7 September 2011—As smartphones and other portable gadgets push the limits of handheld computing, their hunger for electricity has only increased—with no end in sight. A new technology aims to address this issue, not by seeking bigger and better batteries but by looking instead to the shoes on our feet.
When we walk, our bodies create up to 40 watts of mechanical power as heat when our feet strike the ground. A special electricity-generating cushion placed inside the soles of a regular pair of shoes can transform some of that footfall power into several watts of electricity. Over the course of a single day, the generated energy, which gets stored in a small battery in the sole, provides enough electricity for a pedestrian to extend her smartphone’s battery life, for a soldier to augment his portable power needs in the field, or for someone in a developing nation without an electrical grid to power a night’s worth of LED home light use.
The idea of harvesting body energy for portable electronics is certainly not new, although some of this technology is. In 1996, Thad Starner at the MIT Media Lab calculated (PDF) that piezoelectric generators—solids that generate tiny currents when stretched or stressed—could theoretically generate up to 5 W of electricity at a brisk walking pace.
Starner’s forecasts have proved optimistic. Today’s best known piezoelectric footwear—Nike+ running shoes—aren’t really harvesting energy at all. A 2007 teardown by SparkFun Electronics of a Nike+ piezoelectric pedometer, for instance, reveals that even though the pedometer’s chips consume only tens of milliwatts of power, they still run on a separate battery. The piezoelectric part of the device is used only as a sensor, not to produce power.
By contrast, says Tom Krupenkin, associate professor of mechanical engineering at the University of Wisconsin–Madison, recent breakthroughs in microfluidics can fulfill or even exceed Starner’s power projections. The key involves the properties of liquid metals such as mercury and Galinstan, a gallium indium tin alloy. When set on a dielectric-coated conductive substrate with a voltage applied across it, a droplet of liquid metal deforms and spreads across the substrate. When the process is reversed, and the liquid metal in a microfluid device is moved, it induces a voltage.
In August, Krupenkin reported in Nature Communications a proof of principle for this phenomenon ("reverse electrowetting-on-dielectric" or REWOD) as a means of harvesting mechanical energy. InStep NanoPower, his start-up company in Madison, Wis., is now working to commercialize a REWOD-based technology, called the human gait energy scavenger.
According to Krupenkin, InStep is developing a shoe sole that would store the energy from each footfall in an embedded battery. InStep says it would provide up to 10 W from each foot—enough to power a mini Wi-Fi hot spot that communicates with your smartphone via Bluetooth and handles the phone’s biggest battery-draining function: long-range communication with cellphone towers.
"When you analyze the cellphone power budget," Krupenkin says, "you discover that the lion’s share goes into the high-power RF signal. This is 1 to 2 W. The cellphone by itself often uses only tens of milliwatts."
Raziel Riemer, a lecturer in industrial engineering at Ben-Gurion University of the Negev, in Israel, says he has run the numbers and remains skeptical that InStep’s technology could harvest anything close to 20 W of power. But, Riemer adds, "even if they don’t achieve [this]…it’s not such a big deal compared to the innovation of their new method."
This article was updated on 20 September 2011.
About the Author
Mark Anderson, a writer in Northampton, Mass., is a regular contributor to IEEE Spectrum. In the August 2011 issue he waded into Wikipedia’s role in the debate over who authored the plays attributed to Shakespeare.