Researchers have developed an energy-capturing knee brace can generate enough electricity from walking to operate a portable GPS locator, a cell phone, a motorized prosthetic joint or an implanted neurotransmitter.

The wearable mechanism works much like regenerative braking charges a battery in some hybrid vehicles where they collect the kinetic energy that would otherwise be dissipated as heat when a car slows down. This knee brace harvests the energy lost when a human brakes the knee after swinging the leg forward to take a step, researchers at the University of Michigan said in a release.

The scientists tested the knee brace on six men walking on a treadmill at 2.2 miles per hour. They measured the subjects' respiration to determine how hard they were working. A control group wore the brace with the generator disengaged to measure how the weight of the 3.5-pound brace affected the wearer, researchers said.

Test subjects walking with one device on each leg produced an average of 5 watts of electricity, which is about 10 times that of shoe-mounted devices.In the mode in which the brace is only activated while the knee is braking, the 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, researchers said.

"The body is clever. In a lot of places where it could be dissipating energy, it may actually be storing it and getting it back elastically. Your tendons act like springs. In many places, we're not sure whether the energy is really being dissipated or you're just storing it temporarily. We believe that when you're slowing down the knee at the end of swinging the leg, most of that energy normally is just wasted,” said Arthur Kuo, an associate professor of mechanical engineering at Michigan and an author of a paper on the device that appears in the Feb. 8 issue of the journal Science.

Researchers said the prototype device is bulky and heavy and that they want to develop a lighter version that would be helpful to hikers or soldiers who don't have easy access to electricity. And the scientists say similar mechanisms could be built into prosthetic knees other implantable devices such as pacemakers or neurotransmitters that today require a battery, and periodic surgery to replace that battery.