When a patient go to a clinic for an ultrasound of their stomach, they lie on crumpled paper on an exam table. A clinician spreads a thick gunk on their abdomen and 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 over the person’s stomach, a blurry black-and-white image appears on the screen for the doctor 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, wants to miniaturize and simplify the whole thing — and make it portable. In a paper published today in Science, Zhao and his team describe their development of a small ultrasound patch that, when applied to the skin, can provide high-resolution images of what lies beneath. The scientists hope the technology could lead to ultrasound becoming comfortable for longer-term monitoring — perhaps even at home rather than a doctor’s office.
Because ultrasound equipment is so large and requires an office visit, Zhao says, its imaging capabilities are often “short-lived, 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 during an office visit. To address these issues, the scientists designed a patch — about 1 square inch in size and a few millimeters thick — that can be placed practically anywhere on the body and worn for a few days. “It looks like a postage stamp,” Zhao says.
The patch is multi-layered, like a candy wafer, with two main components: an ultrasonic probe stacked on top of a couplant, a material that facilitates the transmission of acoustic waves from the probe to the body. The scientists designed the probe to be thin and rigid, using a 2D array of piezoelectric elements (or transducers) sandwiched between two circuits. Chonghe Wang, one of the study’s co-authors, says these elements “can convert electrical energy into mechanical vibrations.” These vibrations travel in the body as waves and reflect back to an external imaging system to be translated into an image. Those vibrations, Wang adds, “are completely non-invasive. Humans cannot feel them at all.”
To create the ultrasonic probe, the scientists used 3D printing, laser micromachining and photolithography, which uses light 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, such as sweat. Because these techniques have a high throughput, the scientists say, one device can be manufactured in about two minutes.
The jelly-like couplant layer helps those ultrasonic 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 blend that acts like a strong glue-like substance to help the whole thing stick. The scientists found that the patch can be left on the skin for at least 48 hours, can be removed without leaving a residue and is resistant to water.
The MIT team is among a small group of labs that have produced similar miniaturized ultrasonic devices in recent years. Labs at UC San Diego and the 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 larger than the size of a postage stamp.