Optical Rheology of Biological Cells

A step stress deforming suspended cells causes a passive relaxation, due to a transiently cross-linked isotropic actin cortex underlying the cellular membrane. The fluid-to-solid transition occurs at a relaxation time coinciding with unbinding times of actin cross-linking proteins. Elastic contributions from slowly relaxing entangled filaments are negligible. The symmetric geometry of suspended cells ensures minimal statistical variability in their viscoelastic properties in contrast with adherent cells and thus is defining for different cell types. Mechanical stimuli on time scales of minutes trigger active structural responses.

Figure 1

An NIH/3T3 cell is trapped between two horizontally opposing, divergent laser beams. In a step stress experiment it is stretched as the laser power is increased and relaxes when the power is reduced. (a) Phase contrast images are used to analyze the radial deformation. (b)–(d) Development of axial strain $\gamma =\frac{\Delta r}{r}$ with time $t$ when stretching an NIH/3T3 cell for 0.2 s (b), 2.5 s (c), and 10 s (d). Equations (2, 3) were fitted to the data (solid lines).

Figure 2

Storage $G^{\prime }(\omega )$ and loss modulus $G^{\prime \prime }(\omega )$ for an NIH/3T3 fibroblast (left) and an SV-T2 fibroblast (right). The insets display the differences in actin distribution and concentration between NIH/3T3 and SV-T2 cells fluorescently labeled with TRITC-Phalloidin at a concentration of $3\mu \mathrm{g}/\mathrm{m}\mathrm{l}$.

Figure 3

An NIH/3T3 cell is stretched for 60 s with ${\sigma }_0=6\mathrm{P}\mathrm{a}$ and given a 5 min relaxation phase, during which minimal trapping forces are applied. This process is repeated 3 times. (a) Even at low trap powers, where the optical surface stress is too small to stretch the cell, active extension along the laser axis is observed. (b) Phase contrast images of the temporal development of the elongating cell studied in (a).