Simultaneous Separating, Splitting, Collecting, and Dispensing by Droplet Pinch-Off for Droplet Cell Culture.

Simultaneous separating, splitting, collecting, and dispensing a cell suspension droplet has been demonstrated by aspiration and subsequent droplet pinch-off for use in microfluidic droplet cell culture systems. This method is applied to cell manipulations including aliquots and concentrations of microalgal and mammalian cell suspensions. Especially, medium exchange of spheroid droplets is successfully demonstrated by collecting more than 99% of all culture medium without damaging the spheroids, demonstrating its potential for a 3D cell culture system. Through dimensional analysis and systematic parametric studies, it is found that initial mother droplet size together with aspiration flow rate determines three droplet pinch-off regimes. By observing contact angle changes during aspiration, the difference in the large and the small droplet pinch-off can be quantitatively explained using force balance. It is found that the capillary number plays a significant role in droplet pinch-off, but the Bond number and the Ohnesorge number have minor effects. Since the dispensed droplet size is mainly determined by the capillary number, the dispensed droplet size can be controlled simply by adjusting the aspiration flow rate. It is hoped that this method can contribute to various fields using droplets, such as droplet cell culture and digital microfluidics, beyond the generation of small droplets.