2.4 3D printing of A-SA-Gel hydrogel scaffolds
To fabricate the A-SA-Gel hydrogel scaffolds, an in-house built extrusion-based 3D printing system was employed. The schematic representation of the material preparation and printing process is shown in Figure 1A. The system and solution parameters were optimized prior to 3D printing. Briefly, the printing system was designed with a computer controlled automated X–Y–Z stage, a receiving platform, a nozzle mounting block, a printer head (syringe needle), and a pneumatic (pressured-air) system, which consist of compressor, air tube, and control valve. The syringe needle was directly connected with the pneumatic system through air tubes. The pneumatic controller provided air pressure for extruding hydrogel material. The receiving platform was mounted in the X-Y plane that was capable of printing with high precision; the syringe needle mounted to a solo motorized linear Z-axis, and it can be controlled to move up and down. The prepared A-SA-Gel hydrogel was gently added to a 50 ml syringe, and a 22G needle was used for extrusion printing.
In order to print a grid-structured scaffold, as a model scaffolding system, having the size 20 ×20 × 5 mm (L×W×H), and the following parameters were used. The interval of parallel-arranged filaments within the grid structure was kept 0.6 ± 0.1 mm in each layer, and adjacent layers were perpendicularly stacked to construct the porous structure. During the printing process, the prepared A-SA-Gel hydrogel was extruded from the syringe needle to generate continuous filament at room temperature, and the extruded filaments deposited layer-by-layer to form the grid-structured A-SA-Gel hydrogel. A stable air pressure value of 2.8 ± 0.1 Psi was applied to extrude hydrogel. After printing, the hydrogel was crosslinked with CaCl2 solution (4%, w/v), followed by oven-dried for 3 h at 37 °C.
In addition, a thick scaffold structure (10(L)×10(W)×10(H) mm), blood vessel structure (12mm in diameter, 15mm in height), the abbreviations of Shanghai University (SHU) and Vellore Institute of Technology (VIT), and human ear models were also printed under the optimal conditions, in order to further evaluate the 3D printability and self-standing ability of the A-SA-Gel hydrogel, in terms of various shapes, sizes and structures.