3.1 Cell culture
HUVECs were purchased from Beijing Oligobio (China). The cells were cultivated in RPMI 1640 medium containing 10% foetal bovine serum (FBS) at 37 ˚C under a 5% CO2 atmosphere. The culture medium was refreshed every 2 days. Before mixing with the cells, the composite hydrogel was autoclaved and irradiated with ultraviolet light. The cells were subculture to 80% confluence and mixed with ink at a density of 3 × 104 cell/cm2 in 24-well plates. The printed structure was then placed in a constant temperature incubator at 37˚C under a 5% CO2 atmosphere. After 6 h of culturing, a fresh medium was replaced to wash out the unadhered cells. The culture medium was changed daily.