Active matter extracts energy from its surroundings at the single particle level and transforms it into mechanical work. Examples include cytoskeleton biopolymers and bacterial suspensions. Here, we review experimental, theoretical and numerical studies of active nematics - a type of active system that is characterised by self-driven units with elongated shape. We focus primarily on microtubule–kinesin mixtures and the hydrodynamic theories that describe their properties. An important theme is active turbulence and the associated motile topological defects. We discuss ways in which active turbulence may be controlled, a pre-requisite to harvesting energy from active materials, and we consider the appearance, and possible implications, of active nematics and topological defects to cellular systems and biological processes.

Achiral diprotonated porphyrins, forming homoassociates in aqueous solution, lead to spontaneous chiral symmetry breaking. The unexpected result is that the chirality sign of these homoassociates can be selected by vortex motion during the aggregation process. This result is confirmed by means of circular dichroism spectra. These experimental findings are rationalized in terms of the asymmetric influence of macroscopic forces on bifurcation scenarios and by considering the specific binding characteristics of the porphyrin units to form the homoassociates.

Natural systems are undeniably subject to random fluctuations, arising from either environmental variability or thermal effects. The consideration of those fluctuations supposes to deal with noisy quantities whose variance might at times be a sizable fraction of their mean levels. It is known that, under these conditions, noisy fluctuations can interact with the system's nonlinearities to render counterintuitive behavior, in which an increase in the noise level produces a more regular behavior. In systems with spatial degrees of freedom, this regularity takes the form of spatiotemporal order. An overview is presented of the mechanisms through which noise induces, enhances, and sustains ordered behavior in passive and active nonlinear media, and different spatiotemporal phenomena are described resulting from these effects. The general theoretical framework used in the analysis of these effects is reviewed, encompassing the theory of stochastic partial differential equations and coupled sets of ordinary stochastic differential equations. Experimental observations of self-organized behavior arising out of noise are also described, and details on the numerical algorithms needed in the computer simulation of these problems are given.

Activity and autonomous motion are fundamental in living and engineering systems. This has stimulated the new field of "active matter" in recent years, which focuses on the physical aspects of propulsion mechanisms, and on motility-induced emergent collective behavior of a larger number of identical agents. The scale of agents ranges from nanomotors and microswimmers, to cells, fish, birds, and people. Inspired by biological microswimmers, various designs of autonomous synthetic nano-and micromachines have been proposed. Such machines provide the basis for multifunctional, highly responsive, intelligent (artificial) active materials, which exhibit emergent behavior and the ability to perform tasks in response to external stimuli. A major challenge for understanding and designing active matter is their inherent nonequilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Unraveling, predicting, and controlling the behavior of active matter is a truly interdisciplinary endeavor at the interface of biology, chemistry, ecology, engineering, mathematics, and physics.

Winfree turbulence of scroll waves is a special kind of spatiotemporal chaos that exists exclusively in three-dimensional excitable media and is currently considered one of the principal mechanisms of cardiac fibrillation. A chaotic wave pattern develops through the negative-tension instability of vortex filaments, which tend to spontaneously stretch, bend, loop, and produce an expanding tangle that fills up the volume. We demonstrate that such turbulence can readily be controlled by weak nonresonant modulation of the medium excitability. Depending on the forcing frequency, either suppression or induction of turbulence can be achieved.

We show that DNA-linked anisotropic doublets composed of paramagnetic colloidal particles can be endowed with controlled propulsion when floating above a flat plate and subjected to a magnetic field precessing around an axis parallel to the plate. The propulsion mechanism for this artificial swimmer does not involve deformations, and it makes use of the minimal two degrees of freedom needed to propel it at low Reynolds numbers. We combine experimental observations with a theoretical analysis that fully characterizes the propulsion velocity in terms of the strength and frequency of the actuating magnetic field.

ature uses self-assembly to create a widespread variety of complex structures with elaborate geometries and outstanding properties 1 such as hierarchical order, adaptability, selfhealing and bioactivity. Developing new bioinspired processes based on dynamic self-assembly could facilitate the fabrication of synthetic three-dimensional (3D) materials with enhanced complexity, dynamic properties and functionality 2 . Proteins are particularly attractive building blocks because of their versatility and biofunctionality 3 . Elastin-like polypeptides (ELPs) 4 are recombinant proteins that have generated great interest 5 as a result of their modular structure, bioactivity, ease of design and production, and the possibility to create robust and elastic materials 5,6 . ELPs allow for a tunable molecular design 7 and are based on the tropoelastin recurrent motif Val-Pro-Gly-X-Gly (VPGXG), in which X is any amino acid other than proline 7 . This repeating pentapeptide provides ELPs with a thermoresponsive behaviour. Below a critical transition temperature (T t ), the ELP molecule undergoes a reversible-phase transition wherein the protein is soluble in aqueous solution and becomes highly solvated, surrounded by clatharate-like water structures. Above the T t , the hydrophobic domains dehydrate and the protein chain hydrophobically collapses and aggregates to form a phaseseparated state 8 .The use of natural and synthetic proteins to create functional materials has been hindered by the difficulty in controlling their conformation and nanoscale assembly with the precision required to form macroscopic materials. This limitation has driven the development of simpler and more-predictable peptide-based materials 9,10 . Peptide amphiphiles (PAs), for example, are synthetic molecules that can self-assemble into nanofibres and create functional 3D hydrogels that emulate the fibrous architecture of the extracellular matrix (ECM) 11,12 . Nonetheless, most peptide and/or protein materials are formed through equilibrium-based self-assembly approaches that are capable of generating stable supramolecular structures, but with limited hierarchy and spatiotemporal control, which has hindered their functionality 2 .Novel approaches based on the dynamic self-assembly of inorganic building blocks [13][14][15] , actin self-organization 16 and the combination of top-down processes with peptide self-assembly have been reported recently 17 . In particular, Stupp and co-workers have described a self-assembling membrane system obtained through strong electrostatic interactions between PAs and oppositely charged polysaccharides 18 . However, the possibility to exploit the unique structural and functional properties of proteins to create dynamic hierarchical materials remains an elusive target. In this study, we attempt to overcome this hurdle by using self-assembling peptides to promote protein conformational changes and guide their assembly into complex, yet functional, materials. We report the discovery and development of a protein/peptide system t...

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...