Earlier this year, we heard how scientists from the University of California San Diego had developed a flexible ultrasound patch that allows users to see the inner structure of irregular-shaped objects. Well, now they've made one that measures a patient's blood pressure from deep within the body.
When you get your blood pressure taken with an arm cuff, what's being measured is known as peripheral blood pressure. This is different than central blood pressure, which is the pressure within the central blood vessels that deliver blood straight from the heart to other major organs. Not only is central blood pressure more accurate than peripheral, but it's also said to be a better indicator of impending heart disease.
Unfortunately though, the standard method of measuring central blood pressure involves inserting a catheter into a blood vessel in the neck, groin or arm and then guiding it to the heart.
There's also a non-invasive method in which a tonometer probe is held against the skin above a major blood vessel, although readings can be affected by the steadiness and angle at which the device is held, along with the amount of pressure that the user applies to the patient's skin. Additionally, patients must remain very still throughout the procedure.
That's where the new ultrasound patch comes in.
It consists of a thin silicone elastomer sheet that's patterned with "islands" of electrodes and piezoelectric transducers, linked together by springy copper wire "bridges" – this setup allows it to stretch, bend and twist without damage to the electronics, and is known as an island-bridge structure.
Adhered to the skin and hard-wired to a power source and data processing unit, it produces ultrasound waves that continuously monitor the diameter of pulsing major blood vessels located up to 4 cm (1.6 inches) beneath the skin. The data processor translates that data into a real-time central blood pressure reading.
To test the device, a volunteer wore one on his foot, neck, wrist and forearm, both while sitting still and performing exercise. In all cases, it obtained readings that were more accurate than those gathered with a tonometer, and similar to those from a blood vessel-inserted catheter.
"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," says Prof. Sheng Xu, corresponding author of the study. "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 researchers now plan on integrating a power source, processor and wireless communications system into the patch, so it can measure blood pressure and transmit its readings as a standalone device. Ultimately, it may someday be used to continuously monitor patients with heart or lung disease, or who are undergoing surgery.
A paper on the research was recently published in the journal Nature Biomedical Engineering.
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