Berlin 2008 – scientific programme

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BP: Fachverband Biologische Physik

BP 22: Cell Mechanics

BP 22.4: Talk

Thursday, February 28, 2008, 11:15–11:30, PC 203

Stress relaxation, stiffening and fluidization of adherent cells — •Philip Kollmannsberger and Ben Fabry — Physics Department, University Erlangen-Nuremberg, Henkestr. 91, 91052 Erlangen

The linear rheology of adherent cells is characterized by a wide distribution of relaxation times, as seen by a creep or stress relaxation response that follows a weak power law over several time decades. However, stress relaxation of living cells in the non-linear range where stress stiffening occurs has been poorly characterized and are not well understood. We used a magnetic tweezer setup with real-time force control to apply forces of more than 20 nN to beads bound to the cytoskeleton of adherent cells. Deformations in response to stepwise increasing and repeated force application were analyzed using a nonlinear superposition model that allowed us to dissect stress relaxation processes from stiffening responses. Results show that the creep modulus becomes nonlinear and decreases with increasing force. In addition, stresses relaxed in most beads according to a power-law in time with a slope between 0.2 and 0.3 independent of the stress magnitude. Force-induced fluidization and yielding leads to an increase in the power-law exponent. This was indicative either of a disruption of the beads when the force was further increased, or of a substantial plastic deformation after the force was removed. We interpret our results in terms of a model where dynamic stability and turnover of molecular interactions carrying the mechanical stress are determined by an energy landscape with a wide distribution of energy well depths and associated trap stiffnesses.