Tom Krupenkin knows he’s not the first person to try making shoes that generate electricity. In 1998 researchers at the MIT Media Lab rigged a pair of Nikes to broadcast a stride-powered radio signal. Three years later, Trevor Baylis, the inventor of a popular wind-up radio, made a boot-heel insert that charged his cell phone during a 100-mile trek across the Namib Desert in southern Africa. “The idea itself goes a very long time back, arguably to the beginning of the 20th century,” says Krupenkin, an engineering professor at the University of Wisconsin at Madison.
Those earlier efforts were based on a technology called piezoelectrics—tiny crystals that create a current as they compress and expand—and didn’t make enough power to be practical. MIT’s Nikes generated a few thousandths of a watt, and Baylis had to walk for days to charge his cell phone. Krupenkin and his lab partner, J. Ashley Taylor, have discovered a more efficient approach called reverse electrowetting.
Normal electrowetting consumes a small electrical charge as it manipulates tiny droplets on a conductive surface. That’s handy for controlling chemical reactions in medical devices and has potential for use in e-ink displays and cellphone screens. Krupenkin and Taylor, who had worked on electrowetting camera lenses at Bell Labs in New Jersey, found that by running the process backward they could create enough electricity to charge a smartphone. “Like an electrical motor,” Krupenkin says, “in reverse you’re going to get a generator.”
A small prototype generated a few milliwatts via the back-and-forth movement of tiny liquid-metal droplets in an electrode-lined channel. With a millimeter-wide channel embedded in a shoe sole, each step could push 1,000 or so droplets past the electrodes and generate up to 10 watts of power. “It’s a very clever idea,” says Jason Heikenfeld, director of the Novel Devices Laboratory at the University of Cincinnati. Using fluids to generate power, Heikenfeld says, offers more flexibility in design than piezoelectric crystals do, an advantage he says Krupenkin will need as he seeks to create a marketable product.
In 2009, Krupenkin and Taylor co-founded InStep NanoPower to commercialize the technology. They are working on a new prototype the size and shape of a shoe sole that could have a USB-type outlet in the heel. For consumers reluctant to plug wires into their shoes, Krupenkin envisions a mobile hotspot in the toe. The shoe would then handle the high-power signal needed to communicate with cell towers, while the phone would only need to send a signal to the shoe. The savings, says Krupenkin, could extend the life of phone batteries by a factor of 10.
While a shoe-ready insert is still a couple years away, Krupenkin is looking for shoemakers to partner with. “It’s one thing to embed some device,” he says. “It’s a very different story to design footwear in a way that people would love to wear it.”