Nanoliter-scale liquid metering and droplet generation based on a capillary array for high throughput screening

Herein, we developed a simple approach for quantitative metering of nanoliter-scale liquids in parallel based on a capillary array and applied it in high throughput screening protein crystallization conditions. The quantitative metering of liquids was achieved by using capillary force to spontaneously introduce the liquids into short capillaries with fixed length and inner diameter, and the nanoliter-scale droplets were generated by using a pneumatic pump to deliver liquids out from the capillary channels. We adopted measures of sharpening the capillary tips and performing a hydrophobic treatment on the tip surface to significantly reduce the capillary residues during the liquid aspirating and dispensing process, and thus improved the precision to 0.2%-3.5% relative standard deviations (RSD, n = 3) in metering droplets in the range of 280 pL-90 nL. We evaluated the performance of the system in metering liquids of different surface tensions and viscosity. On the basis of this approach, we built a capillary array system with 12 capillaries, by which parallel generation of 12 nL droplets of 12 samples could be achieved in 40 s with a relative standard deviation (RSD) of 1.2%. We applied the system in the screening of lysozyme crystallization conditions of 48 precipitants with 7.5 nL precipitant and 7.5 nL protein solutions in each crystallization droplet reactor, to demonstrate its potentials in large-scale high-throughput screening and analysis with different samples.