Creep function of a single living cell

We used a novel uniaxial stretching rheometer to measure the creep function J(t) of an isolated living cell. We show, for the first time at the scale of the whole cell, that J( t) behaves as a power-law J( t) = At-a. For N = 43 mice myoblasts (C2-7), we find alpha = 0.24 +/- 0.01 and A = ( 2.4 +/- 0.3) 10(-3) Pa-1 s(-alpha). Using Laplace Transforms, we compare A and a to the parameters G(0) and beta of the complex modulus G*(omega) = G(0)omega(beta) measured by other authors using magnetic twisting cytometry and atomic force microscopy. Excellent agreement between A and G(0) on the one hand, and between alpha and beta on the other hand, indicated that the power-law is an intrinsic feature of cell mechanics and not the signature of a particular technique. Moreover, the agreement between measurements at very different size scales, going from a few tens of nanometers to the scale of the whole cell, suggests that self-similarity could be a central feature of cell mechanical structure. Finally, we show that the power-law behavior could explain previous results first interpreted as instantaneous elasticity. Thus, we think that the living cell must definitely be thought of as a material with a large and continuous distribution of relaxation time constants which cannot be described by models with a finite number of springs and dash-pots.