Nonlinear Mechanics and Stress Propagation in Out-of-Equilibrium Cytoskeleton Composites

The cytoskeleton is a system of interest to materials researchers because it is able to tune its material properties in response to its environment in a controlled way in part due to active reconfiguration of its constituents by ATP-consuming motor proteins. While methods for generating in vitro motor-driven composites of cytoskeleton polymers and measuring their dynamics have been established, advances in understanding of how motor activity affects mechanical properties of in vitro composites have been limited due to violation of the steady-state requirement of the generalized Stokes-Einstein relationship. We circumvent GSER by performing optical tweezers microrheology experiments on a set of kinesin-driven actin-microtubule composites carefully tuned to exhibit quasi-steady-state dynamics on the timescale of our measurements. We measure the nonlinear force response of active composites subject to cyclic straining to characterize how motor-driven restructuring alters the composite elasticity, stiffness, and length scale-dependent mechanics of the composites. We uncover a strain-rate dependent transition from dissipative to stiffening force response at mesoscopic lengthscales, and a non-monotonic dependence of stiffness and stress propagation on kinesin concentration.