Shape of things to comeKnee brace acts as charger for cell phones, artificial limbs

Published 8 February 2008

Canadian researcher develops new device which harnesses the energy of a person’s leg as it swings forward during a step; running the generator constantly through the stride generated 7 watts of power

Talk about infusing the term “power walk” with a new meaning: A knee-mounted brace which generates electricity from a person’s stepping action, and requires little extra effort, could be used to power cellphones, prosthetic limbs, medical implants, or first response and military gear. The New Scientist’s Mason Inman writes that the new device harnesses the energy of a person’s leg as it swings forward during a step. The knee straightens towards the end of the leg-swing before the foot hits the ground. That spins gears inside the device, which drive an electric generator. The generator does not significantly increase the effort required for walking, says Max Donelan of Simon Fraser University in Burnaby, Canada, who led its development. :Muscles spend about the same amount of time working as brakes as they do working as motors,” he explains. The device is designed so it only generates electricity during the “braking” phase of each step. This is when the leg is being unbent and is decelerating, just before the foot touches the ground. The device works similarly to hybrid and electric cars, Donelan points out. They boost efficiency by generating electricity from energy expended during braking — known as regenerative braking.

Six men tested one of the generators by walking on a treadmill. Results showed that walking with the device when the electric generator inside was disengaged took about 20 percent more effort. When the generator was engaged only during the braking phase, it added fractionally more effort, and generated about 5 watts of power. Running the generator constantly through the stride generated 2 watts more than the braking phase. Because it increased the load on the walker, however, the efficiency is lowered, requiring 2.3 watts of additional effort for each watt of power produced. Using it only in the braking phase requires less than 1 watt of effort for every watt of electricity. The leg-flexing action is much more efficient at generating electricity than turning a hand crank like those used in some radios or flashlights, says Donelan. They typically require 8 watts of effort to produce a watt of electricity, he says. Most of the extra effort from wearing the device comes from expending energy to move its weight. The researchers plan to make it lighter to address the issue. Already, the 5-watt electricity output from one knee generator is enough to power 10 cellphones at once, says Donelan. The device could be used to drive other gadgets too, and could be useful for soldiers on long missions or for others far from the power grid, such as hikers, or emergency rescue teams.

Such devices might also provide a way to power futuristic bionic limbs. Donelan has created a spin-off company, Bionic Power, to develop the knee generator into a commercial product. “For people to want to use [the knee generator], they have to try to make it so it feels like it’s not even there,” says Larry Rome of the University of Pennsylvania in Philadelphia, who has developed a backpack that generates power from walking. First developed in 2005, improved designs have since reduced the backpack’s weight by two-thirds and increased power output by a factor of three, Rome says. He thinks it is feasible for Donelan’s team to make big improvements in the knee generator too. The knee device does not generate as much electricity as the 20 watts produced by Rome’s current backpack, he says, but it might be more attractive in some situations, he adds. “The backpack was made for people who were already carrying around a heavy load.”