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.

3D Cell Culture Method in Channel-Free Water-in-Oil Droplets.

A new channel-free water-in-oil (WO) droplet 3D cell culture method is proposed to address the challenges while maintaining the advantages of the conventional 3D cell culture methods. The proposed WO method can fundamentally solve the constraint of spheroids size, a common challenge in conventional 3D culture, by using droplet size controllability. The 3D cell culture performance of the WO method is verified by comparing it with the conventional 3D cell culture methods. A systematic investigation of the culture conditions of the WO method confirms the working range of cell concentration and droplet size, as well as the scalability of spheroid size. Adjusting droplet size and cell concentration enables rapid spheroid formation with large and high cell concentration droplets or fast spheroid growth with small and low cell concentration droplets, providing control over the spheroid size and growth rate according to the purpose. Furthermore, long-term culture is demonstrated for 1 month with the proposed method, showing the largest spheroid culture and demonstrating the possibility that this method can be used not only for spheroid formation but also for organoid studies. Finally, if a WO-based automated 3D cell culture system is developed, it will be a useful tool for organoid research.

Safe and efficient RNA and DNA introduction into cells using digital electroporation system.

We systematically compared the delivery and expression efficiencies according to cell types (plant and animal cells) and genetic materials (RNA and DNA) to deliver RNA using a digital electroporation system. Despite the significantly lower RNA delivery in Chlamydomoans reinhartii than DNA delivery due to RNA secondary structure and cell wall, the expression/delivery ratio of RNA was significantly higher than that of DNA (up to 90%), confirming the generally known fact that RNA is more favorable for expression than DNA. On the other hand, in K562 cells, the difference in RNA and DNA delivery efficiency was negligible. Therefore, structural differences between DNA and RNA affect delivery efficiency differently depending on the cell type. RNA delivery efficiency of K562 cells was high, but expression efficiency was much lower than that of microalgae. According to the proposed strategy, compatibility between K562 cells and the nucleic acids used in this study is presumed to be one of the reasons for this low expression efficiency. Gene regulation by delivering small interfering RNA (siRNA) was demonstrated in K562 cells, confirming the feasibility of the digital electroporation system for RNA interference (RNAi) research as a safe and efficient delivery system.

Optimization of the droplet electroporation method for wild type Chlamydomonas reinhardtii transformation.

We performed the transformation of a wild type Chlamydomonas reinhardtii by optimizing previously developed droplet EP method. For more effective and faster optimization, we used DNA dying fluorescent molecule (Yo-Pro-1) for finding optimal EP conditions instead of using protein expression based evaluation method. By examining wider range of electrical parameter space together with the analysis of total current flow of EP process, we found optimal EP conditions. The obtained optimal EP conditions were verified by CFP transgene expression experiments. By applying the optimal EP conditions to the transformation of C. reinhardtii, we obtained transformants and analyzed them using PCR. Finally, implications and future work are discussed.