The cytoskeleton is an active composite of filamentous proteins that dictates diverse mechanical properties and processes in cells. By combining optical tweezers microrheology, fluorescence microscopy, simulations and modeling, the mechanics of actin and microtubule networks restructured was revealed by kinesin motors.

Rich force response behaviors–elastic, yielding, and stiffening–with properties tuned by motor concentration and strain rate were discovered. Moreover, intermediate kinesin concentrations elicit emergent mechanical stiffness and resistance while higher and lower concentrations exhibit softer, more viscous dissipation.

It was found that composites transition from well-mixed interpenetrating networks to de-mixed states of microtubule-rich aggregates surrounded by relatively undisturbed actin phases. It is this de-mixing that leads to the emergent mechanical response.  These results and models shed important light on how to engineer and tune composite systems to exhibit emergent mechanics.