As far as RBC stiffness is concerned, this hypothesis was recently confirmed in Ref. [6]. The high resolution nanoscale mapping of E values acquired in physiological solution unveiled that healthy erythrocytes are stiffer in their centre and softer at the cell periphery [6]. Therefore we probed the local response of RBCs by acquiring force-distance curves at different positions over the cell surfaces and evaluating the perceptual of energy dissipated during the indentation process, trough the estimation of H (Eq. (2.1)). In Fig. 2.2a, b we reported two hysteresis maps acquired on an healthy (a) and pathological red blood cell (b). H values range between 0 and 0.8 for both cells, indicating that a perceptual energy ranging from 0 to 80 % is dissipated during the indentation process. As far as the healthy RBC is concerned, the nanoscale mapping of H shows a cylindrical symmetry. The cell centre behaves approximately as a pure elastic body, showing H values ranging from 0 to 0.1. An increase in H values can be observed at the cell periphery, where H can be as high as 0.8. The cylindrical distribution of H values appears to be strongly correlated with the typical biconcave shape of healthy red blood cells. Moreover such distribution well correlates with the Young’s modulus distribution detected in healthy red blood cells [6]. A significantly different result is obtained in the pathological case. In this case, the nanoscale map appears to be homogeneously brighter than that of the healthy RBCs indicating that a larger amount of energy is dissipated during the indentation process. Moreover, the cylindrical distribution of H values on healthy RBCs is not observed. Conversely, an almost uniform distribution can be detected. Fig. 2.1 Two representative approach-retract cycles acquired on healthy (a) and pathological (b) RBCs. The green coloured area of the cycle represent the hysteresis Fig. 2.2 High-resolution hysteresis maps of healthy (a) and pathological (b) RBCs. Both maps are represented with the same colour scale. These maps display the presence of a cylindrical distribution of H for the healthy RBC, correlated with the biconcave shape of the cell, and the lack of this spatial symmetry for the pathological one. Indeed normal RBC is characterized by value of H in the range 0–0.1 in the centre, and an increase of H in the periphery, while the pathological erythrocyte shows higher value of H, uniformly distributed 2 Nano-Mechanical Response of Red Blood Cells 13
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