Analysis of the blood flow behavior through microchannels by a confocal micro-PIV/PTV system

• Main Author: Lima, Rui A.
• Publication Date: 2007
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10198/1263
• topic: Confocal micro-PIV; Blood Flow; Microcirculation; Biomicrofluidics

Over the years, various experimental methods have been applied in an effort to understand the blood flow behavior in microcirculation. Most of our current knowledge in microcirculation is based on macroscopic flow phenomena such as Fahraeus effect and Fahraeus-Linqvist effect. The development of optical experimental techniques has contributed to obtain explanations on the way the blood flows through microvessels. Although the past results have been encouraging, detailed studies on the flow properties of blood in the microcirculation has been limited by several technical factors such as poor spatial resolution and difficulty to obtain quantitative detailed measurements at such small scales. Therefore, there is still a lack of knowledge on the microscale flow behavior of blood cells through microvessels. In recent years, due to advances in computers, optics, and digital image processing techniques, it has become possible to combine a particle image velocimetry (PIV) system with a conventional microscope. As a result, this combination, known as a micro-PIV, has greatly increased the resolution of the conventional PIV. Although the conventional micro-PIV technique has proven to be useful in measuring the flow behavior in microfluidics devices, the entire flow field is illuminated and consequently the out-of-focus emitted light can result in high levels of background noise, which degrades the measured velocity fields. More recently, considerable progress in the development of confocal microscopy and the advantages of this technique over conventional microscopy have led to a new technique known as confocal micro-PIV. This technique combines the conventional PIV system with a spinning disk confocal microscope (SDCM), which has the ability to obtain in-focus images with an optical thickness less than 1 m (optical sectioning effect) and also to improve the particle image contrast, definition, and spatial resolution. This emerging technique has been successfully applied to measure homogenous fluids, however the question whether a confocal micro-PIV system is a suitable technique to study the blood flow behavior through microchannels remains. In our work, a confocal micro-PIV system and also a confocal micro-particle tracking velocimetry (PTV) system are used, for the first time, to investigate in vitro blood flow through microchannels. By using these systems we have aimed to obtain further insights into the complex flow properties of blood in the microcirculation with the expectation that a better understanding on the blood flow phenomena will make a contribution to the prevention, diagnosis, and treatment of vascular diseases. The validity of our confocal micro-PIV system is performed by comparing the experimental results of pure water seeded with tracer particles with an established analytical solution for a steady flow in a long straight microchannel. Good agreement was obtained, especially around the centre of the microchannel, with errors on the order of 5% or less. Furthermore, we have also demonstrated, for the first time, the ability of the confocal micro-PIV to generate 3D velocity profiles of a blood cell suspension fluid ( 4% haematocrit (Hct)). The confocal system is used to determine both ensemble and instantaneous velocity profiles for in vitro blood (haematocrit up to 17%), flowing through a 100-m square glass microchannel at a constant flow rate and low Reynolds number (Re = 0.025). It was observed small fluctuations in the instantaneous velocity profiles which were found to be closely related to the increase with the Hct. Although the micro-PIV community tend to ignore these fluctuations, this study shows that the root mean square (RMS) values increase with the haematocrit implying that the presence of RBCs within the plasma flow strongly influences the measurements of the instantaneous velocity fields. Consequently, information provided by instantaneous velocities should be taken into account. Furthermore, by measuring the velocity profiles in vitro blood (20% Hct) in a rectangular (300 m wide, 45 m deep) polydimethysiloxane (PDMS) microchannel, small fluctuations were also found in the velocity profiles. Therefore, our results clearly show evidence that the encountered fluctuations are closely related to the motion and interaction of RBCs when flowing in a crowded environment. We show that the confocal micro-PIV system is able to measure with good accuracy the blood plasma flow with Hct up to 9%, in a 100　m square microchannel. However, for Hct bigger than 9%, the light absorbed and scattered by the RBCs contributes to diminish the concentration of tracer particles in the acquired confocal images. Hence, a novel approach was implemented to the confocal system in order to obtain more reliable quantitative measurements on the motion of blood cells at high suspensions of RBCs. By using labeled RBCs and Dextran-40, a confocal micro-PTV system was employed, for the first time, in an effort to track individual tracer cells at high concentration suspensions of RBCs. The ability of the confocal system to generate thin in-focus planes has allowed both qualitative and quantitative measurements in flowing blood at concentrated suspensions (up to 35% Hct) of cell-cell hydrodynamic interaction, RBC orientation and RBC radial dispersion at different depths. Such data is thought to be extremely relevant to elucidate the blood transport mechanisms and associated diseases such as thrombosis and atherosclerosis. By using the confocal micro-PTV system, the RBCs radial dispersion coefficient (Dyy) was measured in the middle plane of a 50m and 100 m glass capillaries in both diluted and concentrated suspensions (Hcts up to 35%) at low Reynolds numbers (Re from 0.003 to 0.005). There is evidence that the RBCs Dyy tends to increase with the Hct but, at Hct of about 25%, it tends to level off. This finding suggests that, at moderate Hcts, the development of the plasma layer and the consequent decrease of the local cell density, surrounding the RBCs, may enhance the radial dispersion of RBCs. In addition, we have also found that Dyy is greatest at radial positions between 0.4R to 0.8R, whereas at locations adjacent to wall (0.8R to 1R) and around the middle of the capillary (0R to 0.2R) the paths of the tracer RBCs tend to exhibit lower radial displacements. Furthermore, our results also provide evidence that RBCs Dyy tends to decrease with the diameter. This phenomenon is believed to be due to Hct reduction with the diameter (Faharaeus effect) and also to geometry constrictions which limit the amplitude of the RBCs radial displacements. Hence, this finding seems to indicate that the reduction of RBC radial dispersion may be linked to the decrease in apparent viscosity with decreasing diameter (Faharaeus-Lindqvist effect). The work reported in this thesis represents the first application of a confocal micro-PIV/PTV system to study the blood flow behavior through microchannels. The confocal system proves to be able to eliminate the problems and concerns of the experimental techniques used in the past and provide additional detailed description on the RBC motion not obtainable by other conventional methods. Finally, the research carried out throughout this thesis provides the basis not only to obtain further insights on the blood mass transport mechanisms under both physiological and pathological conditions but also to improve the existing theories, models, and computer simulations on the blood flow at both micro and macroscale levels.