Cell proliferation assay
The Cell Counting Kit-8 (CCK-8; DOJINDO) was used to analyse cell proliferation in the collagen/gelatin/alginate materials in either the absence or presence of sodium citrate in culture solutions according to the manufacturer’s instructions. Briefly, cells/hydrogel constructs with or without sodium citrate were washed with PBS three times. Next, 0.1 ml of CCK-8 and 1 ml of culture medium were added to each culture dish and incubated in the dark for 2 h at 37 °C. After incubation, 0.5 ml of the culture medium was transferred to a 96-well plate, and the optical density (OD) at 450 nm was immediately read. The 3D constructs without cells were treated the same way, and the CCK-8 medium was used as the blank control. Three samples were tested for each group.