Stathoulopoulos, Antonios; (2024) Erythrocyte properties and microscale transport phenomena. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Blood is a complex, multiphase suspension of various components, with red blood cells (RBCs) being the most abundant. Their deformability and propensity to aggregate play a crucial role in how blood flows, affecting how it moves through small vessels. Conditions like malaria, sickle cell disease, diabetes, and sepsis can impair the deformability of RBCs, altering their distributions in the microvasculature. Recently, the use of statins for cholesterol treatment was correlated with the softening of the RBC membrane, implying that drug therapies may affect the mechanical properties of RBCs. In vitro studies using microfluidics can provide insight into the role of RBC properties in microvascular flows. The present study uses microfluidic devices and advanced imaging techniques to explore how RBC properties affect blood flow in the microvasculature. Specifically, it focuses on how RBCs mechanical properties impact RBC velocity and haematocrit distributions in these vessels. Human RBC suspensions, at different flow rates and haematocrit levels, were perfused in a Y-shaped bifurcation as well as in two different pillar array microfluidic geometries. RBC properties were altered by glutaraldehyde and statins and their flow was visualised using brightfield, microscopic Particle Image Velocimetry (micro-PIV) methods. The acquired images were processed using MATLAB routines to determine velocity and haematocrit distributions. The study first examined how healthy and hardened RBCs behaved in the pillar array and Y-shaped channel. In the pillar array, the flow of RBCs formed two distinct lanes with a region in the middle where few cells were present. Interestingly the reduction in RBC deformability did not significantly affect the velocity distributions. Significant differences in the cell distribution were revealed with hardened RBCs entering the cell-depleted regions in the rear and front stagnation points of the pillars more frequently than the healthy ones and producing more uniform interstitial distributions. Around the apex of the Y-shaped channel, RBCs showed notable differences in the shape of the haematocrit distributions, when comparing healthy and hardened cells. Healthy RBCs tended to accumulate near the inner walls of the channels, forming sharper peaks in their concentration. In contrast, the distribution was more uniform for the hardened RBCs. Although the haematocrit distributions differed locally, similar partitioning characteristics were observed for both suspensions. Comparisons with a T-shaped bifurcation showed that the bifurcation angle affects the haematocrit distributions of the healthy RBCs and not the hardened ones. When statin-treated RBCs were perfused in the pillar array, they followed less tortuous tracks and travelled with slower velocities than healthy RBCs. An increased retention time of statin-treated cells was reported implying that softer RBCs have different shapes compared to the healthy ones, in the area around pillar walls, leading to an increase in the probability of near-zero velocities in the examined domain. Softer cells were found to distribute more evenly in the examined domain whereas healthy RBCs were tightly concentrated in the interstitial spaces between the pillars. In summary, the thesis complements efforts in understanding spatial RBC organisation in complex microvascular networks, elucidating the role of RBC biomechanical properties in partitioning, and highlights the challenges heterogeneous RBC distributions pose on haemodynamic modelling of pathological conditions.

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