TECH SPECIAL...Stretchy battery can bend and twist

Stretchy battery can bend and twist  Researchers have made a stretchable battery that expands to 3 times its size and can be recharged wirelessly. The battery will be used for future medical implants and for wearable electronics     Yonggang Huang from the Northwestern University and John Rogers from the University of Illinois are the first to demonstrate a stretchable lithium-ion battery – a flexible device capable of powering future stretchable electronics.     No longer needing to be connected by a cord to an electrical outlet, the stretchable electronic devices now could be used anywhere, including inside the human body. The implantable electronics could monitor anything from brain waves to heart activity.     Huang and Rogers have demonstrated a battery that continues to work – powering a LED – even when stretched, folded, twisted and mounted on a human elbow. The battery can work for eight to nine hours before it needs recharging, which can be done wirelessly.     The new battery enables true integration of electronics and power into a small, stretchable package. Details are published in the journal N a t u r e C o m m u nic a tio n s.     “We start with a lot of battery components sidebysideinaverysmallspace,andweconnect themwithtightlypacked,longwavylines,”said Huang, a corresponding author of the paper.     “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.”     Thepowerandvoltageofthebatteryaresimilar to a conventional lithium-ion battery of the same size, but the flexible battery can stretch up to 300 per cent and still function.     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, highpowered battery. HOW IT WORKS For their stretchable circuits, the two developed “pop-up” tech that allows circuits to bend, stretch and twist. They created an array of tiny circuit s connected by metal wire “pop-up bridges.”Whenthearrayisstretched,thewires–not the rigid circuits – pop up.     This approach works for circuits but not for a stretchablebattery.Alotofspaceisneededinbetween components for the “pop-up” interconnect to work. Circuits can be spaced out enough in an array, but battery components must be packed tightly to produce a powerful but small battery. There is not enough space between battery components for the “pop-up” tech 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 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.     “We call this ordered unraveling,” Huang said. “And this is how we can produce a battery that stretches up to 300 per cent of its size.”     The stretching process is reversible, and the battery can be recharged wirelessly. The battery’s design allows for the integration of stretchable, inductive coils to enable charging wirelessly through an external source.     Huang, Rogers and their teams found the battery capable of 20 cycles of recharging with little loss in capacity.

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