Now, charge your mobiles just by walking

Washington, February 8: University of Michigan scientists have found that a new energy-capturing knee brace has the potential to generate enough electricity from walking to operate a portable GPS locator or to charge a cell phone.

Arthur Kuo, an associate professor of mechanical engineering at the university, has revealed that the wearable mechanism works much like regenerative braking charges a battery in some hybrid vehicles.

While regenerative brakes collect the kinetic energy that would otherwise be dissipated as heat when a car slows down, the knee brace harvests the energy lost when a human brakes the knee after swinging the leg forward to take a step.

"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," said Kuo, who called the device "a cocktail-napkin idea".

"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," Kuo added.

During a study, reported in the journal Science, the scientists tested the knee brace on six men walking leisurely 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.

The study report reveals that 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.

By contrast, says the report, a typical hand-crank generator takes an average of 6.4 watts of metabolic power to generate one watt of electricity because of inefficiencies of muscles and generators.

"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," Kuo said.

The authors believe that a lighter version of the knee brace would be helpful to hikers or soldiers who do not have easy access to electricity. They also envision its application in implantable devices such as pacemakers or neurotransmitters, which today require a battery and periodic surgery to replace that battery.

"A future energy harvester might be implanted along with such a device and generate its own power from walking," Kuo said.