## 3 Results and Discussion
### Scaffolds Porosity Affective Analysis  
Porosity is a crucial index for evaluating scaffolds [27]. The size of porosity determines the growth ability and mechanical properties of cells, while the porosity of scaffolds mainly depends on fiber spacing. On the premise of guaranteeing the quality and mechanical properties of the support, the main purpose of this section is to adjust the size of the porosity by changing the line spacing; the results are shown in Figure 11.  
The printing parameters were set as follows: air pressure 1.8 bar, needle outlet diameter 400 um, needle speed 10.7 mm/s, thickness 0.32 mm. When the fiber spacing is too small, as shown in Figure 12a,  
Figure 11: The relationship between fiber spacing and porosity. Figure 10: Micro-view of midline of scaffold at different moving speeds. (**a**) extrusion line with moving speed 4 mm/s; (**b**) extrusion line with moving speed 7 mm/s; (**c**) extrusion line with moving speed 10 mm/s; (**d**) extrusion line with moving speed 13 mm/s; (**e**) extrusion line with moving speed 16 mm/s; (**f**) extrusion line with moving speed 20 mm/s. the gap between the extrusion lines is very small, and even sticks together because of the swelling effects. After cross-linking, the pore structure becomes even smaller, and some are even blocked. The permeability of the scaffold structure decreases, which is not conducive to cell growth. When the fiber spacing is appropriate, as shown in Figure 12b-d, there is a certain gap among the extrusion lines deposited on the substrate, which can form a better pore structure. On this basis, the printed support has good permeability and a high mechanical strength. When the interval between fibers is too large, the lower layer cannot provide enough support force to the next layer, so the collapse occurs. With an increase in the number of printing layers, the collapse phenomenon becomes more serious. The \(z\)-axis direction pore is completely blocked, which reduces the overall structural strength of the support. This phenomenon occurs in some areas of Figure 12e-f. The porosity of scaffolds was measured by the liquid displacement method [28]. The specific test methods were as follows: using electronic balance to weigh the specific gravity bottle filled with ethanol, the quality was \(Z_{1}\); weighing the composite support after freeze-drying, the quality was \(Z_{2}\); putting the composite support after freeze-drying into the specific gravity bottle, adding ethanol to make it filled with the specific gravity bottle and weighing, the quality was \(Z_{3}\): using tweezers to remove the support (the pore of the support is filled with ethanol) from the bottle, weighing the specific gravity bottle after removing the support (there is residual ethanol in the bottle), the quality was \(Z_{4}\). The porosity \(Z\) of the scaffold is as following:  
\[K=\frac{Z_{3}-Z_{4}-Z_{2}}{Z_{1}-Z_{4}} \tag{19}\]  
The theoretical porosity can be calculated by using the foregoing mathematical model of porosity prediction, and the theoretical value of porosity can be compared with the experimental data, as shown in Table 3.  
Table 3 shows that when the fibers are spaced in the range of 0.6-1.4 mm, the theoretical and the measured porosity are basically the same. Scaffolds with 50-70% porosity can be printed out, and the porosity is very neat, which confirms the correctness of the related mathematical model. However,  
Figure 12: Print scaffolds at different intervals. (**a**) scaffold structure with fiber spacing 0.6 mm; (**b**) scaffold structure with fiber spacing 0.8 mm; (**c**) scaffold structure with fiber spacing 1.0 mm; (**d**) scaffold structure with fiber spacing 1.2 mm; (**e**) Scaffold structure with fiber spacing 1.4 mm; (**f**) Scaffold structure with fiber spacing 1.6 mm. when the interval is 1.4 mm or more, the collapse of fibers occurs in some areas of the scaffolds.