This patch looks into the body

A new ultrasound patch can increase the activity of the heart, lungs and other organs for 48 hours straight.

Image: Ultrasound patch stuck to the skin. Credit: Wang et al.

Ultrasound scanners that visualize the inside of the human body are a life-saving medical device. Researchers have now reduced the portable ultrasound probe – which normally requires a trained technician to move across the skin – into a flat chip the size of a postage stamp, which is stuck to the skin with a special bioadhesive. The new device can take two days of high-resolution videos of the heart and blood vessels swelling during exercise, or of the stomach expanding and contracting as subjects drink and digest juice.

“The great thing about this is that you can attach this ultrasound probe, this thin ultrasound speaker, to the body for 48 hours,” said Xuanhe Zhao, a mechanical engineer at the Massachusetts Institute of Technology and co-author of a paper in science describes the new device. By taking pictures and videos of internal organs during this time, a portable imaging device can be used to diagnose heart attacks and malignant tumors, test the effectiveness of drugs, and assess general heart, lung or muscle health. “This could change medical imaging by enabling long-term continuous imaging,” Zhao adds, “and it could change the field of wearable devices.”

Close-up of the ultrasound patch. Credit: Felice Frankel

Portable, mobile and accessible

Traditional ultrasound is very suitable for looking under the skin without causing damage to the body, but access to such scans is limited. “Conventional portable ultrasound requires well-trained technicians to properly place the probe on the skin and apply some liquid gel between the probe and the skin,” said Nanshu Lu, a mechanical engineer at the University of Texas at Austin who was not involved. was with the new study, but an associated analysis entered science. “And as you can imagine, it’s very short-lived and very limited.” Because they require an experienced human operator, Lu explains, these scans are expensive and cannot be used in tests where the subject is exercising or putting their body under stress from heat or extreme environments. “Conventional ultrasound has many limitations,” she says. ‘If we can make ultrasonic sensors portable, mobile and accessible, it opens up a lot of new possibilities.’

Thanks to their potential versatility, other researchers have also attempted to create ultrasonic patches. But to stick to soft, stretchy skin, they designed devices that stretch themselves. This form factor degraded image quality because it could not accommodate enough “transducers”—devices that, in this case, convert electrical energy into sound waves at frequencies too high for the human ear to perceive. An ultrasound probe sends these waves through a layer of sticky gel into the human body, where they bounce off organs and other internal structures and then return to the transducers. This converts the mechanical waves back into electrical signals and sends them to a computer for translation into images. The more transducers, the better the image quality.

Comparison of a conventional ultrasound probe with gel and the ultrasound patch. Credit: Wang et al.

“It’s very similar to a camera,” said Philip Tan, an electrical engineer and graduate of Lu’s lab at UT Austin, who was also not involved in the new study but contributed to the analysis piece. A stretchable ultrasound probe, which must be able to bend with every movement of the skin, cannot count that many transducers. And as the wearer moves, the configuration of the transducers changes, making it difficult to take stable images.

Instead of making the device itself stretchable, Zhao and his team attached a three-millimeter rigid probe to a flexible adhesive layer. Replacing the sticky liquid placed between a traditional ultrasound wand and the skin, this adhesive is a hybrid of an aqueous polymer – a hydrogel – and a rubbery material – an elastomer. “It’s a piece of solid hydrogel that contains more than 90 percent water, but it’s in a solid state like pudding,” says Zhao. ‘We cover the surface of this pudding with this very thin elastomer membrane so that the water in the pudding doesn’t evaporate from it.’ This bioglue not only adheres the probe to the skin for 48 hours, but also provides a cushioning layer that protects the rigid electronics from bending skin and muscles.

The bioglue sticks the probe to the skin for 48 hours and protects the electronics from bending skin and muscles. Credit: Wang et al.

To image different body systems, Zhao’s team tested versions of the probe that produce waves of different frequencies and penetrate the body to different depths. For example, a high frequency such as 10 megahertz can penetrate as far as several centimeters under the skin. The researchers used this frequency to record the function of blood vessels and muscles when the subjects sat down and stood up again, or when they exerted themselves. A lower frequency of three megahertz goes deeper, about six centimeters, to capture internal organs. Using this frequency, the researchers imaged the four chambers of one subject’s heart and recorded how another subject’s stomach emptied after digesting a few glasses of juice. The researchers also compared the images collected with their rigid ultrasound probe to the images taken with a stretchable ultrasound device, Zhao says. ‘You can see that our resolution is almost an order of magnitude [10 keer] higher than for the stretchable ultrasound device,” he adds.

Detection and diagnosis

An imaging device that continuously monitors specific areas of the body can be used to monitor and diagnose a variety of conditions. Doctors could closely monitor the growth of a tumor over time. A person at high risk of hypertension may wear an ultrasound patch to measure their high blood pressure, to alert them when the pressure is rising, or to see if a drug is working. A covid patient could stay at home knowing that an imaging device would alert him if his illness causes a lung infection severe enough to require hospitalization. Perhaps its most important application could be in the detection and diagnosis of heart attacks. “Cardiovascular disease is the leading cause of death worldwide,” says Zhao.

Our bodies are full of data. The question is how can we achieve them reliably and continuously’

Heart health is on the radar of other wearable device developers. For example, smartwatches like the Apple Watch are able to track the electrical signals that indicate heart activity with a so-called electrocardiogram (EKG or EKG). This can be used to diagnose heart attacks – at least in some cases. “There are already studies showing that an EKG can only diagnose about 20 percent of heart attacks. The majority of heart attacks actually require imaging modalities, such as ultrasound imaging, to make a diagnosis,” says Zhao. Continuous imaging of a patient’s heart could capture symptoms and provide an early diagnosis.

“The big selling point of this new device is that it enables new kinds of medical diagnosis that cannot be performed in a static environment,” says Tan. To assess heart health, for example, it is useful to measure the organ’s activity during exercise – but it is difficult to hold an ultrasound rod against the sweat-covered chest of a running subject. “With a wearable ultrasound patch where you don’t have to hold the transducer on the person, they were able to show that you can get very high quality images of the heart even while moving,” added Tan.

Wearables monitor health

However, the device is not yet ready for use. First, it still needs to be physically connected to a computer that can collect and analyze the data the probe produces. “We connect this probe via a wire to a data acquisition system,” says Zhao. “But my group is working hard to miniaturize everything and integrate it into our wireless device.” He plans to eventually upgrade the patch with a miniaturized power source and wireless data transmission system. This is an achievable goal, Lu and Tan agree, thanks to shrinking electronic components and manufacturing methods that make it possible to combine these functions in an ‘ultrasound on a chip’. Lu suggests that if the field can attract federal and private investment, such a device could be feasible within five years, though it still requires approval from federal regulators.

Ultimately, ultrasound patches could join the ranks of wearables that monitor human health, including existing devices that collect information about heart rate, sleep quality and even stress. “Our bodies radiate a lot of very personal, very continuous, distributed and multimodal data about our health, our emotions, our attention, our readiness and so on. So we’re full of data,” says Lu. “The question is how we can get it reliably and continuously.”

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