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.
A team of researchers led by the University of California San Diego described their work in a paper published Sept. 11 in Nature Biomedical Engineering.
"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.
"We are adding a third dimension to the sensing range of wearable electronics," added Xu, who is also affiliated with the Center for Wearable Sensors at UC San Diego.
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.
The device measures central blood pressure—which differs from the blood pressure that's measured with an inflatable cuff strapped around the upper arm, known as peripheral blood pressure. 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.
The UC San Diego-led team has developed 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.
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 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.
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.
Find more information on the study here.