The human body requires a lot of energy to run, but efforts are being made to claim some of that back to power electronic devices. Kinetic energy, created from movement, has been harnessed to charge batteries in the battlefield, and thermoelectric generators (TEG), which generate electricity from body heat, have powered flashlights and wearable devices. Now researchers at North Carolina State University (NCSU) have created a new wearable prototype that's lightweight, comfortable and much more efficient than existing TEGs.

TEGs work by harnessing the temperature difference between the wearer's skin and the air around them, and the NCSU team embedded its new design into t-shirts and armbands, discovering the prime location for body heat harvesting in the process.

"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)," explains Daryoosh Vashaee, co-author of the study. "Our technology generates up to 20 µW/cm2 and doesn't use a heat sink, making it lighter and much more comfortable."

The team's TEG is small – only 1 sq cm (0.15 sq in) and 2 mm (0.08 in) thick – and flexible, allowing it to stick to the skin without getting in the way. In the device, a thermally conductive layer makes contact with the skin and absorbs its heat, directing it into the small TEG itself. A polymer layer keeps the heat from dissipating before it passes through the generator. Any heat that makes it out the other side uncaptured is then allowed to dissipate through another layer of thermally conductive material.

Like other wearable TEGs, the electricity generated isn't a huge amount, but could be enough to run low-power devices like health-monitoring wearables, especially if scaled up.

"The goal … 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," says Vashaee. "To do that, we want to make devices that don't rely on batteries. And we think this design and prototype moves us much closer to making that a reality."

The researchers reports that the t-shirt-embedded generators performed reasonably well, generating around 6 µW/cm2 under normal conditions, or up to 16 µW/cm2 during exercise. But the armband TEGs proved more viable overall, with the upper arm found to be the most efficient place to stick them, hitting around 20 µW/cm2.

The wrist has a higher skin temperature, the team found, but the efficiency of generators placed there suffered thanks to the irregular shape, which limited the surface area for the skin and the TEG to make contact. And a chest band didn't allow enough air to flow, as it would normally be covered by clothing.

The research appeared in the journal Applied Energy.

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