US researchers said Tuesday they have developed a way to harness water evaporation as a cheap and planet-friendly way of powering engines.

A team from Columbia University in New York and Loyola University Chicago built tiny experimental gadgets that operate autonomously in the presence of moisture in the air.

The key, the team reported in the journal Nature Communications, is the use of another pervasive natural resource -- bacterial spores.

The harmless spores puff up when exposed to water molecules and shrink when they are dry.

Like flexing and contracting biceps, the movement can be exploited to power machines.

"Water in nature is in constant transformation -- that process has enormous power, enormous energy in it," said lead author Ozgur Sahin, an associate professor of biological sciences at Columbia.

"So far, we've been able to capture energy as water comes down from the clouds, but we would like to capture energy from evaporation, which makes water go into the air, into the atmosphere," he explained in a video distributed by Nature.

"That process is very powerful, (but) we haven't been able to capture energy from that effectively. Until now."

The team built tiny engines made of thin plastic tape strips treated with spores, which they used to power a 100-gram (3.5-ounce) car and light-emitting diodes (LEDs).

Exposed to moisture, the spores expand and the tape stretches.

But it contracts very quickly when the source of moisture is removed -- an up-and-down motion that can drive wheels and pistons.

"When you combine many, many tapes together, then you can increase the force they are producing," said Sahin.

The team's two creations so far are an "evaporation engine" and a "moisture mill".

The first is a type of bio-engineered "muscle" comprised of a parcel of spore-treated tape ribbons encased by plastic walls and an open bottom over a container of water.

The tape "muscle" is connected to shutters at the top of the device which open and close as the strips relax and contract.

The moisture mill, in turn, consists of a wheel with small wings of spore-treated tape all around it.

The wheel runs on an axle attached to a water-treated plastic cover over half the wheel, the other half is outside in drier air.

With the spores inside expanding and those outside contracting, a tiny mass imbalance on the wheel causes it to turn for as long as the inside is kept moist through evaporation.

The technology is still very much in the lab.

But it may one day be used to power prosthetics or robot limbs, batteries and generators or in sportswear that responds to sweat -- the more you perspire, the more energy you produce.

The Nature video can be watched here.
Source: AFP