Printing With Sound: The Wave of the Future

To eject droplets such as these, acoustophoretic printing uses airborne ultrasounds, which are virtually material independent. Even liquid metal can be easily printed. Daniele Foresti, Jennifer A. Lewis, Harvard University

Harvard University announced that its researchers have developed a way to print objects using sound. Called "acoustophoretic printing," the method "could enable the manufacture of many new biopharmaceuticals, cosmetics, and food, and expand the possibilities of optical and conductive materials," according to the press release dated August 31, 2018.

Printing with liquid, such as ink, has become a way of life, thanks to the inkjet printing process. But what if you wanted to print living cells or other biological materials? What if you wanted to print liquid metal? With inkjets, the ability of a printer to pull a substance out of a nozzle grinds to a halt as the substance becomes thicker. But now, though it is still very early in the experimental phase of the process, the team of scientists at Harvard has announced significant progress in the creation of sound fields that can pull viscous substances, such as liquid metal, honey and even living cells, from the nozzle of a printer.

It begins with gravity. Simple gravity is what causes liquid to drip. How fast or often it drips depends on its viscosity — its thickness and resistance to shearing and tensile stresses. Water, for example, is far less viscous than corn syrup. Corn syrup is far less viscous than honey. The more viscous a fluid is, the longer it takes for gravity to produce a droplet. Printing systems, such as inkjet printing, typically use a droplet method of transferring a liquid material to a medium, such as paper. The more viscous a material is, however, the more difficult it is to manipulate for printing.

"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 research associate in materials science and mechanical engineering at Harvard.

This is where sound comes in.

Foresti and his fellow researchers began experimenting with the pressures of sound waves on liquids in order to give gravity a boost. They built a "subwavelength acoustic resonator" designed to produce tightly controlled acoustic fields that effectively increase the relative gravity at the printing nozzle. According to the release, the researchers have been able to generate pulling forces "100 times the normal gravitation forces (1G) of the printer nozzle," more than four times the gravity of the sun. The size of the droplet is simply determined by the amplitude of the soundwave — the higher the amplitude, the smaller the drop. Here is an explanatory video from the research team at Harvard:

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

A wide range of materials have been used to test this new printing method, including honey, stem-cell inks, biopolymers, optical resins and liquid metals. Because sound waves don't pass through materials, using sound to create droplets won't harm the material itself, which is important for printing with living cells.

Dr. Jennifer Lewis, professor of biologically inspired engineering at Harvard, stated, "Our technology should have immediate impact on the pharmaceutical industry. However, we believe that this will become an important platform for multiple industries."

More to Explore