Imagine never having to charge your Fitbit or Apple Watch. How cool would that be? Except that it wouldn’t be, because the energy for charging your wearable device would come from your body heat (Ed: Ba-dum-tsh). This is exactly what a group of researchers from Northwestern University and Donghua University in China have achieved by creating the first ever working, wearable thermoelectric generators (TEGs). They recently published their findings in the journal Nature Communications.

Research on wearable TEGs has been underway for about a decade, although existing prototypes need to be perfected for practical use. Fundamentally, TEGs are made up of a conducting polymer, such as carbon nanotubes, and are able to convert absorbed heat into electric energy which then fuels battery power. But the currently available designs are too bulky to be integrated into fabric.

The scientists behind this new breakthrough generated TEG models using a special type of carbon nanotube fibers that's as thin and flexible as thread and could be woven into a fabric. Their TEGs have amazing stretch capabilities, capturing adequate thermal energy because they are aligned with the direction of heat flow from the body to the device. Lastly, their new TEG was able to transform this heat into battery power efficiently without being hampered by bodily movements. It showed a power density of 70 milliwatts per square meter for a 44 Kelvin temperature difference, one of the highest power outputs reported yet.

Tapping on keys, achieving steps and exercise goals all cause an increase in body heat. That day isn’t far when, the more we use our devices the more charged they will be. Until then, don’t forget your charging cords!