A device has been created which allows people to generate electricity simply by taking a walk to the shops to fetch the newspaper.

The invention generates enough electricity from the motion of the legs to power the walker’s mobile telephone and MP3 player while they’re out and about.

Researchers who developed the mechanism, an adapted knee brace, said it has the potential to be used in prosthetic limbs and pacemakers, or by anyone without easy access to electricity.

They expect to produce a commercial version of the generator, one that could be used by field workers in remote regions, within five years, and more advanced models for medical use within a decade.

It is attached to the walker’s leg and extracts energy from the knee as it slows the swing with each step taken, just as some hybrid cars and trains can generate electricty during braking.

During the slow-down stage of each swing of the leg the body has to disipate kinetic energy and it is this that the knee brace extracts before it can be wasted.

Six volunteers walking on a treadmill at a sedate 2.2mph and wearing a brace on each leg were found to be able to generate 5 watts of electrical power - enough for ten mobile phones simultaneously - without needing to break into a sweat.

During experiments to test the mechanism one of the volunteers was asked to run and managed to generate 54W - enough to light an incandescent 40W lamp with some left over.

The device, including a simple motor and a single-gear clutch mechanism, was put together by a team of scientists from Canada and the United States who identified the knee as the most productive part of the body for generating electricity. The research was led by Dr Max Donelan of the Simon Fraser University in Canada.

Dr Arthur Kuo, of the University of Michigan and one of the inventors, said: “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. 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.”

The effort each user had to put into generating electricty from the knee brace was judged too much for it to be put straight into production but the researchers were confident dramatic improvements in efficiency could be made relatively easily.

By creating and testing the device successfully they had been able to prove the concept worked but said: “This preliminary deminstration could be improved substantially.”

Measurements taken during the experiments, reported in the journal Science, established that the effort involved in generating electricity meant the metabolic costs of walking rose by about 20 per cent.

The method was much more productive than other devices aiming to harness the body’s wasted kinetic energy. Most have concentrated on the impact of the foot when it hits the ground, and the best generated only 0.8W compared to the knee brace’s 5W.

Volunteers were found to have expended just under a watt of additional metebolic power for each watt of electricity that they generated. A typical wind-up generator takes 6.4W of metabolic power to generate a single watt of electricity.

With 5W being twice what is needed for computers in developing regions the invention is thought to have huge potential for by the hundreds of millions of people around the world who have yet to get ready access to electrical supplies. It could equally well have uses for hitch-hikers, campers and soldiers who need to use much of their hardware away from plug sockets.

In the more immediate future the team forecast uses in driving motors in prosthetic limbs which are limited at present by their battery power. Similarly, it could power medical implants and reduce the need for surgery to change the batteries.

Dr Kuo added: “We’ve demonstrated proof of concept. The prototype device is bulky and heavy, and it does affect the wearer just to carry. But the energy generation part itself has very little effect on the wearer, whether it is turned on or not. We hope to improve the device so that it is easier to carry, and to retain the energy-harvesting capabilities.”

Researchers expect to have the next generation prototype ready in a year and believe the first commercial models will be on the market within five years.

Field workers in remote regions are expected to be the first to benefit from the technology. Models suitable for medical uses and powering MP3 and other small gadgets should be ready within the decade.

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