Engineers 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 could be used by people with underlying medical conditions to monitor their own health on a regular basis. It could also be used for remote patient monitoring, especially during the COVID-19 pandemic when people are minimizing in-person visits to a healthcare provider.
Completely different sensors are merged together on a single small platform as small as a postage stamp. Each sensor provides a separate picture of a physical or chemical change. Integrating them all in one wearable patch allows those different pictures to be used together to get a more comprehensive overview of what’s going on in the body.
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 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.
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.
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.
They 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 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. Instead, the researchers used a solid ultrasound gel that 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.