Newswise – MIT engineers have developed a small ultrasound sticker that can monitor the stiffness of organs deep within the body. The sticker, which is about the size of a postage stamp, can be worn on the skin and is intended to detect signs of illness such as liver and kidney failure as well as the progression of solid tumors.

In an open access study to be published in Scientific advancesThe team reports that the sensor can send sound waves through the skin and into the body, where the waves reflect from internal organs and travel back to the sticker. The pattern of reflected waves can be read as a sign of organ stiffness, which the sticker can measure and track.

“When some organs become diseased, they can stiffen over time,” says study lead author Xuanhe Zhao, a professor of mechanical engineering at MIT. “This wearable sticker allows us to continuously monitor changes in stiffness over long periods of time, which is critical for early detection of internal organ failure.”

The team showed that the sticker can continuously monitor the stiffness of organs over 48 hours and detect subtle changes that could indicate disease progression. In preliminary experiments, researchers found that the adhesive sensor can detect early signs of acute liver failure in rats.

Engineers are working on adapting the design for use on humans. They imagine the sticker could be used in intensive care units (ICUs), where the unobtrusive sensors could continuously monitor patients recovering from an organ transplant.

“We imagine that we could stick this sticker on a patient immediately after a liver or kidney transplant and observe how the organ’s stiffness changes over days,” says lead author Hsiao-Chuan Liu. “If acute liver failure is diagnosed early, doctors can take immediate action instead of waiting for the disease to become severe.” Liu was a visiting scientist at MIT at the time of the study and is currently an assistant professor at the University of Southern California.

MIT co-authors of the study include Xiaoyu Chen and Chonghe Wang, as well as collaborators at USC.

Perception wobbles

Like our muscles, the tissues and organs in our body stiffen as we age. In certain diseases, the stiffening of the organs may be more pronounced, indicating a potentially drastic decline in health. Doctors currently have ways to measure the stiffness of organs such as the kidneys and liver using ultrasound elastography – a technique similar to ultrasound imaging in which a technician passes a hand-held probe or wand over the skin. The probe sends sound waves through the body, causing internal organs to vibrate slightly and sending out waves in return. The probe detects the induced vibrations of an organ and the pattern of vibrations can be translated into how wobbly or stiff the organ must be.

Ultrasound elastography is typically used in the intensive care unit to monitor patients who have recently undergone an organ transplant. Technicians regularly examine a patient shortly after surgery to quickly examine the new organ and look for signs of stiffening and possible acute failure or rejection.

“After an organ transplant, the first 72 hours in the intensive care unit are the most important,” says another senior author, Qifa Zhou, a professor at USC. “With traditional ultrasound, you have to hold a probe to your body. But you can’t do that permanently and permanently. Doctors could miss a crucial moment and realize too late that the organ is failing.”

The team realized they could potentially offer a more continuous, portable alternative. Your solution expands one Ultrasound sticker They were previously developed to image deep tissues and organs.

“Our image sticker captured longitudinal waves, while this time we wanted to capture shear waves, which tell you the stiffness of the organ,” explains Zhao.

Existing ultrasound elastrography probes measure shear waves, or the vibration of an organ in response to sound pulses. The faster a shear wave propagates through the organ, the stiffer the organ is interpreted to be. (Think of the rebound of a water balloon compared to a soccer ball.)

The team wanted to miniaturize ultrasound elastography so that it could fit on a postage stamp-sized sticker. They also wanted to maintain the same sensitivity of commercial handheld probes, which typically contain about 128 piezoelectric transducers, each of which converts an incoming electric field into outgoing sound waves.

“We used advanced manufacturing techniques to cut small transducers from high-quality piezoelectric materials, which allowed us to design miniaturized ultrasound stickers,” says Zhou.

The researchers precisely manufactured 128 miniature transducers, which they integrated onto a 25 millimeter chip. They covered the bottom of the chip with an adhesive made of hydrogel – a sticky and stretchy material made from a mixture of water and polymer that allows sound waves to travel in and out of the device with almost no loss.

In preliminary experiments, the team tested the stiffness sensor sticker on rats. They found that the stickers were able to provide continuous measurements of liver stiffness over 48 hours. Using the data collected from the sticker, researchers identified clear and early signs of acute liver failure, which they later confirmed using tissue samples.

“Once the liver fails, the stiffness of the organ increases many times over,” notes Liu.

“You can go from a healthy liver that is as shaky as a soft-boiled egg to a diseased liver that looks more like a hard-boiled egg,” adds Zhao. “And this sticker can detect these differences deep inside the body and raise an alarm when organ failure occurs.”

The team is working with doctors to adapt the sticker for use on patients recovering from an organ transplant in the intensive care unit. In this scenario, they don’t expect any major changes to the sticker’s current design, as it can be stuck to a patient’s skin and any sound waves it sends and receives can be emitted and collected by the electronics connected to the sticker, similar to electrodes and ECG machines in a doctor’s office.

Researchers also hope to transform the sticker into a more portable, self-contained version, with all associated electronics and processing miniaturized to fit into a slightly larger patch. They then imagine that the sticker could be worn by patients at home to continuously monitor conditions over longer periods of time, such as the progression of solid tumors, which are known to harden with increasing severity.

“We believe this is a life-saving technology platform,” says Zhao. “We believe that in the future people will be able to stick a few stickers on their bodies to measure many vital signals and map and track the health of important organs in the body.”

This work was supported in part by the National Institutes of Health.


Written by Jennifer Chu, MIT News

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