title: Generation of chemical concentration gradients in mobile droplet arrays via fragmentation of long immiscible diluting plugs.
abstract: We report a one-step passive microfluidic technique to generate arrays of moving droplets containing variation of chemical concentration between individual drops. We find that a concentration gradient can be established in a long diluting plug by on-chip dilution and extraction of samples via orthogonal coalescence of the plug with a static array of sample drops. The diluting plug containing the gradient is subsequently fragmented by a droplet generator. We show that the technique is flexible, as the dilution range can be tuned by a variety of control parameters including the carrier fluid flow rate, volume of diluting plugs, and stationary drops. We also find that the concentration gradients have a fine resolution and are reproducible to within 2% relative standard deviation. As one demonstrative application, we show the suitability of the technique for generating a dose-response curve for an enzyme inhibition assay. Because of the ability to inject multiple plugs, our technique has the potential for unlimited as well as sequential dilution of a series of substrates. Thus, our method could be valuable as a high-throughput and high-resolution screening tool for assays that require interrogation of the response of one or more target species to numerous distinct chemical concentrations.
authors.affiliation: Department of Chemical Engineering, Texas Tech University, Lubbock, Texas 79409, United States. meng.sun@ttu.edu
authors.forename: Meng
authors.initials: M
authors.lastname: Sun
authors.forename: Siva A
authors.initials: SA
authors.lastname: Vanapalli
mesh_terms: D057075:Enzyme Assays; D019793:Fluorescein / Q000737:chemistry / Q000378:metabolism; D005690:Galactose / Q000031:analogs & derivatives / Q000378:metabolism; D046210:Microfluidic Analytical Techniques / Q000295:instrumentation / Q000379:methods*; D004369:Pentetic Acid / Q000031:analogs & derivatives / Q000737:chemistry; D001616:beta-Galactosidase / Q000378:metabolism*
chemical_list: C099911:diethylenetriaminepentaacetic acid stearylamide; D004369:Pentetic Acid; D001616:beta-Galactosidase; D019793:Fluorescein; D005690:Galactose