Gas tight

For these experiments, ThermoFisher Scientific Matrigel^® (LOT 0202003, Corning, NY, USA) was used. It was maintained at 4 °C to ensure liquid-state and proper loading inside a 1 mL glass syringe (Gas Tight, Hamilton Company, Reno, NV, USA) minimizing the presence of air bubbles. Then, it was placed into an incubator at 37 °C for 15 min to obtain complete Matrigel^® gelation. Finally, the syringe was mounted on Prusa i3 adapter for bioprinting.

The microchannel was set up for droplet formation as follows. All microchannels had a depth of 100 μm, with injection and drain channel widths of 100 μm and 300 μm, respectively. Each microchannel inlet/outlet was connected to a syringe (10 mL gastight syringe, Hamilton Company, USA) via polytetrafluoroethylene (PTFE) tubing (1.59 mm/0.5 mm outer/inner diameter, Flon Industry, Japan). To form droplets, each solution was filled into the syringes and introduced into each microchannel at constant flow rates using a syringe pump (KDS200, KD Scientific, USA). Droplet formation was observed using a high speed camera (FASTCAM-MAX 120K, Photron, Japan) mounted on an upright microscope (BX-50, Olympus, Japan).