Hematocrit, viscosity and velocity distributions of aggregating and non-aggregating blood in a bifurcating microchannel

Abstract

Microscale blood flow is characterised by heterogeneous distributions of hematocrit, viscosity and velocity. In microvascular bifurcations, cells are unevenly distributed between the branches, and this effect can be amplified in subsequent branches depending on a number of parameters. We propose an approach to infer hematocrit profiles of human blood flowing through a bifurcating microchannel. The influence of aggregation, induced by the addition of Dextran 2000 to the samples, is also considered. Averaged values indicate plasma skimming, particularly in the presence of red blood cell (RBC) aggregation. Using an empirical model, the hematocrit profiles are used to estimate local relative viscosity distributions. Simulations are used to predict how the non-uniform viscosity influences the velocity profiles. Comparing these data to velocity profiles of RBCs measured using particle image velocimetry provides validation of the model. It is observed that aggregation blunts velocity profiles after a long straight section of channel. Downstream of the bifurcation, skewing of the velocity profiles is detected, which is enhanced by aggregation. The proposed methodology is capable of providing hitherto unreported information on important aspects of microscale blood rheology.

Going beyond 20 μm-sized channels for studying red blood cell phase separation in microfluidic bifurcations

“…Note that in larger channels, such oscillations are not present but interpolation of the OD profile must anyway be performed in the vicinity off the walls. 47 Here, one can safely extract from this kind of data a single optical density value, determined as the average of OD(x) for x values in the range of 5-15 lm, i.e., in the center of the channel where OD is observed to be roughly constant. The tube hematocrit H t is then assumed to be constant over the channel width and related to this mean OD value.…”

“…2,8,10,15,37,[45][46][47] On one hand, despite the development of microfluidics in the last decade, in vitro experiments have not yet permitted to explore phase separation in bifurcations with branches of characteristic size smaller than 20 lm nor in asymmetric channels with at least one branch smaller (both in depth and width) than the other ones. In addition, "room-sized" experimental models, designed according to the principles of kinematic and dynamic similarity, cannot reproduce the complex rheological properties of RBCs.…”

“…This should ultimately allow a better understanding of the physics at play and a parametrical characterization of the phase-separation effect in steady state, potentially improving the phenomenological description of Pries et al 33,37 The aim of the present paper is to present the experimental methodologies and measurement techniques developed for that purpose, especially in the case of channels of capillary size or slightly larger (typically below 20 lm), including asymmetric bifurcations, in the full physiological hematocrit range. The main originalities of the experiment are the following: first, we design polydimethylsiloxane (PDMS) micro-bifurcations made of square channels with different sizes, while most of the recent previous studies used bifurcations made of rectangular channels with unique depth, 15,45,47 which are easier to fabricate; 48 second, we are interested in regimes which are seldom studied (moderately to highly concentrated RBC suspension flows in small micro-channels), while in most previous studies, channels with sizes superior or equal to 20 lm have been used; 2,8,10,15,[45][46][47] and third, we simultaneously aim at a rigorous control of the experimental conditions, including the possibility of varying independently the inlet hematocrit and the flow rate ratio between both daughter branches, and at an in situ quantitative measurement of the flow parameters. This is extremely challenging in the considered flow regimes and necessitates a combination of various metrologies, including the comparison with reference measurements, in order to validate the results.…”

On the flow topology inside droplets moving in rectangular microchannels

“…The experiments were conducted using a µPIV system developed by Sherwood et al 26 for studies of microscale blood flow. The system makes use of alternate strobe and laser illumination and is amenable to imaging two phase flows, particularly suspensions.…”

Red blood cells in retinal vascular disorders

“…In vessels of a certain size, a spatial distribution of viscosity can be described [15], at least in a time-averaged sense. This definition of viscosity refers to the local viscosity of the fluid, if considered as a continuum, but including the spatial distribution of haematocrit and shear rate.…”