"When people walk with our device on, the generator engages at that time and assists those muscles in slowing down the leg and produces electricity at the same time," Donelan said.
"We can get that electricity without increasing the effort by the user."
Lawrence Rome is a biologist at the University of Pennsylvania in Philadelphia. In previous work, he led a team that developed a backpack that generates electricity from the up and down movement of the load.
He said the essentially effortless energy conversion process of the knee brace is "particularly clever."
As reported in Science in 2005, his backpack required a load weighing between 44 and 84 pounds (20 to 38 kilograms) to produce up to seven watts of electricity.
An improved version generates three times the energy—up to 20 watts—for a given weight, Rome said, but the backpack still requires a load.
The heft of the knee brace, by contrast, is negligible.
"It provides you a way to generate power if you don't happen to have a big backpack on your back," Rome said. "So, it's quite useful from that standpoint."
According to Donelan and colleagues' calculations, the knee brace is also a more practical means of generating electricity than other people-powered devices like hand-crank generators and windup radios.
Every watt of electricity generated with those technologies costs about eight watts of human energy, Donelan said.
The knee brace only requires one extra watt of human energy to produce a watt of electricity.
What's more, "you can do it in the background while you're doing everyday activities," Donelan noted.
"Depending upon what you do, you could walk for eight hours a day, but you're not going to crank the hand generator for eight hours a day."
In addition to amputees and soldiers, Donelan said, the knee brace may prove useful to field and emergency workers in remote areas.
It could also serve as an electricity source for the energy-efficient computers that are becoming popular with schoolchildren in developing countries.
He and his colleagues have spun off a company from their lab called Bionic Power to further develop and commercialize the knee brace.
"Its mission is to provide power for people: people power," Donelan said.
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