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While the skin temperature is higher around the wrist, the irregular contour of the wrist limited the surface area of contact between the TEG band and the skin. This forces the body heat to pass through a centrally-located TEG that is one cm2.“Previous approaches either made use of heat sinks — which are heavy, stiff and bulky — or were able to generate only one microwatt or less of power per centimeter squared (µW/cm2).. To do that, we want to make devices that don’t rely on batteries,” he says. The entire system is thin — only 2 millimeters — and flexible.“Wearable thermoelectric generators (TEGs) generate electricity by making use of the temperature differential between your body and the ambient air,” says Windproof lighter Daryoosh Vashaee, an associate professor of electrical and computer engineering at NC State and corresponding author of a paper on the work.In addition, the researchers incorporated the TEG into T-shirts. Heat that is not converted into electricity passes through the TEG into an outer layer of thermally conductive material, which rapidly dissipates the heat.”

The new design begins with a layer of thermally conductive material that rests on the skin and spreads out the heat.“The goal of ASSIST is to make wearable technologies that can be used for long-term health monitoring, such as devices that track heart health or monitor physical and environmental variables to predict and prevent asthma attacks. The researchers also found that the upper arm was the optimal location for heat harvesting. Meanwhile, wearing the band on the chest limited air flow — limiting heat dissipation — since the chest is normally covered by a shirt. The conductive material is topped with a polymer layer that prevents the heat from dissipating through to the outside air. Our technology generates up to 20 µW/cm2 and doesn’t use a heat sink, making it lighter and much more comfortable. “In this prototype, the TEG is only one centimeter squared, but we can easily make it larger, depending on a device’s power needs,” says Vashaee, who worked on the project as part of the National Science Foundation’s Nanosystems Engineering Research Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) at NC State. “T-shirt TEGs are certainly viable for powering wearable technologies, but they’re just not as efficient as the upper arm bands,” Vashaee says.