Each time you take a step, you waste around 20 watts of power—around 20 times more than it takes to run a typical smartphone. But even though researchers have been chasing the idea of an energy-harvesting shoe since the early 20th century, long before any addiction to mobile gadgets, it's only now that it's actually feasible to turn footsteps into a power source.
A prototype shoe using new energy technology can generate enough power to charge a dead phone, via a USB port on the side of the sneaker. It can also run electronics embedded in the shoe itself, like a Wi-Fi hotspot or a tracker that could be used to located someone in lost in rubble after an earthquake.
Unlike some past attempts at energy-harvesting shoes—like DARPA-sponsored boots for soldiers, which were uncomfortable to wear—the new shoe is supposed to feel like an ordinary sneaker. Walking in it shouldn't take any extra effort.
That's partly because of the new technology and partly because of the way the researchers incorporated it in the shoe. "Essentially, it all depends on how you design the system," says the shoe's developer Tom Krupenkin, a mechanical engineer at the University of Wisconsin-Madison. "If you design the system incorrectly, what's going to happen is that you will feel there's something moving in the shoe as you walk. And we wanted to avoid that."
Krupenkin first started working on the shoe with his team in 2011, after discovering a new way to generate electricity and realizing that it was ideally suited for human locomotion. "This particular way of converting mechanical energy to electrical energy is specifically interesting for energy harvesting, because it allows you to create devices which are very flexible in terms of their shape, size, form factor, and power," he says.
In 2015, the researchers improved the technology to harness more energy. Inside the sole of the shoe, two flat plates are filled with a nontoxic, conductive liquid, and as someone walks, tiny holes in the bottom plate let some of the liquid escape to form bubbles. The movement of the micro-bubbles generates electricity, rather than the footsteps themselves. That makes it possible to end up with more power.
While the shoe can be used to charge a phone or other gadgets directly, Krupenkin thinks the most useful application will be charging electronics inside the shoe; different gadgets can be swapped in and out of the sole.
With an embedded Wi-Fi hotspot, the shoe can help cell phones can save battery life. "A very substantial part of the energy in the smartphone battery is spent on radio communication with the cell-phone tower," he says. "Short range communication such as Bluetooth requires much much less energy, literally 10 times less." The shoe could use a Bluetooth-enabled hotspot to send signals between the phone and the tower, so the phone can stay on longer.
A tracking device could be used for children or someone with a medical condition. Because the sole touches someone's foot, it can also monitor vital signs and send alerts if something is wrong. A firefighter might use the tracking device in their boots. Tracking could also be useful for someone who just needs better directions inside a GPS dead spot; the tracking function works both with and without GPS.
In a disaster, the shoe could help first responders find victims. "In the first five to six hours the chances of survival are the highest, then they very rapidly drop," says Krupenkin. "So finding people is very difficult, and if someone is wearing a shoe like that, then the signal from the shoe would point operators to where he is." The shoe could also indicate which victims are still alive, so rescuers reach them first.
"There are many more applications," he says. "We're designing the system so the electronics module is replaceable by the user, so if you want a new function, you can pull the existing module out of the shoe and plug in the new one."
The researchers, through a new startup called InStepNanoPower, are working with Vibram to commercialize the new sole.