Given the right circumstances, we’ve probably all seen a piece of paper curl up when wet. Could this motion be put to work, that is, generate electricity? Scientists have noticed this wet curling effect in various polymers, but never something that could be said to perform work, such as with polypyrrole.
Robert Langer at Massachusetts Institute of Technology [MIT] took the material one step further by embedding chains of polypyrrole in polyol-borate, mimicking the structure of muscle tissue and some plants that bend in response to changes in humidity.
The resulting thin film, with the appearance of plain black plastic, looks sufficiently inert, but when placed on a wet surface, flips itself over and over. When the material soaks up some of the water, it curls away from it and almost immediately dries, uncurling. As long as there is sufficient water, the process repeats itself, curling and uncurling. Langer found that a strip 0.03 mm thick and weighing 25 mg, could lift nearly 10,000mg a height of 2mm. In human terms, this would be like is like Hossein Rezazadeh, a 340-pound Olympic weightlifter, lifting eight African elephants over his head.
Putting this power to use, though, was Langer’s next step. Laminating piezoelectric film and polypyrrole-polyol-borate material, he wired it up to see how much electricity it would generate. The composite material, weighing about 100 mg, generates 5.6nW when it curls. This may not seem like much, but it is still enough to power a microchip in sleep mode. Large-scale generation will be impractical at 5.6mW/kg, but such thin film composites [hydroelectronics?] could be put to use powering environmental sensors or even biomedical sensors, powered only by sweat.
