Numerical investigation on red blood cell dynamics in microflow: Effect of cell deformability.

The radial dispersion of red blood cells (RBCs) near the vessel wall can significantly affect the transport dynamics in small vessels. The radial dispersion of RBCs is mainly caused by collisions between RBCs and this can be enhanced by aggregation. The objective of this study is to numerically investigate on the effect of RBC deformability on the radial motion of individual RBCs in a range of flow rates. Immersed Boundary - Lattice Boltzmann Method was utilized to study the radial motion of RBCs in a two-dimensional flow domain. The RBC flow simulations were performed at 40% hematocrit in a microvessel with diameter of 25µm and length of 100µm. The dispersion of less deformable RBCs was notably greater than that of normal RBCs at all flow rates and this effect seemed to be more pronounced when the flow rate was increased. The cell dispersion was higher near the vessel wall than the flow center regardless of flow rate and RBCs deformability. Thus, the dispersion of RBCs could be enhanced with flow rate and RBC rigidity. Our findings would be especially useful in investigating blood flows in arterioles and venules.

Recovery of cell-free layer and wall shear stress profile symmetry downstream of an arteriolar bifurcation.

Unequal RBC partitioning at arteriolar bifurcations contributes to dissimilar flow developments between daughter vessels in a bifurcation. Due to the importance of the cell-free layer (CFL) and the wall shear stress (WSS) to physiological processes such as vasoregulation and gas diffusion, we investigated the effects of a bifurcation disturbance on the development of the CFL width and WSS in bifurcation daughter branches. The analysis was performed on a two-dimensional (2-D) computational model of a transverse arteriole at three different flow rates corresponding to parent branch (PB) pseudoshear rates of 60, 170 and 470s(-1), while maintaining a 2-D hematocrit of about 55% in the PB. Flow symmetry was defined using the statistical similarity of the CFL and WSS distributions between the two walls of the vessel branch. In terms of the flow symmetry recovery, higher flow rates caused larger reductions in the flow symmetry indices in the MB and subsequently required longer vessel lengths for complete recovery. Lower tube hematocrits in the SB led to complete symmetry recovery for all flow rates despite the higher initial asymmetry in the SB than in the MB. Arteriolar bifurcations produce unavoidable local CFL asymmetry and the persistence of the asymmetry downstream may increase effective blood viscosity which is especially significant at higher physiological flow rates.

A review of numerical methods for red blood cell flow simulation.

In this review, we provide an overview of the simulation techniques employed for modelling the flow of red blood cells (RBCs) in blood plasma. The scope of this review omits the fluid modelling aspect while focusing on other key components in the RBC-plasma model such as (1) describing the RBC deformation with shell-based and spring-based RBC models, (2) constitutive models for RBC aggregation based on bridging theory and depletion theory and (3) additional strategies required for completing the RBC-plasma flow model. These include topics such as modelling fluid-structure interaction with the immersed boundary method and boundary integral method, and updating the variations in multiphase fluid property through the employment of index field methods. Lastly, we summarily discuss the current state and aims of RBC modelling and suggest some research directions for the further development of this field of modelling.

Effect of deformability difference between two erythrocytes on their aggregation.

In this study, we investigated the rheology of a doublet that is an aggregate of two red blood cells (RBCs). According to previous studies, most aggregates in blood flow consist of RBC doublet-pairs and thus the understanding of doublet dynamics has scientific importance in describing its hemodynamics. The RBC aggregation tendency can be significantly affected by the cell's deformability which can vary under both physiological and pathological conditions. Hence, we conducted a two-dimensional simulation of doublet dynamics under a simple shear flow condition with different deformability between RBCs. To study the dissociation process of the doublet, we employed the aggregation model described by the Morse-type potential function, which is based on the depletion theory. In addition, we developed a new method of updating fluid property to consider viscosity difference between RBC cytoplasm and plasma. Our results showed that deformability difference between the two RBCs could significantly reduce their aggregating tendency under a shear condition of 50 s(-1), resulting in disaggregation. Since even under physiological conditions, the cell deformability may be significantly different, consideration of the difference in deformability amongst RBCs in blood flow would be needed for the hemodynamic studies based on a numerical approach.

Two-phase model for prediction of cell-free layer width in blood flow.

This study aimed to develop a numerical model capable of predicting changes in the cell-free layer (CFL) width in narrow tubes with consideration of red blood cell aggregation effects. The model development integrates to empirical relations for relative viscosity (ratio of apparent viscosity to medium viscosity) and core viscosity measured on independent blood samples to create a continuum model that includes these two regions. The constitutive relations were derived from in vitro experiments performed with three different glass-capillary tubes (inner diameter=30, 50 and 100 µm) over a wide range of pseudoshear rates (5-300 s(-1)). The aggregation tendency of the blood samples was also varied by adding Dextran 500 kDa. Our model predicted that the CFL width was strongly modulated by the relative viscosity function. Aggregation increased the width of CFL, and this effect became more pronounced at low shear rates. The CFL widths predicted in the present study at high shear conditions were in agreement with those reported in previous studies. However, unlike previous multi-particle models, our model did not require a high computing cost, and it was capable of reproducing results for a thicker CFL width at low shear conditions, depending on aggregating tendency of the blood.