You won't feel comfortable or fashionable walking around with Max Donelan's invention strapped to your knee. The bulky 3.5-lb. (1.6 kg) gadget "is not that pleasant," says Arthur Kuo, a biomedical engineer at the University of Michigan, who co-wrote an article on the brace that appeared in Science last month. But Donelan's device pays off in other ways. Using the same principles that allow hybrid cars to recycle energy created in braking, braces worn on both knees can generate 5 watts of electricity by harvesting the energy inherent in a walker's stride. That may not sound like much, but it's enough to charge 10 cell phones, and it's absolutely free. "People like the idea of generating their own power," says Donelan, a kinesiologist at Simon Fraser University in British Columbia. "If you do things in a clever way, you can get energy cheaply."
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