Harvard University researchers have developed a new printing method that uses soundwaves to generate droplets from liquids with an unprecedented range of composition and viscosity. This technique could finally enable the manufacturing of many new biopharmaceuticals, cosmetics, and food and expand the possibilities of optical and conductive materials.
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, makes it ever more difficult to optimize printing parameters to control droplet sizes.
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 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.
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