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NOVEMBER 2018 I PCB007 MAGAZINE 31 flatable cuff strapped around the upper arm. Medical experts consider central BP—the pres- sure in the central blood vessels that sends blood directly from the heart to other major organs throughout the body—more accurate than peripheral BP and better at predicting heart disease. However, measuring central BP isn't typi- cally done in routine exams. The state-of-the- art clinical method is invasive and involves a catheter to be inserted into a blood vessel in a patient's arm, groin, or neck, and guiding it to the heart. While a non-invasive method ex- ists, it cannot 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. "It's highly operator dependent. Even with the proper technique, if you move the tonom- eter tip just a millimeter off, the data get dis- torted. And if you push the tonometer down too hard, it will 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. 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. The UC San Diego-led team's alternative— a soft, stretchy ultrasound patch that can be worn on the skin—provides accurate, precise readings of central BP each time, even while the user is moving. The patch can also get a good reading through fatty tissue. Making Ultrasound Wearable "A major advance of this work is it trans- forms ultrasound technology into a wearable platform. This is important because now we can start to do continuous, non-invasive moni- toring of major blood vessels deep underneath the skin—not just in shallow tissues," said Wang. The patch is a thin sheet of silicone elasto- mer patterned with what's called an island- bridge structure—an array of small electron- Figure 2: BP patch on a neck. Figure 3: Ultrasound patch islands and bridges.