How bacteria sweat could one day power a robot

Water, plastic and bacterial spores might become the power source of the future. The spores soak up moisture and release vapor, and researchers at Columbia University have built a device to tap the energy released by this evaporation.

“Evaporation is a big source of power in nature, and it’s been neglected because there isn’t a clear demonstration of how to use it yet,” said Columbia University biologist Ozgur Sahin, who led the study published June 16 in Nature Communications.

When a bacterial spore takes on water, it swells, which is known as hygroscopy. But upon entering a drier climate, the spore dehydrates and shrinks back to its original size. The switch from bloated to shriveled can happen within a fraction of a second.

Sahin and colleagues placed these sponge-like spores onto long pieces of kapton tape, which are commonly used by engineers. When the tape gets exposed to water or humidity, the spores expand and the tape stretches. As the spores dry, the tape scrunches together. In the end, this stretching and contracting process resembles a muscle — hence the name “hygroscopy driven artificial muscle” or HYDRA.

Depending on what the tape is attached to, the spores can pack a punch. These microbes can turn a rotor to drive a slow car. The evaporation engines can also pull on shudders or lift a pole weighing three thousandths of an ounce.

Generating power from steam isn’t new. The steam engine was invented in 1698, and the evaporation-powered dippy the bird stole the hearts of children in 1945. But the steam engine needs coal, and good ole’ dippy needs fancy chemicals to get moving. Sahin’s new technology only needs an open water source, making it a great option for off-the-grid applications. It cost Sahin’s lab roughly $5 to make each of their initial fuel-free prototypes.

Sahin acknowledges the technology isn’t quite there yet. The team paired the spore engine with a generator, the same device that turns the mechanical energy of a windmill into electricity. But the sweaty microbes only generate electricity on the scale of microwatts — about enough to run a flashing LED light. Bacterial spores could also be difficult to kill if left uncontrolled and free to multiply in the environment. But with the right design, evaporation driven engines could charge a battery or power a robot, the team reports.