Open Access

Shape, dynamics, and viscoelastic properties of eukaryotic cells are primarily governed by a thin,reversibly cross-linked actomyosin cortex located directly beneath the plasma membrane. We obtaintime-dependent rheological responses of fibroblasts and MDCK II cells from deformation-relaxationcurves using an atomic force microscope to access the dependence of cortex fluidity on pre-stress. We introduce a viscoelastic model that treats the cell as a composite shell and assumes thatrelaxation of the cortex follows a power law giving access to cortical pre-stress, area compressibilitymodulus, and the power law (fluidity) exponent. Cortex fluidity is modulated by interferingwith myosin activity. We find that the power law exponent of the cell cortex decreases withincreasing intrinsic pre-stress and area compressibility modulus, in accordance with previousfinding for isolated actin networks subject to external stress. Extrapolation to zero tension returnsthe theoretically predicted power law exponent for transiently cross-linked polymer networks. In contrast to the widely used Hertzian mechanics, our model provides viscoelastic parametersindependent of indenter geometry and compression velocity.