from Nature 522, 259 (18 June 2015) doi:10.1038/522259b
An engine fuelled only by water evaporation can power a miniature car and lights.
Ozgur Sahin at Columbia University in New York and his colleagues applied bacterial spores to thin plastic strips. The spores absorb and release water with changes in relative humidity, so the strips curl and straighten. The team stacked the strips and formed them into a water-containing engine so that the strips were exposed to recurring periods of high and low humidity, acting like oscillators to power the engine. When attached to a generator, the engine powered light-emitting diodes. A rotary version attached to two pairs of wheels (pictured, left) pushed a 100-gram car forwards (pictured, right).
The engine could be used in devices in areas that have scarce electricity, the authors say.
P.S.: interessting: the news is close, the paper is open!