An innovative technology that enables a footwear-embedded mechanism to capture energy during walking and store it for later use could be used to power mobile devices.

“Human walking carries a lot of energy,” says Tom Krupenkin, professor of mechanical engineering at UW-Madison, who developed the technology with J. Ashley Taylor, senior scientist in the university's Mechanical Engineering Department. “Theoretical estimates show that it can produce up to 10 watts per shoe, and that energy is just wasted as heat."

The energy harvester is embedded in the shoe's sole. Image credit: UW-Madison College of Engineering. Tapping into just a small amount of that energy is enough to power a wide range of mobile devices, including smartphones, tablets, laptop computers and flashlights, Krupenkin says. A typical smartphone, for example, requires less than two watts.

However, traditional approaches to energy harvesting and conversion don’t work well for the relatively small displacements and large forces of footfalls, he adds. “So we’ve been developing new methods of directly converting mechanical motion into electrical energy that are appropriate for this type of application.”

The researchers’ energy-harvesting technology takes advantage of “reverse electrowetting,” in which a conductive liquid interacts with a nanofilm-coated surface to convert mechanical energy directly into electrical energy. While reverse electrowetting can generate usable power, it requires an energy source with a reasonably high frequency—such as a mechanical source that vibrates or rotates quickly.

To overcome this, the researchers developed a "bubbler" device comprising two flat plates separated by a small gap filled with a conductive liquid. The bottom plate is covered with tiny holes through which pressurized gas forms bubbles. The bubbles grow until they are large enough to touch the top plate, which causes the bubble to collapse. The speedy, repetitive growth and collapse of bubbles pushes the conductive fluid back and forth, generating electrical charge.

The proof-of-concept bubbler device generated around 10 watts per square meter, and theoretical estimates show that up to 10 kilowatts per square meter might be possible, according to Krupenkin. “For this type of mechanical energy harvesting, the bubbler has a promise to achieve by far the highest power density ever demonstrated,” he says.

Krupenkin and Taylor are seeking to partner with industry and commercialize a footwear-embedded energy harvester through their startup company, InStep NanoPower. Their harvester could directly power various mobile devices through a charging cable, or it could be integrated with a broad range of electronic devices embedded in a shoe, such as a Wi-Fi hot spot that acts as a “middleman” between mobile devices and a wireless network.

Power-generating shoes could be useful for the military, as soldiers currently carry heavy batteries to power their radios, GPS units and night-vision goggles in the field. The advance could also provide a power source to people in remote areas and developing countries that lack adequate electric power grids.