The stress and strain absorbed by your backpack could one day recharge your cell phone.

Researchers at Michigan Technological University have designed a strap that will capture the energy generated by the up-and-down movement of a hiker's pack and turn it into enough voltage to power small electrical devices.

"It's pretty cool," says Henry Sodano, an adjunct professor of engineering "mechanical engineering -- engineering mechanics, who recently accepted a faculty appointment at Arizona State University. "We are harnessing free energy that would normally be lost."

With mechanical engineering graduate students Jonathan Granstrom and Joel Feenstra, Sodano designed straps made of a piezoelectric material that can convert mechanical strain into electrical energy.

You probably wouldn't be able to plug a TV into your backpack; the system is designed for use with devices that require small amounts of electricity, such as a GPS unit. Alternatively, a hiker could charge up a headlamp while walking during the day and then turn it on after dark. Or the backpack could generate enough power to recharge a handheld computer.

The straps are made of a nylon-like polymer that produces a fluctuating, AC current that could be stored in a battery or a capacitor. The researchers teamed up with the Blacksburg, Va., company NanoSonic Inc. to develop a specialized electrode grown on the surface of the strap using nanotechnology.

The beauty of the design is that it requires no extra effort on the part of the user, unlike other devices that transform mechanical energy into electricity, such as wind-up flashlights. It's part of a new field called "energy harvesting."

"We're trying to capture free power. You don't need watts of energy for many modern electronics," Sodano said. "We're not trying to generate significant levels of power, just enough to perform a useful function."

Someone shouldering a heavy pack, such as a soldier in the field, could generate 45.6 milliwatts of power walking two or three miles per hour. That's enough wattage to power small electronics. Or, it could be accumulated for later use.

“In general, we want to accumulate the power before using it; for example you could walk for 20 minutes then have enough power to talk for 2.5 minutes on your cell phone," Sodano says.

The research was funded by the Office of Naval Research, which is investigating power sources for Marines in the field. The researchers hope to receive additional support to develop a prototype and then to commercialize their innovation.