With exquisite precision and reproducibility, cells orchestrate the cooperative action of thousands of nanometer-sized molecular motors to carry out mechanical tasks at much larger length scales, such as cell motility, division and replication1. Besides their biological importance, such inherently non-equilibrium processes are an inspiration for developing biomimetic active materials from microscopic components that consume energy to generate continuous motion2–4. Being actively driven, these materials are not constrained by the laws of equilibrium statistical mechanics and can thus exhibit highly sought-after properties such as autonomous motility, internally generated flows and self-organized beating5–7. Starting from extensile microtubule bundles, we hierarchically assemble active analogs of conventional polymer gels, liquid crystals and emulsions. At high enough concentration, microtubules form a percolating active network characterized by internally driven chaotic flows, hydrodynamic instabilities, enhanced transport and fluid mixing. When confined to emulsion droplets, 3D networks spontaneously adsorb onto the droplet surfaces to produce highly active 2D nematic liquid crystals whose streaming flows are controlled by internally generated fractures and self-healing, as well as unbinding and annihilation of oppositely charged disclination defects. The resulting active emulsions exhibit unexpected properties, such as autonomous motility, which are not observed in their passive analogues. Taken together, these observations exemplify how assemblages of animate microscopic objects exhibit collective biomimetic properties that are starkly different from those found in materials assembled from inanimate building blocks, challenging us to develop a theoretical framework that would allow for a systematic engineering of their far-from-equilibrium material properties.

Transport of fluid through a pipe is essential for the operation of macroscale machines and microfluidic devices. Conventional fluids only flow in response to external pressure. We demonstrate that an active isotropic fluid, comprised of microtubules and molecular motors, autonomously flows through meter-long three-dimensional channels. We establish control over the magnitude, velocity profile and direction of the self-organized flows, and correlate these to the structure of the extensile microtubule bundles. The inherently three-dimensional transition from bulk-turbulent to confined-coherent flows occurs concomitantly with a transition in the bundle orientational order near the surface, and is controlled by a scale-invariant criterion related to the channel profile. The non-equilibrium transition of confined isotropic active fluids can be used to engineer self-organized soft machines.One Sentence Summary: An isotropic fluid composed of nano-sized motors organizes into an autonomous machine that pumps fluid through long channels. Recent studies have revealed emergence of diverse complex patterns in synthetic systems of active matter (21-24). The next step is to elucidate conditions that transform chaotic dynamics of these systems into coherent long-ranged motion that can be used to harvest energy and thus power various micromachines (25-29).Here, we study 3D active fluids and demonstrate an essential difference with their conventional counterparts. The Navier-Stokes equations dictate that a conventional fluid comprised of inanimate constituents will flow only in response to an externally imposed body force, or stress and pressure gradients (29). This is no longer true for active fluids. Indeed, in living organisms, the entire cellular interior can assume large-scale coherent flows in absence of any externally imposed stresses, a phenomenon known as cytoplasmic streaming (30-32). Despite recent advances using living bacterial suspensions (13,14,33,34), creating tunable synthetic active 3 fluids that exhibit autonomous long-ranged flows on length scales large compared to constituent units remains a challenge. We use a 3D microtubule-based isotropic active fluid whose bulk turbulent flows are driven by continuous injection of energy through the linear motion of the constituent kinesin motors (24, 35). We find that confinement robustly transforms locallyturbulent dynamics of such active fluids into globally-coherent flows that persist on meter scales.Our experiments demonstrate that non-equilibrium transitions of synthetic active materials can be used to engineer self-organized machines in which nanometer sized molecular motors collectively propel fluid on macroscopic scales. Microtubule-based active isotropic fluids:The active fluid we study is comprised of microtubule filaments, kinesin motor clusters and depleting polymer (Fig. 1A) (24, 35). Kinesin motors are bound into synthetic clusters with tetrameric streptavidin (36, 37). The depleting polymer induces microtubule bundling (38), while the kinesin c...

The motility of cilia and flagella is driven by thousands of dynein motors that hydrolyze adenosine triphosphate (ATP). Despite decades of genetic, biochemical, structural, and biophysical studies, some aspects of ciliary motility remain elusive, such as the regulation of beating patterns and the energetic efficiency of these nanomachines. In this study, we introduce an experimental method to measure ATP consumption of actively beating axonemes on a single-cell level. We encapsulated individual sea urchin sperm with demembranated flagellum inside water-in-oil emulsion droplets and measured the axoneme's ATP consumption by monitoring fluorescence intensity of a fluorophore-coupled reporter system for ATP turnover in the droplet. Concomitant phase contrast imaging allowed us to extract a linear dependence between the ATP consumption rate and the flagellar beating frequency, with ∼2.3 × 10(5) ATP molecules consumed per beat of a demembranated flagellum. Increasing the viscosity of the aqueous medium led to modified beating waveforms of the axonemes and to higher energy consumption per beat cycle. Our single-cell experimental platform provides both new insights, to our knowledge, into the beating mechanism of flagella and a powerful tool for future studies.

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