  Cell mechanical functions like locomotion, contraction and division are
controlled by the cytoskeleton, a dynamic biopolymer network whose mechanical
properties remain poorly understood. We perform single-cell uniaxial stretching
experiments on 3T3 fibroblasts. By superimposing small amplitude oscillations
on a mechanically prestressed cell, we find a transition from linear
viscoelastic behavior to power-law stress stiffening. Data from different cells
over several stress decades can be uniquely scaled to obtain a master-relation
between the viscoelastic moduli and the average force. Remarkably, this
relation holds independently of deformation history, adhesion biochemistry, and
intensity of active contraction. In particular, it is irrelevant whether force
is actively generated by the cell or externally imposed by stretching. We
propose that the master-relation reflects the mechanical behavior of the force
bearing actin cytoskeleton, in agreement with stress stiffening known from
semiflexible filament networks.
