Mechanics and Stress Propagation in Non-Equilibrium Cytoskeleton Composites

The cytoskeleton is able to access a wide range of material properties to facilitate cellular processes in a controlled and precise manner via active restructuring and reconfiguration of its constituents by ATP-consuming motor proteins. In vitro cytoskeleton composites have been shown to exhibit tunable mechanical properties, and the addition of force-generating motors to these composites induces non-equilibrium dynamics and restructuring. We characterize the effects of non-equilibrium activity on the mechanical properties of actin-microtubule composites driven by variable concentrations of kinesin motor clusters by performing optical tweezers microrheology experiments. We measure the nonlinear force response of the composites subject to cyclical strain at varied strain rates. Additionally, we measure stress propagation through the composites due to locally imposed strain using differential dynamic microscopy (DDM). We uncover a transition from dissipative to stiffening force response of the composites at mesoscopic lengthscales, with a non-monotonic dependence on kinesin concentration. Our work sheds light on how active restructuring by motor proteins tunes the material response of the cytoskeleton to local stresses, enabling diverse processes such as shape change, motility, and repair.