Wearable devices have been limited to sensing signals either on the surface of the skin or right beneath it. A new wearable ultrasound patch non-invasively monitors blood pressure in arteries as deep as four centimeters (more than one inch) below the skin, helping to detect cardiovascular problems earlier on and with greater precision. In tests, the patch performed as well as some clinical methods to measure blood pressure.

The wearable ultrasound patch tracks blood pressure in a deep artery or vein. (Image: Chonghe Wang/Nature Biomedical Engineering)

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. 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 that sends blood directly from the heart to other major organs throughout the body. Medical experts consider central blood pressure more accurate than peripheral blood pressure and also say it's better at predicting heart disease.

Measuring central blood pressure isn't typically done in routine exams. The state-of-the-art clinical method is invasive, involving 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 it can't consistently produce accurate readings. 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. But this can vary between tests and different technicians. Tonometers also require the patient to sit still — which makes continuous monitoring difficult — and are not sensitive enough to get good readings through fatty tissue.

When worn on the neck, the device records central blood pressure in the carotid artery (CA), internal jugular vein (Int JV), and external jugular vein (Ext JV).

The new soft, stretchy ultrasound patch can be worn on the skin and provides 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 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 that produce ultrasound waves when electricity passes through them. The bridges connecting them are made of thin, springlike 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, represent a specific activity or event in the heart. These signals provide detailed information to doctors for assessing a patient's cardiovascular health. They can be used to predict heart failure, determine if blood supply is sufficient, etc.

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