Active viscoelastic composites generate self-regulating asters with programmable lifetimes*

The out-of-equilibrium active reorganization of cytoskeletal networks by molecular motors is necessary for fundamental life processes, such as cell division, cell motility, and environmental sensing. While the passive structure and mechanics of such materials have been well documented, the effects of their steady-state out-of-equilibrium reorganization is a site of current research. In this work, we introduce an active cytoskeletal composite material whose viscoelasticity is controlled by the actin filament concentration. Three qualitatively different states are observed: (1) an extensile fluid phase, (2) localized aster-like contractile structures in coexistence with an extensile fluid, and (3) a bulk contractile gel. The aster-like state consists of locally contracted heterogeneous structures that maintain their complex layered structure over a range of sizes. While the actin concentration triggers a contractile state in coexistence with the active fluid, the resultant filament-rich structures are transient and their lifetimes increase with actin concentration.  These results demonstrate that self-organized dynamical states and patterns, evocative of those observed in the cytoskeleton, do not require precise biochemical regulation but can arise due to purely mechanical interactions of actively driven filamentous materials