Researchers at North Carolina State University have developed a new lightweight prototype for harvesting body heat and converting it into electricity for use in wearable electronics.

The project, which is part of the National Science Foundation’s Nanosystems Engineering Research Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST), aims to make wearable technologies that can be used for long-term health monitoring without relying on batteries, such as devices that track heart health or monitor physical and environmental variables to predict and prevent asthma attacks.

According to the findings published in the journal Applied Energy, the researchers say they have developed a much lighter and more efficient wearable thermoelectric generator (TEG) that can generate “far more electricity” than previous versions.

“TEGs generate electricity by making use of the temperature differential between the body and the ambient air,” said Corresponding Author and Associate Professor of Electrical and Computer Engineering at NC State, Dr Daryoosh Vashaee.

“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 centimetre squared (µW/cm2). Our technology generates up to 20 µW/cm2 and doesn’t use a heat sink, making it lighter and much more comfortable,” continued Dr Vashaee.

The new design is built with a layer of thermally conductive material, which rests directly on the skin and spreads out the heat. The conductive material is topped with a polymer layer that prevents the heat from dissipating away from the body, which forces the body heat to pass through a small TEG to be converted into electricity.

Excess heat that is not used in the conversion process is passed through the device onto an outer layer of material, which allows the heat to dissipate away from the body. The new TEG system is just 2 millimetres thick, is flexible and can be worn on different areas of the body without being bulky or disruptive to regular activities, say the researchers.

The researchers also found that the upper arm was the optimal location for heat harvesting. 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.

According to the team, testing of the device on both the arm and the chest of users determined that the upper arm was the best location for harvesting heat with the TEG. Mounting the TEG on the chest, where it would normally be covered by a shirt, limited its efficiency, but the team also tested the device when incorporated into a T-shirt, where it was found to be able to generate up to 16 µW/cm2 if the wearer was running.

By building better TEGs, the team is helping to move that goal forward, and because the devices can be scaled up, a mobile power supply running on body heat is that much closer to becoming a viable option.

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