Power laws in microrheology experiments on living cells:: Comparative analysis and modeling

We compare and synthesize the results of two microrheological experiments on the cytoskeleton of single cells. In the first one, the creep function J(t) of a cell stretched between two glass plates is measured after applying a constant force step. In the second one, a microbead specifically bound to transmembrane receptors is driven by an oscillating optical trap, and the viscoelastic coefficient G(e)(omega) is retrieved. Both J(t) and G(e)(omega) exhibit power law behaviors: J(t)=A(0)(t/t(0))(alpha) and parallel to G(e)(omega)parallel to=G(0)(omega/omega(0))(alpha), with the same exponent alpha approximate to 0.2. This power law behavior is very robust; alpha is distributed over a narrow range, and shows almost no dependence on the cell type, on the nature of the protein complex which transmits the mechanical stress, nor on the typical length scale of the experiment. On the contrary, the prefactors A(0) and G(0) appear very sensitive to these parameters. Whereas the exponents alpha are normally distributed over the cell population, the prefactors A(0) and G(0) follow a log-normal repartition. These results are compared with other data published in the literature. We propose a global interpretation, based on a semiphenomenological model, which involves a broad distribution of relaxation times in the system. The model predicts the power law behavior and the statistical repartition of the mechanical parameters, as experimentally observed for the cells. Moreover, it leads to an estimate of the largest response time in the cytoskeletal network: tau(m)similar to 1000 s.