Researches led by the University of California San Diego have invented a new wearable ultrasound patch that monitors blood pressure in arteries deep beneath the skin. The non-invasive method could help with the detection of cardiovascular anomalies.
The work, published in the Nature Biomedical Engineering, reported that in tests, the patch performed as well as some clinical methods to measure blood pressure.
Applications include real-time, continuous monitoring of blood pressure changes in patients with heart or lung disease, as well as patients who are critically ill or undergoing surgery. 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 nano-engineering at the UC San Diego Jacobs School of Engineering and an 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.”
“We are adding a third dimension to the sensing range of wearable electronics,” he continued.
The patch can continuously monitor central blood pressure in major arteries as deep as four centimeters below the skin.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. It differs from peripheral blood pressure, which is usually measured with an inflatable cuff strapped around the upper arm. Medical experts consider central blood pressure more accurate than peripheral blood pressure and also say it’s better at predicting heart disease.
“This has the potential to be a great addition to cardiovascular medicine,” said Dr. Brady Huang, a co-author on the paper and radiologist at UC San Diego Health. “In the operating room, especially in complex cardiopulmonary procedures, accurate real-time assessment of central blood pressure is needed—this is where this device has the potential to supplant traditional methods.”
The clinical method of measuring central blood pressure 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.A non-invasive method exists, but accurate readings are highly inconsistent as it involves holding a pen-like probe, called a tonometer, on the skin directly above a major blood vessel. To get a good reading, the tonometer must be held steady, at just the right angle and with the right amount of pressure each time.
In contrast, the soft, stretchy ultrasound patch can be worn on the skin and provide accurate, precise readings of central blood pressure each time,through fatty tissue, and even while the user is moving.
The patch was tested on a male subject, who wore it on his 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 co-author Chonghe Wang.
The patch is a thin sheet of silicone elastomer patterned with 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 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.
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.
The team is looking to collaborate with experts in data processing and wireless technologies for the next phase of the project.
This project was supported by the National Institutes of Health, and the Center for Wearable Sensors at UC San Diego.