A knee brace which can generate electricity as you walk, creating enough power to charge mobile phones and medical devices, has been developed by scientists.

The device is inspired by technology used in hybrid cars which store the power from braking to generate electricity. So-called regenerative brakes can collect energy that would otherwise be dissipated as heat when a car slows. In the same way, the knee brace uses the energy dissipated at the knee as the leg slows after a step.

In tests, volunteers generated five watts of power when they wore a brace on each leg. The researchers said it was enough to operate a portable GPS navigation unit, a robotic prosthetic joint or a heart pacemaker.

Describing the invention in the journal Science, Dr Arthur Kuo, of the University of Michigan in Ann Arbor, said that people were a rich source of energy. "An average-sized person stores as much energy in fat as a 1,000kg battery. People use muscle to convert this stored chemical energy into positive mechanical work ... This work can be performed at a high rate, with 100W easily sustainable."

Many devices already take advantage of the human-power capacity to produce electricity, including hand-cranked generators as well as wind-up flashlights, radios and mobile phone chargers. "A limitation of these conventional methods is that users must focus their attention on power generation. For electrical power generation over longer durations, it would be desirable to harvest energy from everyday activities such as walking," wrote Kuo.

He said the knee was a good place to harvest energy from the body. "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 prototype device was tested by the researchers on men walking at a leisurely 2.2mph on a treadmill. This version is bulky, however, and Kuo hopes to improve the design.

Alok Jha

http://www.guardian.co.uk/science/2008/feb/08/energy.research 

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