Engineers and researchers in Illinois have developed a battery that can not only be bent but also stretched up to three times its original size. It could assist in tech as diverse as a smartphone wristwatch and a heart monitor.

The battery is the work of Northwestern University’s Yonggang Huang and the University of Illinois’ John Rogers along with a team of researchers. They’ve been working on the problem that although making technology smaller has proven possible, it’s still often not durable enough for uses involving the human body.

Their battery design is remarkably simple in concept, if not execution. It’s made up of tiny lithium-ion batteries that have been printed on a polymer sheet. Each individual battery is connected by coiled wires that can stretch out and straighten (creating an effect like netting) but also return to their original shape.

The coil is a serpentine connection, meaning that not only does each individual loop in the coil form an S shape, but the coil itself is an S shape. Both the coil as a whole and the individual loops straighten out during expansion, but only the coil becomes taut. The individual loops retain their “slack”, which allows the process to be reversible.

For now its a prototype proof of concept. Testing shows the basic idea works and the power supplied is similar to a standard lithium-ion battery of the same (unstretched) size. However, the current model only lasts around eight hours before needing a recharge and begins losing capacity after about 20 charging cycles.

One of the big benefits of the design is that the charging should work wirelessly over a short distance. That greatly increases the viability of using it inside the body, such as in a heart monitor.

It’s also been suggested the system could work well in a flexible “smartwatch” that needed a battery the same size as one found in a phone but in a way that could wrap around a wrist (and cope with movement) for comfort.

(Image credit: Northwestern University)