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.