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In-shoe tech scavenges energy, generates electricity as you walk

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Thanks to a technique called reverse electrowetting, one day you can completely charge your cellphone with a 2-hour stroll.

Just how much oomph is in your step? Up to 20 watts of power is lost as heat each time your foot hits the ground.

But wait! Scientists have found a way to harvest energy from metal microdroplets in the soles of shoes. Talk about power walking.

Not only would it encourage Thomas Jefferson’s preferred form of exercise, this method for collecting and storing energy may also provide a cheap and environmentally friendly alternative to batteries. Portable electronics like cellphones and cameras use between 1 and 15 watts… making foot power pretty awesome if we could harvest it.

"Humans, generally speaking, are very powerful energy-producing machines," says study researcher Tom Krupenkin from the University of Wisconsin–Madison. "While sprinting, a person can produce as much as a kilowatt of power."

Currently, environmentally friendly, portable high-power energy harvesting – by converting mechanical motion into electrical power – suffers from poor conversion rates. That means they get mere milliwatts (although, that’s just fine for my kinetic watch).

To get around this, Krupenkin and UW-Madison’s Ashley Taylor developed a new method called reverse electrowetting – converting the energy of moving liquid droplets into an electrical current. ScienceNOW explains:

They started with a conductive solid substrate, which they topped with droplets of an electrically conductive liquid. On top of it they placed a metal electrode coated with a 10- to 50-nanometer-thick film of an insulating material… The bottom conductive substrate and the top electrode were then connected into a circuit. So when the solid electrode was pushed down, compressing the liquid droplets, or pushed laterally over the top of them, the device produced… voltage.

Then they tested the idea using electrodes coated thinly with something called dielectric tantalum oxide and microscopic drops of mercury and gallium-based alloy.

According to Nature News, 150 droplets produced a few milliwatts of power. But after extrapolating the data, they calculated that 1,000 droplets, which would fit in a 40-square centimeter area, could generate up to 10 watts per step.

That would be enough to power a cellphone, military radio, GPS device, or even a small laptop. In your feet, you would have "a mobile electronic unit with you that is always ready,” Krupenkin says.

You could completely recharge a standard cellphone battery by going for a 2-hour stroll, he adds.

Getting energy to your portable device will be tricky. They’re pondering having the electricity-generating device connected to a shoe-bound wireless transmitter. Since the biggest drain on your battery is radio communication, the battery would last tens of times longer, according to Krupenkin.

The duo have already patented the idea and are scaling up the device and designing a shoe to contain it. They formed a company – InStep NanoPower – to commercialize their new in-shoe tech. Expect a prototype that fits into the sole of a shoe (pictured) in a couple years.

Potential customers, MSNBC describes, range from military personnel who now carry 20 pounds of batteries in the field to keep their gadgets running to people in developing countries who have inadequate access to electrical grids for recharging their cellphones.

It’s not quite a totally inexhaustible energy source, but it’ll sort of be like having an electrical power generator under your foot.

The work was published in Nature Communications today.

Images: InStep NanoPower / T. Krupenkin & J.A. Taylor

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Janet Fang

Contributing Editor

Janet Fang has written for Nature, Discover and the Point Reyes Light. She is currently a lab technician at Lamont-Doherty Earth Observatory. She holds degrees from the University of California, Berkeley and Columbia University. She is based in New York. Follow her on Twitter. Disclosure