Electricity has been squeezed from a damp surface for the first time, thanks to a polymer film that curls up and moves – a bit like an artificial muscle – when exposed to moisture. The film could be used to run small, wearable devices on nothing but sweat, or in remote locations where conventional electricity sources aren't available.
When a dry polymer absorbs water, its molecular structure changes. This can, in principle, be converted into larger-scale movement, and in turn electricity. But previous attempts at creating a material powered by a moisture gradient – the difference in chemical potential energy between a wet region and a dry region - failed to produce a useful level of force.
These unsuccessful tries used a polymer called polypyrrole. Now Robert Langer and colleagues at the Massachusetts Institute of Technology have turned to the material again, embedding chains of it within another material, polyol-borate. This more complex arrangement mimics structures found in muscles as well as in plant tissues that bend in response to changes in humidity.
Flipping film
The result looks like an ordinary piece of thin black plastic, but when placed on a wet surface, something extraordinary happens. As the material absorbs water, its end curls away from the surface and the film becomes unstable, so it flips over. The ends have now dried out, so they are ready to absorb more water, and the whole process repeats itself. This continuous flipping motion lets the film travel across a suitably moist surface unaided.
Langer found that a 0.03-millimetre-thick strip, weighing roughly 25 milligrams, could curl up and lift a load 380 times its mass to a height of 2 millimetres. It was also able to move sideways when carrying a load about 10 times its mass.
To extract energy from this effect, Langer's team added a layer of piezoelectric material – one which produces electricity when squeezed. When this enhanced film, weighing about 100 milligrams, flipped over, it generated an output of 5.6 nanowatts – enough to power a microchip in sleep mode.
Electricity from sweat
Though the output is small, it is proof that electricity can be extracted from a water gradient. "To the extent of our knowledge, we are the first to utilise a water gradient, without a pressure gradient, to generate electricity," says Langer.
Large-scale energy harvesting is unlikely as the size of the device needed would be impractical, but it could be used to power small devices such as environmental monitoring systems in remote locations. "It will be interesting for applications where the amount of energy needed may be low but where access to energy may be difficult," says Peter Fratzl at the Max-Planck Institute of Colloids and Interfaces in Potsdam, Germany, who was not involved in the work.
Another application, Langer suggests, would be to place the film inside the clothing of joggers or athletes. The evaporation of sweat could generate enough electricity to power sensors monitoring blood pressure and heart rate.
Journal reference: Science, DOI 10.1126/science.1230262
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