An In Vitro Hemodynamic Flow System to Study the Effects of Quantified Shear Stresses on Endothelial Cells

Numerous in vitro systems have previously been developed and employed for studying the effects of hemodynamics on endothelial cell (EC) dysfunction. In the majority of that work, accurate flow quantification (e.g., uniformity of the flow over the ECs) remains elusive and wall shear stress (WSS) quantifications are determined using theoretical relationships (without considering the flow channel aspect ratio effects). In addition, those relationships are not applicable to flows other than steady laminar cases. The present work discusses the development of a novel hemodynamic flow system for studying the effects of various well-quantified flow regimes over ECs. The current work presents a novel hemodynamic flow system applying the concept of a parallel plate flow chamber (PPFC) with live microscopy access for studying the effects of quantified WSS on ECs. A range of steady laminar, pulsatile (carotid wave form) and low-Reynolds number turbulent WSSs were quantified through velocity field measurements by a laser Doppler velocimetry (LDV) system, to validate the functionality of the current hemodynamic flow system. Uniformity of the flow across the channel width can be analyzed with the current system (e.g., the flow was uniform across about 65-75% of the channel width for the steady cases). The WSS obtained from the experiments had higher values in almost all of the cases when compared to the most commonly-used theoretical solution (9% < error < 16%), whereas another relationship, which considers the channel dimensions, had better agreement with the experimental results (1% < error < 8%). Additionally, the latter relationship predicted the uniform flow region in the PPFC with an average difference of <5% when compared to the experimental results. The experimental data also showed that the WSS at various locations (D, E and F) at the test section differed by less than 4% for the laminar cases representing a fully developed flow. WSS was also determined for a low-Re (Re = 2750) turbulent flow using (1) the Reynolds shears stress and (2) the time-averaged velocity profile gradient at the wall, with a good agreement (differences <16%) between the two where the first method returned a higher value than the second. Porcine aortic endothelial cell (PAEC) viability in the system and morphological cell response to laminar WSS of about 11 dyne/cm(2), were observed. These results provide performance validation of this novel in vitro system with many improved features compared to previous similar prototypes for investigation of flow effects on ECs. The integration of the LDV technique in the current study and the comparison of the results with those from theory revealed that great care must be taken when using PPFCs since the commonly used theoretical relation for laminar steady flows is unable to predict the flow uniformity (which may introduce significant statistical bias in biological studies) and the predicted WSS was subjected to greater error when compared to a more comprehensive equation presented in the current work. Moreover, application of the LDV technique in the current system is essential for studies of more complex cases, such as disturbed flows, where the WSS cannot be predicted using theoretical or numerical modelling methods.