Liquid flow and control without solid walls.

When miniaturizing fluidic circuitry, the solid walls of the fluid channels become increasingly important1 because they limit the flow rates achievable for a given pressure drop, and they are prone to fouling2. Approaches for reducing the wall interactions include hydrophobic coatings3, liquid-infused porous surfaces4-6, nanoparticle surfactant jamming7, changes to surface electronic structure8, electrowetting9,10, surface tension pinning11,12 and use of atomically flat channels13. A better solution may be to avoid the solid walls altogether. Droplet microfluidics and sheath flow achieve this but require continuous flow of the central liquid and the surrounding liquid1,14. Here we demonstrate an approach in which aqueous liquid channels are surrounded by an immiscible magnetic liquid, both of which are stabilized by a quadrupolar magnetic field. This creates self-healing, non-clogging, anti-fouling and near-frictionless liquid-in-liquid fluidic channels. Manipulation of the field provides flow control, such as valving, splitting, merging and pumping. The latter is achieved by moving permanent magnets that have no physical contact with the liquid channel. We show that this magnetostaltic pumping method can be used to transport whole human blood with very little damage due to shear forces. Haemolysis (rupture of blood cells) is reduced by an order of magnitude compared with traditional peristaltic pumping, in which blood is mechanically squeezed through a plastic tube. Our liquid-in-liquid approach provides new ways to transport delicate liquids, particularly when scaling channels down to the micrometre scale, with no need for high pressures, and could also be used for microfluidic circuitry.

Insights into Chemically Fueled Supramolecular Polymers.

Supramolecular polymerization can be controlled in space and time by chemical fuels. A nonassembled monomer is activated by the fuel and subsequently self-assembles into a polymer. Deactivation of the molecule either in solution or inside the polymer leads to disassembly. Whereas biology has already mastered this approach, fully artificial examples have only appeared in the past decade. Here, we map the available literature examples into four distinct regimes depending on their activation/deactivation rates and the equivalents of deactivating fuel. We present increasingly complex mathematical models, first considering only the chemical activation/deactivation rates (i.e., transient activation) and later including the full details of the isodesmic or cooperative supramolecular processes (i.e., transient self-assembly). We finish by showing that sustained oscillations are possible in chemically fueled cooperative supramolecular polymerization and provide mechanistic insights. We hope our models encourage the quantification of activation, deactivation, assembly, and disassembly kinetics in future studies.

Photoinitiated Transient Self-Assembly in a Catalytically Driven Chemical Reaction Cycle.

Chemically fueled chemical reaction networks (CRNs) are essential in controlling dissipative self-assembly. A key challenge in the field is to store chemical fuel-precursors or "pre-fuels" in the system that are converted into activating or deactivating fuels in a catalytically controlled CRN. In addition, real-time control over catalysis in a CRN by light is highly desirable, but so far not yet achieved. Here we show a catalytically driven CRN that is photoinitiated with 450 nm light, producing activated monomers that go on to perform transient self-assembly. Monomer activation proceeds via photoredox catalysis, converting the monomer alcohol groups into the corresponding aldehydes that self-assemble into large supramolecular fibers. Monomer deactivation is achieved by organometallic catalysis that relies on pre-fuel hydrolysis to release formate (i.e. the deactivating fuel). Additionally, irradiation with 305 nm light accelerates the release of formate by photo-uncaging the pre-fuel, leading to a factor of ca. 2 faster deactivation of the monomer. Overall, we show transient self-assembly upon visible light photoactivation, and tunable life-times by ultraviolet light.

Redox-Controlled Shunts in a Synthetic Chemical Reaction Cycle.

Shunts, alternative pathways in chemical reaction networks (CRNs), are ubiquitous in nature, enabling adaptability to external and internal stimuli. We introduce a CRN in which the recovery of Michael-accepting species is driven by oxidation chemistry. Using weak oxidants can enable access to two shunts within this CRN with different kinetics and a reduced number of side reactions compared to the main cycle that is driven by strong oxidants. Furthermore, we introduce a strategy to recycle one of the main products under flow conditions to partially reverse the CRN and control product speciation throughout time. These findings introduce new levels of control over artificial CRNs, driven by redox chemistry, narrowing the gap between synthetic and natural systems.

