No power outlet? No problem! If a pair of University of Wisconsin–Madison engineers has its way, juicing up a dying smartphone could soon be as easy as plugging it into your shoe. In a paper published in the November 16, 2015 issue of Scientific Reports, Tom Krupenkin, a professor of mechanical engineering, and J. Ashley Taylor, a senior scientist, detail a new technology that’s “particularly well-suited” for harvesting energy from human motion to power mobile electronics. “Human walking carries a lot of energy,” Krupenkin said in a statement. “Theoretical estimates show that it can produce up to 10 watts per shoe, and that energy is just wasted as heat. A total of 20 watts from walking is not a small thing, especially compared to the power requirements of the majority of modern mobile devices.”


Considering that an average smartphone operates on less than two watts, even a small proportion of that energy could keep myriad portables, including laptops, humming along nicely.

Power-generating shoes could be particularly useful in the military, since soldiers typically carry heavy batteries to power devices such as radios, GPS units, and night-vision goggles in the field. A footwear-embedded energy harvester, Krupenkin and Taylor say, could also provide a source of power for people in remote regions or developing countries with inadequate electrical grids.

While the concept itself isn’t new, the researchers’ approach takes advantage of “reverse electrowetting,” a mechanical-to-electrical energy conversion behavior that Krupenkin and Taylor observed in 2011.

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The phenomenon, which occurs when a conductive liquid interacts with a nanofilm-coated surface, can generate usable power, but only if the mechanical source is vibrating or rotating “reasonably” quickly.

To enhance the effect, Krupenkin and Taylor created a device that generates an electrical charge by spawning and then collapsing tiny bubbles through a conductive fluid.

Though their proof-of-concept “bubbler” generated around 10 watts per square meter in preliminary experiments, theoretical estimates show that up to 10 kilowatts per square meter might be possible, according to Krupenkin.

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Krupenkin and Taylor are currently looking for industry partners to help them commercialize their footwear-embedded energy harvester through their startup company, InStep NanoPower.

There are a couple of ways their harvester could power mobile devices: directly via a cable or by embedding electronics such as a Wi-Fi hotspot.

The latter tack could even extend the life of a cellphone battery by as much as 10 times between charges.

“For a smartphone, just the energy cost of radio-frequency transmission back and forth between the phone and the tower is a tremendous contributor to the total drain of the battery,” Krupenkin said.

+ University of Wisconsin–Madison