Abstract
Microfluidic methods for the formation of single and double emulsion (DE) droplets allow for the encapsulation and isolation of reactants inside nanoliter compartments. Such methods have greatly enhanced the toolbox for high-throughput screening for cell or enzyme engineering and drug discovery. However, remaining challenges in the supply of reagents into these enclosed compartments limit the applicability of droplet microfluidics. Here, a strategy is introduced for on-demand delivery of reactants in DEs. Lipid vesicles are used as reactant carriers, which are co-encapsulated in double emulsions and release their cargo upon addition of an external trigger, here the anionic surfactant sodium dodecyl sulfate (SDS). The reagent present inside the lipid vesicles stays isolated from the remaining content of the DE vessel until SDS enters the DE lumen and solubilizes the vesicles' lipid bilayer. The versatility of the method is demonstrated with two critical applications chosen as representative assays for high-throughput screening: the induction of gene expression in bacteria and the initiation of an enzymatic reaction. This method not only allows for the release of the lipid vesicle content inside DEs to be synchronized for all DEs but also for the release to be triggered at any desired time.
Highlights
The encapsulation of cells within gel microbeads partially overcomes these limitations.[14]
The protocol developed for surfactant-mediated cargo delivery in double emulsions consists of three parts (Figure 1): 1) encapsulation by double emulsion generation, 2) cargo release, and 3) reaction and downstream analysis
We developed a novel technique for the synchronized release of liposomal cargo in nanoliter double emulsions (DEs) vessels and illustrated the versatility of this method using different reaction systems and analytical tools
Summary
The encapsulation of cells within gel microbeads partially overcomes these limitations.[14]. Schmitt Department of Biosystems Science and Engineering Bioprocess Laboratory ETH Zürich Mattenstrasse 26, Basel CH-4058, Switzerland Another promising approach for compartmentalization uses water-in-oil-in-water double emulsions (DEs).[22,23,24] In contrast to single emulsions, DEs are compatible with routine highthroughput screening instruments such as fluorescence-activated cell sorters (FACS).[25,26,27] This convenient use of standard instruments makes DEs an ideal tool for screening large variant. The addition of sodium dodecyl sulfate (SDS), which can permeate across the thin oil shell of the double emulsion, causes the solubilization of the liposome membrane and subsequent cargo release.[37] The membrane rupture and successive delivery of the encapsulated reactant, triggering the reaction start, can be precisely controlled. The analysis of the DE droplets is carried out via flow cytometry and fluorescence microscopy
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