Structure, dynamics, and mechanics of fiber networks assembled by an active fluid

In living systems, active forces interact with viscoelastic media to robustly generate complex structure and function. Inspired by such processes, we construct an in vitro system in which an active fluid assembles and actuates a soft elastic sheet, generating an interplay of structure, dynamics, and mechanics across multiple length and time scales. We prepare a spatially homogeneous suspension of initially inactive kinesin motors, microtubule-PRC1 bundles, and actin-fascin bundles. Upon activation, kinesin motors slide along microtubules, generating chaotic extensile flows that advect the actin bundles, causing them to gradually coarsen and percolate into an elastic network. The activity level of the microtubule fluid controls the timescale, pathway, and degree of structural reorganization of the actin network. Together with microrheology measurements, these observations suggest that the active fluid simultaneously restructures and stiffens the actin network until the network starts to push back on the active stresses. Once formed, the network contracts into a sheet suspended at the sample midplane. While the active fluid exhibits a correlation length of ~100 µm, it drives elastic deformations of the actin sheet that, under some conditions, spontaneously synchronize into shear oscillations that span the centimeter-scale system size. These experiments demonstrate that active matter can both assemble and drive multiscale mechanical structures, and are an initial step towards rationally designing materials that mimic complex biological processes.