  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.
