SHEETS of material that produce voltage when flexed could generate power from the motion of the human body.
Previous materials were either too rigid or too inefficient to be practical as pliable power generators. Now two research teams have solved the problem using different approaches. The materials could allow future medical implants to harvest their own power, by using the pulsing of arteries, for example.
Yi Qi and of Princeton University developed a way to soften up the usually inflexible crystal lead-zirconate-titanate (PZT), which is one of the most efficient piezoelectric materials known.
Advertisement
鈥淧eople thought, 鈥榯his is a crystal鈥�; they never thought about whether they could make it flexible,鈥� says McAlpine. But he and Qi found that when an extremely thin film of the ceramic is grown on a solid substrate and cut into strips about 5 micrometres thick, the resulting material can flex (see diagram).
These 鈥渘anoribbons鈥� are like fibre-optic cable made using glass, says McAlpine. Being long and thin, they can still bend despite being made of a material that is rigid in bulk.
The strips were attached to conducting silicone rubber to produce a flexible sheet that converts motion to electricity about half as well as traditional, rigid PZT ().
In contrast, Chieh Chang and of the University of California at Berkeley created fibres from a piezoelectric polymer called PVDF. The polymer is usually made in sheets, but the researchers spun it into fibres by drawing the molten material through a nozzle using an electric field.
This technique usually results in a fibre inside which the charged domains responsible for the material鈥檚 useful properties are randomly oriented, leading them to cancel out one another鈥檚 output. The Berkeley team used a strong electric field and the mechanical stress of the spinning process to line up those domains and ensure they work in unison.
When more than 40 samples were tested the fibres proved capable of converting 12.5 per cent of the mechanical energy used to deform them into electricity. Some recovered 20 per cent (). Lin says this makes them competitive with a conventional film of rigid PZT.
鈥淯sing flexible materials will open up a new field of mechanical energy harvesting,鈥� says at the University of Wisconsin, Madison, who says 鈥渨aste鈥� movement is often overlooked as an energy source.
McAlpine says flexible piezomaterials of either kind could be used to make motion-powered generators to extend the battery life of medical devices like pacemakers. 鈥淵ou could even eliminate the battery altogether.鈥� he says.
