Microwell Array Method for Rapid Generation of Uniform Agarose Droplets and Beads for Single Molecule Analysis

Compartmentalization of aqueous samples in uniform emulsion droplets has proven to be a useful tool for many chemical, biological, and biomedical applications. Herein, we introduce an array-based emulsification method for rapid and easy generation of monodisperse agarose-in-oil droplets in a PDMS microwell array. The microwells are filled with agarose solution, and subsequent addition of hot oil results in immediate formation of agarose droplets due to the surface-tension of the liquid solution. Because droplet size is determined solely by the array unit dimensions, uniform droplets with preselectable diameters ranging from 20 to 100 μm can be produced with relative standard deviations less than 3.5%. The array-based droplet generation method was used to perform digital PCR for absolute DNA quantitation. The array-based droplet isolation and sol-gel switching property of agarose enable formation of stable beads by chilling the droplet array at -20 °C, thus, maintaining the monoclonality of each droplet and facilitating the selective retrieval of desired droplets. The monoclonality of droplets was demonstrated by DNA sequencing and FACS analysis, suggesting the robustness and flexibility of the approach for single molecule amplification and analysis. We believe our approach will lead to new possibilities for a great variety of applications, such as single-cell gene expression studies, aptamer selection, and oligonucleotide analysis.