Simulation of a detoxifying organ function: Focus on hemodynamics modeling and convection-reaction numerical simulation in microcirculatory networks

When modeling a detoxifying organ function, an important component is the impact of flow on the metabolism of a compound of interest carried by the blood. We here study the effects of red blood cells (such as the Fahraeus-Lindqvist effect and plasma skimming) on blood flow in typical microcirculatory components such as tubes, bifurcations and entire networks, with particular emphasis on the liver as important representative of detoxifying organs. In one of the plasma skimming models, under certain conditions, oscillations between states are found and analyzed in a methodical study to identify their causes and influencing parameters. The flow solution obtained is then used to define the velocity at which a compound would be transported. A convection-reaction equation is studied to simulate the transport of a compound in blood and its uptake by the surrounding cells. Different types of signal sharpness have to be handled depending on the application to address different temporal compound concentration profiles. To permit executing the studied models numerically stable and accurate, we here extend existing transport schemes to handle converging bifurcations, and more generally multi-furcations. We study the accuracy of different numerical schemes as well as the effect of reactions and of the network itself on the bolus shape. Even though this study is guided by applications in liver micro-architecture, the proposed methodology is general and can readily be applied to other capillary network geometries, hence to other organs or to bioengineered network designs.

A computational model for microcirculation including Fahraeus-Lindqvist effect, plasma skimming and fluid exchange with the tissue interstitium

We present a two-phase model for microcirculation that describes the interaction of plasma with red blood cells. The model takes into account of typical effects characterizing the microcirculation, such as the Fahraeus-Lindqvist effect and plasma skimming. Besides these features, the model describes the interaction of capillaries with the surrounding tissue. More precisely, the model accounts for the interaction of capillary transmural flow with the surrounding interstitial pressure. Furthermore, the capillaries are represented as one-dimensional channels with arbitrary, possibly curved configuration. The latter two features rely on the unique ability of the model to account for variations of flow rate and pressure along the axis of the capillary, according to a local differential formulation of mass and momentum conservation. Indeed, the model stands on a solid mathematical foundation, which is also addressed in this work. In particular, we present the model derivation, the variational formulation, and its approximation using the finite element method. Finally, we conclude the work with a comparative computational study of the importance of the Fahraeus-Lindqvist, plasma skimming, and capillary leakage effects on the distribution of flow in a microvascular network.

The Role of Microsystems Integration Towards Point-of-Care Clozapine Treatment Monitoring in Schizophrenia

We present a perspective on microsystems integration aspects for concurrent cellular and molecular sensing in a lab-on-a-chip device. While of interest for a range of applications, very few - narrowly focused - examples of such devices can be found in the literature. Here, we approach the challenge from a systems level, considering sensor integration both in parallel and in series. Our study is specifically geared toward schizophrenia treatment, where concurrent blood monitoring of the antipsychotic clozapine and white blood cells could lead to improved treatment outcomes. We evaluate the critical system components for either design, namely plasma skimming (parallel) and in-blood clozapine detection (series). We find that plasma skimming is infeasible, but for the first time demonstrate direct detection of clozapine in whole blood. With a corresponding series-integrated microsystem, we finally demonstrate downstream white blood cell analysis on the same samples using impedance cytometry. We thus present the first lab-on-a-chip device capable of label- and reagent-free concurrent sensing of cellular and molecular markers.

Hematocrit distribution and tissue oxygenation in large microcirculatory networks

OBJECTIVE

Oxygen tension in the brain is controlled by the microcirculatory supply of RBC, but the effect of non-Newtonian blood flow rheology on tissue oxygenation is not well characterized. This study assesses different biphasic blood flow models for predicting tissue oxygen tension as a function of microcirculatory hemodynamics.

METHODS

Two existing plasma-skimming laws are compared against measured RBC distributions in rat and hamster microcirculatory networks. A novel biphasic blood flow model is introduced. The computational models predict tissue oxygenation in the mesentery, cremaster muscle, and the human secondary cortex.

RESULTS

This investigation shows deficiencies in prior models, including inconsistent plasma-skimming trends and insufficient oxygen perfusion due to the high prevalence (33%) of RBC-free microvessels. Our novel method yields physiologically sound RBC distributions and tissue oxygen tensions within one standard deviation of experimental measurements.

