When a patient goes into a clinic for an ultrasound of their stomach, they lie down on crinkly paper atop an exam table. A clinician spreads a thick goo on their abdomen, then presses a small probe into it to send acoustic waves into the patient’s body. These waves bounce off their soft tissues and body fluids, returning to the probe to be translated into a 2D image. As the probe moves over the person’s stomach, a blurry black-and-white picture appears onscreen for the clinician to read.
While ultrasound technology is a staple in many medical settings, it is often big and bulky. Xuanhe Zhao, a mechanical engineer at the Massachusetts Institute of Technology, aims to miniaturize and simplify the entire thing—and make it wearable. In a paper published today in Science, Zhao and his team describe their development of a tiny ultrasound patch that, when stuck to the skin, can provide high-resolution images of what lies underneath. The scientists hope that the technology can lead to ultrasound becoming comfortable for longer-term monitoring—maybe even at home rather than at a doctor’s office.
Because ultrasound equipment is so large and requires an office visit, Zhao says, its imaging capabilities are often “short term, for a few seconds,” limiting the ability to see how an organ changes over time. For example, physicians might want to see how a patient’s lungs change after taking medication or exercising, something that is difficult to achieve within an office visit. To tackle these problems, the scientists designed a patch—approximately 1 square inch in size and a few millimeters thick—that can be placed practically anywhere on the body and worn for a couple of days. “It looks like a postage stamp,” Zhao says.
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The patch is multi-layered, like a candy wafer, with two main components: an ultrasound probe which is stacked on top of a couplant, a material that helps facilitate the transmission of acoustic waves from the probe into the body. The scientists designed the probe to be thin and rigid, using a 2D array of piezoelectric elements (or transducers) stuck between two circuits. Chonghe Wang, one of the coauthors on the study, says that these elements can “transform electrical energy into mechanical vibrations.” These vibrations travel into the body as waves and reflect back to an external imaging system to be translated into a picture. Those vibrations, Wang adds, “are fully noninvasive. The human cannot feel them at all.”
To create the ultrasound probe, the scientists used 3D printing, laser micromachining, and photolithography, in which light is used to create a pattern on a photosensitive material. The probe is then coated with a layer of epoxy, which helps protect it from water damage, like from sweat. Because these techniques are high-throughput, the scientists say, one device can be manufactured in approximately two minutes.
The jellylike couplant layer helps those ultrasound waves travel into the body. It contains a layer of hydrogel protected by a layer of polyurethane to hold in water. All of this is coated with a thin polymer mixture that acts as a strong gluelike substance to help the entire thing stick. The scientists found that the patch can cling to skin for at least 48 hours, can be removed without leaving residue, and can withstand water.
The MIT team is among a small group of labs that have produced similar miniaturized ultrasound devices over the past few years. Labs at UC San Diego and the University of Toronto are working on related projects—Wang produced an earlier patch model at UCSD. But these were often limited in their imaging capabilities or were larger than postage-stamp-sized.
The new design—with a rigid probe on top of a stretchy couplant layer—is a detour from other patches, says Zhao, which often made the actual probe flexible. A flexible probe creates a problem, he says: “The ultrasound probe is similar to the imaging sensor of your camera. Imagine if you distort that imaging sensor; then the images captured will be distorted and the resolution will be lost.” By keeping the probe rigid but letting the couplant layer bend and stretch, the scientists were able to achieve a higher resolution with better imaging quality. Their version also lets them customize the imaging depth—seeing as far as 20 centimeters below the skin—and resolution.
To measure wearability, they placed the patch on 15 human subjects for 48 hours. Only one person noted slight itchiness. The scientists also stuck the patches on themselves to get firsthand feedback. “I forgot that it was there,” says Xiaoyu Chen, another coauthor on the paper. “It’s very comfortable.” Wang agrees, adding that it’s much more pleasant than traditional ultrasound gel, which “will make a mess on your skin—it’s cold and itchy.”Their current design has one big drawback: It’s not wireless. That meant that to test the imaging capabilities of each patch over that two-day period, the subject had to agree to stay hooked up to a conventional laboratory ultrasound imaging system through a cable. The cable was long enough that the subject could still “move around, walk around; for example, they can also walk on a treadmill or bike on a cycling machine,” Zhao says.
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By sticking the patch on different parts of the subject’s body, the researchers could get images of the stomach, muscles, blood vessels, lungs, and heart. After the subject exercised, the scientists showed that the left ventricle of the heart expanded and the blood-flow rate in the carotid artery increased. In another set of images, the scientists found that the subject’s stomach would expand as they drank juice, then contract as the juice was processed. “We also imaged the bladder, but we didn’t put that data inside this paper,” Wang quips.
Chandra Sehgal, a radiology researcher at the University of Pennsylvania, notes that the miniature nature and user-friendliness of a patch like this could help clinicians feel confident that any changes observed in the images are actually due to the patient changing their behavior and not operator error. “Ultrasound is known for its variability and user-dependence,” he says. For example, accidentally moving the probe a smidge to the side can make a vein look larger than it is. With the patch, it would be easier to tell if this apparent vein expansion was a mistake or could be attributed to something real, like the patient lying down. “You can do this measurement in a more reliable way,” he adds.
This work “is very exciting,” says Lawrence Le, who runs a laboratory focused on ultrasound imaging and technology development at the University of Alberta. He notes, though, that cables and wires are still needed to connect the patch to an external imaging system. “In the future, I think it’s possible that this data can be wirelessly sent out,” Le says, given recent advances to miniaturize and integrate the imaging system. “It’s getting there.”
Zhao and his team are already envisioning how this patch can be used in medical settings. One application, he says, could be for monitoring the lung function of a Covid patient at home—seeing how it changes over time. Another could be for measuring blood pressure and heart function in people with cardiovascular diseases. Zhao says that it could also be used to supplement something like an EKG, which records electrical signals from the heart but not images, to give a fuller picture of what is going on inside the body.
While the scientists have demonstrated that the patch works, they agree with Le that it would be better if it were wireless so that the patient would not need to be constantly hooked up to a machine. They are also working on further improving the image resolution with the goal of “reaching or exceeding the resolution of point-of-care ultrasound,” Zhao says. A patch that users could wear for long periods opens up the possibility of long-term continuous imaging, he adds: “We have the opportunity to obtain huge amounts of data of different organs.” And so, he says, it will be important to build algorithms to process that data, so that clinicians can potentially diagnose conditions from the images.
In the meantime, though, the team is thrilled that a stamp-sized patch can actually visualize a person’s organs. Being able to “see something inside my body in the moment,” Chen says, is “amazing.”