Efficient Photoredox Cycles to Control Perylenediimide Self-Assembly.

Photoreduction of perylenediimide (PDI) derivatives has been widely studied for use in photocatalysis, hydrogen evolution, photo-responsive gels, and organic semiconductors. Upon light irradiation, the radical anion (PDI⋅- ) can readily be obtained, whereas further reduction to the dianion (PDI2- ) is rare. Here we show that full 2-electron photoreduction can be achieved using UVC light: 1) in anaerobic conditions by 'direct photoreduction' of PDI aggregates, or 2) by 'indirect photoreduction' in aerobic conditions due to acetone ketyl radicals. The latter strategy is also efficient for other dyes, such as naphthalenediimide (NDI) and methylviologen (MV2+ ). Efficient photoreduction on the minute time-scale using simple LED light in aerobic conditions is attractive for use in dissipative light-driven systems and materials.

Chemically Fueled Autonomous Sol→Gel→Sol→Gel→Sol Transitions.

Complex non-equilibrium phase behaviors are a hallmark of natural self-assembling systems. Here we show how intricate phase transitions can be achieved through a chemically fueled reaction cycle to yield autonomous sol→gel→sol→gel→sol transitions. A relay of chemical transformations based on thiazinane metathesis leads to two consecutive transient gelations in a closed system. Within seconds of fuel addition to deactivated thiazinane monomers, an imine-based hydrogel forms that consists of fibrillar microspheres. This gel quickly loses its mechanical strength and forms a solution, from which a second aldehyde-based gel nucleates and remains stable for over one day. Overall, our reaction cycle gives rise to two consecutive re-entrant phase transitions without any experimental intervention.

Chemically Fueled Self-Sorted Hydrogels.

Narcissistic self-sorting in supramolecular assemblies can help to construct materials with more complex hierarchies. Whereas controlled changes in pH or temperature have been used to this extent for two-component self-sorted gels, here we show that a chemically fueled approach can provide three-component materials with high precision. The latter materials have interesting mechanical properties, such as enhanced or suppressed stiffness, and intricate multistep gelation kinetics. In addition, we show that we can achieve supramolecular templating, where pre-existing supramolecular fibers first act as templates for growth of a second gelator, after which they can selectively be removed.

Fluid Drag Reduction by Magnetic Confinement.

The frictional forces of a viscous liquid flow are a major energy loss issue and severely limit microfluidics practical use. Reducing this drag by more than a few tens of percent remain elusive. Here, we show how cylindrical liquid-in-liquid flow leads to drag reduction of 60-99% for sub-mm and mm-sized channels, regardless of whether the viscosity of the transported liquid is larger or smaller than that of the confining one. In contrast to lubrication or sheath flow, we do not require a continuous flow of the confining lubricant, here made of a ferrofluid held in place by magnetic forces. In a laminar flow model with appropriate boundary conditions, we introduce a modified Reynolds number with a scaling that depends on geometrical factors and viscosity ratio of the two liquids. It explains our whole range of data and reveals the key design parameters for optimizing the drag reduction values. Our approach promises a new route for microfluidics designs with pressure gradient reduced by orders of magnitude.

Magnetic Control over the Fractal Dimension of Supramolecular Rod Networks.

Controlling supramolecular polymerization is of fundamental importance to create advanced materials and devices. Here we show that the thermodynamic equilibrium of Gd3+-bearing supramolecular rod networks is shifted reversibly at room temperature in a static magnetic field of up to 2 T. Our approach opens opportunities to control the structure formation of other supramolecular or coordination polymers that contain paramagnetic ions.

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents.

Spin orbit torque driven switching is a favorable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here, we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in situ characterization using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, with multilevel switching demonstrated. The result is that spin-switching current densities, down to 800 kA cm-2, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements but also neuromorphic and probabilistic computing.

