Discrete and Process Automation

Wearable Patch Uses Ultrasound Waves to Monitor Central Blood Pressure

13 September 2018

Researchers from the University of California San Diego have created a wearable patch that non-invasively monitors central blood pressure four centimeters below the surface of the skin.

A wearable ultrasound patch tracks blood pressure in a deep artery or vein. Source: Chonghe WangA wearable ultrasound patch tracks blood pressure in a deep artery or vein. Source: Chonghe Wang

The new patch could help doctors diagnose cardiovascular problems earlier than any other blood pressure monitoring methods. It can be used to continuously monitor blood pressure in heart or lung disease patients.

"Wearable devices have so far been limited to sensing signals either on the surface of the skin or right beneath it," said Sheng Xu, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering and the corresponding author of the study. "But this is like seeing just the tip of the iceberg. 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," said Xu, who is also affiliated with the Center for Wearable Sensors at UC San Diego.

The patch is made out of a thin sheet of silicone with elastomer patterns and an island-bridge structure of electronics parts and spring-shaped wires. Each of the islands has piezoelectric transducers that produce ultrasound waves that monitor the pressure. The ultrasound capability could be used to monitor many other physiological body signs.

"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."

Measuring central blood pressure is incredibly important for monitoring heart issues. Central blood pressure is widely believed to be more accurate and a better indicator for predicting heart disease than peripheral blood pressure.

Current methods for measuring central blood pressure is very invasive. A catheter inserted in blood vessels in the patient’s arm, groin or neck. This is an uncomfortable process for the patient and can be difficult.

There is a non-invasive method to measure central blood pressure, but it is not accurate or consistent. A pen-like probe called a tonometer is placed on the skin perfectly above a major blood vessel. The tonometer needs to be held perfectly steady at just the right angle with the right pressure and the patient must stay still. This makes it difficult to achieve the necessary accuracy. Furthermore, the tonometer isn’t sensitive enough to get readings through fatty tissue.

"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 the tonometer down too hard, it'll put too much pressure on the vessel, which also affects the data," said co-first author Chonghe Wang, a nanoengineering graduate student at UC San Diego.

The new wearable ultrasound patch is soft and stretchy. The patient wears it on the skin and the patch gathers blood pressure information over time. After testing, the patch could gather information about blood pressure with accuracy and precision, even when the patient is moving, and it works through fatty tissue.

The patch was tested on a male patient. He wore it on his forearm, wrist, neck and foot. There were two tests performed, one when he was still and one when he was exercising. During both tests, the patch proved to be more consistent and precise than the tonometer and comparable to the traditional probe technique.

"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 ultrasound waves continuously record the diameter of a pulsing central blood vessel. The ultrasound waves represent a specific activity or heart event, which provides information about the heart, even predicting a heart attack.

There are some improvements that need to be made before the patch is made available clinically. The researchers say the power source, data processing units, wireless communication capabilities all need to be improved.

"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 paper on this technology was published in Nature Biomedical Engineering.

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