Photo-Illustration: InStep NanoPower
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 Labcalculated (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 2007teardown 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.
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