2.2. Preparation of bioink and cells for 3D bioprinting
Carrageenan, which is mainly composed of κ-Carr (Solarbio Inc., Beijing, China), it was mixed at 1.5% w/v concentration with sodium alginate (Solarbio Inc.) solution[36], and the concentration of the alginate solution was fixed at 0.5% w/v, 1% w/v, and 1.5% w/v in this study. MCNTs were purchased from TIMENANO Inc. China and fixed at 0.5% w/v, 1% w/v, and 1.5% w/v). Calcium chloride (CaCl2, Solarbio Inc.) was used as the cross-linking agent for sodium alginate solution. The preparation method of carrageenan composite hydrogel is as follows: (1) kc: κ–carrageenan (1.5 g) was dissolved in double-distilled water (100 mL) in a 3-necked round bottom flask and stirred under a constant flow of nitrogen for 4 h. (2) kc-s: carrageenan (1.5 g) and sodium alginate were dissolved in double-distilled water (100 mL) in a 3-necked round bottom flask and stirred under a constant flow of nitrogen gas for 4 h. (3) kc-c: 0.05 g of MCNTs were placed into 50-mL deionised water and subjected to ultrasonic dispersion for 10 min to obtain a carbon nanotube water dispersion system. A certain amount of carbon nanotube dispersion solution was added to the carrageenan solution to obtain a carrageenan/MCNT hydrogel system (carrageenan concentration 1.5% w/v, carbon nanotube concentration 0.01% w/v). All of the prepared hydrogels were aseptically processed before use. Particularly, in this case, we used a high pressure for 1 h and ultraviolet light for 2 h. To prepare the cell-loaded biological ink, we mixed human umbilical vein endothelium cells (HUVECs; Beijing Oligobio, Beijing, China) into a hydrogel for printing.