Wearable health tech gets efficiency upgrade

In a paper published in Applied Energy, the NC State researchers report significant enhancements to the flexible body heat harvester they first reported in 2017.

The harvesters use heat energy from the human body to power wearable technologies – think of 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 desired for use with wearable technologies. Mehmet Ozturk, an NC State professor of electrical and computer engineering and corresponding author of the paper, mentioned superior skin contact with flexible devices, as well as the ergonomic and comfort considerations to the device wearer, as the core reasons behind building flexible thermoelectric generators, or 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 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.

The proof-of-concept TEG originally reported in 2017 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 here is using a high thermal conductivity silicone elastomer doped with graphene flakes and EGaIn,” Ozturk said. The elastomer provides mechanical robustness against punctures while improving the device's performance.

“Using this elastomer allowed us to boost the thermal conductivity – the rate of heat transfer – by six times, allowing improved lateral heat spreading,” he said.

Ozturk added that one of the strengths 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. Ozturk said future work will focus on further improving the efficiencies of these flexible devices.

Yasaman Sargolzaeiaval, Viswanath P. Ramesh, Taylor V. Neumann, Veena Misra, Michael Dickey and Daryoosh Vashaee co-authored the paper. The group also has a recent patent on the technology.

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Funding for the work comes from the NC State's National Science Foundation-funded Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) Center under grant EEC1160483. The mission of the ASSIST Center is to create self-powered wearables capable of long-term multi-modal sensing without having to replace or charge the batteries.

Note to editors: An abstract of the paper follows.

“Flexible Thermoelectric Generators for Body Heat Harvesting – Enhanced Device Performance Using High Thermal Conductivity Elastomer Encapsulation on Liquid Metal Interconnects”

Authors: Yasaman Sargolzaeiaval, Viswanath P. Ramesh, Taylor V. Neumann, Veena Misra, Daryoosh Vashaee, Michael D. Dickey and Mehmet C. Ozturk, North Carolina State University

Published: Jan. 30, 2020, online in Applied Energy

DOI: 10.1016/j.apenergy.2019.114370

Abstract: This paper reports flexible thermoelectric generators (TEGs) employing eutectic gallium indium (EGaIn) liquid metal interconnects encased in a novel, high thermal conductivity (HTC) elastomer. These TEGs are part of a broader effort to harvest thermal energy from the body and convert it into electrical energy to power wearable electronics. The flexible TEGs reported in this paper employ the same thermoelectric 'legs' used in rigid TEGs, thus eliminating the need to develop new materials specifically for flexible TEGs that often sacrifice the so-called 'figure of merit' for flexibility.

Flexible TEGs reported here embed rigid thermoelectric 'legs' in soft and flexible packaging, using stretchable EGaIn interconnects. The use of liquid metal interconnects provides ultimate stretchability and low electrical resistance between the thermoelectric legs. The liquid metal lines are encased in a new stretchable silicone elastomer doped with both graphene nano-platelets and EGaIn to increase its thermal conductivity.

This high thermal conductivity elastomer not only reduces the parasitic thermal resistance of the encapsulation layer but it also serves as a heat spreader, leading to 1.7X improvement in the output power density of TEGs compared to devices fabricated with a conventional elastomer. The device performance is further improved by a thin Cu layer acting as a heat spreader providing an additional 1.3X enhancement in the output power at 1.2 m/s air velocity (typical walking speed). Worn on the wrist, our best devices achieve power levels in excess of 30 ?W/cm2 at an air velocity of 1.2 m/s outperforming previously reported flexible TEGs.