If you're cursing the hot, sticky summer weather today, you can take comfort in the fact that the same humidity could someday be used as fuel.
Scientists at Columbia University have built a tiny vehicle that is powered by the moisture in the air.
The technology isn't quite as far along as electric or self-driving cars, but the miniature car proves that humidity could possibly be used as a fuel in the future. The Columbia researchers published a study in Nature Communications, describing how they used microscopic, moisture-sucking spores to power a tiny vehicle weighing just 100 grams (3.5 oz).
The bacterial spores expand and contract as they absorb and release moisture when moved from humid air to dry air and back again. The spores are glued to a series of polymer sheets that lengthen as the spores expand. When the polymers contract again, they lift tiny weights that are used to generate small amounts of electricity, which could power resistors or an LED.
For the miniature car, polymers were mounted on a Ferris wheel-like device, with half of the wheel covered by a damp cloth. The spores under the cloth expand with the humidity, which causes the polymers to stretch and increase in weight. This extra weight is enough to rotate the wheel, which in turn is used to power the vehicle.
The spores only expand and contract by six percent, but it is enough to make a series of polymers quadruple in length when moved from 30 percent humidity to 80 percent. The study proved that spores could expand and contract more than one million times without changing their range of motion and showed that contracting polymer strips could lift up to 50 times their own weight, albeit very slowly.
The spores used come from a bacteria called Bacillus subtilis, which is found in dirt and human intestines.
"Evaporation-driven engines may find applications in powering robotic systems, sensors, devices and machinery that function in the natural environment," says the study.
The power produced is so small that this technology isn't likely to ever power a life-sized car, judging by the speeds reached in the video below (which is actually running at double speed), so you certainly wouldn't want to be in a rush to get anywhere. However, it is comforting that the sticky weather that can make summer unbearable in some parts of the world can at least be put to some constructive use.