Direct observation of deformation of individual red blood cells in oscillatory fluid flow produced using a generator of precise sinusoidal shear flow

We report the development of a precision sinusoidal shear flow generator that creates an oscillatory shear flow in the narrow gap between two parallel glass plates moving in opposite directions, thereby allowing direct observation of the cyclical deformation and recovery of a single red blood cell (RBC). The system is used to demonstrate that RBCs change their shape with cyclical elongation and shape recovery and align with the fluid streamlines in the direction of laminar Couette shear flow. From six repetitions, it can be seen that the curvature showing the time series of the elongation index (EI) of an RBC in sinusoidal shear flow in the present device was highly symmetrical and there were no significant differences at a 95% confidence interval. Moreover, the system provides details about the deformation characteristics of an RBC, which have four phases: (i) low deformation, in which the EI is minimal and the RBC mostly retains its original circular shape; (ii) shape elongation, in which the RBC loaded with shear forces begins to change its shape dynamically from circular to oval; (iii) steady deformation, in which the EI is constant and the shape of the RBC is elliptical throughout; and (iv) shape recovery, in which the EI decreases and the RBC becomes oval with trailing endpoints. Along with this information, the developed measurement system has potential application in clinical and biological analyses of RBC deformability.