## 4 Conclusions  
This study demonstrated different designed and simulated microfluidic channel geometries and the influence of fluid dynamic parameters within their microenvironment. It has emerged that various designed geometrical parameters coupled with fluid dynamic parameters have a significant impact on the wall shear stress and velocity magnitude. Subsequent numerical simulations identified the width and height as geometrical parameters capable of controlling both the velocity profile and the wall shear stress within the microfluidic channels. The simulation of microchannels indicated that as the width of the microchannels decreases, the velocity and wall shear stress values tend to increase, whilst as the height of the microchannels decreases, the velocity and wall shear stress values increase, demonstrating that the width and height of microchannels contribute to the control of shear stress. Moreover, utilising six different flow rates, we showed another way to control the wall shear stress within the microchannels. Furthermore, the pressure was shown to be inversely proportional to the width and directly proportional to the flow rate within the microfluidic channels. The simulations also showed that the velocity and wall shear stress assumed different values at different flow rates. Shear stress, flow rate, and pressure parameters play an important role in controlling or predicting the cellular microenvironment. This method can be applied to optimise the microchannel geometries and to keep these parameters within physiological values, which can enhance nutrition and oxygen delivery, thus increasing cell morphology, proliferation, function and the long-term survival of many cell types. In particular, the magnitude of shear stress regulates cell morphology, phenotype transformation, and gene expression. Likewise, pressure plays an important role in cell functions and can cause damage to cells, resulting in apoptosis. The physiological values of shear stress in the human vascular system are in the range of 0.05 to 12 Pa, so when the shear stress values are below this range, the cells can undergo apoptosis, and if these values are above this range, the cells can be washed away by the fluid flow. In future work, fabrication of the above-designed microfluidic devices with an in vitro 3D cell culture applying the simulated fluid dynamic and geometrical parameters to be compared with this study could provide a quantitative measure of the efficiency of  
Figure 11: The variations of the velocity profile for different values of the aspect ratio of height (h) and width (w).  
the analysed geometries. This will reveal a suitable strategy to predict or control the effects of these forces within the different designs of microfluidic channels and the potential as an alternative to conventional cell culture, which could transform many areas of basic research and drug development.  
Conceptualisation, M.R.; Methodology, F.P. and M.R.; Software, F.P.; Formal analysis, F.P. and M.R.; Writing--original draft, F.P. and M.R.; Reviewing and editing, F.P., W.B., M.R.; Supervision, M.R. All authors have read and agreed to the published version of the manuscript.  
No funding was received for conducting this study.  
The authors would like to thank Brunel University Library for the technical help with SolidWorks and COMSOL Software.  
The authors declare no conflict of interest.