CONCLUSIONS

A simple, novel biphasic blood flow model is introduced with equal or better predictive power when applied to historic raw data sets. It can overcome limitations of prior models pertaining to trifurcations, anastomoses, and loops. This new plasma-skimming law eases the computations of bulk blood flow and hematocrit fields in large microcirculatory networks and converges faster than prior procedures.

Hematocrit distribution and tissue oxygenation in large microcirculatory networks

OBJECTIVE

Oxygen tension in the brain is controlled by the microcirculatory supply of RBC, but the effect of non-Newtonian blood flow rheology on tissue oxygenation is not well characterized. This study assesses different biphasic blood flow models for predicting tissue oxygen tension as a function of microcirculatory hemodynamics.

METHODS

Two existing plasma-skimming laws are compared against measured RBC distributions in rat and hamster microcirculatory networks. A novel biphasic blood flow model is introduced. The computational models predict tissue oxygenation in the mesentery, cremaster muscle, and the human secondary cortex.

RESULTS

This investigation shows deficiencies in prior models, including inconsistent plasma-skimming trends and insufficient oxygen perfusion due to the high prevalence (33%) of RBC-free microvessels. Our novel method yields physiologically sound RBC distributions and tissue oxygen tensions within one standard deviation of experimental measurements.

CONCLUSIONS

A simple, novel biphasic blood flow model is introduced with equal or better predictive power when applied to historic raw data sets. It can overcome limitations of prior models pertaining to trifurcations, anastomoses, and loops. This new plasma-skimming law eases the computations of bulk blood flow and hematocrit fields in large microcirculatory networks and converges faster than prior procedures.

Effect of erythrocyte aggregation on spatiotemporal variations in cell-free layer formation near on arteriolar bifurcation

OBJECTIVE

To investigate how red blood cell aggregation could modulate the spatial variations in cell-free layer formation in the vicinity of an arteriolar bifurcation.

METHODS

Visualization of blood flow was performed in upstream and downstream vessels of arteriolar bifurcations in the rat cremaster muscles under reduced flow conditions before and after induction of red blood cell aggregation to both physiological normal- and pathological hyperlevels seen in humans.

RESULTS

Large asymmetries of layer widths on opposite sides of the downstream vessel were attenuated along the vessel and this effect could be prominently enhanced by the hyperaggregation due to a higher formation rate of the layer which was greater on one side than the other of the vessel. The proportion of downstream layer formation constituted by the smaller downstream vessel generally increased with a thicker layer width at the wall of the upstream vessel adjacent it. A greater tendency of the layer formation in the smaller downstream vessel was found under the hyperaggregating condition than normal-aggregating and nonaggregating conditions.

CONCLUSION

Red blood cell aggregation could attenuate the asymmetry in cell-free layer formation on opposite sides of the downstream vessel, but enhances the heterogeneity of the layer formation between downstream vessels.

Blood-plasma separation in Y-shaped bifurcating microfluidic channels: a dissipative particle dynamics simulation study

The motion of a suspension of red blood cells (RBCs) flowing in a Y-shaped bifurcating microfluidic channel is investigated using a validated low-dimensional RBC model based on dissipative particle dynamics. Specifically, the RBC is represented as a closed torus-like ring of ten colloidal particles, which leads to efficient simulations of blood flow in microcirculation over a wide range of hematocrits. Adaptive no-slip wall boundary conditions were implemented to model hydrodynamic flow within a specific wall structure of diverging three-dimensional microfluidic channels, paying attention to controlling density fluctuations. Plasma skimming and the all-or-nothing phenomenon of RBCs in a bifurcating microfluidic channel have been investigated in our simulations for healthy and diseased blood, including the size of a cell-free layer on the daughter branches. The feed hematocrit level in the parent channel has considerable influence on blood-plasma separation. Compared to the blood-plasma separation efficiencies of healthy RBCs, malaria-infected stiff RBCs (iRBCs) have a tendency to travel into the low flow-rate daughter branch because of their different initial distribution in the parent channel. Our simulation results are consistent with previously published experimental results and theoretical predictions.