University of California San Diego device
monitors cardiovascular signals and multiple biochemical levels.
Engineers at the University of California
San Diego have developed a soft, stretchy skin patch that can be worn on the
neck to continuously track blood pressure and heart rate while measuring the
wearer’s levels of glucose as well as lactate, alcohol or caffeine. It is the
first wearable device that monitors cardiovascular signals and multiple
biochemical levels in the human body at the same time.
“This type of wearable
would be very helpful for people with underlying medical conditions to monitor
their own health on a regular basis,” said Lu Yin, a nanoengineering Ph.D.
student at UC San Diego and co-first author of the study. “It would also serve
as a great tool for remote patient monitoring, especially during the COVID-19
pandemic when people are minimizing in-person visits to the clinic.”
Such a device could benefit individuals
managing high blood pressure and diabetes—individuals who are also at high risk
of becoming seriously ill with COVID-19. It could also be used to detect the
onset of sepsis, which is characterized by a sudden drop in blood pressure
accompanied by a rapid rise in lactate level.
One soft skin patch that can do it all
would also offer a convenient alternative for patients in intensive care units,
including infants in the NICU, who need continuous monitoring of blood pressure
and other vital signs. These procedures currently involve inserting catheters
deep inside patients’ arteries and tethering patients to multiple hospital
monitors.
“The novelty here is
that we take completely different sensors and merge them together on a single
small platform as small as a stamp,” said Joseph Wang, a professor of
nanoengineering at UC San Diego and co-corresponding author of the study. “We
can collect so much information with this one wearable and do so in a
non-invasive way, without causing discomfort or interruptions to daily
activity.”
The new patch is a product of two
pioneering efforts in the UC San Diego Center for Wearable Sensors, for which
Wang serves as director. Wang’s lab has been developing wearables capable of
monitoring multiple signals simultaneously—chemical, physical and
electrophysiological—in the body. And in the lab of UC San Diego
nanoengineering professor Sheng Xu, researchers have been developing soft,
stretchy electronic skin patches that can monitor blood pressure deep inside
the body. By joining forces, the researchers created the first flexible,
stretchable wearable device that combines chemical sensing (glucose, lactate,
alcohol and caffeine) with blood pressure monitoring.
“Each sensor provides
a separate picture of a physical or chemical change. Integrating them all in
one wearable patch allows us to stitch those different pictures together to get
a more comprehensive overview of what’s going on in our bodies,” said Xu, who
is also a co-corresponding author of the study.
Patch of All Trades
The patch is a thin sheet of stretchy
polymers that can conform to the skin. It is equipped with a blood pressure
sensor and two chemical sensors—one that measures levels of lactate (a
biomarker of physical exertion), caffeine and alcohol in sweat, and another
that measures glucose levels in interstitial fluid.
The patch is capable of measuring three
parameters at once, one from each sensor: blood pressure, glucose, and either
lactate, alcohol or caffeine. “Theoretically, we can detect all of them at the
same time, but that would require a different sensor design,” said Yin, who is
also a Ph.D. student in Wang’s lab.
The blood pressure sensor sits near the
center of the patch. It consists of a set of small ultrasound transducers that
are welded to the patch by a conductive ink. A voltage applied to the
transducers causes them to send ultrasound waves into the body. When the
ultrasound waves bounce off an artery, the sensor detects the echoes and
translates the signals into a blood pressure reading.
The chemical sensors are two electrodes
that are screen printed on the patch from conductive ink. The electrode that
senses lactate, caffeine and alcohol is printed on the right side of the patch;
it works by releasing a drug called pilocarpine into the skin to induce sweat
and detecting the chemical substances in the sweat. The other electrode, which
senses glucose, is printed on the left side; it works by passing a mild
electrical current through the skin to release interstitial fluid and measuring
the glucose in that fluid.
The researchers were interested in
measuring these particular biomarkers because they impact blood pressure. “We
chose parameters that would give us a more accurate, more reliable blood
pressure measurement,” said co-first author Juliane Sempionatto, a
nanoengineering Ph.D. student in Wang’s lab.
“Let’s say you are
monitoring your blood pressure, and you see spikes during the day and think
that something is wrong. But a biomarker reading could tell you if those spikes
were due to an intake of alcohol or caffeine. This combination of sensors can
give you that type of information,” she said.
Engineering Challenges
One of the biggest challenges in making the
patch was eliminating interference between the sensors’ signals. To do this,
the researchers had to figure out the optimal spacing between the blood
pressure sensor and the chemical sensors. They found that one centimeter of
spacing did the trick while keeping the device as small as possible.
The researchers also had to figure out how
to physically shield the chemical sensors from the blood pressure sensor. The
latter normally comes equipped with a liquid ultrasound gel in order to produce
clear readings. But the chemical sensors are also equipped with their own
hydrogels, and the problem is that if any liquid gel from the blood pressure
sensor flows out and makes contact with the other gels, it will cause
interference between the sensors. So instead, the researchers used a solid
ultrasound gel, which they found works as well as the liquid version but
without the leakage.
“Finding the right
materials, optimizing the overall layout, integrating the different electronics
together in a seamless fashion—these challenges took a lot of time to
overcome,” said co-first author Muyang Lin, a nanoengineering Ph.D. student in
Xu’s lab. “We are fortunate to have this great collaboration between our lab
and Professor Wang’s lab. It has been so fun working together with them on this
project.”
Next Steps
The team is already at work on a new
version of the patch, one with even more sensors. “There are opportunities to
monitor other biomarkers associated with various diseases. We are looking to
add more clinical value to this device,” Sempionatto said.
Ongoing work also includes shrinking the
electronics for the blood pressure sensor. Right now, the sensor needs to be
connected to a power source and a benchtop machine to display its readings. The
ultimate goal is to put these all on the patch and make everything wireless.
“We want to make a
complete system that is fully wearable,” Lin said.
