One of the ways we experience the world around us is through our skin.
From sensing temperature and pressure to pleasure or pain, the many nerve
endings in our skin tell us a great deal.
Our skin can
also tell the outside world a great deal about us as well. Moms press their
hands against our foreheads to see if we have a fever. A date might see a blush
rising on our cheeks during an intimate conversation. People at the gym might
infer you are having a good workout from the beads of sweat on you.
But Caltech’s
Wei Gao, assistant professor in the Andrew and Peggy Cherng department of
Medical Engineering wants to learn even more about you from your skin, and to
that end, he has developed an electronic skin, or e-skin, that is applied
directly on top of your real skin. The e-skin, made from soft, flexible rubber,
can be embedded with sensors that monitor information like heart rate, body
temperature, levels of blood sugar and metabolic byproducts that are indicators
of health, and even the nerve signals that control our muscles. It does so
without the need for a battery, as it runs solely on biofuel cells powered by
one of the body’s own waste products.
“One of the
major challenges with these kinds of wearable devices is on the power side,”
says Gao. “Many people are using batteries, but that’s not very sustainable.
Some people have tried using solar cells or harvesting the power of human
motion, but we wanted to know, ‘Can we get sufficient energy from sweat to
power the wearables?’ and the answer is yes.”
Gao explains
that human sweat contains very high levels of the chemical lactate, a compound
generated as a by-product of normal metabolic processes, especially by muscles
during exercise. The fuel cells built into the e-skin absorb that lactate and
combine it with oxygen from the atmosphere, generating water and pyruvate,
another by-product of metabolism. As they operate, the biofuel cells generate
enough electricity to power sensors and a Bluetooth device similar to the one
that connects your phone to your car stereo, allowing the e-skin to transmit
readings from its sensors wirelessly.
“While
near-field communication is a common approach for many battery-free e-skin
systems, it could be only used for power transfer and data readout over a very
short distance,” Gao says. “Bluetooth communication consumes higher power but
is a more attractive approach with extended connectivity for practical medical
and robotic applications.”
Devising a power
source that could run on sweat was not the only challenge in creating the
e-skin, Gao says; it also needed to last a long time with high power intensity
with minimal degradation. The biofuel cells are made from carbon nanotubes
impregnated with a platinum/cobalt catalyst and composite mesh holding an
enzyme that breaks down lactate. They can generate continuous, stable power
output (as high as several milliwatts per square centimeter) over multiple days
in human sweat.
Gao says the
plan is to develop a variety of sensors that can be embedded in the e-skin so
it can be used for multiple purposes.
“We want this
system to be a platform,” he says. “In addition to being a wearable biosensor,
this can be a human-machine interface. The vital signs and molecular
information collected using this platform could be used to design and optimize
next-generation prosthetics. “
Journal article: https://robotics.sciencemag.org/content/5/41/eaaz7946
Source: https://myfusimotors.com/2020/04/25/electronic-skin-fully-powered-by-sweat-can-monitor-health/
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