Increase in aggregation size did not translate into a linear increase of the blood viscosity.
The viscosity was found to increase with decreasing shear rate and increasing hematocrit, exemplifying the established non-Newtonian shear-thinning behaviour of blood. For the 10% H suspensions, in contrast, lowering the shear rate below 10 s -1 resulted in a significant increase of RBC aggregate sizes. For the samples suspended at 5% H, aggregate size was not strongly correlated with shear rate. Aggregate sizes were determined using image processing techniques, while apparent viscosity was measured using optical viscometry. RBC aggregate sizes were quantified in controlled and measurable shear rate environments for 5, 10 and 15% hematocrit. Experiments were performed using a micro particle image velocimetry (μPIV) system for shear rate measurements, coupled with a high-speed camera for flow visualization. These microchannels were fabricated using standard photolithography methods. Suspensions of human blood were flowed and observed in vitro in poly-di-methyl-siloxane (PDMS) microchannels to characterize RBC aggregates. The main objective of this work is to quantify and characterize RBC aggregates in order to enhance the current understanding of the non-Newtonian behaviour of blood in microcirculation. The theory and mechanics behind aggregation are, however, not yet completely understood. Remarkably RBCs deform and bridge together to form aggregates under very low shear rates. The vortical structures were detected at the end, but also at the entrance, of the narrow channel and their formation is a direct result of the viscoelastic flow properties of blood plasma.Red blood cells (RBCs) are the most abundant cells in human blood. In one of their experiments, the research team let plasma flow through a narrow channel of the kind found in stenotic (constricted) arteries or in a stent (a medical implant inserted into constricted blood vessels).
These vortices may facilitate the formation of deposits on blood vessel walls which could influence blood clot formation. "An important part of our study was developing microfluidic instruments sensitive enough to pick up the small differences in viscosity that are the signature of non-Newtonian fluids," explained Professor Arratia.Įxperiments performed by Professor Wagner's team in Saarbrücken also showed that blood plasma influences the creation of vortices in flowing blood. Their measurements showed that blood plasma exhibits a flow behavior different to that of water and that plasma can demonstrate a substantially higher flow resistance. The studies by Professor Arratia and his team at the University of Pennsylvania involved a microfluidic approach in which they developed a model of a microvascular system in order to study the flow properties of blood plasma. The plasma shows "viscoelastic" properties, which means that it exhibits both viscous and elastic behavior when deformed, forming threads that are typical of non-Newtonian fluids. "Our experiments showed that the blood plasma forms threads, that is, it exhibits an extensional viscosity, which is something we do not observe in water," explained Professor Wagner. The work at Saarland University involved experiments in which the blood plasma was allowed to form drops inside a specially built apparatus equipped with high-speed cameras fitted with high-resolution microscope lenses to analyze drop formation.
Arratia have studied the flow dynamics of blood experimentally. The research team around experimental physicist Christian Wagner and engineer Paulo E.
The results from this research may well help to improve computer simulations of this kind of pathological process. The results demonstrate that blood plasma is itself a non-Newtonian fluid.Īccording to the study's findings, the complex flow behavior of blood plasma could play a crucial role with respect to vascular wall deposits, aneurysms or blood clots. But results from researchers at Saarland University and at the University of Pennsylvania have now shown that plasma is a very special fluid that plays a crucial part in determining how blood flows. After all, plasma, the liquid in which the blood cells are suspended, consists to 92 percent of water. For decades, researchers have assumed that blood plasma flows like water. Blood plasma was generally regarded simply as a spectator that played no active role. Up until now it has been assumed that the special flow characteristics exhibited by blood were mainly due to the presence of the red blood cells, which account for about 45 percent of the blood's volume.
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