  We measure the dynamical mechanical properties of human red blood cells.
Single cell response is measured with optical tweezers. We investigate both the
stress relaxation following a fast deformation, and the effect of varying the
strain rate. We find a power law decay of the stress as a function of time,
down to a plateau stress, and a power law increase of the cell's elasticity as
a function of the strain rate. Interestingly, the exponents of these quantities
violate the linear superposition principle, indicating a nonlinear response. We
propose that this is due to breaking of a fraction of the crosslinks during the
deformation process. The Soft Glassy Rheology Model accounts for the relation
between the exponents we observe experimentally. This picture is consistent
with recent models of bond remodeling in the red blood cell's molecular
structure. Our results imply that the blood cell's mechanical behavior depends
critically on the deformation process.
