This article was posted on 01/16/2013

Material generates power from water vapor — enough to lift 380 times its own weight

MIT engineers design breakthrough material that could be used in future nano devices and microelectronics

Engineers from MIT have created a unique polymer film that can generate electricity from water vapor.

Thin, polymer film can generate electricity from just a small amount of water vapor.

The material is made from an interlocking networking of two polymers — polypyrrole and polyol-borate. The former is made up of a hard yet flexible matrix and serves the purpose of providing support to the film. The latter material, meanwhile, is a soft gel that swells when it absorbs water.

When the MIT team put the two polymers together, they found that the material changed its shape whenever it absorbed small amounts of water vapor; specifically, it curled up and down, and even somersaulted itself forward. This continuous motion, the group proved, can be used to power a bevy of micro- and nanoelectric devices or, on a grander scale, even drive robotic limbs.

Mingming Ma, a postdoc at MIT’s David H. Koch Institute for Integrative Cancer Research and lead author of the group’s paper, expounds upon the potential that this material can offer: “With a sensor powered by a battery, you have to replace it periodically. If you have this device, you can harvest energy from the environment so you don’t have to replace it very often.”

Diving into the study

The group’s polymer film draws upon energy found in the water gradient between dry and water-rich environment. When a 20-micrometer-thick film was exposed to a moist surface, the bottom polyol-borate layer absorbed the evaporated water, forcing the film to curl away from the surface. As this happened, the bottom of the film was exposed to air, which caused it to quickly release the moisture. As this happened, the polypyrrole layer took over, somersaulting the material forward and back to its original position. When this happened, the bottom layer was exposed to the moisture and — you guessed it — it started to curl up again.

This cycle was repeated again . . . and again . . . and again. It’s entertaining to watch (video below) but what we are actually seeing is the film actively converting the chemical energy of the water gradient into mechanical energy.

Potential uses

There are two ways to use this film: as an actuator or as a generator. When used as an actuator, the engineers reported some pretty impressive findings. For one, they were able to demonstrate that a 25-milligram film was able to lift a glass of slides 380 times its own weight. The film also demonstrated being able to transport a load of silver wires 10 times its weights.

Polymer film transporting silver wires.

With such impressive demonstrations, it’s hard not to imagine how the material could be used if it were ever scaled up. The group states the case that the film could replace the electricity-powered generators used to control robotic limbs, and instead only draw upon water as a source for energy.

“It doesn’t need a lot of water,” Ma says. “A very small amount of moisture would be enough.”

Now, while the film could potentially be used to replace the components in an electricity-powered robotic limb, on the flip side of things, it can also be used to generate electricity. You see, the film can be coupled with a piezoelectric material so that the mechanical stress of the constant somersaulting can be converted into electric charge. When this idea was tested out, the group was able to generate an average power of 5.6 nanowatts.

Now sure, this might not sound like a lot, but it is enough power to store in capacitors for use in ultra-low-power microelectronic devices like temperature and humidity sensors. It could also be used to power microelectricalmechanical systems or nanoelectronics.

Furthermore, if and when the material is able to generate electricity on a larger scale, it could be used to harvest energy directly from the environment and even be attached to clothing, where the evaporation of a person’s body sweat could power physiological sensors.

The MIT engineers are now working on improving the efficiency of the conversion of mechanical energy to electrical energy, so that smaller films can be used to power larger devices.


Below is a video of the polymer film in action:

Read the two-page report

Along with Ma, other authors of the paper are Koch Institute postdoc Liang Guo and Daniel Anderson, the Samuel A. Goldblith Associate Professor of Chemical Engineering and a member of the Koch Institute and MIT’s Institute for Medical Engineering and Science.

The paper was published in the January 11th issue of Science. You can download it (for free) below. ■

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