The patch uses ultrasound, so it could potentially be used to non-invasively track other vital signs and physiological signals from places deep inside the body. “Wearable devices have so far been limited to sensing signals either on the surface of the skin or right beneath it. But this is like seeing just the tip of the iceberg,” said Sheng Xu, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering and the corresponding author of the study. “By integrating ultrasound technology into wearables, we can start to capture a whole lot of other signals, biological events and activities going on way below the surface in a non-invasive manner.”
Central blood pressure is the pressure in the central blood vessels, which send blood directly from the heart to other major organs throughout the body. Medical experts consider central blood pressure more accurate than peripheral blood pressure and also say it’s better at predicting heart disease. However, measuring central blood pressure isn’t typically done in routine exams, because it is invasive. It involves a catheter inserted into a blood vessel in a patient’s arm, groin or neck and guiding it to the heart.
The UC San Diego-led team has developed a convenient alternative—a soft, stretchy ultrasound patch that can be worn on the skin and provide accurate, precise readings of central blood pressure each time, even while the user is moving. And it can still get a good reading through fatty tissue.
Measuring central blood pressure
The new ultrasound patch can continuously monitor central blood pressure in major arteries as deep as four centimeters (more than one inch) below the skin. “We are adding a third dimension to the sensing range of wearable electronics,” said Xu.
The patch was tested on a male subject, who wore it on the forearm, wrist, neck and foot. Tests were performed both while the subject was stationary and during exercise. Recordings collected with the patch were more consistent and precise than recordings from a commercial tonometer. The patch recordings were also comparable to those collected with a traditional ultrasound probe.
“A major advance of this work is it transforms ultrasound technology into a wearable platform. This is important because now we can start to do continuous, non-invasive monitoring of major blood vessels deep underneath the skin, not just in shallow tissues,” said said co-first author Chonghe Wang, a nanoengineering graduate student at UC San Diego.
The patch is a thin sheet of silicone elastomer patterned with what’s called an “island-bridge” structure—an array of small electronic parts (islands) that are each connected by spring-shaped wires (bridges). Each island contains electrodes and devices called piezoelectric transducers, which produce ultrasound waves when electricity passes through them. The bridges connecting them are made of thin, spring-like copper wires. The island-bridge structure allows the entire patch to conform to the skin and stretch, bend and twist without compromising electronic function.
The patch uses ultrasound waves to continuously record the diameter of a pulsing blood vessel located as deep as four centimeters below the skin. This information then gets translated into a waveform using customized software. Each peak, valley and notch in the waveform, as well as the overall shape of the waveform, represents a specific activity or event in the heart. These signals provide a lot of detailed information to doctors assessing a patient’s cardiovascular health. They can be used to predict heart failure, determine if blood supply is fine, etc.
Researchers note that the patch still has a long way to go before it reaches the clinic. Improvements include integrating a power source, data processing units and wireless communication capability into the patch. “Right now, these capabilities have to be delivered by wires from external devices. If we want to move this from benchtop to bedside, we need to put all these components on board,” said Xu.