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Gradient generation in MDA
Gradient generation in MDA
MDA stands for Mobile Droplet Array. Instead of generating a concentration gradient in an SDA, this study is focused on producing mobile droplets with varying concentrations i.e. a gradient in MDA by chopping a long diluting plug, which carries a dynamic concentration gradient as a result of continuously coalescing with 10 uniform and pre-trapped static droplets.
After the fragmentation of the gradient plug into individual isolated droplets, the dynamic gradient can be fixed and preserved for the downstream mixing of the chemically distinctive droplets with other reagents or target species. The whole process was recorded in a video on the Movie page.
Multiple gradients generation
Multiple gradients generation
By increasing the oil flow rate, the static drops can be washed out from the hydrodynamic traps through the resistance channels under enhanced Laplace pressure. It allows for replacing previous diluted static drops with new screen targets to be tested sequentially in varied concentrations. The left images demonstrate that ten 16-nL drops are depleted from the 320-µm traps at a flow rate of 30 µL/min in less than 10 s.
Compared to SDA, MDAs are amenable to higher throughput analysis. Approximately 6,500 droplets with varying concentrations can be generated in an hour at a fairly high resolution. Unlimited dilution and a wide range of concentration gradient can be achieved by delivering multiple diluting plugs to the stationary drops.
Laser-induced fluorescence detection of droplets in gradients
Laser-induced fluorescence detection of droplets in gradients
The concentrations of the dye in each droplet can be measured in grayscale intensity values using a custom-written MATLAB routine. The fluorescent intensities can be measured by a laser-induced fluorescence (LIF) detector. The plot above shows the varying concentrations of fluorescence in three MDAs, generated by fragmentation of three diluting plugs sequentially. Each peak represents a single drop. The height of the peak indicates the fluorescence intensity (i.e. the concentration) of the droplet. The baseline indicates the value of the carrier oil. The concentration profile can be flexibly adjusted by altering the flow rate, or the volumes of either the static drops or the diluting plugs.
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