Regensburg 2007 – scientific programme
Parts | Days | Selection | Search | Downloads | Help
BP: Fachverband Biologische Physik
BP 24: Cell Mechanics (in vivo)
BP 24.1: Talk
Thursday, March 29, 2007, 10:00–10:15, H44
Nonlinear creep measurements in living fibroblasts — •Philip Kollmannsberger, Claudia T. Mierke, and Ben Fabry — ZMPT, Biophysics Group, University of Erlangen-Nuremberg
The linear viscoelasticity of adherent cells and biological tissue is characterized by a wide distribution of relaxation times and shows a power-law creep response or a power-law viscoelastic spectrum over several decades of time or frequency. In addition, single cells and tissue exhibit a highly nonlinear stress-strain relationship. The viscoelastic behavior of cells in the non-linear regime is unknown, however, but is of particular interest to test different conflicting theories. Here we measured the viscoelastic behavior of a variety of adherent cells in the linear and non-linear regime using magnetic tweezers with real-time force feedback. We imposed a staircase-like sequence of 1 nN force steps up to a maximum force of 10 nN onto 4.5 µm fibronectin-coated magnetic beads bound to the cytoskeleton via integrins. For each stress level σ, the differential creep response of single cells followed a power law: J(σ,t) = J0(σ)(t/t0)b, however the differential creep modulus J0(σ) decreased with stress, equivalent to stress stiffening. The power-law creep exponent b showed no systematic stress dependence, although in some cells b increased at high forces, consistent with yielding and disruption events. Static stress stiffening is predicted by models of semiflexible polymers and can be modelled using Fung’s theory of quasilinear viscoelasticity for biological tissues, whereas a speed-up of relaxation processes due to yielding and structural changes is consistent with soft glassy rheology.