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.
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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.
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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.
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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.
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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.
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Source: Ultrasound Enables Remote 3-D Printing–Even in the Human Body | Scientific American
Robin Edgar
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