C3PE: counter-current continuous phase extraction for improved precision of in-droplet chemical reactions

To improve tools for controlling and optimizing miniaturized chemistry, a novel oil extraction architecture, designated as the Counter-Current Continuous Phase Extraction ((CPE)-P-3) module, was developed to enable precise control over reaction incubation in water-in-oil droplet microfluidic reactors. Using a symmetric pillar array coupled to adjustable oil flows prevented sample loss and droplet breakup, even at high final volume fractions, and cross-flow added novel stabilization of oil extraction against instability in control pressures. By integrating this dynamic functionality, (CPE)-P-3 enabled rational selection of the oil extraction magnitude across a range of achievable final droplet volume fractions (up to 85%) when processing droplets at 40-200 Hz. Further, this versatile device handled many droplet sizes (70-500 pL demonstrated here). Next, this approach to controlling droplet volume fraction enabled incubation time monitoring and optimization when coupled to a K-channel direct injection feature to label selected droplets. In profiling system characteristics like volume fraction, channel geometry, and continuous phase viscosity, this technique provided a powerful tool to control, measure, and improve incubation performance. Finally, applying (CPE)-P-3 principles to an in-droplet beta-galactosidase enzyme reaction (useful in immunoassay systems) increased product formation while significantly decreasing variance in product yield among droplets relative to a non-extracted comparison. We envision that this method will inform future design and implementation of high precision in-droplet chemistry while being of general interest in the study of emulsion fluid dynamics.