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 ten times more viscous than water. Many fluids of interest to researchers are far more viscous; for example, biopolymer and cell-laden solutions that 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, making it more difficult to optimize printing parameters to control droplet sizes.
A new printing method was developed that uses sound waves to generate droplets from liquids with an unprecedented range of composition and viscosity. This technique could enable the manufacture of many new biopharmaceuticals, cosmetics, and food, and expand the possibilities of optical and conductive materials.
Thanks to gravity, any liquid can drip. With gravity alone, droplet size remains large and drop rate difficult to control. To enhance drop formation, the new technique generates sound waves that assist gravity — acoustophoretic printing. The idea is to generate an acoustic field that literally detaches tiny droplets from the nozzle, much like picking apples from a tree.
A sub-wavelength acoustic resonator was built 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 — 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 sound waves, the smaller the droplet size, irrespective of the viscosity of the fluid.
The process was tested on a range of materials, from honey to stem-cell inks, biopolymers, optical resins, and even liquid metals. 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.