It's not exactly going to replace the fuel in your Hummer, but this new engine -- which costs just $5 to build -- could represent the first step toward devices powered by nothing but bacterial spores and evaporating water.

In a study published Tuesday in Nature Communications, Columbia University researchers present several devices powered by the shrinking and swelling of bacterial spores in response to changes in humidity. The initial prototypes may look like child's play, but the researchers are convinced that they've only just begun to tap the energy potential of the technology.

The work started nearly a decade ago, when lead author and associate professor Ozgur Sahin participated in a project studying bacterial spores. Sahin was contributing a high-tech microscope his lab had developed, but what he learned about the spores inspired him to turn his focus to them.

"People before us had shown that the spores change shape in response to humidity," Sahin told The Post. "They shrink when they're dry and expand when exposed to moisture. But in our studies, we found them to be surprisingly rigid. That told us that this shape change must come with a lot of energy. In the beginning, I was just amazed at this biological substance. But then I thought, there must be applications for this."

Sahin has shown in previous studies that the spores are capable of harvesting large quantities of energy from their interaction with water. But he realized he'd need to show the energy storage and release in action if he wanted the work to move forward.

"I sketched out this idea, where a device was placed on the surface of a reservoir of water," he said. "If you made shutters that could allow moisture through or block it, you could control the humidity of the spores inside, making them cyclically expand and contract."

And if the spores were rigged to be attached to the shutters, he realized, the device could be self-regulating. When spores got too dry, they'd shrink up and close the shutters, creating a moist environment that lead the spores to expand and open the shutters, ad infinitum.

In the new study, his team has cobbled together devices that create lifting and piston-like motions by harnessing the natural tendency of the spores -- which are commonly produced in great quantities for probiotic supplements -- to expand and contract. The devices contain strips of plastic coated in the spores to make their reaction easier to harness.

The devices already generate enough power to turn on light bulbs, so Sahin thinks that the technology could be used to power lights on devices that sit on the sea -- hydrothermal generators or oil rigs, for example. And he points out that his team cut a lot of corners to produce a prototype that worked, so they could see a big uptick in the energy output pretty quickly. They currently use regular Elmer's glue to adhere the spores to the tape, for example, and that bond can't actually withstand the power that the spores are capable of tugging it with. Switching to a better adhesive could make the machines work more efficiently.

Sahin and his team will work on scaling up the engines. In theory, he said, bacterial spores could produce more energy per square foot than wind farms, for a much lower cost. But for now this hasn't been demonstrated in a real device. Still, he hopes the low cost of the prototypes he's produced will inspire some clever uses of the (very cool) phenomenon.

"Our devices may seem like toys, but a lot of big technologies start out that way," Sahin said.