FIGURE 6.

Measurement of the nonlinear creep modulus. (Top) An example of the displacement of a bead on a F9 wild-type cell in response to a staircaselike force pattern. The measured displacement values are shown as a thick line, and the fits of Eq. 2 to the data are shown as thin lines. (Center) The differential cell stiffness (geometric mean ± geometric SE) increased with force by a similar factor in all the cell lines, as indicated by the nearly parallel lines of stiffness versus force in a log-log plot. (Bottom) The power-law exponent, b, was force-independent in all cell lines. (Note: the stiffness values and exponents at 0.5 nN are not identical with the values in Fig. 4 because detached beads were excluded from this analysis.)