Blood-mimicking fluid for biotechnological applications. Fluid dynamic behavior of red blood cells and droplets under confined shear flow

In spite of the development of cardio-vascular prosthetic devices, prosthetic heart valves, pulsatile blood pumps, models of the circulatory system, and sophisticated studies of flow through the complex geometries of the circulation, an analog test fluid with the rheological properties of blood is still lacking. The properties of blood pose several problems for quantitave applications as a test fluid: composition differs from donor to donor, precluding a fixed standard; properties change with time because of metabolic processes; opacity restricts optical studies of flow; blood samples can carry disease, thus presenting a biological hazard to workers; blood is a two phase, unstable system subject to sedimentation, and it can clot, thus blocking its fluid-like flow; the volume which can be withdrawn from a single donor is limited, restricting high volume applications. To formulate a model blood fluid it is necessary to understand the rheological and fluidodynamic properties of blood and in particular of red blood cells (RBCs), because they form the major and the largest constituent of blood components. One of the most striking properties of red blood cells is their high deformability, which allows them to flow through vessels with radius smaller than cell size in microcirculation and is essential to maintain optimal blood circulation and gas transfer between blood and tissues. In this work, the attention was focussed on microconfined shear flow of RBCs in capillaries with radius comparable to average cell size. The main fluidodynamic observables are shape and velocity of the flowing RBCs, which depend on capillary radius, pressure drop, and RBC volume fraction in the capillary (referred to as tube hematocrit HT). The microcapillaries are placed in a rectangular flow cell, where a suspension of RBCs is fed under the action of a liquid head in the physiological range. Video microscopy images of the flowing RBCs are acquired at high magnification and later processed by an automated image analysis macro. It was found that RBCs from healthy donors exhibit the classical parachute shape observed in vivo. Because the ability of red blood cell to deform and pass through capillaries is essential for continual flow of blood in the microvasculature, in this work was presented a novel tool for evaluating the impact of impaired deformability of RBCs on the flow of blood in the microvasculature by directly measuring perfusion of a test microchannel network (made by soft-lithography technique) with dimensions and topology similar to the real microcirculation. Furthermore, was analyzed the flow of deformable droplets of one Newtonian fluid suspended in another Newtonian fluid flowing in glass microcapillaries were. The resultant shapes of the liquid drops are similar to the shapes of red blood cells that have been observed in narrow glass capillaries as well as in blood vessels.