Heat-shock transformation of Escherichia coli in nanolitre droplets formed in a capillary-composited microfluidic device

Abstract

This work describes an improved method for monodispersed water-in-oil droplet formation and collection using a composite microfluidic device composed of a poly(dimethylsiloxane) (PDMS) microfluidic device and a commercially available quartz capillary. The application of the method to chemical heat-shock (CaCl₂-dependent) transformation of Escherichia coli (E. coli) is also presented. With this approach, tunable and uniform different-sized droplets were generated and conveniently collected into a capillary for subsequent experiments. Characterization of droplet size and formation frequency exhibits that droplet behavior is strongly dependent on the ratio (R) of aqueous phase flow rate (Q_aq) to oil phase flow rate (Q_o). An increase in R induces droplet size and droplet formation frequency increase, which agrees well with a theoretical calculation. To illustrate the application of this droplet-based device in biological fields, as a case study, we also apply this device to the study of heat-shock E. coli transformation. Results demonstrate that plasmid DNA can be effectively transformed into E. coli, and a similar transformation efficiency with the traditional tube-based method can be obtained. This technique provides a new way for droplet generation and easy collection, as well as functional genomics studies by taking advantage of the high throughput of droplet microfluidics.