Active restructuring of kinesin-driven cytoskeletal composites with hydrogel inclusions*

The cytoskeleton is a dynamic composite network - comprised of  protein filaments such as semiflexible actin and rigid microtubules, as well as motor proteins, including kinesin - that enables vital cellular processes such as mitosis, movement, and growth. The versatile restructurability of the cytoskeleton makes it an ideal platform for designing materials that combine programmability, flexibility, and resilience with applications from soft tissue repair to biodegradable plastics and rubbers. Here, we create actin-microtubule composites, driven by kinesin motors, and with structural and dynamical properties that are tuned by different types of crosslinkers. We find that composites reconfigure into diverse structures ranging from homogeneous clusters to heterogeneous threadlike networks, at rates that are programmed by crosslinker and kinesin concentrations. We simultaneously image actin, microtubules and kinesin motors in the composites and use particle image velocimetry and particle-tracking to characterize the non-equilibrium dynamics and structure. Moreover, we show that microscale hydrogel inclusions alter the structure, dynamics and resilience of the active composites, dependent on the size, stiffness and concentration of the inclusions. In turn, we show that we can tune the composites to generate compressive forces sufficient to flatten, shrink, and asymmetrically deform hydrogel inclusions.