Ozgur Sahin, Columbia University, New York, NY, USA, and colleagues have developed two devices that are directly powered from evaporation by using bacterial spores that shrink and swell with changing humidity. The researchers hope to scale up this mechanism to produce large amounts of energy.

The researchers glued spores to both sides of a thin, double-sided plastic tape to create a dashed line of spores in a way that dashes on one side overlapped with gaps on the other side. When dry air shrinks the spores, the spore-covered dashes curve and shorten the tape. By anchoring the ends of the tape, a kind of artificial muscle was build that is controlled by changing humidity. The team placed many of these tapes next to each other inside a floating plastic case topped with shutters. Evaporating water made the air inside the case humid. The tapes became longer and opened the shutters, and allowed the air to dry out. The spores shrunk again and by this closed the shutters. This generated a self-sustaining cycle of motion. Coupled to a generator, it produced enough electricity to cause a small light to flash.

A second device, the Moisture Mill, contains a plastic wheel with tabs of tape covered on one side with spores. Half of the wheel sits in dry air, causing the tabs to curve, and the other half sits in humid environment, where the tabs straighten. As a result, the wheel rotates continuously, effectively acting as a rotary engine. The device was able to power a small toy car.

• Scaling up nanoscale water-driven energy conversion into evaporation-driven engines and generators,
  Xi Chen, Davis Goodnight, Zhenghan Gao, Ahmet H. Cavusoglu, Nina Sabharwal, Michael DeLay, Adam Driks, Ozgur Sahin,
  Nature Communic. 2015, 6.
  DOI: 10.1038/ncomms8346