The impact of channel geometry and flow regime on endothelial orientation and morphology in vessel-on-chip

This study investigates the impact of channel geometry and applied flow on the morphology and function of endothelial cells (ECs) within vessel-on-chip (VoC) models. Traditional organ-on-chip models often utilize rectangular cross-section channels, resulting in flat walls, sharp corners, and non-uniform wall shear stress profiles, which do not accurately mimic physiological conditions. Tubular channels with a circular cross-section provide a more in vivo-like geometry and result in a physiological uniform wall shear stress. Here, sugar 3D-printing is used for fabricating tubular channels, and stereolithography 3D-printing is applied for making rectangular channels. Using these models, the effects of both channel geometry and various flow conditions on the orientation and morphology of ECs, from both blood and lymph vessels, is explored. The research demonstrates that ECs in tubular channels exhibit a more uniform coverage and circumferential alignment/migration compared to rectangular channels. Unidirectional and bidirectional flow conditions induce alignments parallel to flow, overruling the circumferential alignment induced by curvature in tubular channels. The combination of tubular geometry and pulsatile flow induces enhanced circumferential orientation, different from rectangular channels in which cells remain primarily aligned along the flow direction under pulsatile flow. In summary, our systematic study shows that the channel curvature determines the cell orientation in static conditions, and that adding flow results in a competing effect between geometry and flow for which the resulting cell alignment depends on the flow conditions. Additionally, the study examines the initial adhesive interactions between monocytes and ECs, revealing that EC orientation induced by different flow regimes impacts monocyte rolling velocities. This finding is important for understanding immune cell motility and adhesion in healthy and diseased states. This study underlines the importance of the combined effect of channel geometry and flow conditions in VoC models, and it provides a strong motivation for the continued development of advanced OoC systems with ever more representative geometrical designs and flow control, to maximize the potential of OoC to revolutionize biomedical research and personalized medicine by providing more accurate and functional models of human physiology.