## 4 Discussion  
The application of physiological laminar shear stress on cellular models has been shown to optimize the cell differentiation and ameliorate their predictive value through reproduction of the in vivo shear force. To culture the BMEC under flow, we can benefit today from a wide range of dynamic devices whose features and possible uses greatly differ [48]. Below is an overview of the major parameters of existing devices created to perform permeability studies. A first approach was to integrate the fluidic component into the classically used inserts. A few devices were engineered by implementing fluid movement either inside or below Transwell(r) inserts [49; 50]. However, these complex architectures make the shear stress impossible to calculate, and are prone to turbulences [51]. These models based on Transwell(r) inserts therefore do not allow to endure physiological shear stress ranges. Then, microfluidic devices were created to reproduce the architecture of two compartments separated by BMEC for analyzing the movement of substances across the BBB. According to the disposition of the channels, these devices can be classified as: horizontally aligned channels, crossed channels, or vertically aligned channels. In side-by-side configurations, channels are next to each other in the same horizontal plane. BMEC are seeded on the entire channel surface, but only a portion of them is in contact with the adjacent compartment. Consequently, the exchanges with the other compartment are only on one side. In addition, this configuration does not allow to visualize the cell-cell contacts at this interface along the culture time [36; 39; 40; 52; 53]. Devices based on crossed channels, i.e., perpendicularly superimposed, have the benefit of allowing the visualization of the entire cellular interface between the compartments. However, the overlapping area between the two compartments is too small compared to the entire seeding surface, resulting in a minor number of cells in contact with the other compartment [30; 54; 34]. Finally, vertically superimposed channels represent the perfect configuration combining a larger contact surface between the two compartments, with the possibility of morphological examination on the entire cell monolayer. A few published devices present this ideal configuration but show some drawbacks on the experimental setups [32; 35; 37; 38; 55], discussed below. The equation derived from the Haagen-Poiseuille formula \(\left(\tau=6\mu 0/wh^{2}\right)\) is widely used to configure the flow rate depending on the desired shear stress in microfluidic models. It depicts an average value specifically in case of special channel dimensions, but does not consider the heterogeneity of shear stress distribution [56]. Actually, cells can be subjected to \begin{table}
\begin{tabular}{c c c} \hline
**Culture Condition** & **Recovery (\%)** & _Papp_ **(10\({}^{-6}\) cm\(\cdot\)s\({}^{-1}\))** \\ \hline Cells in static culture & 87.3 \(\pm\) 0.36 & 14.2 \(\pm\) 0.66 \\ Cells in dynamic culture & 86.6 \(\pm\) 2.99 & 10.2 \(\pm\) 1.11 \({}^{*}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Results from lucifer yellow (LY) permeability measurement in hCMEC/D3 cells maintained for seven days in our barrier device. Experiment was performed in duplicate on cells cultured in static or dynamic condition (under shear stress of 10 dyn\(\cdot\)cm\({}^{-2}\)). Results are given as means \(\pm\) standard deviation. Recovery percentages estimated the loss of LY during the experiment due to LY non-specific binding. Apparent endothelial permeability (_Papp_) was calculated as previously described [18]. \({}^{*}\): Student _p_-value \(<\) 0.05.  
strong variations along the channel width. To prevent this effect, dimensions of the fluidic channel must be defined to form a rectangular cross section (\(w>>h\)) [48]. Furthermore, it is important to predict the flow profile according to the channel dimensions to configure reliable flow rate values [42].