Hydrodynamic deformation reveals two coupled modes/time scales of red blood cell relaxation

We study the mechanical relaxation behavior of human red blood
cells by observing the time evolution of shape change of cells
flowing through microchannels with abrupt constrictions. We observe
two types of relaxation processes. In the first fast process
(tau(1) similar to 200 ms) the initially parachute shaped cells
relax into cup-shaped cells (stomatocytes). These cells relax and
reorient in a second relaxation process with a response time of
tau(1/2) similar to 10 s into the equilibrium discoid shapes. The
values for the relaxation times of single red blood cells in the
population scatter significantly within the cell population between
0.11 s < tau(1) < 0.52 s and 9 s < tau(1/2) < 49 s,
respectively. However, when plotting tau(1/2) against tau(1), we
find a linear relationship between the two timescales and are able
to relate both to the elastic properties of the spectrin
cytoskeleton underlying the red cell's plasma membrane. Adenosine
Triphosphate (ATP) enhances dissociation of spectrin filaments
resulting in a reduced shear modulus. We modify the cytoskeleton
connectivity by depletion and repletion of ATP and study the effect
on relaxation. Both the linear relationship of timescales as well
as the ATP dependence can be understood by theoretical models.