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Biophys J. 2014 Feb 4;106(3):526-34. doi: 10.1016/j.bpj.2013.12.031.

Determinants of fluidlike behavior and effective viscosity in cross-linked actin networks.

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Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Indiana.
Institute for Biophysical Dynamics, James Franck Institute, and Department of Physics, University of Chicago, Chicago, Illinois. Electronic address:
Institute for Biophysical Dynamics, Computation Institute, and Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois. Electronic address:


The actin cortex has a well-documented ability to rapidly remodel and flow while maintaining long-range connectivity, but how this is achieved remains poorly understood. Here, we use computer simulations to explore how stress relaxation in cross-linked actin networks subjected to extensional stress depends on the interplay between network architecture and turnover. We characterize a regime in which a network response is nonaffine and stress relaxation is governed by the continuous dissipation of elastic energy via cyclic formation, elongation, and turnover of tension-bearing elements. Within this regime, for a wide range of network parameters, we observe a constant deformation (creep) rate that is linearly proportional to the rate of filament turnover, leading to a constant effective viscosity that is inversely proportional to turnover rate. Significantly, we observe a biphasic dependence of the creep rate on applied stress: below a critical stress threshold, the creep rate increases linearly with applied stress; above that threshold, the creep rate becomes independent of applied stress. We show that this biphasic stress dependence can be understood in terms of the nonlinear force-extension behavior of individual force-transmitting network elements. These results have important implications for understanding the origins and control of viscous flows both in the cortex of living cells and in other polymer networks.

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