Engineers at the University of California, San Diego, have developed a soft and flexible ultrasound patch that can be worn on the skin to monitor blood flow in major arteries and veins deep inside the human body.

Knowing how fast and how much blood flows through a patient’s blood vessels is important because it can help doctors diagnose a variety of cardiovascular diseases, including blood clots, heart valve problems, circulatory disorders in the extremities or blockages in the arteries that can lead to a stroke or heart attack.

The new ultrasound patch, developed at the University of California, San Diego, can continuously monitor blood flow — as well as blood pressure and heart function — in real time. Wearing such a device could make it easier to detect cardiovascular problems early on.

The team, led by Sheng Xu, a professor of nanoengineering at the Jacobs School of Engineering at the University of California, San Diego, described the patch in a July 16 paper published in Nature Biomedical Engineering.

The patch can be worn around the neck or chest. What makes the patch special is that it can noninvasively sense and measure cardiovascular signals up to 14 centimeters deep inside the body. In doing so, it can do so with high accuracy.

This type of wearable device can give you a fuller, more accurate picture of what’s going on in the deep tissues and critical organs like the heart and brain, all from the surface of your skin.”

Sheng Xu, professor of nanoengineering, Jacobs School of Engineering at the University of California, San Diego

“Sensing signals at this depth is extremely challenging for wearable electronics. However, this is where the body’s most important signals and central organs are located,” says Chongge Wang, a former graduate student in nanoengineering in Sheng Xu’s lab and co-author of the first study. “We have developed a wearable device that can penetrate this deep into the tissues and sense vital signals far beneath the skin. This technology could provide new insights into health care.”

Another innovative feature of the patch is that the ultrasound beam can be tilted at different angles and directed to areas of the body that are not directly under the patch.

This is a first in the field of wearable devices, Xu explained, because existing wearable sensors usually only monitor areas that are directly below them. “If you want to sense signals elsewhere, you have to move the sensor to that location. With this patch, we can probe areas that are wider than the footprint of the device. It can open up a lot of possibilities.”

How it works

The patch consists of a thin sheet of flexible, stretchable polymer that adheres to the skin. The patch incorporates an array of millimeter-sized ultrasound sensors. Each is individually controlled by a computer – this type of array is known as an ultrasonic phased array. This is a key part of the technology because it gives the patch the ability to penetrate deeper and wider.

The phased array has two basic modes of operation. In one mode, all transducers can be synchronized to transmit ultrasonic waves together, creating a high-intensity ultrasonic beam that focuses on a single area of the body up to 14 centimeters deep. In another mode, the transducers can be programmed to transmit asynchronously, producing ultrasound beams that can be directed at different angles.

“With phased array technology, we can manipulate the ultrasonic beam the way we want it,” says Muyang Lin, a graduate student in the Department of Nanoengineering at the University of California, San Diego, who is also one of the original authors of the study. “This gives our device a lot of capabilities: central organ monitoring as well as high-resolution blood flow. This would not have been possible with just one sensor.”

The phased array consists of a 12-by-12 grid of ultrasound sensors. When electricity passes through the sensors, they vibrate and emit ultrasonic waves that pass through the skin and penetrate deep into the body. When the ultrasound waves pass through a large blood vessel, they collide with the movement of red blood cells flowing inside it. This motion alters or shifts the way ultrasound waves reflect off the patch, an effect known as a Doppler frequency shift. This shift in reflected signals is picked up by the patch and used to create a visual record of blood flow. The same mechanism can also be used to create moving images of the walls of the heart.

Potential for change in the clinic setting

For many people, blood flow is not something that is measured during a routine visit to the doctor. It is usually evaluated after a patient shows signs of cardiovascular problems or if the patient is at high risk.

The standard blood flow study itself can be time-consuming and labor-intensive. A trained technician presses a hand-held ultrasound probe against the patient’s skin and moves it from one area to another until it is directly over a major blood vessel. This may seem simple, but results can vary from test to test and from specialist to specialist.

Because the patch is easy to use, it can solve these problems, says Sai Zhou, a graduate student in the Department of Materials Science and Engineering at the University of California, San Diego, and a co-author of the study. “Just stick it on your skin and then read the signals. It’s operator-independent and doesn’t create extra work or burden for technicians, clinicians or patients,” he said. “In the future, patients will be able to wear something like this for spot or continuous home monitoring.”

In tests, the patch showed the same results as a commercial ultrasound transducer used in the clinic. It accurately recorded blood flow in large blood vessels such as the carotid artery, the artery in the neck that supplies blood to the brain. Being able to track changes in that blood flow could, for example, help determine whether a person is at risk for a stroke long before symptoms appear.

The researchers note that the patch still has a long way to go before it is ready for use in the clinic. Currently, it must be connected to a power supply and tabletop equipment in order to work. Xu’s team is working on integrating all the electronics into the patch to make it wireless.

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