  We report the simultaneous characterization of time- and force-dependent
mechanical properties of adherent cells in the physiologically relevant regime
of large forces. We used magnetic tweezers to apply forces to magnetic beads
bound to the cytoskeleton, and recorded the resulting deformation (creep
response). The creep response followed a weak power law at all force levels.
Stress stiffening and fluidization occurred simultaneously and were quantified
by the force-dependence of the creep compliance and the power law exponent. The
amount of stiffening and fluidization in response to force was controlled
solely by the contractile prestress of the cell and followed a simple
relationship. This behavior may be of fundamental importance in biological
processes that involve a mechanical interaction between cells and their
environment.
