A newly-developed, stretchable lithium-ion battery could be used in the human body to power bionic implants and monitor brain or heart activity.
Northwestern University's Yonggang Huang and the University of Illinois' John A. Rogers have demonstrated a battery that continues to work, even when stretched up to three times its original size.
The power and voltage of the stretchable battery are much the same as a conventional lithium-ion battery of the same size. It can work for up to nine hours before it needs recharging, which can be done wirelessly.
"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," says 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."
The biggest challenge was designing a cordless power supply. In the past, the pair developed 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.
But this approach wouldn't work for a stretchable battery, as a lot of space is needed in between components for the pop-up interconnect to work - a problem when designing a small battery. The answer is to use metal wire interconnects that are long, wavy lines, filling the small space between battery components.
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' stretches out and disappears, leaving a line of small squiggles. These in turn disappear as the interconnect between electrodes becomes taut.
"We call this ordered unraveling," says Huang. "And this is how we can produce a battery that stretches up to 300 percent of its original size."