Forming a Large-Scale Droplet Array in a Microcage Array Chip for High-Throughput Screening

Forming a large-scale droplet array plays an important role for microfluidic droplet-based high-throughput screening and analysis. Herein, we describe a simple and rapid method to form a large-scale two-dimension (2D) droplet array by using a microcage array chip. Differing from the previous droplet array formation methods, microcages formed by being surrounded by multiple micropillars could rapidly spread the oil phase through the gaps between the micropillars and trap droplets with fast speed and convenient operation. We formed a large-scale 2D monolayer droplet array containing approximately 1 000 000 droplets on a 5.5 cm x 5.5 cm microcage array chip within 90 s. The droplets in the droplet array could be further incubated for performing biochemical reactions and detected by a fluorescence microscope in real time. Due to the exact trapping and positioning functions of the microcages to the droplets, single targeted fluorescent droplets in the array could be individually picked out and transferred to culture medium by a microfluidic droplet-handling robot with a success rate of 100% and a picking operation time of 2.0 s for one droplet under the optimized conditions. This system was validated in the screening of the bacterium expressing the esterase AFEST from a mixture of AFEST-expressing and phosphotriesterase-expressing E. coli cells, achieving a success rate of 100% for single-droplet picking while maintaining the bacterial cell viability. The present system has the potential to be applied in high-throughput screening and analysis, such as single cell analysis, directed evolution, and drug screening.