Fig. 4.

Cytoplasmic modulus under different effective strain rates and probe sizes. (A) Dependence of the apparent cytoplasmic modulus on normalized loading speed and probe size. Each modulus value (black circles) is averaged over at least 20 experiments; the SE bar has a length smaller than the size of the symbols in the plot. The 3D surface is obtained by piecewise linear interpolation of the measured apparent cytoplasmic modulus (E_A). The color of the surface represents the value of E_A. (B) E_A at different loading conditions does not change with the area of the probe bead at low effective strain rate regime, since viscoelasticity is dominating in this regime. However, E_A increases with the area of the bead at the high effective strain rate regime, since poroelasticity is dominating in this regime. (C) E_A is solely determined by V/a at low effective strain rate regime (0.1 s⁻¹ < V/a < 2 s⁻¹); this reinforces that V/a is the control parameter of the mechanical resistance at the low effective strain rate regime where viscoelasticity dominates. At the high effective strain rate regime (V/a > 5 s⁻¹), E_A obtained with different probe size starts to deviate even at the same V/a. Instead, E_A has the same value when the same Va is maintained (indicated by the horizontal dashed lines); this reinforces that Va is the control parameter of the resistance at the high effective strain rate regime where poroelasticity dominates. (Inset) The logarithmic plot of the dependence of E_A on V/a, which exhibits a seeming power-law behavior with a power of ∼0.25 at the low-speed regime.