Scientists have developed a stretchable lithium-ion battery that can stretch up to 300 per cent of its original size and still function. Northwestern University's Yonggang Huang and the University of Illinois' John A Rogers have demonstrated the battery that continues to work - powering a commercial light-emitting diode (LED) - even when folded, twisted and mounted on a human elbow.
The flexible battery which can be charged wirelessly is capable of powering innovative stretchable electronics. The implantable electronics can be used anywhere, including inside the human body, scientists say.
They could monitor anything from brain waves to heart activity, succeeding where flat, rigid batteries would fail.
"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," said Huang.
"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," Huang said in a statement.
Huang and Rogers have been working together for the last six years on stretchable electronics, and designing a cordless power supply has been a major challenge.
Now they have solved the problem with their clever "space filling technique," which delivers a small, high-powered battery.
For their stretchable electronic circuits, the two developed "pop-up" technology that allows circuits to bend, stretch and twist. They created an array of tiny circuit elements connected by metal wire "pop-up bridges".
When the array is stretched, the wires - not the rigid circuits - pop up. This approach works for circuits but not for a stretchable battery. There is not enough space between battery components for the "pop-up" technology to work. Huang's design solution is to use metal wire interconnects that are long, wavy lines, filling the small space between battery components.
The unique mechanism is a "spring within a spring": The line connecting the components is a large "S" shape and within that "S" are many smaller "S's".
When the battery is stretched, the large "S" first stretches out and disappears, leaving a line of small squiggles. The stretching continues, with the small squiggles disappearing as the interconnect between electrodes becomes taut.
"We call this ordered unravelling. And this is how we can produce a battery that stretches up to 300 per cent of its original size," Huang said.
The stretching process is reversible, and the battery can work for eight to nine hours before it needs recharging.
The details of the battery were published in journal Nature Communications.