BioEd Online

A new device that straps to your leg can extract enough energy from your walking motion to power ten mobile phones — and the developers say that you'll barely notice the extra effort it requires.

Max Donelan of Simon Fraser University in Burnaby, Canada, and his colleagues say that their device should capture even more power, at a lower energy cost to the wearer, once it is improved beyond the current prototype.

It could eventually be used to power medical implants, eliminating the need for surgery to replace batteries. The device might also drive robotic prosthetic limbs by harnessing the movements of the wearer, and could power communications technology in parts of the world that lack electricity supplies.

Watt's new?

The knee brace is by no means the first device that scavenges energy from human movement. Wind-up flashlights and radios are now common, as are watches that are powered by everyday arm movements. Researchers have also made systems that generate electricity by compression of the soles of shoes, and 3 years ago a team at the University of Pennsylvania created a backpack that produces more than 7 watts of power as the load slides up and down on a frame¹.

Test subjects walking with the new device strapped on both legs produced about 5 watts of electricity². That's less than is produced by the backpack, but ten times more than is typically produced by shoe-mounted energy harvesters. Importantly, the cost in 'metabolic power' to the subjects was insignificant - also just about 5 watts.

The knee device works so well because it can be programmed to engage in energy generation only during that part of the walking cycle when leg muscles are being used to slow down the swinging lower leg.

In other words the device can be set so that it doesn't impede a user swinging their lower leg forward during the first half of a step, but only to help slow the lower leg down during the second half of a step, when the foot comes to rest again. In this part of the step, the bending motion of the knee brace is translated into energy through a system of gears hooked up to a generator.

Applying the breaks

This is comparable to ‘regenerative braking’ used in some electric and hybrid motor cars to extract energy as the vehicle brakes. “Walking is a lot like stop-and-go driving,” Donelan says. “Within each stride, muscles are continually accelerating and decelerating the body. In hybrid-electric cars, energy normally dissipated as heat during braking drives a generator instead. We have applied this same principle to walking. That way, the device generates substantial electricity without increasing the effort required to walk.”

The researchers are already working hard on an improved design. “We are about a year into the next generation of the device,” says Donelan. “We are making it lighter and shifting much of the remaining mass further up the leg so that it feels lighter still.” Weights lower down the leg are a greater burden as they have to be swung along during walking.

The tricky — and "fun", says Donelan — part is trying to develop a system that automatically senses the walking speed and style of the user on a particular terrain, and adapts itself to extract the maximum amount of power for the minimum user effort.