## 3 Results and Discussions
### Velocity Effects on Unit Cell-Structured Bone Tissue Repair Scaffolds
And the WSS distribution on the surface of the S-0.7 stent shows the same trend at different flow velocities, with the high WSS region distributed on the wall close to the main flow, while the pore wall parallel to the main flow direction has a lower WSS value. Unlike the pressure distribution, the WSS distribution between the cells of each layer does not show a decreasing trend layer by layer, and the WSS distribution between the cells of each layer is relatively uniform; it can be hypothesized that the velocity between different layers also shows a uniform distribution. The flow entrance and exit pressure differences were calculated for four sets of simulations with inlet flow velocities of 0.3 mm/s, 0.5 mm/s, 1 mm/s, and 1.2 mm/s, respectively, and the permeability was derived to obtain Figure 9a. As well as the average flow rates between the cell layers of different heights of the S-0.7 stent were statistically calculated for different inlet flow velocities and plotted in Figure 9b. With the increase in velocity, the differential pressure of fluid at the entrance and exit of the S-0.7 stent is linear, while the permeability remains constant, a trend that is consistent with the definition of permeability and indicates the validity of the simulation model. In the control group with different flow velocities, the average flow velocities between the cell layers were not much affected by the height of the stent, and they were all slightly lower than the inlet velocity, and the distribution of the flow velocities between the cells in each layer was similar, which was consistent with the previous speculation and reflected the reasonableness of the modulation of the distribution of the WSS on the wall of the bone tissue repair stent by the velocity. The WSS frequency distribution of S-0.7 scaffolds with different velocity control groups is plotted in Figure 10.  
Comparison of the control groups with inlet velocities of 1.0 mm/s and 1.2 mm/s, respectively, revealed that the frequency distribution of WSS on the stents had a similar trend, with WSS distributed most between 0 and 10 mPa, followed by 10-20 mPa, and the frequency distribution between 20 and 30 mPa appeared to have a very small point before increasing slightly. The overall WSS frequency distribution showed an equiproportional scaling trend with decreasing velocity. The frequency difference of WSS in the 10-30 mPa interval was not significant in the four control scaffolds, indicating that the S-0.7 scaffolds  
Figure 9: (**a**) Pressure difference and permeability of S-0.7 scaffold at different flow velocities; (**b**) average interlayer velocity between different unit cells. had strong potential for cartilage differentiation. At an inlet velocity of 1.2 mm/s, 10% of the scaffolds of the S-0.7 scaffolds showed a WSS of more than 50 mPa, which was close to the threshold for cell death and, therefore, the S-0.7 scaffolds were used for the differentiation of cartilage, and the WSS of these scaffolds was not significant. Therefore, the velocity should be controlled not to be higher than 1.2 mm/s when using S-0.7 scaffolds for perfusion culture. When the velocity was 1.0 mm/s, only 0.49% of the WSS exceeded 50 mPa, but it should be noted that at this time, the fine [22] cells will be differentiated towards bone, cartilage, and fibrous tissues. When the inlet velocity is 0.5 mm/s and less, the distribution of WSS on the scaffolds is almost always less than 30 mPa, and at this time the MSCs will not differentiate towards fibrous tissue. To increase the proportion of cells differentiating toward osteogenesis, the velocity of the perfused bioreactor should be adjusted within 0.1-0.3 mm/s.