## 3 Results and Discussion
### Scaffolds Mechanical Properties Analysis  
A shared drawback of porous hydrogel scaffolds is their relatively weak mechanical properties. By adjusting the proper internal structures, the mechanical properties of scaffolds can be improved [25]. The essence is to change the contact area between two adjacent layers by adjusting the different angles of extruded gel, thereby obtaining a different Young's modulus and dynamic modulus of scaffolds. In this paper, the SA-GEL solution made from 3% sodium alginate and 8% gelatin was selected and the printing parameters were set as the extrusion pressure being 1.8 bars, the moving speed of the nozzle being 10 mm/s, and the layer height being 0.32 mm. Not only were the 0-90 degree scaffolds with 1 and 1.2 mm spacing printed, but also the scaffolds with internal 0/45/135 degrees and 0/45 degrees were printed. The printed paths were just as Figure 13 shows--the scaffold in Figure 13a was a 0/90 degree scaffold with 1 mm spacing, the scaffold in Figure 13b was a 0/90 degree scaffold with 1.2 mm spacing, the scaffold in Figure 13c was a 0/45 degree scaffold with 1.2 mm spacing, the scaffold in Figure 13d was a 0/45/135 degree scaffold with 1.2 mm spacing, and the scaffold in Figure 13e was a staggered scaffold with 1.2 mm spacing. The size of sample model is (12 x 12 x 3) mm\({}^{3}\) for each condition. Figure 13: Different internal structure patterns of scaffolds. (**a**) 0/90 degree scaffolds with 1 mm spacing; (**b**) 0/90 degree scaffolds with 1.2 mm spacing; (**c**) 0/45 degree scaffolds with 1.2 mm spacing; (**d**) 0/45/135 degree scaffolds with 1.2 mm spacing; (**e**) staggered scaffolds with 1.2 mm spacing. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Fiber Distance (mm)** & **Theoretical Porosity** & **Experimental Porosity** \\ \hline
0.6 & 32.3 & 31.8 \\
0.8 & 49.6 & 48.9 \\
1 & 60.3 & 60.7 \\
1.2 & 66.4 & 66.5 \\
1.4 & 71.0 & 68.8 \\
1.6 & 75.5 & 72.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The theoretical porosity and the measured porosity of scaffolds. The printing results are shown in Figure 14, which are all highly consistent with the predictions of the theoretical model. Then, the mechanical properties of hydrogel scaffolds cross-linked with different internal structures were studied, including Young's modulus and the dynamic modulus. Scaffolds with different internal structures were tested as follows: the compressive modulus of the scaffolds was measured by the cross-head velocity of 0.01 m/s, and the Young's modulus was calculated based on the linear region of the stress-strain curves of all the samples. The dynamic test was carried out by applying 15% sinusoidal deformation at a frequency of 1 Hz. After more than 20 cycles of steady-state response, the dynamic modulus was determined from the ratio of stress amplitude to strain amplitude. Under the condition of 50% deformation, a 24-h creep recovery test was carried out in PBS at 37 \({}^{\circ}\)C, and the stress was removed, and the samples were allowed to recover within 24 h.  
It can be clearly seen from Figures 15 and 16 that the scaffold with an internal structure of 0/90 is the group with the smallest middle distance of all scaffolds. The Young's modulus is the highest, reaching 113.64 \(\pm\) 12.83 KPa, and the dynamic modulus is also the highest, at 841.5 \(\pm\) 27.9 KPa, but the porosity of the scaffolds is the lowest. When the fiber spacing increases to 1.2 mm, just make the internal structure as 0/90 degree with 1.2 mm spacing; the Young's and dynamic moduli of samples were 69.7 \(\pm\) 4.5 KPa and 502.3 \(\pm\) 63.5 KPa, respectively, which decreased significantly. Larger fiber spacing reduces the mechanical properties of the scaffolds, which is in line with the expected relationship between fiber spacing, porosity and mechanical properties of the composite scaffolds. That is, the larger the fiber spacing is, the higher the porosity is, and the worse the mechanical properties of the scaffolds are.