Nanotechnology promises to enable tiny, intricate circuits powering devices on any surface. But unless they’re harvesting energy from something like a heartbeat, the devices can only be as small as the smallest battery.

Now researchers at Rice University have combined the two, packing an entire lithium-ion battery into a single nanowire. The developers say it’s as small as such a device can possibly get.

Researchers led by Rice professor Pulickel Ajayan built a hybrid energy storage device, which serves as a battery and a supercapacitor. The first version sandwiched an electrolyte between a nickel/tin anode and a cathode made of a polymer called polyaniline. The cathode also served as a supercapacitor, storing lithium ions in bulk, as this writeup by Rice University explains. The prototype proved that lithium ions would move through the electrolyte and into the cathode.