## 3 Results and Discussions
### Effect of Microchannel Height within a Microfluidic Device  
The height of a microfluidic device is another parameter that has been studied to evaluate how this geometric feature can influence the fluid dynamic parameters such as velocity and wall shear stress within a microenvironment. For this study, we compared the velocity and wall shear stress results obtained for the circular, longitudinal, and three parallel channel geometries with a height of 200 \(\upmu\)m to those obtained within the same devices but with a height of 60 \(\upmu\)m. This particular height (60 \(\upmu\)m) was chosen based on the literature where similar geometries were analysed [22; 26; 55].  
The circular microchannel device (Figure 10a,b) demonstrated that as the height decreases, both the velocity and wall shear stress profiles tend to show a plateau region. As mentioned above, this is due to the ratio between the width of the cultivation chamber (diameter = 1 mm) and the height (60 \(\upmu\)m), which tends to decrease the ratio between these two parameters (width and height), leading to a plateau region in the velocity and wall shear stress profiles. Figure 10b shows the different values of wall shear stress moving from the bottom (h = 0 \(\upmu\)m, solid lines) to the middle (h = 30 \(\upmu\)m, dotted lines) of the microchannel.  
Furthermore, similar results were found for the longitudinal microchannel device (Figure 10c,d), where the ratio between the width (250 \(\upmu\)m) and the height (60 \(\upmu\)m) decreased, demonstrating a plateau region for the velocity and wall shear stress profiles (bottom and middle) within the microchannel. Figure 10d shows the values of wall shear stress at the bottom (h = 0 \(\upmu\)m, solid lines) and in the middle (h = 30 \(\upmu\)m, dotted lines) of the microchannel. Furthermore, the results of the velocity magnitude and wall shear stress of the designed three parallel microchannels (Figure 10e,f) showed that decreasing the height (60 \(\upmu\)m) of the microchannels resulted in the formation of a plateau region for both the velocity and wall shear stress profiles within the side microchannels. The different values of wall shear stress moving from the bottom (h = 0 \(\upmu\)m, solid lines) to the middle (h = 30 \(\upmu\)m, dotted lines) of the microdevice are shown in Figure 10f.  
Figure 10: The influence of height in microfluidic devices using different heights of 30 and 60 \(\upmu\)m. The velocity magnitude and wall shear stress profile at the bottom microchannels (h = 0 \(\upmu\)m) and in the middle of the microchannels (h = 30 \(\upmu\)m). (**a,b**) the circular microchamber; (**c,d**) the longitudinal microchannel; and (**e,f**) the three parallel microchannels with a height of 60 \(\upmu\)m using flow rates of 2, 6, and 10 \(\upmu\)L/min for the side channels. Red circles in (**e,f**) highlight the velocity and shear stress fluctuations near the micropillar structures.  
Figure 11 shows the aspect ratio between the height (h) and width (w). The figure demonstrates that by increasing the aspect ratio between the h and w of the channel, the parabolic profile changes, becoming more horizontal [46]. Additionally, based on Equation (5) when the height of the microchannels reduced, the shear stress as well as the velocity profile was increased, indicating that they are directly proportional [22; 26].