RALEIGH, N.C.—North Carolina State University engineers have improved the efficiency of a flexible device worn on the wrist that harvests heat energy from the human body to monitor health.
The researchers reported "significant enhancements" in preventing heat leakage in the flexible body heat harvester they've been working on since 2017.
The harvesters use the body's heat energy to power items such as smart watches that measure heart rate, blood oxygen, glucose and other health parameters. The devices, which never need to have their batteries recharged, rely on the same technological principles used on rigid thermoelectric harvesters that convert heat to electrical energy.
Originally, the devices employed semiconductor elements that were connected electrically in series using liquid-metal interconnects embedded in a stretchable silicone elastomer.
The interconnects are made of EGaIn, an alloy of gallium and indium that exhibits metal-like electrical conductivity and stretchability.
Researchers improved the energy delivery by using a higher thermal-conductivity version of the silicone elastomer used to encapsulate the EGaIn interconnects.
In the latest iteration, aerogel flakes have been added to the silicone to further optimize the elastomer's thermal conductivity. Tests show that this can reduce the heat leakage through the elastomer by as much as 50 percent.
"The addition of aerogel stops the heat from leaking between the device's thermoelectric 'legs,'" Mehmet Ozturk, an N.C. State professor of electrical and computer engineering, said in a release. "The higher the heat leakage, the lower the temperature that develops across the device, which translates to lower output power.
"The flexible device ... is performing an order of magnitude better than the device we reported in 2017 and continues to approach the performance of rigid devices."