Many different types of biological cells have the capability of sensing and generating mechanical forces. These biophysical properties of cells are utilized for many different aspects of cell physiology, including cell migration and division as well as building multi-cellular tissue. To a large degree, the active mechanical behavior of cells is regulated by the filamentous actin (F-actin) cytoskeleton. F-actin is a semi-flexible biopolymer that forms the basis of larger length scale structures in the cell through the action of other proteins that regulate assembly, cross-linking and force generation. Nearly all of these processes are driven far from thermal equilibrium by processes that rely on the consumption of chemical energy to regulate the spatial and temporal organization of network mechanics and force generation. To elucidate the physical properties of the actin cytoskeleton, we have studied the dynamics and biophysical properties of actin networks formed with myosin motors both in live cells and reconstituted networks of purified proteins. In these systems, we observe that the stability and mechanics of actomyosin networks is highly tuned based on the amount of tension. These behaviors enable rapid remodeling under low tension, but stabilize the structures as forces are increased. Thus, cellular materials provide insight into design principles that are utilized by highly adaptive matter.

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