Actively stressed marginal networks
Chase Broedersz, Princeton University
The mechanical properties of cells are regulated in part by internal stresses generated actively by molecular motors in the cytoskeleton. Experiments on reconstituted intracellular F-actin networks with myosin motors show that such active contractility dramatically affects network elasticity. There is also experimental evidence that cytoskeletal networks in living cells may be unstable or only marginally stable in the absence of motor activity. We study the impact of active stresses on the mechanics of disordered, marginally stable networks using a simple model for networks of fibers with linear bending and stretching elasticity. Motor activity controls the elasticity in an anomalous fashion close to the point of marginal stability by coupling to critical network fluctuations. In addition, such motor stresses can stabilize initially floppy networks, extending the range of critical behavior to a broad regime of network connectivities below the marginal point. Away from this regime, or at high stress, motors give rise to a linear increase in stiffness with stress. The remarkable mechanical response of these actively stressed networks is captured by a simple, constitutive scaling relation, which highlights the important role of nonaffine strain fluctuations as a susceptibility to motor stress.