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Parking droplets on plastic pillars offers a new twist on microwell plates

Relying on evaporation to mix droplets could offer advantages over microplates used for high-throughput screening and bioassays

by Melissa Pandika, special to C&EN
August 1, 2016 | A version of this story appeared in Volume 94, Issue 31

Pipetting tiny droplets on top of microscale plastic pillars could offer an advantage over microwell plates for carrying out diagnostic assays (Anal. Chem. 2016, DOI: 10.1021/acs.analchem.6b01657). Mixing and sample evaporation are potential problems when working with microliter volumes of samples in microplates. So researchers led by Jose L. Garcia-Cordero of National Polytechnic Institute, in Monterrey, Mexico, designed an alternative approach that exploits rather than prevents evaporation. Evaporation can cause surface tension differences in a droplet that spur Marangoni currents, which result from liquid flowing to areas of higher surface tension. The team used a milling machine to fashion 800-µm-diameter, 1-mm-tall pillars on an acrylic plate. The researchers then conducted a colorimetric assay by pipetting an enzyme solution onto the top of a pillar and letting it evaporate to concentrate it before adding a glucose solution. When the enzymes acted on the glucose, the sugar underwent a set of reactions to form a colored product. The team monitored reactions in both high and low humidity for 20 minutes, taking photographs at intervals. At high humidity, the droplets on the pillars didn’t evaporate or substantially change color, suggesting that evaporation provides the mixing necessary for the reaction to occur. But as the droplets evaporated at low humidity, their color changed.

A series of images shows a chemical reaction taking place in a tiny liquid droplet.
Credit: Anal. Chem.
Researchers pipette an enzyme solution (top, blue) onto a plastic pillar and allow it to concentrate via evaporation. Then they add a sample solution (yellow). Evaporation-driven currents mix the sample, promoting a reaction and producing a color change.


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