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 helpful for people with underlying medical conditions
to monitor their own health on a regular basis.
The study is published in the journal Nature Biomedical Engineering.
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 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, 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.
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.
The combination of the sensors helps to judge whether any sudden spike in Blood pressure
is due to an intake of alcohol or caffeine.
In tests, subjects wore the patch on the neck while performing various combinations of many
tasks: exercising on a stationary bicycle; eating a high-sugar meal; drinking an alcoholic
beverage; and drinking a caffeinated beverage.
Measurements from the patch closely matched those collected by commercial monitoring devices
such as a blood pressure cuff, blood lactate meter, glucometer and breathalyzer.
Measurements of the wearers' caffeine levels were verified with measurements of sweat samples
in the lab spiked with caffeine.
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 which 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.
Ongoing work 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.