Effect of in-plane and out-of-plane bifurcated microfluidic channels on the flow of aggregating red blood cells

The blood flow through our microvascular system is a renowned difficult process to understand because the complex flow behavior of blood is intertwined with the complex geometry it has to flow through. Conventional 2D microfluidics has provided important insights, but progress is hampered by the limitation of 2-D confinement. Here we use selective laser-induced etching to excavate non-planar 3-D microfluidic channels in glass that consist of two generations of bifurcations, heading towards more physiological geometries. We identify a cross-talk between the first and second bifurcation only when both bifurcations are in the same plane, as observed in 2D microfluidics. Contrarily, the flow in the branch where the second bifurcation is perpendicular to the first is hardly affected by the initial distortion. This difference in flow behavior is only observed when red blood cells are aggregated, due to the presence of dextran, and disappears by increasing the distance between both generations of bifurcations. Thus, 3-D structures scramble in-plane flow distortions, exemplifying the importance of experimenting with truly 3D microfluidic designs in order to understand complex physiological flow behavior.

Selective internal radiation therapy: quantifying distal penetration and distribution of resin and glass microspheres in a surrogate arterial model

PURPOSE

To experimentally investigate the effects of microsphere density and diameter on distal penetration.

MATERIALS AND METHODS

A surrogate hepatic arterial system was developed to replicate the hemodynamics (pressures, flow rates, pulsatile flow characteristics) and anatomic geometry (vessel diameters) proximal and distal to the microsphere injection point. A planar tumor model, placed distal to the injection point, allowed visualization of deposited microspheres. Bland resin and glass microspheres, with physical characteristics approximating the characteristics of commercially available products, were injected into the surrogate system. Microsphere type, injection rate, systemic flow rate, and tumor model inclination were varied among tests (glass, n = 7; resin, n = 6) with replicates for 2 conditions. After injection, 254 micrographs were obtained at previously defined locations throughout the tumor model to document microsphere distributions. Average microsphere distributions and mass measurements of microspheres collected at the tumor outlet were analyzed to quantify distal penetration for each case.

RESULTS

Across all test conditions, average penetration depths of resin microspheres were higher compared with glass microspheres (45.1 cm ± 11.8 vs 22.3 cm ± 9.9). The analysis of variance indicated that the observed difference between microsphere type (glass vs resin) was significant (P = .005, df = 1,2). The observed distance means did not differ significantly across flow rate or inclination angle.

CONCLUSIONS

Penetration depths of resin microspheres were significantly higher than penetration depths of glass microspheres in the surrogate hepatic arterial system.

'Go with the flow ': a review of methods and advancements in blood flow imaging

Physics has delivered extraordinary developments in almost every facet of modern life. From the humble thermometer and stethoscope to X-Ray, CT, MRI, ultrasound, PET and radiotherapy, our health has been transformed by these advances yielding both morphological and functional metrics. Recently high resolution label-free imaging of the microcirculation at clinically relevant depths has become available in the research domain. In this paper, we present a comprehensive review on current imaging techniques, state-of-the-art advancements and applications, and general perspectives on the prospects for these modalities in the clinical realm.

Extracorporeal bypass model of blood circulation for the study of microvascular hemodynamics

Many studies have been performed to better understand the hemodynamics in microvessels, such as arterioles and venules. However, due to the heterogeneous features of size, shape, blood-flow velocity, and pulsatility of microvessels, conducting a systematic study on these factors has been almost impossible. Although in vitro studies have been performed for this purpose, the usefulness of in vitro data is limited by the fact that the rheological properties of blood are changed as blood is exposed to in vitro environments. The purpose of the present study is to investigate the feasibility of a rat extracorporeal bypass model that combines in vivo and in vitro models. An arteriovenous shunt loop with a sub-bypass loop of fluorinated ethylene propylene (FEP) microtube was constructed between the jugular vein and femoral artery of a rat. Three pinch valves were installed in the main loop. Microscopic images of the blood flow in the FEP tube were sequentially captured with a high-speed camera, and the whole velocity field information was obtained using a micro-particle image velocimetry technique. Experimental results reveal that the velocity fields of the blood flow inside the microtube are well measured because the FEP tube is transparent and has nearly the same refractive index as water. The flow velocity and the pulsatility index of the blood flow in the microtube can be controlled by adjusting the three pinch valves installed upstream, midstream, and downstream of the bypass loop. This hybrid model that combines in vivo and in vitro models can be useful in studying microvascular hemodynamics.

