Ultrasonic waves print single drops of viscous fluids

Scientists have wielded ultrasonic waves to print droplets of liquids that are more than 1,000 times as viscous as the inks jetting from standard printers (Sci. Adv. 2018, DOI: 10.1126/sciadv.aat1659). Dubbed acoustophoretic printing by Daniele Foresti and Jennifer A. Lewis of Harvard University, Dimos Poulikakos of ETH Zurich, and coworkers, the technique offers access to a broader range of sticky substances than standard printing methods. The new method works by focusing sonic waves at the liquid-expelling nozzle to create pressures up to 100 times as great as those caused by gravity­—strong enough to pull down resistant droplets. Researchers can fine-tune the droplet size (100 µm to 1 mm in diameter) by adjusting the sonic blasts’ volume, with louder volumes making smaller droplets. Using its sound system, the team created a pattern of UV-curable adhesives, deposited bioinks laced with live stem cells into 48-well plates, and printed liquid-metal electrodes made of a gallium-indium alloy on cloth. The method prints droplets as fast as ink-jet printers do by volume, the authors say. Public University of Navarre’s Asier Marzo Perez says the technique is an impressive application of acoustic radiation forces that provides a simple yet effective solution for printing bio-materials and liquid metals.