Generation of micro-droplet on demand with reduced sizes by a hybrid pneumatic electrohydrodynamic method

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 t; gas of high pressure P0 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 P0 or increasing t the conduction time of the solenoid valve can change the volume of droplet in a wider range.