Last updated October 30, 2018 at 8:48 am
A new printing method uses sound waves to create ultra-precise liquid droplets.
Video credit: Daniele Foresti, Leah Burrows, Jennifer A. Lewis, Harvard University.
Harvard University researchers have utilised the force of sound waves to control liquid droplets, in an effort to make printing even more precise and improve manufacturing processes.
Liquid droplets are used in many applications from basic inkjet printing to microcapsules for drug delivery.
“By harnessing acoustic forces, we have created a new technology that enables myriad materials to be printed in a drop-on-demand manner,” said Jennifer Lewis, the Hansjorg Wyss Professor of Biologically Inspired Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences and the senior author of the paper, published in Science Advances.
Sticky business
At its core, printing is a liquid being dispensed from a nozzle. However, the behaviour and sizes of droplets of the liquid are determined by several factors such as the viscosity of the liquid.
For example, traditional inkjet printers found in homes are only suitable for liquids that are roughly 10 times more viscous than water. In other applications like biopharmaceuticals and bioprinting which use biopolymer and cell-laden solutions, the liquids can be up to 100 times more viscous than water. Sugar-laden honey, on the other hand, is 25,000 times for viscous than water.
Other factors that can clog up the printing process are temperature and composition, both which can impact liquid viscosity.
“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, first author of the paper, the Branco Weiss Fellow and Research Associate in Materials Science and Mechanical Engineering at SEAS and the Wyss Institute.
Pendant drop: In acoustophoretic printing, sound waves 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 A. Lewis, Harvard University
Sound it out
Left alone to gravity, droplet size remains large and the drop rate is difficult to control.
The researchers used sound waves to assist gravity, calling this new method acoustophoretic printing.
They built a subwavelength acoustic resonator that creates a pressurised field, allowing them to generate a pulling force exceeding 100 times the normal gravitation forces (1 G) at the tip of the printer nozzle – more than four times the gravitational force on the surface of the sun.
Without getting stuck in a bother, this adjustable force pulls each droplet off the nozzle when it reaches a specific size and ejects it towards the printing target.
The higher the amplitude of the sound waves, 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 their set up on different materials, including honey, stem-cell inks, biopolymers, optical resins, and liquid metals.
The sound waves don’t travel through the droplet itself, making the method safe to use even with sensitive biological cargo, such as living cells or proteins.
This new technology opens up new fields for printing applications, even beyond biopharmaceuticals, cosmetics, and food manufacturing. It extends the options to creating optical and conductive materials.
By controlling the target position, the ejected droplets can be carefully deposited and patterned anywhere. In this example, honey drops are patterned on a glass substrate. Credit: Daniele Foresti, Jennifer A. Lewis, Harvard University
