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Wearable Ultrasound Patch Tracks Blood Pressure

The pressure of blood flowing through arteries and other blood vessels can harm organs when it remains too high for too long. High blood pressure, or hypertension, can lead to heart attack, stroke, kidney disease, and other health problems. But this common disease usually doesn’t cause warning symptoms, so many people don’t know they have it.

When worn on the neck, the device records central blood pressure in the carotid artery, internal jugular vein and external jugular vein (Image courtesy of Chonghe Wang/Nature Biomedical Engineering)

A blood pressure test is the only way to detect hypertension. The usual method is with a blood pressure cuff on the arm. Another method is with a small device called a tonometer pressed against the skin over a blood vessel. For patients who are critically ill, health care providers can obtain even more accurate blood pressure readings by inserting a special tube inside an artery near the heart. This method is called cardiac catheterization.

To develop an accurate but less invasive technique, a research team led by Dr. Sheng Xu at the University of California, San Diego, set out to develop a thin, wearable blood pressure sensor using ultrasound transducers. Transducers make high frequency sound waves that bounce off the blood vessel. The transducer then receives the echo patterns and sends them to a computer to create a representation of the vessel’s changing diameter, called a waveform. When calibrated to a patient’s blood pressure, these waveforms can be used to monitor changes in blood pressure.

The team engineered a stretchable, flexible device that can be worn as a skin patch. It has an array of ultrasound transducers so that the one that’s in the best position over an artery can be selected. A layer of silicone beneath the patch eliminates the need for the gel used with conventional ultrasound probes. The work was supported in part by NIH’s National Institute of Biomedical Imaging and Bioengineering (NIBIB). Results were published in Nature Biomedical Engineering on September 11, 2018.

The patch could produce waveforms for vessels as deep as 4 centimeters below the skin surface. The researchers compared the skin patch to a pen-like tonometer for measuring blood pressure on the neck, wrist, arm, and foot of a healthy 22-year-old man. When the man moved or twisted, the readings were more stable with the skin patch than with the tonometer. The team found better precision and accuracy with the patch as well. 

“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,” Xu says. “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 precision and accuracy of the skin patch hasn’t yet been compared to the invasive catheterization method of measuring blood pressure. For the next phase of development, the team would like to engineer a way to make the skin patch wireless. 

—by Geri Piazza

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