Re-programming Hydrogel Properties Using a Fuel-Driven Reaction Cycle.

Nature uses catalysis as an indispensable tool to control assembly and reaction cycles in vital non-equilibrium supramolecular processes. For instance, enzymatic methionine oxidation regulates actin (dis-)assembly, and catalytic guanosine triphosphate hydrolysis is found in tubulin (dis-)assembly. Here we present a completely artificial reaction cycle which is driven by a chemical fuel that is catalytically obtained from a "pre-fuel". The reaction cycle controls the dis-assembly and re-assembly of a hydrogel, where the rate of pre-fuel turnover dictates the morphology as well as the mechanical properties. By addition of additional fresh aliquots of fuel and removal of waste, the hydrogels can be re-programmed time after time. Overall, we show how catalytic fuel generation can control reaction/assembly kinetics and materials' properties in life-like non-equilibrium systems.

Mechanical chiral resolution.

Mechanical interactions of chiral objects with their environment are well-established at the macroscale, like a propeller on a plane or a rudder on a boat. At the colloidal scale and smaller, however, such interactions are often not considered or deemed irrelevant due to Brownian motion. As we will show in this tutorial review, mechanical interactions do have significant effects on chiral objects at all scales, and can be induced using shearing surfaces, collisions with walls or repetitive microstructures, fluid flows, or by applying electrical or optical forces. Achieving chiral resolution by mechanical means is very promising in the field of soft matter and to industry, but has not received much attention so far.

Systems chemistry.

Building on our ability to design and synthesise molecules and our understanding of the noncovalent interactions between these molecules, the chemical sciences are currently entering the new territory of Systems Chemistry. This young field aims to develop complex molecular systems showing emergent properties; i.e. properties that go beyond the sum of the characteristics of the individual consituents of the system. This review gives an impression of the state of the art of the field by showing a diverse number of recent highlights, including out-of-equilibrium self-assembly, chemically fuelled molecular motion, compartmentalised chemical networks and designed oscillators. Subsequently a number of current challenges related to the design of complex chemical systems are discussed, including those of creating concurrent formation-destruction systems, continuously maintaining chemical systems away from equilibrium, incorporating feedback loops and pushing replication chemistry away from equilibrium. Finally, the prospects for Systems Chemistry are discussed including the tantalizing vision of the de novo synthesis of life and the idea of self-synthesising and self-repairing chemical factories.

Non-equilibrium supramolecular polymerization.

Supramolecular polymerization has been traditionally focused on the thermodynamic equilibrium state, where one-dimensional assemblies reside at the global minimum of the Gibbs free energy. The pathway and rate to reach the equilibrium state are irrelevant, and the resulting assemblies remain unchanged over time. In the past decade, the focus has shifted to kinetically trapped (non-dissipative non-equilibrium) structures that heavily depend on the method of preparation (i.e., pathway complexity), and where the assembly rates are of key importance. Kinetic models have greatly improved our understanding of competing pathways, and shown how to steer supramolecular polymerization in the desired direction (i.e., pathway selection). The most recent innovation in the field relies on energy or mass input that is dissipated to keep the system away from the thermodynamic equilibrium (or from other non-dissipative states). This tutorial review aims to provide the reader with a set of tools to identify different types of self-assembled states that have been explored so far. In particular, we aim to clarify the often unclear use of the term "non-equilibrium self-assembly" by subdividing systems into dissipative, and non-dissipative non-equilibrium states. Examples are given for each of the states, with a focus on non-dissipative non-equilibrium states found in one-dimensional supramolecular polymerization.

Surface-Assisted Self-Assembly of a Hydrogel by Proton Diffusion.

