Boot-embedded "energy harvesting" for on-the-go power

Need the ability to charge mobile devices on the go? A new energy harvesting system developed by University of Wisconsin-Madison mechanical engineers ensures that battery power is available whenever you need it, and wherever you are, by simply plugging in to your boot.

The energy harvester is embedded within boot soles to capture the energy generated by footsteps. This energy is then conditioned by a tiny electronic chip and stored in a rechargeable battery. A little more than one watt of power is generated per boot – between two to three watts total – which is enough electricity to power a range of mobile devices such as smartphones, GPS, tablets, laptops, and flashlights.

Devices can be powered by connecting to the boots via USB cable. The device can also be integrated with a broad range of electronic devices embedded in boots, such as a Wi-Fi hotspot, to serve as a middleman of sorts between mobile devices and a wireless network.

“It’s typical to also embed electronics that can perform other functions – such as GPS/Bluetooth modules, accelerometers, and gyros,” explains Tom Krupenkin, a professor of mechanical engineering at the University of Wisconsin-Madison. “The embedded electronics consume little of the energy generated, so any extra goes to the battery.”

The energy harvesting technology features “reverse electrowetting,” which was pioneered by Krupenkin and J. Ashley Taylor, a senior scientist in the mechanical engineering department at UW.

Figure 1: Shoes that harness the energy generated by footsteps could be used to power mobile devices on the go. (Credit: University of Wisconsin-Madison College Of Engineering.)
(Click graphic to zoom by 1.9x)

How does the technology work? A flow of conductive liquid interacts with a nanofilm-coated surface, and the mechanical energy of the fluid flow is directly converted into electrical energy.

For this boot-specific application, Krupenkin and Taylor had to develop a high-frequency energy source “bubbler” device that combines reverse electrowetting with bubble growth and collapse.

The bubbler device design consists of two flat plates separated by a small gap filled with conductive liquid; it contains no moving parts. Its bottom plate is covered with tiny holes through which pressurized gas forms bubbles. These bubbles grow until they’re large enough to touch the top plate, then the bubbles collapse.

This repetitive growth and collapse of bubbles pushes the conductive fluid back and forth to generate an electrical charge. So the high frequency needed for efficient energy conversion doesn’t come from a mechanical energy source; instead it’s delivered via an internal property of the bubbler approach.

One common misconception about the technology is that you need to walk first to power the battery – but that isn’t true for the majority of scenarios. “In most cases, it’s already charged and ready to go,” Krupenkin says.

If you need to charge a dead cellphone, it can be recharged via boot in the “same amount of time as from a wall outlet … but as soon as a bit of electricity comes into the phone from the boot, the phone should be operational,” he adds. In the unlikely event that the battery inside the boot ever becomes completely drained, it will take about four hours to recharge it – the same amount of time as a standard battery used in a typical cellphone.

How reliable and rugged is the technology in extreme conditions? “The system is hermetically sealed, so it isn’t affected by factors such as humidity or even salt in underwater conditions,” Krupenkin says. “In terms of temperature ranges, it can handle the same range of extreme conditions – Arctic or desert – that the footwear can sustain. You wouldn’t want to leave the boots in -50 °F temperatures overnight and then try to put them on your feet … it won’t be a pleasant experience. But the system will operate as soon as the heat from your foot warms it up a bit.”

Before ditching your heavy battery packs, it’s important to note that this technology is intended as a supplement. “It can mitigate dependence on batteries; it isn’t a complete replacement,” Krupenkin notes. “But if you’re on a mission that lasts longer than expected, or if you have a dead battery, it will help.”

While scaling the technology to 10 watts is possible, it wouldn’t be useful in this case because it would become “too bulky and expensive,” he points out.

Right now, embedding electronics in footwear is uncommon because there hasn’t been a good way to power it. GPS, in particular, makes sense in footwear, however: “Once energy harvesting becomes common, this might change – within the military realm as well,” Krupenkin says.

Krupenkin and Taylor have launched a startup, InStep NanoPower, and are working with Vibram to develop the first practical footwear energy harvester (Figure 1). For more information, visit