U.S. researchers say they've developed stretchable batteries that can be used to power a new generation of flexible electronics.

Scientists at Northwestern University and the University of Illinois say the batteries will allow stretchable electronic devices to be used anywhere, including inside the human body where they could monitor anything from brain waves to heart activity, succeeding where flat, rigid batteries would fail.

Yonggang Huang of Northwestern and John A. Rogers of the University of Illinois report they've developed a battery that continues to work even when stretched, folded, twisted and mounted on a human elbow.

The battery can work for eight to nine hours and can then be recharged wirelessly, they said.

"We start with a lot of battery components side by side in a very small space, and we connect them with tightly packed, long wavy lines," Huang said. "These wires provide the flexibility. When we stretch the battery, the wavy interconnecting lines unfurl, much like yarn unspooling. And we can stretch the device a great deal and still have a working battery."

The stretchable battery can deliver power and voltage similar to a conventional lithium-ion battery of the same size, the researchers said, but the flexible battery can stretch up to 300 percent of its original size and still function.