When a patient walk into a clinic for an ultrasound of their abdomen, they lie down on crumpled paper on top of an exam table. A clinician spreads a thick goo on their stomach, 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 and return to the probe to be translated into a 2D image. As the probe moves across the person’s abdomen, a blurry black-and-white image appears on the screen for the clinician to read.
While ultrasound technology is a staple in many medical settings, it is often large and bulky. Xuanhe Zhao, a mechanical engineer at the Massachusetts Institute of Technology, aims to miniaturize and simplify it all—and make it portable. 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 beneath. The researchers hope that the technology could lead to ultrasound becoming comfortable for longer-term monitoring—perhaps even at home instead of in 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, doctors may 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 researchers designed a patch—about 1 square inch in size and a few millimeters thick—that can be placed virtually anywhere on the body and worn for a few days. “It looks like a postage stamp,” says Zhao.
The patch is multi-layered, like a candy wafer, with two main components: an ultrasound probe, which is stacked on top of a coupling, a material that helps facilitate the transmission of acoustic waves from the probe into the body. The researchers 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 co-authors of 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 an image. These vibrations, Wang adds, “are completely non-invasive. Humans cannot feel them at all.”
To create the ultrasound probe, the researchers used 3D printing, laser micromachining and photolithography, where light is used to create a pattern on a light-sensitive material. The probe is then coated with a layer of epoxy, which helps protect it from water damage, e.g. from sweat. Because these techniques are high-throughput, the researchers say, a device can be manufactured in about two minutes.
The jelly-like coupling layer helps these ultrasound waves travel into the body. It contains a layer of hydrogel protected by a layer of polyurethane to retain water. All of this is coated with a thin polymer mixture that acts as a strong glue-like substance that helps it all stick. The researchers found that the patch can stick to the skin for at least 48 hours, can be removed without leaving a 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 University of Toronto 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 sizes.