(a) Schematic of cell seeding experiments wherein a concentrated HUVEC solution is placed on top of the pre-made scaffolds, followed by immediate confocal imaging. (b) HUVECs seeded on top of the B-GelMA readily transfer into the micropores of the scaffold in less than 5 min (left panel), shown in the confocal microscope images; whereas, the bulk GelMA (20% w/v) does not support immediate cell infiltration. B-GelMA scaffolds (thickness 0.5-1 mm) were seeded from the top and imaged from the bottom. Cells were able to penetrate all the way through the scaffold. The images only show a depth of field ~ 250 μm from the bottom. For the bulk gel, the scaffold thickness ~ 250 μm, and the image presents the whole 3D sample. Image dimensions ~ 1550 μm × 1550 μm × 254 μm.