You could soon be charging your mobile phone using your shoe thanks to researchers in the US who have created a tiny device that generates electric power as it is stretched and then released.

The device, based on a zinc-oxide wire, can create an alternating current of about 8 pA and a voltage of about 50 mV, which is at least 15 times greater than other tiny generators. The team claims that the process is efficient enough to be used to power remote sensors or even electronic equipment.

The new technology has been developed by Zhong Lin Wang and colleagues at the Georgia Institute of Technology and researchers at the University of Dayton. Earlier this year, Wang unveiled a zinc-oxide nanowire fabric that, when stretched or crumpled, could generate enough electricity to charge a mobile phone.

Their design relied on harvesting the piezoelectricity created by millions of tiny wires that were bent as they gnashed together within the fabric. However, in practice such fabrics would not last long because the gnashing would quickly damage the wires — and it is very difficult to make the fabric such that the wires gnash together properly in the first place.

Improved design

Now, Wang claims to have improved on his design by focusing on the electricity that is generated when individual wires are stretched and released (Nature Nanotechnology DOI:10.1038/NNANO.2008.314).

Wang’s team made the new generator by bonding a single zinc-oxide wire — 4 µm thick and 200 µm long — onto a flexible insulating film made of polyimide. The wire is stretched by simply bending the film. The resulting voltage — caused by the piezoelectric effect — is measured by completing the circuit using an ammeter.

To make an AC generator, the team connected one end of the wire to a “Schottky barrier” — a semiconductor device that allows current to flow in one direction through the wire, but not the other. When the wire is stretched, a voltage builds up across the wire, then when the wire is allowed to relax, a similar but opposite voltage appears across the wire.

Connecting in series

Wang told that an AC voltage of about 50 mV is generated when the wire is stretched by about 0.1% and then released at a rate of 22 cycles per minute by a motor-driven mechanical arm. The team also connected two such wires in series and found that the two output voltages added together when the wires were bent simultaneously — suggesting that a number of generators could be connected together to create higher voltages.

The team also found that increasing the rate at which a wire is strained boosts both the output voltage and current.

The device converts mechanical energy to electrical energy with an efficiency as high as 6.8%. While this is less one tenth the efficiency of a hydroelectric turbine, for example, Wang believes that it is good enough for practical applications because many wires can be integrated together to boost both the power and voltage output.

He also says that his team are investigating how the shape of the wires and the materials used can be modified to maximize efficiency. “I have 15 people working on various parts of this technology," said Wang, who has filed several patents on using zinc-oxide wires for generating electricity.

Wang believes that the wires could be used to power medical sensors, for example, by implanting them into muscle tissue. They could also be fitted into shoe pads, where they would generate power while the wearer is walking — something that could prove invaluable to soldiers on long missions, who currently have to carry large supplies of batteries.