Many researchers across the world are developing flexible electronics for a myriad of reasons including greater durability and use as medical implants. Powering these devices can be tricky though, because your standard battery is quite rigid and could potential rupture if bent too much. Researchers at Northwestern University and the University of Illinois have changed that though by creating a flexible, stretchable battery that can be pulled to three times its normal size.
This team of researchers has been working on flexible electronics for some time and had previously developed a 'pop-up bridge' system for them. Unfortunately that same technique will not work with batteries because battery components must be tightly packed to be effective. Instead the researchers had to develop a spring design for the wires connecting the battery components within the flexible membrane. This design utilizes two orders of "S" shapes, with the larger curve being made up of multiple, smaller "S's," so when you pull on the wires, the curves straighten out.
The battery the researchers built had similar power and voltage to a conventional lithium-ion battery of the same (non-stretched) size. The researchers can also integrate coils that will allow for wireless charging of the battery, but unfortunately the battery was only able to survive 20 charging cycles before it started losing efficacy, though further research should improve that.
Source: Northwestern University