An open-pattern droplet-in-oil planar array for single cell analysis based on sequential inkjet printing technology.

Cellular heterogeneity represents a fundamental principle of cell biology for which a readily available single-cell research tool is urgently required. Here, we present a novel method combining cell-sized well arrays with sequential inkjet printing. Briefly, K562 cells with phosphate buffer saline buffer were captured at high efficiency (74.5%) in a cell-sized well as a "primary droplet" and sealed using fluorinated oil. Then, piezoelectric inkjet printing technology was adapted to precisely inject the cell lysis buffer and the fluorogenic substrate, fluorescein-di-β-D-galactopyranoside, as a "secondary droplet" to penetrate the sealing oil and fuse with the "primary droplet." We thereby successfully measured the intracellular β-galactosidase activity of K562 cells at the single-cell level. Our method allows, for the first time, the ability to simultaneously accommodate the high occupancy rate of single cells and sequential addition of reagents while retaining an open structure. We believe that the feasibility and flexibility of our method will enhance its use as a universal single-cell research tool as well as accelerate the adoption of inkjet printing in the study of cellular heterogeneity.