Flow of red blood cells in microvessels

The study and modeling of the red blood cells flow are of particular importance in medicine and biomedical engineering since they can provide essential information concerning the treatment strategy and the design of micro-scale medical devices. The objectives of the present study are twofold. Firstly, we aim to compare two tracing methods for measuring the blood velocity in-vitro using μ-PIV and secondly we attempt to measure the cell free layer (CFL) during blood flow inside microvessels, whose hydraulic diameter is less than 300μm, i.e., when the Fahraeus-Lindqvist effect is significant. The cell-free layer is a near-wall layer of plasma, lacking red blood cells, which in this case tend to migrate towards the center of the microvessel. All experiments are conducted in two channels (160X180μm, 100X100μm) etched on a PMMA plate and sealed with the same material. Since conducting experiments using real blood is difficult to accomplish, we used artificial “blood”, a fluid that has similar rheological properties. This “blood” is a suspension of healthy human red blood cells (RBC) in saline water (0.9 % w/w sodium chloride) while two different hematocrits (Hct) 10 and 20% are studied. The addition of a 0.5 % v/v EDTA solution hinders the coagulation of the RBCs. The “blood” flow rate is controlled by a syringe-pump (Aladdin, Al-2000). In the first part of our study a μ-PIV system is used for measuring the axial velocity of the blood resembling fluid. The measuring section is illuminated by a double cavity Nd:YAG laser emitting at 532nm, while the flow is recorded using a high sensitivity CCD camera, connected to a Nikon microscope. A 20X air immersion objective with NA=0.45 is also used resulting to 2.8μm depth of field. As it is known the μ-PIV method is based on measuring the velocity of tracing particles. In our study we choose to use two kinds of tracer, i.e., either micro fluorescent polystyrene particles (with mean diameter of 1μm) or the RBCs themselves after colouring them with a fluorescence dye (Rhodamine B). For the flow rate range used, i.e. 0.5 to 5ml/h, it was found that, regardless of the tracing method employed, the blood velocity distribution in the middle of the microchannel is practically the same (Fig. 1). However, when the mean velocities are of the same order of magnitude with the settling velocity of the RBCs, a slight deviation is observed. This can be attributed to the fact that in this case the settling of the RBCs affects the measurement of the axial velocity, an observation also confirmed by experimental data. In the second part of this study the extent of the CFL is measured. In this case the test section is illuminated by a light source placed under the microchannel. The light intensity is adjusted so as to be fully absorbed by the RBCs when not in motion. When the flow is initiated the CFL develops and is identified as a light stripe at the vicinity of the vessel walls. The flow is recorded by a high speed camera and the width of the CFL is estimated by analyzing the consecutive video frames. Fig. 2 presents typical frames where the width of the CFL can be identified. It is observed that the width of the CFL increases as the diameter of the tube is increased and decreases as the hematocrit is increased. Also for relative low flow rates the width of the CFL is found to increase by increasing the flow rate, while for flow rates higher than 3ml/h it attains a constant value.