[Image from UC San Diego]
The patch can be used for real-time, continuous blood pressure monitoring changes in patients who have heart or lung disease, as well as patients who are critically ill or in surgery. The UCSD-developed patch uses ultrasound to non-invasively track vital signs and physiological signals from places deep within 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,” Sheng Xu, a nano engineering professor at the UC San Diego Jacobs School of Engineering and an author on the study, said in a press release. “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.”
The ultrasound patch is able to continuously monitor central blood pressure in major arteries that are typically 4 cm below the skin. The researchers suggest that the technology could be used in a number of inpatient procedures.
“This has the potential to be a great addition to cardiovascular medicine,” Dr. Brady Huang, co-author on the paper and a radiologist at UC San Diego Health, said. “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.”
Central blood pressure is different from peripheral blood pressure that is measured with an inflatable cuff around the upper arm. Central blood pressure is pressure in the central blood vessels that send blood directly to major organs throughout the body from the heart. The researchers report that central blood pressure can be more accurate than peripheral blood pressure and could be better at predicting heart disease.
The traditional way of measuring central blood pressure is invasive and involves a catheter being inserted into a blood vessel in a patient’s arm, groin or neck and guided into the heart. There is currently non-invasive method available, but the researchers suggest that it is unable to consistently produce accurate readings. It involves holding a tonometer on the skin directly above a major blood vessel. The tonometer has to be held completely steady and at the right angle with the right amount of pressure to get a good reading. Patients also have to sit still for an extended period of time, which makes continuous monitoring difficult.
“It’s highly operator-dependent. Even with the proper technique, if you move the tonometer tip just a millimeter off, the data get distorted. And if you push on the vessel, which also affects the data,” the study’s first author Chonghe Wang said.
The patch is a soft and stretchable ultrasound patch can be worn on the skin to give precise readings of central blood pressure, even while a patient is moving. It is also able to get accurate readings through fatty tissue.
In tests, the patch was worn by male subjects who wore it on their forearm, wrist, neck and foot. The tests were performed while the patient was moving and stationary. The recordings were collected and found to be more consistent and precise than recordings from commercial tonometers. The patch recordings were also comparable to readings collected with an invasive 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 Wang.
The patch consists of a thin sheet of silicone elastomer that is patterned is an island-bridge strutted of small electronic parts that are connected by spring-shaped wires. The islands have electrodes and devices known as piezoelectric transducers that can produce ultrasound waves when electricity passes through them. The thin, spring-like copper wires connect the islands to each other. The island-bridge structure allows for the whole patch to conform to the skin and stretch and bend without the electronic function being compromised.
Ultrasound waves continuously record the diameter of pulsing blood vessels below the skin. Information collected is translated into a waveform using custom software. The outcome is a variety of peaks, valleys and notches in waveform and an overall shape of the waveform to show activity or events in the heart. The signals emitted from the patch can give doctors detailed information about a patient’s cardiovascular health. It can be used to predict heart failure and determine if blood supply is fine, among other things.
The researchers plan to continue working on the patch before it can be used in clinics. They hope to improve the patch by adding a power source, data processing units and wireless communication capabilities.
“Right now, these capabilities have to be delivered by wires from external devices. If we want to move this from bench top to bedside, we need to put all these components on board,” Xu said.
The research was published in the journal Nature Biomedical Engineering and was supported by the National Institutes of Health and the Center for Wearable Sensors at UC San Diego.
