A group of Canadian and U.S. scientists have put a twist on the term "power-walking," building a knee-mounted device they say can harvest enough energy from a person's stride to power multiple cellphones.

Simon Fraser University assistant professor Max Donelan sits behind a prototype of the knee brace.
(Science)

Researchers at Simon Fraser University and two American universities said the technology could potentially be used to help power prosthetic limbs or medical implants, and could provide extra energy for military personnel without the added weight of a backpack battery.

The device, which weighs about 1.6 kilograms, can generate an average of five watts of electricity with minimal extra effort on the part of the walker, said Simon Fraser University assistant professor of kinesiology Max Donelan, the lead author of an article about the device published Thursday in the journal Science.

Generating electricity from human energy is nothing new — stationary bikes as power generators are commonplace and many portable devices, such as flashlights, can be powered using hand cranks.

The difference with the knee-brace device however, is that it gets its energy not from the power used to accelerate or move the leg forward, but rather from the energy put into slowing down the knee joint at the end of a person's step.

Donelan said the device works in a similar manner to hybrid-electric cars, which recycle power from braking.

'You want to go where the muscles are'

"Walking is like having one foot on the gas and one on the brake," said Donelan. "What this device does is take advantage of the uneconomy of walking."

The biomechanical energy harvester comprises an aluminium chassis (green) and generator (blue) mounted on a customized orthopedic knee brace (red), totalling 1.6 kg., with one worn on each leg.
(Science)

Previous efforts to harness walking power in a similar manner have focused on shoe-mounted devices or bouncing a load in a special backpack, said Donelan, but these methods were less efficient.

"To really get the most out of your body, you want to go where the muscles are," he said.

The scientists tested the knee brace on six men walking on a treadmill at 1.5 metres per second. A control group wore the brace with the generator disengaged while another group had the generator activated at a number of settings. In the group where the brace was only activated while the knee was braking, the subjects required less than one watt of extra metabolic power to produce one watt of energy.

Of course, all the participants had to exert extra energy to carry the device itself.

The device is still just a prototype, said Donelan, and while the energy needed to generate electricity is low, the energy needed to lift the bulky brace makes the device less efficient.

But Donelan said Bionic Power Inc., the spin-off company that will continue to develop the product, has been working for a year on making it smaller and lightweight. It's also working on a newer version capable of adapting to different terrain and uneven walking surfaces.

Donelan, who also acts as chief science officer of Bionic Power, said the technology could be used in the future to power portable devices such as iPods, GPS locators or mobile phones.

But in the immediate future, he sees it being used to help power medical devices such as prosthetic limbs or implants such as pacemakers or neurotransmitters. He says those devices in the future could be implanted alongside the energy harvester.

Researchers from the University of Pittsburgh and University of Michigan worked alongside the team of scientists and engineers from Simon Fraser. The research was funded by the Natural Sciences and Engineering Research Council of Canada, Michael Smith Foundation for Health Research and the Canadian Institute of Health Research.