Viscoelastic phenotyping of red blood cells

Red Blood Cells (RBCs) are the simplest cell types with complex dynamical and viscoelastic phenomenology. While the mechanical rigidity and the flickering noise of RBCs have been extensively investigated, an accurate determination of the constitutive equations of the relaxational kinetics is lacking. Here we measure the force relaxation of RBCs under different types of tensional and compressive extension-jump protocols by attaching an optically trapped bead to the RBC membrane. Relaxational kinetics follows linear response from 60pN (tensional) to -20pN (compressive) applied forces, exhibiting a triple-exponential function with three well-separated timescales over four decades (0.01-100s). While the fast timescale ($\tau_F\sim 0.02(1)s$) corresponds to the relaxation of the membrane, the intermediate and slow timescales ($\tau_I=4(1)s$; $\tau_S=70(8)s$) likely arise from the cortex dynamics and the cytosol viscosity. Relaxation is highly heterogeneous across the RBC population, yet the three relaxation times are correlated, showing dynamical scaling. Finally, we find that glucose depletion and laser illumination of RBCs lead to faster triple-exponential kinetics and RBC rigidification. Viscoelastic phenotyping is a promising dynamical biomarker applicable to other cell types and active systems.