Ph.D Thesis

Ph.D StudentBransky Avishay
SubjectA Study of Erythrocytes Mechanical and Rheological
Properties, Using an Automated Rheoscope based on
a Microfabricated Flow Cell
DepartmentDepartment of Biomedical Engineering
Supervisors PROFESSOR EMERITUS Uri Dinnar (Deceased)
Full Thesis textFull thesis text - English Version


The deformability of erythrocytes is of great importance for oxygen delivery in the microcirculation. Reduced RBC deformability is associated with several types of hemolytic anaemias, malaria, sepsis and diabetes. Premature aging (as in hemolytic anaemias) and natural aging of erythrocytes is also associated with loss of deformability as well as reduced cell volume. As RBCs circulate through the fine network of vessels they lose membrane area and consequently also shrink in volume. Thus both the deformability and the volume of RBCs serve as important diagnostic parameters in the clinic.

An automated rheoscope has been developed, utilizing a microfabricated glass flow cell, high speed camera and advanced image-processing software. RBCs suspended in a high viscosity medium were filmed flowing through a microchannel. Under these conditions, erythrocytes take different orientations and undergo varying deformation according to their location in the velocity profile.

The rheoscope system produces valuable data such as velocity profile of RBCs, spatial distribution within the microchannel, cell volume and deformation index (DI) curves.

The variation of DI across the channel height, due to change in shear stress, was measured for the first time. These measurements were taken at a constant flow rate for several distances from the center of flow and cover most of the relevant shear stress spectrum. Such DI curves were obtained for normal and Thalassemia RBCs and their diagnostic potential was demonstrated. The method is an improvement of the existing techniques for deformability measurements in which shear is controlled mechanically.

The spatial distribution and velocity of RBCs and rigid microspheres (1┬Ám) were measured. The maximum slip velocity was found to be linearly correlated to the flow rate, for both cells and microspheres. RBC showed enhanced inward lateral migration compared to the rigid spheres, which is attributed to RBCs deformation and normal stresses. The results demonstrate the coupling between RBCs mechanical properties and their motion in microvessels.

The volume and surface area of the flowing cells have been estimated based on a fluid mechanics model and experimental results and fell within the normal range. The results obtained show that, on average, the deformability of the cells increases with increase in their size. This phenomena, which has not been measured before in single cells, is in agreement with previous findings that mature RBCs are smaller and less deformable.

Hence, the system developed, provides means for examining the behavior of individual RBCs in microchannels, and may serve as a microfabricated diagnostic device for deformability and volume measurements.