Reduced heat leakage improves wearable health device

Date:
March 8, 2021
Source:
North Carolina State University
Summary:
By reducing heat leakage, engineers improve wearable device powered
by body heat.



FULL STORY ========================================================================== North Carolina State University engineers continue to improve the
efficiency of a flexible device worn on the wrist that harvests heat
energy from the human body to monitor health.


==========================================================================
In a paper published in npj Flexible Electronics, the NC State researchers report significant enhancements in preventing heat leakage in the
flexible body heat harvester they first reported in 2017 and updated
in 2020. 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 the 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,
historically trail well behind rigid devices, which have been superior
in their ability to convert body heat into usable energy.

The NC State 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 reported in 2020 employed the same architecture but significantly improved 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 was a high thermal conductivity silicone elastomer -- essentially a type of rubber --
that encapsulated the EGaIn interconnects.

The newest iteration adds aerogel flakes to the silicone elastomer to
reduce the elastomer's thermal conductivity. Experimental results showed
that this innovation reduced the heat leakage through the elastomer
by half.

"The addition of aerogel stops the heat from leaking between the device's thermoelectric 'legs,'" Ozturk said. "The higher the heat leakage, the
lower the temperature that develops across the device, which translates
to lower output power.

"The flexible device reported in this paper is performing an order of
magnitude better than the device we reported in 2017 and continues to
approach the performance of rigid devices," Ozturk added.

Ozturk said that one of the strengths of the NC State-patented technology
is that it employs the very same semiconductor elements used in rigid
devices perfected after decades of research. The approach also provides a low-cost opportunity to existing rigid thermoelectric module manufacturers
to enter the flexible thermoelectric market.

He added that his lab will continue to focus on improving the efficiency
of these flexible devices.

========================================================================== Story Source: Materials provided by
North_Carolina_State_University. Original written by Mick
Kulikowski. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Viswanath Padmanabhan Ramesh, Yasaman Sargolzaeiaval, Taylor
Neumann,
Veena Misra, Daryoosh Vashaee, Michael D. Dickey, Mehmet C. Ozturk.

Flexible thermoelectric generator with liquid metal interconnects
and low thermal conductivity silicone filler. npj Flexible
Electronics, 2021; 5 (1) DOI: 10.1038/s41528-021-00101-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210308084227.htm

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