Bead Encapsulation and Ejection from Droplet for Solid-Phase Synthesis

Solid-phase synthesis techniques are at the heart of synthesis of polymeric molecules like oligonucleotides (DNA and RNA), oligopeptides, oligosaccharides as well as the generation of combinatorial libraries for drug and vaccine development. Droplet microfluidic implementation of solid-phase synthesis has emerged as an important unmet technological challenge that can automate, massively parallelize, enhance throughput, and reduce reagent consumption of these otherwise traditionally expensive, labor, time and cost intensive processes. This can in turn facilitate the synthesis of longer DNA strands. However, this has remained challenging from a physical sample handling standpoint due to the inability to move individual beads across the droplet-medium interfaces. Here, we show the electric field assisted encapsulation and ejection of individual hydrophobic beads from aqueous microdroplets and describe its operation as a voltage dependent change in the electrofluidic potential energy of the system. We discuss the interfacial tension, contact angle, viscosity, bead, and droplet size dependence of the above process. As a proof-of-concept implementation of solid-phase synthesis, we demonstrate the enzymatic coupling of a fluorescently labelled nucleotide to a functionalized bead using the above process.