Power harnessed one step at a time

His team created piezoelectric straps that draw power from the bag's natural bounce. At a normal stride, the stress on the bands can pull in 45.6 milliwatts (mW) – just shy of what's needed to perpetually power an iPod nano MP3 player, and more than enough to keep a Motorola Razr mobile phone charged.

"We could power a Razr in standby using 9mW of power and store the remaining 36.6mW of power, allowing us to talk for one minute for every 10 minutes walked," he says. "Or you could charge an LED headlamp while you walk in the day and use it at night while you camp."

The catch: for the straps to collect the full 45.6mW, they need to support a 100-pound knapsack. "That's a lot," Mr. Sodano admits with a laugh. But he designed the straps for the US military to use. Since soldiers are used to encumbering loads, the special straps in their packs will capture practically free energy, he says.

Finding a balance of how much to leech off a person's movement is the most difficult problem for human-powered technology. All energy has to come from somewhere. So if you're the one charging the device, then, to some extent, you're the one feeling the drain.

Self-winding watches work well because they are so light to begin with that the added weight of the self-winding mechanism is almost unnoticeable. But watches also require barely any power.

"To get anything substantial out of these devices, they would have to weigh a ton, and that's something few consumers will agree to," says Peter Glas­kowsky, a technology analyst for Envisioneering, a market-research firm, in Seaford, N.Y. "The energy you're saving by not using batteries is actually coming from you, and therefore its coming from food. If you add up the energy used to grow, package, ship, and eat, food is an extremely inefficient energy source."

Many plans for battery-charging shoes have been abandoned because walking in them was too taxing, says Mr. Glaskowsky. Most models so far have relied on an extra thick sole that depresses and generates electricity with each step – like a mini version of POWERLeap built into the boot. "A lot of people complain that it feels tiring after a while, like walking in sand," says Glaskowsky.

(Mr. Anton, a graduate student at Virginia Tech, plans to unveil a new shoe design in October that places a piezoelectric charger in a box above the heel. This would add weight, he says, but eliminate the deflating feel of the sole-based style.)

Similarly, the One Laptop per Child foundation ditched their plan to attach an auxiliary hand crank to the side of its "$100 laptops," deciding the strain was not worth what little power it provided. The group then signed on for a "yo-yo" pull-cord charger, but are now leaning toward using simple solar panels.

Simple movements may soon extend battery life

Some piezoelectric engineers have found successful, subtle ways to feed off human motion. A consortium of mostly British companies is on track to release by the end of the year an in-body microgenerator that will convert energy from joint movements, heartbeats, and breathing, says manager Martin McHugh of Zarlink Semiconductors. The tiny device will help both decrease the size and extend the life of batteries attached to pacemakers and other medical instruments, saving patients from costly, surgical replacements.

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