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 Max Donelan and colleagues reasoned that a device that helps the leg decelerate could generate power without requiring much additional energy from the person.
It's sort of like the way that some hybrid-electric cars produce electricity from braking.
With the device, a minute of walking can power a cell phone for 10 minutes, Donelan, of Simon Fraser University in Burnaby, British Columbia, said in a telephone interview. Other potential uses include powering a portable GPS locator, a motorized prosthetic joint or implanted drug pumps.
Donelan and colleagues at the University of Pittsburgh and the University of Michigan report development of the new device in Friday's edition of the journal Science.
The first practical use for the generator is likely to be in producing power for artificial limbs, said Donelan, who with his co-authors has founded a company to develop the device commercially.
The generator weighs about 3.5 pounds so users do burn energy carrying it on their knee, but they don't notice whether it is switched on or off when walking on a treadmill, he said.
However, they miss it when it's removed because they get used to its extra braking action, he added.
With one generator on each knee, people walking on a treadmill were able to generate about 5 watts of power.
Lawrence C. Rome of the University of Pennsylvania called the development "extremely clever."
Other people have thought of rotational devices around joints, Rome said, "but what's really clever is these guys only turn on resistance when person trying to brake, so it helps you."
Rome, who was not part of the research team, previously developed a backpack that generates electricity from the movement of the person carrying it.
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 The backpack was comfortable so it was an easy sell, he said. Whether the new knee generator is practical will depend on it being comfortable so people will want to wear it, he said in a telephone interview.
Arthur Kuo, an associate professor of mechanical engineering at the University of Michigan and a co-author of the report, called the device "a cocktail-napkin idea."
"There is power to be harvested from various places in the body, and you can use that to generate electricity. The knee is probably the best place," Kuo said in a statement. "During walking, you dissipate energy in various places, when your foot hits the ground, for example. You have to make up for this by performing work with your muscles. ... We believe that when you're slowing down the knee at the end of swinging the leg, most of that energy normally is just wasted."
The prototype is bulky, he said, but the energy generation part itself has very little effect on the wearer.
"We hope to improve the device so that it is easier to carry, and to retain the energy-harvesting capabilities," he said.
The research was funded by the National Sciences and Engineering Research Council of Canada, Michael Smith Foundation for Health Research and the Canadian Institute of Health Research.
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On the Net:
Science: http://www.sciencemag.org/
[Associated
Press; By RANDOLPH E. SCHMID]
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