Mechanical engineers Shervin Foroughi and Mohsen Habibi were painstakingly maneuvering a tiny ultrasound wand over a pool of liquid when they first saw an icicle shape emerge and solidify. The pair shrieked so loudly that their colleagues down the hall at Montreal’s Concordia University could hear them. “Well, they would have heard us, if they hadn’t been at home because of COVID,” Foroughi says. Still, a quick video call let the researchers share their excitement: after months of effort, they had 3-D printed a solid object by exposing a liquid to a focused field of sound waves—transmitted through a solid wall.
The Concordia team’s new “direct-sound printing” technology is the first to create a solid structure using sound waves from behind a barrier. And although it has a long way to go to reach commercial viability, the researchers believe their remote-control 3-D printing opens the door to numerous possibilities. It could, they say, potentially enable minimally invasive tissue engineering and bioimplant repair within the human body. It could also support industrial repairs in other difficult-to-access places such as inside an airplane’s fuselage.
Most commercial forms of 3-D printing involve extruding fluid materials—plastics, ceramics, metals or even biological compounds—through a nozzle and hardening them layer-by-layer to form computer-drafted structures. That hardening step is key, and it relies on energy in the form of light or heat. The fluid’s ability to create chemical bonds and thus to solidify is controlled by the amount of energy each molecule receives—and transmitting enough of this energy usually requires direct, highly focused contact between the energy source and the material.
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The Concordia team, including Muthukumaran Packirisamy, a mechanical engineering professor at the university, who works on microelectromechanical systems design, had another idea. “We wanted to do 3-D printing in places that are not accessible to light or heat,” says Habibi, then a postdoctoral researcher at the university. The team realized that sound waves offered a way to rapidly focus and manipulate energy without requiring direct access to the liquid material. “This was the gap we wanted to fill,” Habibi says.
Using ultrasound to trigger chemical reactions in room-temperature liquids isn’t new in itself. The field of sonochemistry and its applications, which matured in the 1980s at the University of Illinois Urbana-Champaign (UIUC), relies on a phenomenon called acoustic cavitation. This happens when ultrasonic vibrations create tiny bubbles, or cavities, within a fluid. When these bubbles collapse, the vapors inside them generate immense temperatures and pressures; this applies rapid heating at minuscule, localized points. The Concordia team sought to unleash the power of sonochemistry as an unconventional way to print conventional materials, as well as ones that are impossible to print with typical energy sources. “Those unimaginable temperatures and pressures generated in just a picosecond create conditions perfect for instantaneous printing,” Habibi says.
In their experiments, which were published in Nature Communications in 2022, the researchers filled a cylindrical, opaque-shelled chamber with a common polymer (polydimethylsiloxane, or PDMS) mixed with a curing agent. They submerged the chamber in a tank of water, which served as a medium for the sound waves to propagate into the chamber (similar to the way ultrasound waves from medical imaging devices travel through gel spread on a patient’s skin). Then, using a biomedical ultrasound transducer mounted to a computer-controlled motion manipulator, the scientists traced the ultrasound beam’s focal point along a calculated path 18 millimeters deep into the liquid polymer. Tiny bubbles started to appear in the liquid along the transducer’s path, and solidified material quickly followed. After fastidiously trying many combinations of ultrasound frequencies, liquid viscosity and other parameters, the team finally succeeded in using the approach to print maple-leaf shapes, seven-toothed gears and honeycomb structures within the liquid bath. The researchers then repeated these experiments using various polymers and ceramics, and they presented their results at the Canadian Acoustical Association’s annual conference this past October.
“Manufacturing using sound is an incredibly innovative idea, and I am delighted to see it,” says UIUC professor William King, who focuses on advanced materials and manufacturing, nanotechnology, and heat transfer and was not involved in the new study. The ultrasound approach, he says, holds intriguing possibilities for rapidly producing complex three-dimensional geometries that may not be possible with other manufacturing processes. Still, he points out that the 3-D printing processes that are now mainstream succeeded by first finding a foothold in one or two niche uses. “I look forward to seeing if sound-based printing can find the necessary application to succeed,” King adds.
For Tiziano Serra, who leads the Sound Guided Tissue Regeneration focus area at the AO Research Institute Davos in Switzerland, one appealing application would be clinical repairs from a distance. That means injecting a biological material—such as gelatin, fibrin (a protein that is important in blood clotting) or a hydrogel embedded with drugs—to some location in the body and then printing it into a structure that would repair musculoskeletal damage or gradually release drugs around a cancerous or infected site. Other bioprinting technologies use ultraviolet light to harden these materials, but this light cannot penetrate an opaque barrier. Ultrasound “could act in situ and give a lot of advances and opportunities,” Serra says. “Injection avoids long surgeries, possibility of infection and health care costs.” He cautions that this technique would not, however, work for printing with live cells. The heat and pressure would kill them.
On the nonbiological front, remote-control printing could aid repairs in the aerospace industry. Habibi says engineers could squeeze liquid plastic into a hard-to-access area of an airplane’s fuselage and then use the new 3-D printing technique to solidify the goo into solid structures—such as, for example, the porous plastic isolators that dampen an aircraft’s vibrations.
A crucial next step for sound-based printing would be to show how this process can function in real applications that meet the strict requirements of engineers and product designers, such as materials strength, surface finish and repeatability.
The research team will soon publish new work that discusses improvements in printing speed and, significantly, resolution. In the 2022 paper the team demonstrated the ability to print “pixels” that measure 100 microns on a side. In comparison, traditional 3-D printing can achieve pixels half that size.
Still, according to Daniele Foresti, a mechanical engineer who works on new approaches to 3-D printing at AcousticaBio, a spin-off company of Harvard University, the difference in resolution isn’t a reason to reject the new technique. After all, there’s always a tendency to compare new technology to well-established tools. “Some things have been around for 30 years,” he says, and researchers have had more time to develop them and improve their performance by, say, increasing the resolution. “When you prove that a new mechanism works and has potential for advancement,” Foresti says, “that’s valuable in itself.”