In their first attempt, researchers used sound waves to print a 3D object from a distance – even with a wall in the way. Mechanical engineering engineers Shiva Foroughi and Mohsen Habibi carefully moved a small acoustic stick over a pool of liquid when they saw an ice shape form and solidify for the first time. The duo shouted so loudly that their colleagues at Concordia University in Montreal were able to hear them. “Well, if they weren’t home because of COVID, they would have heard us,” says Foroughi. However, a quick video call allowed them to share their excitement: after months of effort, they had printed a solid object by exposing a liquid to a concentrated field of sound waves – transmitted through a solid wall.
Direct Acoustic Printing Technology
The new direct acoustic printing technology from the Concordia team is the first technique that uses sound waves to create a solid structure using sound waves from behind a barrier. Although it still has a long way to go to reach commercial viability, the researchers believe that remotely controlled 3D printing opens the door to many possibilities. This technology, according to the researchers, could enable tissue engineering with minimal intervention and repair vital implants within the human body. It could also support industrial repairs in hard-to-reach locations, such as inside an aircraft structure.
Commercial 3D Printing Methods
Most commercial 3D printing methods involve the flow of liquid materials – plastics, ceramics, metals, or even biological compounds – through a nozzle and solidifying them layer by layer to form computer-drawn structures. This final step is crucial and relies on energy in the form of light or heat. The ability of the liquid to create chemical bonds and thus solidify is governed by the amount of energy each molecule receives – and typically requires enough energy transmission to establish direct, concentrated contact between the energy source and the material.
A New Idea for Acoustic 3D Printing
The Concordia team, including reliable Packirisamy, a professor of mechanical engineering at the university who works in the design of micro-electromechanical systems, had another idea. “We wanted to do 3D printing in places that can’t be reached by light or heat,” says Habibi, who was then a postdoctoral researcher at the university. The team realized that sound waves provide a means to focus and manipulate energy quickly without needing direct access to the liquid material. “This is the gap we wanted to fill,” says Habibi.
Using Ultrasound to Stimulate Chemical Reactions
Using ultrasound to stimulate chemical reactions in liquids at ambient temperature is not new in itself. The field of sonochemistry and its applications, which matured in the 1980s at the University of Illinois at Urbana-Champaign (UIUC), relies on a phenomenon called acoustic cavitation. This occurs when ultrasonic vibrations create tiny bubbles, or cavities, within a liquid. When these bubbles collapse, they generate tremendous temperatures and pressures within them; this applies to rapid heating at very small, defined points. The Concordia team sought to harness the power of sonochemistry as an unconventional method for printing traditional materials, as well as those that are impossible to print using standard energy sources. “Those unimaginable temperatures and pressures for a split second create perfect conditions for instant printing,” says Habibi.
Researchers’ Experiments and Results
In their experiments, published in Nature Communications in 2022, the researchers filled a cylindrical chamber with a common polymer (polydimethylsiloxane, or PDMS) mixed with a curing agent. They immersed the chamber in a water tank, which acted as a medium for the propagation of sound waves to the chamber (similar to how ultrasound waves propagate from medical imaging machines through the gel spread on a patient’s skin). Then, using a medical-grade ultrasonic transducer mounted on a computer-controlled motion motor, the scientists traced the focus point of the ultrasound beam along a calculated path 18 mm deep into the liquid polymer. Tiny bubbles began to appear in the liquid along the path of the ultrasonic transducer, quickly followed by the material solidifying. After experimenting with various ultrasound frequencies and liquid viscosities and other parameters, the team successfully used this approach to print maple leaf shapes, seven-toothed gears, and honeycomb structures within the liquid medium. The researchers subsequently repeated these experiments using various polymeric and ceramic materials, and they presented their results at the Canadian Acoustical Association conference last October.
Applications
Printing with Sound
Professor William King from the University of Illinois at Urbana-Champaign, who focuses on advanced materials, manufacturing, nanotechnology, and heat transfer, and did not participate in the new study, says, “Manufacturing with sound is a very innovative idea, and I’m glad to see it.” He notes that the ultrasound approach has exciting potential for producing complex 3D engineering that may not be achievable through other manufacturing processes. However, he points out that the 3D printing processes that have become popular now first found a foothold in two or three uses. “I look forward to seeing if sound printing can find the necessary application for success,” King adds.
Applications of Sound Printing in Clinical Repairs and Aerospace
For Tiziano Serra, who leads the field focused on ultrasound-guided tissue regeneration at the AO Research Institute in Davos, Switzerland, one exciting application would be remote clinical repairs. This means injecting a bio-material – such as gelatin or fibrin (an important protein in blood clotting) or drug-embedded hydrogel – to a site in the body and then printing it into a structure that would repair musculoskeletal damage or provide a gradual release of drugs around a cancerous or infected site. Other bioprinting technologies use ultraviolet light to cure these materials, but this light cannot penetrate a dark barrier. Serra says, “Ultrasound can act in situ and provide a lot of progress and opportunities. “Injections avoid long surgeries and the risk of infection and healthcare costs.” However, he warns that this technology will not work for printing with live cells. Heat and pressure would kill them.
Regarding the non-biological side, remote-controlled printing can help with repairs in the aerospace industry. Habibi states that engineers could inject liquid plastic into hard-to-reach areas in an aircraft structure and then use the new 3D printing technique to cure the viscous material into solid structures – such as porous plastic isolators that dampen aircraft vibrations.
The Next Step for Sound Printing
The next critical step for sound printing will be demonstrating how this process can work in real-world applications that meet the requirements of engineers and product designers, such as material strength, surface finish, and repeatability.
The research team will soon publish new work discussing significant improvements in printing speed and accuracy. In a 2022 paper, the team demonstrated the ability to print “pixels” measuring 100 microns on each side. In comparison, traditional 3D printing can achieve pixels half that size.
However, according to Daniele Foresti, a mechanical engineer working on a novel approach to 3D printing at AcousticaBio, a subsidiary of Harvard University, the difference in accuracy is not a reason to dismiss the new technology. After all, there is always a tendency to compare new technology to already 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, for example, enhancing accuracy. “When you prove that a new mechanism works and has potential for advancement,” says Foresti, “that is value in itself.”
Copyright and Permissions: Rachel Berkowitz is a freelance science writer and contributing editor for Physics Magazine. She is based in Vancouver, British Columbia, and Eastsound, Washington.
Leave a Reply