If you’re just strapping on a wearable for a run or for a quick reading of one or more biometrics, battery life isn’t much of a concern. But what if you need continuous monitoring for optimal health or to predict the onset of attacks or episodes? With predicting asthma attacks and continuous heart monitoring, scientists at NC State focused on developing wearable tech that could recharge batteries while on the body to power wearable Health Tech devices.

The technology, called thermoelectric generators (TEGs), harvests body heat. “Wearable thermoelectric generators (TEGs) generate electricity by making use of the temperature differential between your body and the ambient air,” says Daryoosh Vashaee, associate professor of electrical and computer engineering at NC State. According to Vashee, other wearable heat harvesting technologies use heat sinks, which are heavier, bulkier, and capable of producing much less energy than TEGs. Vanshee said heat sinks can only produce one microwatt or less of power per centimeter squared (µW/cm2), but “Our technology generates up to 20 µW/cm2.” TEGs can produce more power per covered area.

The NC State researchers also explored which parts of the body are best for heat harvesting. Their answer: the upper arm. Wrists and chests have higher temperature, but irregular contours on the wrist diminish available contact space and chest bands don’t work because they’re covered by clothing, therefore restricting exposure to ambient air. So it sounds like the TEG is best as an upper arm band, but works best if you don’t wear long sleeved shirts, sweaters, or jackets. The group also experimented with building the tech into t-shirts. The results were better than heat sinks but not as efficient as bands. TEGs embedded in t-shirts generated up to 6 µW/cm2  in a resting state and 16 µW/cmwhile running.

Vanshee and his group detailed their work in a recently published paper. Applications for continuous battery charging via body heat harvesting could potentially expand beyond heart monitoring and predicting asthma attacks. Any wearable biometric device that could alert wearers or caregivers of impending issues could benefit, such as predicting epileptic seizures or allergic reactions.