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.
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 platform as small as a stamp,” said Joseph Wang, a professor of nanoengineering at UC San Diego. “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 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.
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 onto the patch with 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 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. 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.
In tests, subjects wore the patch on the neck while performing various combinations of the following 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 in the lab by measuring sweat samples 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. The latter normally comes equipped with a liquid ultrasound gel 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
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 all of these on the patch and make everything wireless.