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.
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.