Engineers who design wearable devices that harvest human energy for power face a daunting dilemma: how do you collect a significant amount of power without making the user expend a lot of extra effort? Gadgets like hand-crank generators and windup radios require manual work from a user, and existing shoe-mounted generators produce less than one watt of power.

A team of engineers has developed a modified knee brace that captures energy that would otherwise have been lost while the wearer walks. The generator produces about five watts–enough to power 10 cell phones simultaneously.

“If you want power, go where the muscles are,” says Max Donelan, a professor at Simon Fraser University, in British Columbia, who led the research. “We thought, maybe there’s a smart, selective way to do energy harvesting when muscles are normally decelerating in the body.” Donelan’s research appears in the February 8 issue of the journal Science.

Donelan looked to the legs, which have the largest muscles in the body, and capitalized on a careful understanding of how humans use energy to walk. During an average stride, a person uses her muscles to bend at the knee and swing her leg forward, like a pendulum bob. This is positive work. At the end of the swing, she executes negative work to decelerate her moving leg. She places her foot on the ground, and by then her other leg has begun its swing.

Donelan and his team concentrated on harvesting energy from the end of the stride using their bionic knee brace. When the brace’s generator is engaged, it collects power while slowing down the motion of the leg. As a result, the brace reduces the human effort required at the end of the swing phase.

If the mechanism were continuously engaged, however, it would also impede acceleration at the beginning of the swing and require more energy from the wearer. To solve this problem, Donelan installed a sensor in the device to monitor the knee angle and switch the generator on and off. According to his research, this “generative braking” approach requires only one-eighth the metabolic power of a continuously operating mechanism.

“What’s extremely clever about this device is that it only tries to capture mechanical energy when the muscles would be primed to slow the body down,” says Lawrence Rome, a biology professor at the University of Pennsylvania. Rome, who did not work on the knee brace, recently designed a backpack that converts walking energy into electricity. “[Donelan’s knee brace is] a smart device, and it only works when you’re trying to brake yourself,” says Rome. “It lets the reverse torque of the generator do the work of the muscle.”

If Donelan’s approach to energy harvesting sounds familiar, it’s because he uses the same strategy employed by hybrid automobiles. When a driver applies the brakes of a hybrid, the electric motor begins to act as a generator. The generator slows down the car and at the same time converts kinetic energy into electricity, which is then used to recharge the battery. Conventional braking systems rely on friction to slow down, and the car’s kinetic energy is dissipated as heat.

“Walking is like stop-and-go driving,” Donelan says. “Within every stride, the muscles are accelerating and then decelerating the body. Hybrid cars take energy and give it to the battery.”

Donelan’s prototype weighs in at just over three pounds, and he is currently developing a lightweight model that could be used by prosthetics manufacturers and the military. Demand for human-energy harvesters like Donelan’s knee brace and Rome’s backpack is increasing, thanks in part to the proliferation of small electronics like cell phones and handheld GPS units, especially in the military.

“A soldier with a 24-hour mission [might have to] carry nearly 30 pounds of batteries with him,” Donelan says. “They have to power everything from GPS to communications to night vision.”

In addition, Donelan says that his knee brace has potential in medical markets: it could augment a paralyzed limb or power a prosthetic. “You could take a healthy limb and use it to power the injured limb,” he says.