This is different from those other electric shoes that were making the rounds yesterday. Piezoelectrics are interesting, but simply don’t produce a lot of power for their size. Enough to make a low-power transmitter squirt a few bytes, or light an LED. They’re great for collecting ambient forces like sound and vibration, if you know the direction and type of force you want to harvest, there are better ways to go about it.
In the case of harvesting energy from footsteps, you have a pretty good idea of how the forces will work. And some researchers from the University of Wisconsin Madison are working on a system that might be able to pull as much as 20W from your strut. Why are you strutting, anyway? You think you’re so cool.
Yes, theoretically, you could charge your iPad or power a whole low-power computer of some kind just by walking around. Of course, you could never stop walking, which might end up being a good thing. The researchers, who have privatized to research and sell the technology under the name Instep Nanopower, say that somewhere around 300W of power dissipates as heat alone, and they think they can siphon off a bit without messing with your gait.
What they’ve done is introduce a fluid made up of tiny metal microdroplets into a special substrate. When your foot presses down on it, the droplets are forced through the substrate, which generates electricity by a means known as reverse electrowetting (that’s probably patent pending, but you can read the paper in Nature).
Once the energy has been grabbed, it can be held for drawing off later or used immediately to power a device. They suggest a sort of relay that would do a mobile phone’s high-power transmitting for it, while using a lower-power signal to send stuff to the phone, saving battery life. Kind of an over-specific application, but you get the idea.
They address several potential issues in the FAQ (squishiness, leaks, cost, etc) but don’t mention weight. I’m concerned that the battery and liquid metal would increase the weight of the sure significantly. I like light shoes myself, and I’m not sure I’d want to add another four or five ounces to each foot for a questionably accessible power source.