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Sample deposition based on micro-droplet ejection (MDE) has broad application prospects in the field of biomedicine. As a potential technology option for cell printing, a hybrid pneumatic-electrohydrodynamic (HPEHD) MDE system is built in the laboratory. Strong electric field is established by applying a high voltage between the nozzle and a collector electrode. The pneumatic actuation is realized via a solenoid valve staying outside of the liquid chamber. The solenoid valve is set in conduction for a short period of time Delta t; gas of high pressure P-0 enters the liquid chamber, and produces a pressure pulse, which extrudes the liquid slightly out of the nozzle. The liquid is deformed further in the electric field into a cone shape (Taylor cone), and then the end of the Taylor cone breaks to form a micro-droplet. The ejection process is studied using machine-vision and image processing. With sodium alginate (1.0%) as bioink, single droplet per ejection is realized, and the droplet size is reduced by 50% due to the presence of the electric field. It is found that increasing the voltage has little effect on the size of droplets. In contrast, increasing source pressure P-0 or increasing Delta t the conduction time of the solenoid valve can change the volume of droplet in a wider range.
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