## 3 Results and Discussions
### The Closed-Microchannel Structure
(**a–c**) the fluid flow; (**d–f**) wall shear stress; (**g–i**) pressure simulations in a closed microchannel structure with a height of 200 \(\upmu\)m using six different flow rates; (**a,d,g**) flow rates of 3 \(\upmu\)L/min for the main microchannels and 2 \(\upmu\)L/min for the side microchannels; (**b,e,h**) 5 \(\upmu\)L/min for the main microchannels and 6 \(\upmu\)L/min for the side microchannels; (**c,f,i**) 7 \(\upmu\)L/min for the main microchannels and 10 \(\upmu\)L/min for the side microchannels. (**j**) generated mesh density study for a closed microchannel structure. Figure 9: Cont. As mentioned above, for the three parallel microchannel geometry and for this configuration, it is possible to observe how, in the proximity of the micropillars, the velocity and the shear stress decrease. Furthermore, the presence of flow rate in the main microchannel has led to an increase in the values of both the parameters, velocity and shear stress. In this geometry (Figure 9), the micropillars have larger dimensions and gap space (with a length of 120 \(\upmu\)m, a width of 100 \(\upmu\)m, and a distance of 50 \(\upmu\)m) than those designed previously in three parallel channels (length 100 \(\upmu\)m, width 50 \(\upmu\)m, and distance 20 \(\upmu\)m). This difference led to the formation of fluctuations in the proximity of micropillars, generating a less homogeneous shear stress profile near the micropillars (Figure 9a-c--red circles), which was compared to the one that was obtained in the previous simulation for the parallel channels' microfluidic device (Figure 7g,h--red circles). Therefore, this simulation indicated that both the size and gap space between the micropillars are the two parameters that can be manipulated to control the shear stress within the device. Furthermore, the viscosity of cell suspensions will not influence the flow parameters as the viscosity of the medium is very similar to the water viscosity. According to the Navier-Stokes Equation (2), an increasing cell concentration on the floor of the channel should not change the flow profile [40]. Frequently, in CFD simulation studies of in vitro culture systems, the culture medium is considered a Newtonian fluid for rheological and computational modelling purposes, as the widely used commercial culture media (DMEM and RPMI) supplemented with typical concentrations of FBS exhibit Newtonian behavior [59]. Moreover, it was shown that the viscosity of the culture medium is similar to the Newtonian behaviour of water. However, changes in the culture medium are affected by various experimental conditions such as seeding densities, substrate, the geometry of the device, the media formulation itself, and flow conditions [59]. A similar result was obtained by [PERSON] et al. [30], who studied the effect of gap spacing between the micropillars on wall shear stress within a five parallel channel device separated by rows of micropillars. However, their study aimed to design a microfluidic device with a micropillar gap spacing below 30 \(\upmu\)m that was capable of generating homogeneous shear stress gradients, whereas, by using greater gap distances, the pillars led to oscillations of the shear stress profile. However, in this study, we designed a microfluidic device  
Figure 9: The velocity, wall shear stress (at h = 0 \(\upmu\)m, the bottom) and in the middle (h = 100 \(\upmu\)m) of the microchannel, and pressure profiles using six flow rates within the side and main microchannels versus the width of the microchannel. (**a**) the velocity magnitude; (**b**) wall shear stress at h = 0 \(\upmu\)m and; (**c**) at h = 100 \(\upmu\)m; (**d**) pressure using flow rates of 3 \(\upmu\)L/min (main microchannel) and 2 \(\upmu\)L/min (side microchannels) (blue line); 5\(\upmu\)L/min (main microchannel) and 6 \(\upmu\)L/min (side microchannels) (dotted green line); and 7 \(\upmu\)L/min (main microchannel) and 10 \(\upmu\)L/min (side microchannels) (red line). Red circles highlight the velocity and shear stress fluctuations near the micropillar structures.