Engineers have demonstrated a flexible device that harvests the heat energy from the human body to monitor health. The device surpasses other flexible harvesters that use body heat as the sole energy source.
The harvesters use heat energy from the human body to power wearable technologies — such as smart watches that measure your heart rate, blood oxygen, glucose, and other health parameters — that never need to have their batteries recharged. The technology relies on the same principles governing rigid thermoelectric harvesters that convert heat to electrical energy.
Flexible harvesters that conform to the human body are highly desirable for use with wearable technologies. Superior skin contact with flexible devices, as well as the ergonomic and comfort considerations to the device wearer, are core reasons behind building flexible thermoelectric generators (TEGs).
The performance and efficiency of flexible harvesters, however, currently trail well behind rigid devices, which have been superior in their ability to convert body heat into usable energy. The new flexible device is significantly better than other flexible devices reported to date and is approaching the efficiency of rigid devices.
The proof-of-concept TEG originally employed semiconductor elements that were connected electrically in series using liquid-metal interconnects made of EGaIn — a non-toxic alloy of gallium and indium. EGaIn provided both metal-like electrical conductivity and stretchability. The entire device was embedded in a stretchable silicone elastomer. The upgraded device employs the same architecture but it significantly improves the thermal engineering of the previous version, while increasing the density of the semiconductor elements responsible for converting heat into electricity. One of the improvements is an improved silicone elastomer — essentially a type of rubber — that encapsulates the EGaIn interconnects.
The key is using a high-thermal-conductivity silicone elastomer doped with graphene flakes and EGaIn. The elastomer provides mechanical robustness against punctures while improving the device’s performance. Using this elastomer allowed researchers to boost the thermal conductivity — the rate of heat transfer — by six times, allowing improved lateral heat spreading.
One strength of the technology is that it eliminates the need for device manufacturers to develop new flexible, thermoelectric materials because it incorporates the very same semiconductor elements used in rigid devices. Future work will focus on further improving the efficiencies of these flexible devices.