Living cells sense the mechanical features of their environment and adapt to it by actively remodeling their peripheral network of filamentary proteins, known as cortical cytoskeleton. By mimicking this principle, we demonstrate an effective control strategy for a microtubule-based active nematic in contact with a hydrophobic thermotropic liquid crystal. By using well-established protocols for the orientation of liquid crystals with a uniform magnetic field, and through the mediation of anisotropic shear stresses, the active nematic reversibly self-assembles with aligned flows and textures that feature orientational order at the millimeter scale. The turbulent flow, characteristic of active nematics, is in this way regularized into a laminar flow with periodic velocity oscillations. Once patterned, the microtubule assembly reveals its intrinsic length and time scales, which we correlate with the activity of motor proteins, as predicted by existing theories of active nematics. The demonstrated commanding strategy should be compatible with other viable active biomaterials at interfaces, and we envision its use to probe the mechanics of the intracellular matrix.iquid crystals are viscous fluids that self-assemble into equilibrium molecular arrangements featuring anisotropic physical properties that can be easily tailored by suitable boundary conditions, and reversibly rearranged by using modest electric or magnetic fields (1). These soft matter mesophases are not exclusive of artificial materials, as they are ubiquitous in lipid solutions (2) and concentrated DNA fragments (3), and have been recently obtained by in vitro cytoskeletal reconstitutions based on aqueous suspensions of filamentous proteins cross-linked by compatible molecular motors (4-6). The latter type of materials is referred to as active liquid crystals because, unlike their passive counterparts, they exhibit out-of-equilibrium behavior with supramolecular orientational order that is dynamically self-assembled at the continuous expense of hydrolysable adenosine triphosphate (ATP). Experiments with active soft matter (7-17) reveal self-organizing features that are not present in passive materials. Despite the vast richness of behavior endowed by activity, traditional liquid crystals have a dramatic advantage: their orientation can be easily controlled to switch among different predesigned configurations, which is crucial for the operation of devices, and for fundamental research in partially ordered materials. Contrarily, experiments on active nematics have relied on establishing their composition, confinement geometry, or activity as design parameters, but they lack true control capabilities of the resulting dynamic self-assembly. This limits their potential to serve as in vitro model systems of the intracellular matrix or for the development of new functional biomaterials. Here, by interfacing an active nematic film with a hydrophobic oil that features smectic (lamellar) liquid-crystalline order (18), we reversibly align the originally turbulent f...

Active matter embraces systems that self-organize at different length and time scales, often exhibiting turbulent flows apparently deprived of spatiotemporal coherence. Here, we use a layer of a tubulin-based active gel to demonstrate that the geometry of active flows is determined by a single length scale, which we reveal in the exponential distribution of vortex sizes of active turbulence. Our experiments demonstrate that the same length scale reemerges as a cutoff for a scale-free power law distribution of swirling laminar flows when the material evolves in contact with a lattice of circular domains. The observed prevalence of this active length scale can be understood by considering the role of the topological defects that form during the spontaneous folding of microtubule bundles. These results demonstrate an unexpected strategy for active systems to adapt to external stimuli, and provide with a handle to probe the existence of intrinsic length and time scales.

In materials, long-range order, robust shapes and patterns can emerge from self-organization phenomena driven by interactions between particles, molecules and atoms, rather than being imposed by external constrains. In active materials, additional energy-consuming processes generate forces that can self-organize to give rise to motion and shape. In tissues, the emergence of shapes arises from the coordination of cellular forces and is referred to as morphogenesis. By analogy, morphogenesis is thus thought to be driven by cellular self-organization. However, because development is under the tight control of genetics, discovering the nature of self-organization mechanisms involved in morphogenesis remains challenging, focusing the efforts of many creative studies 1 .Akin to elongated molecules in liquid crystals 2 , elongated cells can self-organize into patterns featuring long-range orientational order [3][4][5][6] . Orientational fields may present topological defects, regions where the orientational order is ill-defined. Still, defects indicate very specific orientational configurations around their cores 2 . In active systems-driven by internal energy-consuming processestopological defects entail characteristic flow and stress patterns that depend on the defects' topological strength s, which indicates the rotation of the orientational field along a path encircling the defect's core 2 . In particular, active half-integer defects (s = ±1/2) have been thoroughly studied [7][8][9][10][11][12][13][14] . In cell monolayers, their position correlates with cell extrusion 12 or changes in cell density 13 .Integer topological defects in cell monolayers, such as vortices, spirals or asters (s = +1), remain less characterized. Yet, they abound in nature, at cellular, tissue and organismal scales 15 . Astral and spiral cellular patterns have been identified in fibroma 16 , brain tumour 17 and corneal epithelia 18 , where they lead to abnormal cell aggregation in the defect core. Because of their symmetry, integer defects may play essential roles in organizing tissue architecture by stabilizing mechanical patterns. Indeed, integer defects colocalize with the mouth, tentacles and foot of hydra during its development 19 . In vitro, cellular integer defects have been generated by imposing their orientational patterns through cell substrate microstructuration 20,21 .Still, their dynamics and mechanics remain unknown, as well as the mechanisms by which active integer topological defects could directly contribute to remodelling tissues.