Figure 4
From: Dynamic monitoring of cell mechanical properties using profile microindentation
BAEC stress relaxation can be described using only two independent mechanical parameters: the apparent stiffness E* and the relaxation parameter α.
(a) Example of raw data extracted from cross-correlation image analysis. We obtain the indentation (d − dc) (black curve) and the deflection (d − z) − (d0 − z0) (grey curve). t = 0 s marks the beginning of the indentation (left arrow). By multiplying the deflection by the indenter’s rigidity kind (nN/μm), we find the applied force F (nN). After the chosen force Fthreshold is attained (right arrow), we maintain the indentation constant, ensuring constant strain during force relaxation. (b) Example force-indentation curve fitted with a single parameter: the apparent stiffness E*. In this example, approach speed is 1.4 μm/s. Data acquisition frequency is approximately 30 Hz. The inset represents a histogram of the apparent stiffness E* of BAECs (n = 20 cells and N = 139 indentation curves) measured with a microindenter whose base is moving at 1.4 μm/s, fitting the first 1.0 μm of the force-indentation curve. (c) Scatter plot of A/(R1/2dmax3/2) as a function of E*, where A is the pre-factor in the force relaxation as given by equation (3), R is the effective radius given by R = 1/(1/ Rprobe + 1/ Rcell) with Rcell ~ 20 μm, dmax is the indentation maintained during relaxation and E* is the apparent stiffness measured by fitting the first 1.0 μm of the force-indentation curve of BAECs (n = 51 cells and N = 191 indentation curves). The very good correlation (correlation coefficient r = 0.95) shows that A and E* are inter-dependent variables, both measuring a cell’s apparent stiffness. (d) Example force-time relaxation curve at fixed indentation. Cell relaxation is observed over 80 s. Time is normalized by t0 = 1 s. Force is normalized by its maximum value attained at the first time point.
