Correlation of bulk rheology of human blood with nano-rheology and deformability of individual erythrocytes

Abstract

It is now widely recognised that the deformability of erythrocyte membranes determines the rheological properties of whole blood and affects its physiological functions. Impaired erythrocyte deformability has been related to a number of cardio-vascular disorders, diabetes, infectious deceases, etc. Macroscopic rheological properties of whole blood have been studied extensively over years. However it is difficult to correlate macroscopic rheological properties of blood, with nono-rheological properties of erythrocytes, as techniques currently used to investigate the deformability of erythrocytes (such as filtration, rheoscopy, ektacytometry, etc) do not provide insights into structural properties of individual cells. In this study we have used a high sensitivity stress controlled rheometer for characterizing the rheological behaviour of erythrocyte suspensions and the Atomic Force Microscope (AFM) to probe the nano-scale deformation and viscoelasticity of individual red blood cells under physiological conditions. Erythrocytes of natural and decreased deformability have been studied. Erythrocyte suspensions were investigated using steady (rotational) and dynamic (oscillatory) modes. The shear thinning nature, yield stress, low and high shear viscosities, dynamic moduli G’ and G” and relaxation phenomena have been ascertained in relation to changing deformability of red blood cells. Interaction forces between a colloid probe and an immobilized red blood cell have been measured using the AFM to determine the deformation and viscoelasticity of the cell membrane. Quantitative interpretation of the interaction force data has been carried out. The effective separation between the cell and the probe has been determined and the level of deformation, or Young’s modulus, of the cells, has been obtained. Hysteresis in the approach/retraction of force versus distance data has then been used to determine the visco-elastic response and relaxation time of blood cells. Correlations between macrorheology of erythrocyte suspensions and nano-rheological behaviour of individual erythrocytes have been established and provide a basis for future investigation into cell disease and the effects of pharmaceutical agents.