers currently use to build actuators, the researchers reported.