As the old joke goes, what do you call a device that stores a lot of energy in a very small space? You call it “a bomb.”

Aside from blowing things up, storing more energy is an important goal in improving everything from wearable health tech devices to electric cars. The more electricity that you can store in a given space — a greater energy density — means that the device will be able to run longer between charges. Batteries have service this function well, but they have limited energy density. In addition, they can take a long time to recharge and can only be recharged a certain number of times.

Capacitors differ from batteries in that they can accept a charge rapidly, and can discharge it just as rapidly. In recent years, supercapacitors — or “supercaps” — have increased their energy density but still remain short of traditional batteries. Exotic materials such as graphene and carbon nanotubes (CNTs) have played a major role in these new developments.

Researchers at Duke University and Michigan State University have developed a new type of supercap that can be stretched without damaging its performance. They start with a “forest” of CNTs that are grown on a silicon substrate. They are then connected by coating the surface with a nanolayer of gold. They then transfer the forest — gold side down — to an elastic substrate that has been stretched in both directions. The substrate is then released, shrinking to one-quarter its original size. The result is a dense, flexible device that is much less prone to physical damage than traditional supercaps. They can also be stretched and bent, making them suitable for flexible applications such as smart garments and other wearable technology.

This new design could be paired with flexible batteries to provide energy storage that could be recharged quickly, as with energy harvesting from body motion, but store lots of power to be released over time by a battery. This hybrid approach could make a wide range of wearable devices practical.