4.4.1 Printability of carrageenan/sodium alginate hydrogel
The printability of the hydrogel was simulated to present the printing conditions with various printing parameters and to predict the optimal printing parameters based on the measured rheological properties. The printability was calculated as shown in Figure 5(b), with the printing speed of 0–18 mm/s and pressure of 10–40 kPa, by using the equations described above. The cross marks on the printability map represent the actual printing conditions shown in Figure 5(a), and they were labelled with the printing numbers from 1 to 16. Moreover, Figure 5(a) shows the microscopic images of the printed struts at various nozzle velocities (1, 2, 4, and 8 mm/s) and pressures (10, 20, 30, and 40 kPa). The obtained microscopic images were quantitatively analysed to measure the printing resolution and the quality shown in Figure 5(c).
As shown in Figure 5(a), the diameter of the printed gel filament was inversely proportional to the printing rate, while the diameter of printed gel filament decreased with increasing printing rate at low printing pressure (10 kPa). In other words, the samples in the higher printing rate (4 mm/s, 8 mm/s) No. 3 and No. 4 did not acquire an integrated cylindrical structure. At the same printing pressure, the samples No. 1 and No. 2 gel filament were printed with a lower printing speed, resulting in a relatively full cylindrical structure. Thus, unmatched printing speed and printing pressure resulted in insufficient flow filling.
The incomplete shape of gel filament caused by increased printing pressure to 20 kPa existed. The sample No. 8 with 8 mm/s was particularly obvious; however, the uniformity of its diameter and shape integrity was significantly better than that of sample No. 4 (i.e. the same printing speed and lower printing pressure), which demonstrated that the coordination effect of printing pressure and printing speed had an obvious influence on the shape and diameter of the extrusion gel filament and that a higher printing rate cannot be applied with lower printing pressure.
Furthermore, the printability values of samples No. 5, 9, 10, 13, 14, 15, and 16 were greater than 0.5. In Figure 5(b), the printability of samples No. 1, 2, 6, 7, 11, and 12 were assigned according to their print parameters, which matched with the values measured and calculated from the actual printing samples. The results demonstrated that the simulation of printability of kc-sa-1.0 hydrogels were basically accurate. As shown in Figure 5(d), the diameter of the printed gel filament was mainly distributed between 1.0 and 1.8 mm.