Researchers use acoustic forces to print droplets that couldn’t be printed before
By Leah Burrows, SEAS Communications
Highlights
- Harvard University researchers have developed a new printing technology that uses sound waves to control the size of liquid droplets independent of fluid viscosity
- This approach could greatly broaden the types of liquids, including biopharmaceuticals, that can be printed drop-on-demand
- The researchers used sound waves to generate a highly confined force at the tip of the printer nozzle, which pulls the droplet. The higher the amplitude of the sound waves, the smaller the droplet size.
Printing with sound
“By harnessing acoustic forces, we have created a new technology that enables myriad materials to be printed in a drop-on-demand manner,” said senior author Jennifer Lewis, Sc.D., Core Faculty Member at the Wyss Institute for Biologically Inspired Engineering. She also is the Hansjörg Wyss Professor of Biologically Inspired Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences and the Jianming Yu Professor of Arts and Sciences at Harvard.
The research is published in Science Advances.
Liquid droplets are used in many applications from printing ink on paper to creating microcapsules for drug delivery. Inkjet printing is the most common technique used to pattern liquid droplets, but it’s only suitable for liquids that are roughly 10 times more viscous than water. Yet many fluids of interest to researchers are far more viscous. For example, biopolymer and cell-laden solutions, which are vital for biopharmaceuticals and bioprinting, are at least 100 times more viscous than water. Some sugar-based biopolymers could be as viscous as honey, which is 25,000 times more viscous than water.
The viscosity of these fluids also changes dramatically with temperature and composition, which makes it ever more difficult to optimize printing parameters to control droplet sizes.
“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, Ph.D., first author of the paper, and a Branco Weiss Fellow and Research Associate in Materials Science and Mechanical Engineering at SEAS and the Wyss Institute.
To do that, the researchers turned to acoustic waves.
Thanks to gravity, any liquid can drip — from water dripping out of a faucet to pitch in the century-long pitch drop experiment. With gravity alone, droplet size remains large and drop rate difficult to control. Pitch, which has a viscosity roughly 200 billion times that of water, forms a single drop per decade.
To enhance drop formation, the research team relies on generating sound waves. These pressure waves have been typically used to defy gravity, as in the case of acoustic levitation. Now, the researchers are using them to assist gravity, dubbing this new technique acoustophoretic printing.
The researchers built a subwavelength acoustic resonator that can generate a highly confined acoustic field resulting in a pulling force exceeding 100 times the normal gravitation forces (1 G) at the tip of the printer nozzle — that’s more than four times the gravitational force on the surface of the sun.
This controllable force pulls each droplet off of the nozzle when it reaches a specific size and ejects it towards the printing target. The higher the amplitude of the soundwaves, the smaller the droplet size, irrespective of the viscosity of the fluid.
“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.
The researchers tested the process on a wide range of materials from honey to stem-cell inks, biopolymers, optical resins and, even, liquid metals. Importantly, sound waves don’t travel through the droplet, making the method safe to use even with sensitive biological cargo, such as living cells or proteins.
“Our technology should have immediate impact on the pharmaceutical industry,” said Lewis. “However, we believe that this will become an important platform for multiple industries.”
“This is an exquisite and impactful example of the breadth and reach of collaborative research,” said Dan Finotello, director of NSF’s MRSEC program. “The authors have developed a new printing platform using acoustic-forces, which, unlike in other methods, are material-independent and thus offer tremendous printing versatility. The application space is limitless.”
1/6 Acoustophoretic printing can also process thick, transparent polymers – widely used in optics. With its unique capabilities of drop generation, microlens arrays – like the ones of a fly – can be promptly printed! Credit: Daniele Foresti, Jennifer Lewis / Harvard University 
2/6 To eject droplets, acoustophoretic printing utilizes airborne ultrasounds - virtually material independent. Even liquid metal can be easily printed! This particular liquid metal forms a solid shell when in contact with the atmosphere, and this particular property makes it easy to pile drops one on top of another. Credit: Daniele Foresti, Jennifer Lewis / Harvard University 
3/6 Honey is a prototypical example for viscous fluid, being 25 thousand times more viscous (i.e. thicker) than water! Acoustophoretic printing enables droplet formation of any material, generating the possibly tiniest single drop of honey…in a tiny honey jar. An instant of the honey drop flight is captured in this photo. Credit: Daniele Foresti, Jennifer Lewis 
4/6 In acoustophoretic printing, soundwaves generate a controllable force that pulls each droplet off of the nozzle when it reaches a specific size and ejects it towards the printing target - much like picking apples from a tree! Credit: Daniele Foresti, Jennifer Lewis / Harvard University 
5/6 In acoustophoretic printing, sound waves don’t travel through the droplet, making the method safe to use even with sensitive biological cargo, such as living cells or proteins. Here, human stem cells are grown within patterned droplets – demonstrating how acoustophoretic printing is a promising platform to handle very delicate cargos. Credit: Daniele Foresti, Jennifer Lewis, Katharina Kroll / Harvard University 
6/6 In acoustophoretic printing, soundwaves generate a controllable force that pulls each droplet off of the nozzle when it reaches a specific size and ejects it towards the printing target - much like picking apples from a tree! Credit: Daniele Foresti, Jennifer Lewis / Harvard University
The Harvard Office of Technology Development has protected the intellectual property relating to this project and is exploring commercialization opportunities.
This research was co-authored by Katharina Kroll, Robert Amissah, Francesco Sillani, Kimberly Homan, Ph.D., and Dimos Poulikakos, Ph.D., Professor at ETH Zurich. It was funded by Society in Science through the Branco Weiss Fellowship and the National Science Foundation through Harvard MRSEC.