Then, 300 µL of the composite hydrogel was poured in each device. Thedevices were then placed in the incubator at 37 °C for 30 min for collagenpolymerization and afterward UV-exposed for 20 s at 850 mW cm −2 ata distance of 1 cm to enable photocrosslinking of GelMA. The PDMStubes were seeded with HUVECs as described above, and 200 µL ofEGM was placed in the device on top of the hydrogel for culture.