(45d) Cell Quakes: Mechanics and Microrheology in Active Gels and Living Cells

The mechanics of the in vivo cytoskeleton is controlled in part

by the details of its non-equilibrium steady-state. In this ``active'' material,

molecular motors (e.g. myosin) exert transient contractile stresses

on the F-actin filament network. Since microrheology traditionally

relies of the linear response properties of the soft materials in

thermal equilibrium, this departure from equilibrium has profound

implications for the interpretation of microrheological data

from the interior of living cells and in vitro active networks.

In active networks, such as the in vitro systems of Mizuno et al.

[Science 315 (5810) pp. 370-373 (2007).] and in living cells,

the underlying theoretical foundation of the interpretation

of microrheology -- the Fluctuation-Dissipation theorem -- does

not apply. New ideas are needed. In this talk, I review microrheology, and then

discuss a new theoretical interpretation of microrheology

in active (i.e. molecular motor driven) networks. I also

explore how molecular motor activity can reversibly

control the elastic properties of these active gels.

The cytoskeleton points towards the development of

new biomimetic materials whose elastic properties

can be tuned by controlling the material's

non-equilibrium steady-state.