Ozgur Sahin, Ph.D., believes that water evaporation is the largest power source in nature. In an effort to demonstrate the potential of this untapped resource, Sahin and his fellow researchers have created prototype electrical generators with rubber sheets that move in response to changes in humidity thanks to a coating of bacterial spores.

In a 2012 study published in the Journal of the Royal Society Interface, Sahin, along with Wyss Institute Core Faculty member L. Mahadevan, Ph.D., and Adam Driks,Ph.D., a professor of microbiology and immunology at Loyola University Chicago Stritch School of Medicine, detailed how a soil bacterium called Bacillus subtilis dries up to become a tough, wrinkled, dormant spore. These spores can then be almost immediately restored to their original shape when taking on water.

With their ability to shrink reversibly, the researchers realized the spores must be storing energy. In an effort to measure the energy of the spores, Sahin coated a tiny, flexible silicon plank in a solution containing the spores with the assumption he would be able to measure the humidity-driven force in a customized atomic force microscope. To his surprise, before he could even get it under the microscope, Sahin could see with the naked eye the plank curving and straightening in response to the subtle humidity changes from his breath.

"I realized then that this was extremely powerful," said Sahin.

Powerful indeed. Sahin discovered that the flexible, spore-coated plank could generate 1,000 times as much force as human muscle when the humidity was increased from that of a dry, sunny day to a humid, misty day – this is 10 times greater than materials currently used to build actuators. Sahin also calculated that moistening 1 lb (0.45 kg) of dry spores would generate enough force to lift a car 3.2 ft (1 m) off the ground.

After testing silicon, rubber, plastic, and adhesive tape, Sahin settled upon rubber as the most promising material for a spore-coated actuator. Using Legos, a miniature fan, a magnet and a spore-coated cantilever, he constructed a simple humidity-driven generator that produces electricity via the rotation of the magnet that is driven by the cantilever flipping back and forth in response to changing humidity levels.

Although the prototype only captures a small percentage of the energy released by evaporation, Sahin says efficiency could be improved by genetically engineering the spores to be stiffer and more elastic. In fact, in early experiments a mutant strain provided by Driks has already been shown to store twice as much energy as normal strains. The researchers believe the technology will one day make it possible to have electrical generators driven by changes in humidity from sun-warmed ponds and harbors.

"Solar and wind energy fluctuate dramatically when the sun doesn't shine or the wind doesn't blow, and we have no good way of storing enough of it to supply the grid for long," said Wyss Institute Founding Director Don Ingber, M.D., Ph.D. "If changes in humidity could be harnessed to generate electricity night and day using a scaled up version of this new generator, it could provide the world with a desperately needed new source of renewable energy."

The team, which also included Xi Chen, a postdoctoral research associate at Columbia University, has had their study published in the journal Nature Nanotechnology.

A spore-covered piece of latex is shown moving in response to changes in humidity in the following video.

Source: Wyss Institute