Using body heat to charge your smart watch

NC State researchers create flexible device that harvests heat from body to stay charged

February 7, 2020 

Wearable health technology like Fitbits require batteries or charging

You’re probably familiar with wearable health technology by now. Think Fitbits or smart watches that measure your heart rate and other health information. It’s all pretty amazing and pretty accurate. There’s only one problem with all of it however; the batteries that power the devices need charging and/or changing. Researchers at North Carolina State University may have an answer.

They’ve demonstrated a flexible device that harvests the heat energy from the human body to monitor health. That’s right, the heat from the body charges and recharges the device, no batteries need to be recharged. It’s not a new technology. In fact, the NC State engineering team first demonstrated the concept in 2017. But in a paper published in Applied Energy, the NC State researchers report significant enhancements to their first flexible body heat harvester.

“Flexible devices that conform to the human body are much better than rigid devices because of superior contact with the skin (and thus better charging capability) plus they are more ergonomic and comfortable to wear,” explains Megmet Ozturk, Ph.D., professor Electrical and Computer Engineering at North Carolina State University and one of the authors of the paper. “However, we are still struggling with efficiency issues.”

Right now, the performance and efficiency of flexible harvesters isn’t very good when compared to rigid devices, which have been superior in their ability to convert body heat into usable energy. But that’s changing. “The flexible device reported in this paper is significantly better than other flexible devices reported to date and is approaching the efficiency of rigid devices, which is very encouraging,” Ozturk said.

How it works

The 2017 model used semiconductor elements that were connected electrically in series using liquid-metal. It was made of what’s called EGaIn – a non-toxic alloy of gallium and indium. EGaln is pretty amazing because it provides both metal-like electrical conductivity and stretchability. The entire device was embedded in a stretchable silicone elastomer. That’s what you see in the picture. The upgraded device has the same architecture, but the thermal engineering is significantly improved along with the density of the semiconductor elements that convert heat into electricity. The silicone elastomer, which encapsulates everything, also gets an upgrade. Researchers are still at the drawing boards and in the labs working to improve efficiency even more, but the days are flexibly, wearable health tech are getting closer.

—Frank Graff 

 Frank Graff is a producer/reporter with UNC-TV, focusing on Sci NC, a broadcast and online science series.