## 3 Results and Discussions  
### Velocity Effects on Unit Cell-Structured Bone Tissue Repair Scaffolds  
Firstly, the analysis was carried out for the flow of 1 mm/s bioreactor inlet velocity in the bone tissue repair scaffolds with unit cell structure. The pressure distribution of S-0.7 is shown in Figure 7a. With the inflow of fluid from the inlet (Y = 0.01 m), the pressure decreases gradually with the flow direction, and the overall pressure distribution is uniform. Using  
Figure 7: (**a**) Contour of pressure distribution on S-0.7 scaffold; (**b**) velocity vector field of internal flow in S-0.7 scaffold; (**c**) WSS distribution diagram of S-0.7 scaffold; (**d**) frequency distribution of WSS for S-0.7 scaffold. Equation (2), the permeability was determined to be 1.23 \(\times\) 10\({}^{-8}\) m\({}^{2}\), which exceeds the reported average permeability of human trabecular bone and can satisfy cellular nutrient supply requirements. Figure 7b presents the velocity vector field within the scaffold. Fluid flow primarily occurred along \(Y\)-axis aligned pores of the scaffold, forming multiple primary flow streams with the highest velocity at the pore center and a gradual velocity decay toward the wall. Flow velocities within \(X\)- and \(Z\)-axis pores were significantly lower, resulting in a periodically regular but non-uniform velocity distribution. This non-uniformity may compromise nutrient supply and cellular metabolism in low-velocity regions of the scaffold. The WSS distribution of the S-0.7 scaffold at 1 mm/s for the scaffold was plotted in Figure 7c. As illustrated in the figure, WSS distribution exhibits similar trends to the velocity field, with high WSS primarily concentrated on the scaffold walls aligned with the main flow direction (\(Y\)-axis), while X- and \(Z\)-axis pore walls experience lower magnitudes. Statistical analysis of WSS values revealed a mean of 15.77 mPa and a maximum of 59.69 mPa. The relative frequency distribution of WSS across different intervals is presented in Figure 7d. As illustrated, at an inlet velocity of 1 mm/s, the WSS distribution along the S-0.7 scaffold walls was non-uniform. The 0-10 mPa WSS range accounted for 53% of the total distribution, and this wall region promotes osteogenic differentiation of MSCs [25]. In contrast, the 10-30 mPa WSS region induces ECM mineralization or cartilaginous tissue formation, while the 30-60 mPa region stimulates fibrous tissue development. Combining these results with the WSS contour plot, the non-uniform WSS distribution can be attributed to heterogeneous fluid flow patterns within the scaffold pores. If all the cells on the scaffold wall are differentiating towards osteoblasts, the scaffold structure or the velocity of the bioreactor needs to be modulated. The velocity was first changed to modulate the scaffold WSS distribution. Numerical simulations were performed for the S-0.7 scaffold at different inlet velocities with the rest of the settings kept constant. The pressure distribution cloud plots and WSS distribution contour plots at three inlet flow velocities of 1.2 mm/s, 1.0 mm/s, and 0.5 mm/s were selected for comparison after comparative analysis of the simulation results. Pressure and WSS contour plots for the S-0.7 scaffold under varying inlet velocities are presented in Figure 8.  
Figure 8: Pressure and WSS contour plots of S-0.7 scaffold under varying flow velocities. Through comparison, it is found that the pressure distribution on the surface of the scaffold with different flow velocities all show the same trend, decreasing layer by layer with the direction of flow inlet (\(Y\)-axis direction), with the maximum pressure in the inlet layer, the minimum pressure in the outlet layer, and the uniform pressure distribution between each layer. With the increase in inlet velocity, the pressure difference between the upper and lower layers between different groups increases.