Migration of blood cells in non-uniform suspension for a dialyzer design
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Hemodialysis is a renal replacement therapy that removes waste solutes from the blood stream using concentration gradients across a membrane. In order to overcome several shortcomings and increase the waste removal rate, a new dialyzer (filter) design is proposed in this study. In the new dialyzer design, the blood concurrently flows with a sheath fluid in a micro-fluidic channel. Because the blood stream directly contacts the sheath stream, it is important to prevent blood cell migration from the blood stream to the sheath stream while providing enough time for the waste solutes to diffuse into the sheath stream. This research was intended to understand the migration behavior of red blood cells (RBC) and platelets in non-uniform suspension flow, where the blood and sheath flows in direct contact, and apply the results to identify the feasible design space of the proposed dialyzer. The effect of different flow conditions and channel geometry on the blood cell extraction ratios (ER), the ratio of cells lost into the sheath stream, in non-uniform suspension flows was parametrically studied using Lattice Boltzmann and Spectrin Link (LB-SL) method based direct numerical simulation (DNS). Analyzing ER over the flow distance showed that the channel size and the area ratio of sheath to channel are the main variables that affect the ER. Based on the relationship found, a meta-model of RBC ER was created, although platelet ERs showed only a general trend. Based on the study, feasible conditions that will retain blood cells in the blood stream were identified. Then, the DNS results of blood cell ER were used with a molecule diffusion model and a hemodialysis system model to study the feasibility of the proposed dialyzer design that maximizes middle molecule filtration with limited blood cell and protein loss. No feasible design was found in the studied range suggesting that relying purely on the diffusion based on the direct contact for the removal of middle molecules is not a feasible solution with the small channel size (~700 µm) due to the loss of protein. It suggested that in order to increase the middle molecule removal while maintain the protein level, clearance ratio of middle molecule to protein should be increased using large channel size, small sheath stream thickness, long tubule length, and slow blood flow velocity. The intellectual merit of this research lies in understanding the migration behavior of blood cells in a non-uniform suspension. This knowledge helped to establish the feasibility of the proposed dialyzer design and can be applied in a variety of applications for the manipulation of cells in a micro-fluidic channel.