In vitro blood flow visualizations and cell-free layer (CFL) measurements in a microchannel network

Abstract

Microvascular networks are not simple straight microchannels but rather complex geometries composed by successive asymmetric divergent and convergent bifurcations. Despite the extensive research work in this field, still lack of knowledge about the blood flow behavior in microvascular networks. The current study applies the most current advanced visualization and microfabrication techniques to provide further insights into to the blood flow in network geometries. Hence, by using a high-speed video microscopy system, blood flow measurements and visualizations of the cell-free layer (CFL) were performed along a microchannel network composed by several divergent and convergent bifurcations. The inlet flow rate was kept constant whereas the hematocrit (Hct) and the depth of the geometry was changed in order to evaluate their effects into the CFL thickness. The results, show clearly that the Hct has a significant impact on the CFL thickness whereas the effect of reducing the depth did not contribute to a noticeable change on the CFL. In addition, the in vitro blood flow results reported here provide for the first time that in microfluidic devices having several asymmetric confluences it is likely to have the formation of several CFLs not only around the walls but also in middle of the main channels just downstream of the last confluence apex. Although, to best of our knowledge there is no evidence that this kind of flow phenomenon also happens in vivo, we believe that for microvascular networks with similar geometries and under similar flow conditions tested in this work, this kind of phenomenon may also happen in vivo. Furthermore, the results from this study could be extremely helpful to validate current numerical microvascular network models and to develop more realistic multiphase numerical models of blood flow in microcirculation.the vessel size dependent effective viscosity (known as Fahraeus-Lindqvist effect) [2,6]. By decreasing the tube diameter there is a decrease of the apparent viscosity, due to the migration of RBCs towards the core flow and consequent increase of the CFL thickness around the walls. These microscale hemodynamic phenomena play an important role in blood mass transport mechanisms [3,11]. The CFL thickness depends of several factors such as cell concentration, deformability, vessel diameter, cell aggregation, flow rate, presence of microbubbles and geometry of the microchannels [12][13][14][15][16][17][18][19][20]. For instance, by increasing the concentration of RBCs, i.e., increasing the hematocrit (Hct), there is a decrease of CFL thickness [17,21].The bifurcations are geometries extremely common in both microvessels and microfluidic devices and it is important to improve our current understanding regarding the influence of the bifurcations and confluences on the blood flow behavior at a microscale level. As a result, several numerical and in vitro blood flow studies have investigated the effect these complex https://doi.

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