Controlling supramolecular growth at solid surfaces is of great importance to expand the scope of supramolecular materials. A dendritic benzene-1,3,5-tricarboxamide peptide conjugate is described in which assembly can be triggered by a pH jump. Stopped-flow kinetics and mathematical modeling provide a quantitative understanding of the nucleation, elongation, and fragmentation behavior in solution. To assemble the molecule at a solid-liquid interface, we use proton diffusion from the bulk. The latter needs to be slower than the lag phase of nucleation to progressively grow a hydrogel outwards from the surface. Our method of surface-assisted self-assembly is generally applicable to other gelators, and can be used to create structured supramolecular materials.

Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles.

Although multiple methods have been developed to detect metal cations, only a few offer sensitivities below 1 pM, and many require complicated procedures and sophisticated equipment. Here, we describe a class of simple solid-state sensors for the ultrasensitive detection of heavy-metal cations (notably, an unprecedented attomolar limit for the detection of CH(3)Hg(+) in both standardized solutions and environmental samples) through changes in the tunnelling current across films of nanoparticles (NPs) protected with striped monolayers of organic ligands. The sensors are also highly selective because of the ligand-shell organization of the NPs. On binding of metal cations, the electronic structure of the molecular bridges between proximal NPs changes, the tunnelling current increases and highly conductive paths ultimately percolate the entire film. The nanoscale heterogeneity of the structure of the film broadens the range of the cation-binding constants, which leads to wide sensitivity ranges (remarkably, over 18 orders of magnitude in CH(3)Hg(+) concentration).

Transport into metal-organic frameworks from solution is not purely diffusive.

Chemistry in motion: a combination of confocal microscopy and reaction-diffusion modeling provided a powerful toolkit with which solution transport into metal-organic framework crystals was studied. Commonly used pure diffusion models are insufficient to describe this process and, instead, it is necessary to account for the interactions of the guest molecules and the MOF scaffold.

Controlled supramolecular oligomerization of C3-symmetrical molecules in water: the impact of hydrophobic shielding.

The supramolecular oligomerization of three water-soluble C(3)-symmetrical discotic molecules is reported. The compounds all possess benzene-1,3,5-tricarboxamide cores and peripheral Gd(III)-DTPA (diethylene triamine pentaacetic acid) moieties, but differ in their linker units and thus in their propensity to undergo secondary interactions in H(2)O. The self-assembly behavior of these molecules was studied in solution using circular dichroism, UV/Vis spectroscopy, nuclear magnetic resonance, and cryogenic transmission electron microscopy. The aggregation concentration of these molecules depends on the number of secondary interactions and on the solvophobic character of the polymerizing moieties. Hydrophobic shielding of the hydrogen-bonding motif in the core of the discotic is of paramount importance for yielding stable, helical aggregates that are designed to be restricted in size through anti-cooperative, electrostatic, repulsive interactions.

Multivalent supramolecular dendrimer-based drugs.

Supramolecular complexes consisting of a hydrophobic dendrimer host [DAB-dendr-(NHCONH-Ad)(64)] as well as solubilizing and bioactive guest molecules have been synthesized using a noncovalent approach. The guest-host supramolecular assembly is first preassembled in chloroform and transferred via the neat phase to aqueous solution. The bioactive guest molecules can bind to a natural (serotonin 5-HT(3)) receptor with nanomolar affinity as well as to the synthetic dendrimer receptor in aqueous solution, going toward a dynamic multivalent supramolecular construct capable of adapting itself to a multimeric receptor motif.

Devising Synthetic Reaction Cycles for Dissipative Nonequilibrium Self-Assembly.

Fuel-driven reaction cycles are found in biological systems to control the assembly and disassembly of supramolecular materials such as the cytoskeleton. Fuel molecules can bind noncovalently to a self-assembling building block or they can react with it, resulting in covalent modifications. Overall the fuel can either switch the self-assembly process on or off. Here, a closer look is taken at artificial systems that mimic biological systems by making and breaking covalent bonds in a self-assembling motif. The different chemistries used so far are highlighted in chronological order and the pros and cons of each system are discussed. Moreover, the desired traits of future reaction cycles, their fuels, and waste management are outlined, and two chemistries that have not been explored up to now in chemically fueled dissipative self-assembly are suggested.