Energy harnessed from humidity can power small devices

Researchers have built devices that harness changes in atmospheric humidity to generate small amounts of electricity, lift tiny weights, and even power a toy car. In the grand scheme of things, that captured energy is not free, but it’s pretty darn close. The study suggests that evaporation could be used to operate a variety of gadgets that don’t require a lot of power, scientists say.

“This is one of the first experiments to show that humidity can be a source of fuel,” says Albert Schenning, a materials scientist at the Eindhoven University of Technology in the Netherlands who wasn’t involved in the new study. The team’s designs, he says, “are very nice and very clever.”

All the gadgets rely on a simple phenomenon—the change in size of bacterial spores as they absorb moisture from the air and then release it, says team leader Ozgur Sahin, a biophysicist at Columbia University. Sahin and his colleagues used the living but dormant spores from Bacillus subtilis, a species of bacteria commonly found in soil and in the human gastrointestinal tract. Each spore typically swells and then shrinks up to 6% when moved from dry air to extremely humid air and then back again, Sahin says. The researchers harnessed that size-changing action by gluing thin layers of spores onto one side of curved sheets of polymer. When the spores swelled, that side of the polymer sheet lengthened—which in turn caused the curved sheet to somewhat straighten out. The stretching and contracting of these spore-coated polymer sheets are the driving force for the team’s devices.

A change in size of 6% may not sound that impressive. But when the researchers strung together a series of these polymer sheets, the “artificial muscles” they created quadrupled in length when relative humidity changed from below 30% to more than 80%, the team reports today in Nature Communications.  

The thicker the spore layer, the longer it would take for the muscles to react to changes in humidity. So, to make sure their artificial muscles were quick-acting, the researchers used spore layers that were extremely thin—no more than 3 micrometers thick, or about 5 spores deep on average, Sahin says. Tests showed that the devices could react to humidity changes within 3 seconds, he notes.

Tests also revealed that the spore-coated polymer strips could expand and shrink for more than 1 million cycles with little change in their range of motion. Other trials showed that the strips, when they shrank, could lift more than 50 times their own weight (video). But they did so much more slowly than an animal’s muscle would, so the power they generated—that is, their rate of energy production—was correspondingly low.

Nevertheless, the team harnessed changes in humidity to perform actual work. In one device, the back-and-forth motion driven by one artificial muscle suspended above a postage stamp–sized patch of water provided enough electrical power to light an LED. In another, the expansion of muscles on one side of a Ferris wheel–like device (where the air was humidified by evaporation from a wet paper towel) but not the other triggered an imbalance that caused the wheel to rotate (video). The team used the motion of a similar wheel to power a 100-gram toy car (video).

“These are fun demonstrations, but they prove the principle,” says Peter Fratzl, a materials scientist at the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, who was not involved with the work. Researchers are constantly looking for sources of energy, even if they’re small, he notes. “It makes sense to use these gradients [in humidity], because they’re everywhere and they’re free.”

The team’s results are a good conceptual starting point, says George Whitesides, a chemist at Harvard University. Such devices could, in theory, generate enough electricity to run a few transistors, he adds. “But it will still be a while before these things are in every child’s bathtub.”