Extended Data Fig. 1: Establishment of systems and conditions for sender-cell and receiver-cell encapsulation in droplets.

a) Visualization of cell–cell fusion using GFP-split complementation system assay. GFP11 in Spike-expressing sender cells and GFP1-10 in ACE2-expressing receiver cells are nonfluorescent by themselves, while coculture and fusion of the sender and receiver cells resulted in reconstituted GFP that becomes fluorescent upon complementation. Created with BioRender.com b) Design of microfluidics device for generation of monodispersed droplets with human cells. Conditions used: continuous phase (Qc): HFE with 0.5% (w/w) of surfactant; dispersed phase (Qd): HEK293T cells (2.5 × 10⁶ cells/ml) in culture medium (18% OptiPrep); flow rate: Qc: Qd = 9,000: 6,000 μL/hour. c) Droplet occupancy increased with the sender/receiver cell concentration used and aligned with Poisson distribution. d) Droplet co-occupancy in different mixing concentrations of the sender and receiver cells.