A knee brace that generates enough power to charge your mobile phone as you walk has been developed by scientists in the US and Canada.

The brace can harvest the energy from knee movement and convert it to electricity. Even a walk down to the shops can generate up to 13W, enough to power 30 minutes of talktime on your mobile.

We humans are a rich source of energy. "We're pretty effective batteries," says Dr Arthur Kuo of the University of Michigan. "In our fat we store the equivalent of a 1,000kg battery."

This energy can be harnessed to power all sorts of devices such as mobile phones and SatNav devices. This is achieved by a system called 're-generative braking' already used in hybrid-electric cars like the Toyota Prius.

'Re-generative braking' collects the dissipating energy caused by braking and channels it to the battery. The knee brace effectively does the same thing by collecting the energy expended by taking a step and channelling it to your mobile device via the knee brace.

"Walking is a lot like stop-and-go driving," explains Dr Max Donelan of Simon Fraser University in Burnaby, Canada. "Within each stride muscles are continuously accelerating and decelerating the body."

Designed primarily to help amputees, the knee brace is quite large and a bit cumbersome, but scientists hope to release a lightweight, slimline version in about 18 months.