Published online 23 August 2011 | Nature | doi:10.1038/news.2011.493

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Deformed droplets offer step-by-step way to charge up personal electronics.

Your shoes could soon be used to keep your electronic gadgets charged on the go. Tom Krupenkin and J Ashley Taylor

Forget to charge your phone? Your MP3 player? Soon, a quick stroll in a special pair of shoes could provide enough power to keep both going for hours.

There's a lot of 'oomph' in a step: up to 10 watts of power is lost as heat each time a foot hits the ground. Mobile devices such as phones and laptops use between 1 and 15 watts, so harnessing our 'foot power' would make a notable difference for consumers. So far, however, attempts to harvest this energy using vibrating plates or piezoelectric materials, which produce electricity when compressed or bent, have mustered only a few milliwatts.

Using as their basis a system published today in Nature Communications1, mechanical engineers Tom Krupenkin and Ashley Taylor at the University of Wisconsin in Madison are now developing 'in-shoe technology' that could generate up to 10 watts of power per footstep.

Krupenkin's inspiration is a technique called electrowetting, in which a conductive liquid droplet, placed on an electrode, is physically deformed by an applied electric charge. The technique depends on the use of a dielectric material — which is usually an insulator but that can be polarized in an electric field — to coat the electrode. When the dielectric is charged the droplet can wet the surface more easily, and deforms. In his system, Krupenkin runs this process backwards, using the changing physical form of liquid drops between dielectric-coated plates to generate charge and therefore electrical power.

To test his idea, Krupenkin placed patches of electrodes coated very thinly with the dielectric tantalum oxide along tiny channels a few millimetres wide. Using a resistor to convert electrical charge to alternating current, Krupenkin was able to harvest electrical energy from drops of either mercury or galinstan, a gallium-based alloy as they were moved along these channels and over the electrodes.

"If you run a motor in reverse you get an electrical generator," says Krupenkin, explaining that this is the same idea.

"Usually in electrowetting we apply voltage to move things. Here it is the other way round," says Frieder Mugele, an expert in electrowetting from the University of Twente, the Netherlands, who was not involved with the work. "I would have liked to have had this idea myself," he concedes.

The amount of power generated increased in proportion to the number of droplets used. So far, Krupenkin has got up to 150 droplets, which produced a few milliwatts of power. But he has extrapolated his experimental data and calculated that in a device with 1,000 droplets, which would easily fit into an area of 40 centimetres squared, up to 10 watts could be generated.

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Ten watts would be enough to power a mobile phone, a military radio, a GPS device, even a small laptop, Krupenkin says. Which could mean an end to curtailed conversations on a phone with a dying battery. In your feet, you would have "a mobile electronic unit with you that is always ready", says Krupenkin. He hopes a device like this could be useful in developing countries, where electricity isn't always as plentiful or accessible as it is in more industrialized parts of the world.

Neil White, an electronics expert at the University of Southampton, UK, says it will be impressive if the scaled-up device really can produce 10 watts, adding: "I haven't seen anything like this before." White remains cautious, though, and would like to see more work looking at whether some of those 10 watts might be coming from the piezoelectric properties of the tantalum oxide, rather than from its interaction with the droplets.

Krupenkin has patented his idea and is now concentrating on scaling up the device and designing a shoe to contain it.