Active assembly in dense suspensions of polar biofilaments

Reconstituted cytoskeletal suspensions serve as quintessential model systems in the research of macroscopic non-equilibrium phenomena. The interplay between active agents found in such systems can produce novel materials though extensile flows or contractile densification. We introduce a system of microtubules and tip-accumulating kinesin-4 motors that evolves through consecutive stages of macroscopic contraction and expansion. Motors generate inter-filament sliding through the system’s entirety, driving large length-scale instabilities in both density and polar filament order. The coordinated interplay of these processes prompts the formation of highly organized structures, analogous to amphiphilic vesicles, sheets, and bilayers. We note striking similarities of this system’s formative dynamics to liquid-liquid demixing, enriched by the non-equilibrium nature of this material’s activity.