Liquid droplets are used for many applications, from printing ink on paper to creating microcapsules for drug delivery. But the most common droplet patterning technique, inkjet printing, is limited to liquids roughly 10 times more viscous than water.

Sound waves are being used to generate a controllable force that can pull droplets, regardless of their viscosity, off a printer nozzle. Source: Daniele Foresti, Jennifer A. Lewis, Harvard UniversityThat doesn’t work for other areas of research interest such as biopharmaceuticals and bioprinting: biopolymer and cell-laden solutions are at least 100 times more viscous than water. Some sugar-based biopolymers with a honey-like consistency are even 25,000 times more viscous than water. Add this to the fact that fluid viscosity changes dramatically with temperature and composition, and it’s clear that an alternate solution is needed.

Harvard University researchers have addressed that challenge with a new printing method that relies upon sound waves to generate liquid droplets. "Our goal was to take viscosity out of the picture by developing a printing system that is independent from the material properties of the fluid," said Daniele Foresti, a materials science and mechanical engineering researcher who served as first author of a paper on the research.

While sound waves have been used to defy gravity (check out this demonstration of acoustic levitation, for example), the researchers used them instead to assist gravity. Any liquid can drip because of gravity, but the rate and size of the droplet is difficult to control. Pitch, the sticky resinous substance used for waterproofing, has a viscosity of roughly 200 billion times that of water — and, as a result, it drips at the rate of one drop per decade.

But through the use of a subwavelength acoustic resonator that generated a force exceeding 100 times normal gravitation, the team was able to pull droplets of specific sizes from the tip of a printer nozzle and eject them toward a printing target. Using higher amplitude sound waves resulted in smaller droplet sizes, and the success of the technique was not hampered by the viscosity of the liquid.

"The idea is to generate an acoustic field that literally detaches tiny droplets from the nozzle, much like picking apples from a tree," explained Foresti.

The researchers dubbed their new technique “acoustophoretic printing,” and they tested it on a wide range of materials from honey to stem-cell inks, biopolymers, optical resins and even liquid metals. The method is safe to use with sensitive biological cargo such as living cells or proteins, because the sound waves don’t travel through the droplet.

This technique could be a game-changer for a number of industries, enabling the manufacture of new biopharmaceuticals, cosmetics and food, and expanding the possibilities of optical and conductive materials.

The research appears in Science Advances.