We present an acoustofluidic device for fluorescently triggered merging of surfactant-stabilized picoliter droplet pairs at high throughput. Droplets that exceed a preset fluorescence threshold level are selectively merged by a traveling surface acoustic wave (T-SAWs) pulse. We characterize the operation of our device by analyzing the merging efficiency as a function of acoustic pulse position, duration and acoustic pressure amplitude. We probe droplet merging at different droplet rates and find that efficient merging occurs above a critical acoustic power level. Our results indicate that the efficiency of acoustically induced merging of surfactant stabilized droplets is correlated with acoustic streaming velocity. Finally, we discuss how both time-averaged and instantaneous acoustic pressure fields can affect the integrity of surfactant layers. Our technique, by allowing the merging of up to 10 5 droplets per hour, shows a great potential for integration into microfluidic systems for high-throughput and high-content screening applications.
Functional screenings in droplet-based microfluidics require the analysis of various types of activities of individual cells. When screening for enzymatic activities, the link between the enzyme of interest and the information-baring molecule, the DNA, must be maintained to relate phenotypes to genotypes. This linkage is crucial in directed evolution experiments or for the screening of natural diversity. Micro-organisms are classically used to express enzymes from nucleic acid sequences. However, little information is available regarding the most suitable expression system for the sensitive detection of enzymatic activity at the singlecell level in droplet-based microfluidics. Here, we compare three different expression systems for L-asparaginase (L-asparagine amidohydrolase, EC 3.5.1.1), an enzyme of therapeutic interest that catalyzes the conversion of L-asparagine to L-aspartic acid and ammonia. We developed three expression vectors to produce and localize L-asparaginase (L-ASNase) in E. coli either in the cytoplasm, on the surface of the inner membrane (display), or in the periplasm. We show that the periplasmic expression is the most optimal strategy combining both a good yield and a good accessibility for the substrate without the need for lysing the cells. We suggest that periplasmic expression may provide a very efficient platform for screening applications at the single-cell level in microfluidics.
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