It took hundreds of failed experiments, sticking gummy gels to all sorts of surfaces, for scientists at Massachusetts Institute of Technology to get what they were looking for: a material so adhesive, it could cement a device to the skin for two full days while still letting sound waves pass through.
The resulting platform, described in new research published in Science, can be used for tools like wearable ultrasound probes that help provide continuous, hands-free monitoring of internal tissue and organs. Where imaging devices were once bulky and stand-alone, they now have the potential to comfortably move with a person for hours or days at a time. Experts said the device has an imaging quality comparable to that of commercially sold systems.
“It’s a very impressive new frontier about how we can use ultrasound imaging continuously to assess multiple organs, organ systems,” said Eric Topol, the founder and director of the Scripps Research Translational Institute, who studies wearables and was not involved in the new study. “48 hours of continuous imaging, you’d have to lock somebody up in a hospital, put transducers on them. This is amazing, from that respect.”
Experts said that the study uses a novel approach to solve the long-standing challenge of attaching an ultrasound to the human body. Ultrasounds are normally fairly large desktop- or phone-sized devices with an associated probe that glides over the area of the body that needs to be imaged. The tip of that probe is covered in what are known as transducers, which send and receive the sound waves of the ultrasound.
“A lot of people actually never thought about using rigid transducers,” said Nanshu Lu, a bioelectronics researcher at the University of Texas at Austin who did not participate in the study. “We always think ‘Oh, rigid transducers, let’s forget about them. There’s no way.’”
To develop the new device, the researchers decided to place a pliable material between the skin and the transducers that send and receive the sound waves of the ultrasound. Importantly, that new material is a hydrogel — a water-filled polymer — encapsulated with a rubbery, bioadhesive elastomer that maintains the shape and gooeyness of the hydrogel. The material also eliminates air bubbles that would otherwise pop up between the ultrasound’s probe and the skin.
Those air pockets are also a problem in conventional ultrasounds, which also rely on a layer of gel slathered on the skin to help the sound waves travel from the ultrasound to the body. But that gel — unlike the hydrogel used in the new technology — is messy and dries quickly, making it impossible to use in a longer-term wearable.
“It’s really like [changing] from toothpaste to modern Jell-O. Now it’s solid, it maintains shape,” said Zhao. The hydrogel is also wrapped in an elastomer “skin” that, as Zhao explained, “really locks the water inside the Jell-O, so the Jell-O wouldn’t dry out over a time of days.”
The robust adhesiveness of the hydrogel-elastomer composite also allows for relatively smooth imaging when the body is in motion. Experts said that this device ushers in a new era of thinking about wearables not just as technology woven into clothing or worn on the wrist, but as patches that can be applied like an EKG electrode and offer detailed insight into what is happening with a patient over time. “It’s a new window into the human body that we’ve never had before,” said Topol. “This is anatomy. That’s very different. We’ve never had a sensor with continuous anatomical imaging.”
The promise of continuous imaging is especially appealing for ultrasounds, which offer the potential for real-time, cross-sectional imaging at a cheaper cost and without the radiation required with other methods. “We’ll benefit from these new ultrasound technologies,” said Michael Forney, a musculoskeletal radiologist at the Cleveland Clinic who was not involved in the new development.
And experts see this type of imaging enabling a far deeper look into the body than many current wearables can provide. Signals on the skin “are important, but they are symptoms,” Sheng Xu, said a wearable ultrasound researcher at the University of California, San Diego who did not participate in the research. He said imaging internally lets researchers and physicians get a better handle on the root causes of those symptoms.
“Wearables going deep is starting the future. No doubt about it,” he said.
Xu has been independently working on a stretchable ultrasound probe that, although it captures images at a lower resolution and is more susceptible to body movement than the new technology, can visualize a larger part of the body.
Physicians like Forney could also benefit from more convenient ultrasound technologies during medical and surgical procedures, where imaging is crucial. As “the tools get developed to do more ultrasound-guided surgeries, it will be a waste of a hand to have to hold a probe,” said Forney. “And this kind of device would solve that problem.”
Forney also needs ultrasound probes to diagnose patients with a variety of conditions, from snapping tendons to muscles that excessively impinge on arteries. For these scenarios, he needs to perform a dynamic exam where the patients are actively contracting their muscles. Keeping a probe on the patient can be difficult, however. “Sometimes we can, and sometimes we can get an answer, but lots of times it’s hard to hold a probe on somebody when they’re trying to walk,” Forney said. “This device would make that diagnosis a lot easier because you would just put it in the area and have the patient, basically, exercise.” He also said anesthesiologists and emergency physicians might benefit from the device when performing a nerve block to relieve pain.
“I look at this as a platform technology, a technology that could monitor many potential things,” said Jeffrey Karp, a bioengineer at Brigham and Women’s Hospital, who was not involved in this research. Since its development in the 1970s, MRI has spread far beyond one of its initial intended uses of detecting cancerous tissue, Karp added, noting that the same is true of any new capability. “If you look at MRI as an example, there was initially some really important use cases, but when it started to become more mainstream, all these other uses started popping up.”
More work needs to be done, including scaling down the signal processing unit that generates the images. “I don’t see that as a major challenge,” Karp said. “And I’m really curious about where this technology is going to go next.”
The next steps could include clinical testing. “The challenge now is, okay, you’ve got the clinical device, now we need some clinical trials,” said Joshua Copel, an OB-GYN and pediatrician at Yale who studies ultrasound technologies and is not associated with the new wearable. “And we need FDA approval and we need clinical trials to compare it to standard care.”
Experts also said that for the device to be impactful, whether at home or in the clinic, there are challenges ahead in analyzing and interpreting the data. There is simply the capacity to image so much more than physicians are used to, and monitoring constantly may require leveraging recent advances in software. “Another application I think would be the use of artificial intelligence,” said Philip Tan, a Ph.D. student at UT Austin who was not involved in the study and who authored a commentary with Lu on the new findings. “An image is only worth something if you can actually diagnose it. So even if we’re able to get all of these images, we still need help getting useful medical diagnoses out of them.”
There’s also a question about how health systems would store the massive amount of data that would be generated by continuous ultrasounds.
“If you do that for 48 hours continuously, that’s a lot of data. Where does that data go?” Copel asked. “How does that get integrated into the health record so that there’s some way of going back and saying ‘Here’s what happened at this point and here’s what I did?’”
Copel, who cares for newborns and infants, said there would be challenges to using this kind of technology in his field, but he has hopes for other uses. “There’s a lot of potential for this,” said Copel. “They could get to that point of something that’s wearable for an athlete.”
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