Knee-Brace Generator Offers Portable "Power Plant"

John Roach
for National Geographic News

February 7, 2008

Inspector Gadget, take note: Researchers have created a knee brace that converts the energy from a walker's stride into electricity.

Results from the device show that if the clumsy cartoon character wore a brace on each leg as he stumbled after his nemesis Dr. Claw, he'd easily generate five watts of electricity.

That's enough juice to power ten cell phones at once—or in the inspector's case, perhaps enough to keep his telescoping appendages functioning for an entire TV episode.

More practical and immediate applications are for "people whose lives depend on portable power," said Max Donelan, a professor of kinesiology at Simon Fraser University in Burnaby, Canada.

For example, he said, the knee brace could help power electronic prosthetic limbs worn by amputees or help soldiers charge batteries for essential battlefield gadgets like night vision goggles.

(Related photo: "Bionic Hand Unveiled in Britain" [July 19, 2007].)

Donelan and his colleagues describe the high-tech knee brace in tomorrow's issue of the journal Science.

Power Walking

Donelan said the contraption is similar, in theory, to a hybrid-electric car, in which energy normally dissipated as heat during braking drives an electric generator instead.

"We use essentially that same principle and apply it to walking," he said.

The knee-brace generator kicks on at the end phase of a stride, when the hamstrings—the muscles at the back of the thigh—normally activate to help brake the leg.

(Watch a video about how the human body works.)

"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."

Practical Generator

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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