A team of scientists at Columbia University has devised tiny engines that are powered by evaporation.

Researchers said the devices generate electricity from the energy produced by bacterial spores known as Bacillus subtilis, which has strong mechanical responses to changing relative humidity.

Further explanting its working, researchers said the spores expand when they absorb water and contract when they dry out.

The research, published on Tuesday in the journal Nature Communications, showed that by controlling the moisture in the air, produced by evaporation, to which the spores are exposed. The devices grab the energy of these expansions and contractions to drive rotary or piston engines.

The idea came from research into the mechanical properties of the subtilis spores, which can exist in a dormant state for hundreds of years, being conducted by a Columbia microbiologist.

Ozgur Sahin, an associate professor of biological sciences at the university, said, "It struck me as amazing how much mechanical energy they seem to have. They are so rigid that as the material's shape changes it produces a lot of energy".

During the study, Sahin glued the spores to a tape made of polyimide and surrounded them with a shutter mechanism that controls the passage of moisture.

Polyimide is a polymer used in fuel cells, computer displays, and various military applications. When the shutter is open, moisture escapes to the air and the spore dries out; when it closes, moisture fills the gap, the humidity increases, and the material expands. An 8-centimeter by 8-centimeter water surface can produce on average about 2 microwatts of electricity, and can burst up to 60 microwatts, said Sahin.

The value of the tiny engines, however, might lie more in demonstrating the ubiquity of natural energy that can, at least in theory, be harnessed by relatively simple and cheap devices, rather than in any practical application in the near term.