A skin patch could monitor blood flow deep within a person's body.
Engineers from the University of California San Diego have developed a stretchy device that senses the activity within major veins and arteries, up to 14cm (5.5 inches) beneath the skin.
Blood flow indicates a person's risk of clots, poor circulation and even blockages that can lead to heart attacks and strokes.
Writing in the journal Nature Biomedical Engineering, the engineers explain how the non-invasive patch can be worn on the chest or neck, enabling medics to identify cardiovascular complications early on.
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The San Diego team has stressed the patch is still in an early stage of development. The engineers are working towards a wireless device, with the existing prototype requiring connection to a power source and bench-top machine.
"This type of wearable device can give you a more comprehensive, more accurate picture of what's going on in deep tissues and critical organs like the heart, all from the surface of the skin", said lead author Professor Sheng Xu.
Co-author Chonghe Wang added: "Sensing signals at such depths is extremely challenging for wearable electronics, yet this is where the body's most critical signals and the central organs are buried.
"We engineered a wearable device that can penetrate such deep tissue depths and sense those vital signals far beneath the skin. This technology can provide new insights for the field of healthcare."
The patch is made up of a flexible compound that sticks to the skin. It also contains an array of millimetre-sized ultrasound transducers.
Electricity flowing through the transducers causes them to vibrate and emit ultrasound waves. These then penetrate the skin, travelling deep into the body.
When the waves pass through a major blood vessel, they encounter movement from the red blood cells – which transport oxygen – flowing inside.
This movement changes how the ultrasound waves echo back to the patch. The shift in the reflected signals then gets picked up by the patch, creating a visual recording of a person's blood flow. The same mechanism can also create moving images of the heart's walls.
All of the patch's transducers can be synchronised to transmit ultrasound waves in unison, producing a high-intensity beam that focuses on one spot up to 14cm deep within the body.
The transducers can also be programmed to transmit waves out of sync, leading to ultrasound beams that can be steered to different angles. Existing sensors generally only monitor the area directly beneath them.
"With this patch, we can probe areas that are wider than the device's footprint," said Professor Xu. "This can open up a lot of opportunities."
Co-author Muyang Lin agreed, adding: "This gives our device multiple capabilities, monitoring central organs as well as blood flow, with high resolution. This would not be possible using just one transducer."
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A person's blood flow is not routinely monitored by their doctor and is generally only assessed in high-risk individuals or after a sign of cardiovascular complications.
When blood flow is monitored, standard examinations involve a trained technician pressing a handheld ultrasound probe against a person's skin, moving it until the device is directly above a major blood vessel.
This can be a complicated and time-consuming process, with the results varying between technicians and different types of monitors.
The San Diego team's simpler patch could overcome these problems.
"Just stick it on the skin, then read the signals," said co-author Sai Zho.
"It's not operator dependent and it poses no extra work or burden to the technicians, clinicians or patients.
"In the future, patients could wear something like this to do point of care or continuous at-home monitoring."
Tests have suggested the patch performs as well as a commercial ultrasound probes that are already being used in clinics. Results show the patch accurately recorded blood flow in major vessels, like the neck's carotid artery, which supplies blood to the brain.
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