Could an energy-harvesting device implanted into the sole of your shoe produce enough juice to power your cellphone, MP3 player or laptop? That's the vision of researchers Tom Krupenkin and J. Ashley Taylor at the University of Wisconsin-Madison, who are developing a system that could do just that.

Their innovation hinges upon an idea called "electrowetting," in which a droplet of a conductive liquid is placed on an electrode and is deformed by the application of electric charge. But Krupenkin took the concept in a different direction, what the researchers call "reverse electrowetting." In this case, microscopic droplets are placed between multilayer thin films; as the droplets move — in this case, from the energy of a shoe landing on the ground — energy is produced. This energy can then be harvested to power just about anything.

Their paper, "Reverse electrowetting as a new approach to high-power energy harvesting," was published on Aug. 23 in the journal Nature Communications.

The idea of capturing the energy you produce from walking is nothing new. For example, a device called the nPower PEG, introduced last year, can be stuck in a backpack or briefcase where it generates emergency power from the motion of the body.  

Another system, first used in 2008, used the energy from dancers' bodies to power the lights in a dance club's floor.

But earlier systems are relatively inefficient, producing only a few milliwatts of electricity. Krupenkin and Taylor say their system could produce up to 10 or even 20 watts of energy, which is more than enough to power most small devices. Shoes could be built with USB ports, so any device could be plugged in and charged from the energy gathered while walking.

Not only that, it could be enough to store and operate a Wi-Fi hotspot right in your shoe, an innovation which they say would help make small devices more efficient and last longer between charges by operating as a kind of "middleman" between devices and networks.

The process would capture energy that would normally be lost as heat. "Humans, generally speaking, are very powerful energy-producing machines," Krupenkin explained in a prepared release. "While sprinting, a person can produce as much as a kilowatt of power."

The duo were inspired by the energy needs of people who would not have ready access to the electric grid, particularly soldiers in the field, who often carry as much as 20 pounds of batteries to power their communications equipment, laptops and night-vision goggles, according to the news release.

Krupenkin and Taylor have already formed a company, InStep NanoPower, LLC, to commercialize the technology.

So far, this is just a concept. The researchers have used 150 droplets to produce a few milliwatts of power. But they have extrapolated the data from those experiments and predict that 1,000 droplets — which would take up just 40 square centimeters — would produce 10 watts of electricity.