Scientists have developed a knee brace that captures energy from a moving knee, much like regenerative braking charges a battery in a Toyota Prius.

The device, jointly (hah!)developed by scientists at three universities, uses kinetic energy to generate electricity.

When the leg extends, energy spins gears on the device, which in turn power a built-in generator. Software determines at which point in each stride the generator should engage, and resistance from the engaged generator aids the wearer in slowing down the leg. See video of the knee in action.

Data on metabolic power supplied by the University of Michigan highlight the device's efficiency:

Subjects required less than one watt of extra metabolic power for each watt of electricity they generated. A typical hand-crank generator, for comparison, takes an average of 6.4 watts of metabolic power to generate one watt of electricity because of inefficiencies of muscles and generators.

Scientists at the University of Michigan claim that walking can generate "enough electricity to operate a portable GPS locator, a cell phone, a motorized prosthetic joint or an implanted neurotransmitter." They are working on trimming it down from its prototype weight of 3.5 pounds -- the current size works as a proof-of-concept, but isn't practical.

The Michigan team and colleagues at Simon Fraser University in Canada and the University of Pittsburgh also will need to work on the name of the device. If they want this thing to sell, they'll have to come up with something snappier than "biomechanical energy harvester." I like "The In-Your-Pants Power Plant" as a tagline -- it's memorable.

Next they should manufacture it in several colors, and make the housing washable. Knees get sweaty, after all. Finally, they should market the device to athletes, hikers/hunters/backpackers, and the military (make sure it comes in camo).