Asymmetry of red blood cell motions in a microchannel with a diverging and converging bifurcation

In microcirculation, red blood cells (RBCs) flowing through bifurcations may deform considerably due to combination of different phenomena that happen at the micro-scale level, such as: attraction effect, high shear, and extensional stress, all of which may influence the rheological properties and flow behavior of blood. Thus, it is important to investigate in detail the behavior of blood flow occurring at both bifurcations and confluences. In the present paper, by using a micro-PTV system, we investigated the variations of velocity profiles of two working fluids flowing through diverging and converging bifurcations, human red blood cells suspended in dextran 40 with about 14% of hematocrit level (14 Hct) and pure water seeded with fluorescent trace particles. All the measurements were performed in the center plane of rectangular microchannels using a constant flow rate of about 3.0 × 10(-12) m(3)/s. Moreover, the experimental data was compared with numerical results obtained for Newtonian incompressible fluid. The behavior of RBCs was asymmetric at the divergent and convergent side of the geometry, whereas the velocities of tracer particles suspended in pure water were symmetric and well described by numerical simulation. The formation of a red cell-depleted zone immediately downstream of the apex of the converging bifurcation was observed and its effect on velocity profiles of RBCs flow has been investigated. Conversely, a cell-depleted region was not formed around the apex of the diverging bifurcation and as a result the adhesion of RBCs to the wall surface was enhanced in this region.

Spatial variation of blood viscosity: modelling using shear fields measured by a μPIV based technique

The spatial characteristics of blood viscosity were investigated by combining a newly developed constitutive equation with shear deformation fields calculated from velocity measurements obtained by a μPIV based technique. Blood at physiological hematocrit levels and in the presence of aggregation was sheared in a narrow gap plate-plate geometry and the velocity and aggregation characteristics were determined from images captured using a high resolution camera. Changes in the microstructure of blood caused by aggregation were observed to affect the flow characteristics. At low shear rates, high aggregation and network formation caused the RBC motion to become essentially two-dimensional. The measured velocity fields were used to estimate the magnitude of shear which was subsequently used in conjunction with the new model to assess the spatial variation of viscosity across the flow domain. It was found that the non-uniform microstructural characteristics of blood influence its viscosity distribution accordingly. The viscosity of blood estimated in the core of the examined flow, using a zero-gradient core velocity profile assumption, was found to be significantly higher than the overall effective viscosity determined using other velocity profile assumptions.

In vitro confocal micro-PIV measurements of blood flow in a square microchannel: The effect of the haematocrit on instantaneous velocity profiles

A confocal microparticle image velocimetry (micro-PIV) system was used to obtain detailed information on the velocity profiles for the flow of pure water (PW) and in vitro blood (haematocrit up to 17%) in a 100-microm-square microchannel. All the measurements were made in the middle plane of the microchannel at a constant flow rate and low Reynolds number (Re=0.025). The averaged ensemble velocity profiles were found to be markedly parabolic for all the working fluids studied. When comparing the instantaneous velocity profiles of the three fluids, our results indicated that the profile shape depended on the haematocrit. Our confocal micro-PIV measurements demonstrate that the root mean square (RMS) values increase with the haematocrit implying that it is important to consider the information provided by the instantaneous velocity fields, even at low Re. The present study also examines the potential effect of the RBCs on the accuracy of the instantaneous velocity measurements.