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Hypothesis\ud \ud Microscopic self-propelled motors (SPMs) are an area of active research, but very little investigation has been conducted on millimetre-scale or macroscopic SPMs and exploring their potential in biomedical research. In this study, we tested if 3D reactive inkjet (RIJ) printing could be used for precise fabrication of millimetre-scale self-propelled motors (SPMs) with well-defined shapes from regenerated silk fibroin (RSF) by converting water soluble RSF (silk I) to insoluble silk fibroin (silk II). Secondly, we compared the different propulsion behaviour of the SPMs to put forward the best geometry and propulsion mechanism for potential applications in enhancing the sensitivity of diffusion-rate limited biomedical assays by inducing fluid flow.\ud \ud \ud \ud Experiments\ud \ud SPMs with four different geometric shapes and propelled by two different mechanisms (catalysis and surface tension gradient) were fabricated by 3D RIJ printing and compared. For bubble propulsion, the structures were selectively doped in specific regions with the enzyme catalase in order to produce motion via bubble generation and detachment in hydrogen peroxide solutions. For surface tension propulsion, PEG400-doped structures were propelled through surface tension gradients caused by leaching of PEG400 surfactant in deionized water.\ud \ud \ud \ud Findings\ud \ud The results demonstrated the ability of 3D inkjet printing to fabricate SPMs with desired propulsion mechanism and fine-tune the propulsion by precisely fabricating the different geometric shapes. The resulting 3D structures were capable of generating motion without external actuation, thereby enabling applications in biomedicine such as micro-stirring small fluid volumes to enhance biological assay sensitivity. The surface tension gradient caused by the leaching of surfactant led to faster propulsion velocities with smooth deceleration, whereas, in comparison, catalysis-induced bubble propulsion tended to be jerky and uneven in deceleration, and therefore less suitable for aforementioned applications. Computational fluid dynamic simulations were used to compare the various experimental SPMs ability to enhance mixing when deployed within 96-well plate microwells, to reveal the effect of both SPM shape and motion character on performance, and show viability for small scale mixing applications.

Understanding the kinetics of the crystallization process for organometal halide perovskite formation is critical in determining the crystalline, nanoscale morphology and therefore the electronic properties of the films produced during thin film formation from solution. In this work, in situ grazing incidence small-angle X-ray scattering (GISAXS) and optical microscopy measurements are used to investigate the processes of nucleation and growth of pristine mixed halide perovskite (MAPbI3-x Cl x ) crystalline films deposited by bar coating at 60 °C, with and without additives in the solution. A small amount of 1,8-diiodooctane (DIO) and hydriodic acid (HI) added to MAPbI3-x Cl x is shown to increase the numbers of nucleation centers promoting heterogeneous nucleation and accelerate and modify the size of nuclei during nucleation and growth. A generalized formation mechanism is derived from the overlapping parameters obtained from real-time GISAXS and optical microscopy, which revealed that during nucleation, perovskite precursors cluster before becoming the nuclei that function as elemental units for subsequent formation of perovskite crystals. Additive-free MAPbI3-x Cl x follows reaction-controlled growth, in contrast with when DIO and HI are present, and it is highly possible that the growth then follows a hindered diffusion-controlled mechanism. These results provide important details of the crystallization mechanisms occurring and will help to develop greater control over perovskite films produced.

The ability to generate rotation in ellipsoidal catalytic micro swimming devices by catalyst deposition at glancing angles allows reliable access to circling trajectories. This behaviour could enable propulsive ellipsoids to gather cargo and enhance mixing at small scales. Catalytically propelled colloidal rotation has been previously achieved in spherical swimming devices by means of neighbour shadowing during catalyst deposition leading to non-symmetrical coatings. However in this work shadowing effects arise due to the ellipsoid's inherent anisotropy, removing the requirement for a closely packed colloidal crystal monolayer. We use geometric analysis of the catalyst deposition process, and mechanistic understanding to propose a link between the observed trajectories and the catalyst distribution and suggest further routes to improve control of rotation rates.

Dipeptide repeat proteins (DPRs) are aggregation-prone polypeptides encoded by the pathogenic G4C2 repeat expansion in the C9orf72 gene, the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD). In this study, we focus on the role of poly-GA DPRs in disease spread. We demonstrate that recombinant poly-GA oligomers can directly convert into solid-like aggregates and form characteristic &[beta]-sheet fibrils in vitro. To dissect the process of cell-to-cell DPR transmission, we closely follow the fate of poly-GA DPRs in either their oligomeric or fibrillized form after administration in the cell culture medium. We observe that poly-GA DPRs are taken up via dynamin-dependent and - independent endocytosis, eventually converging at the lysosomal compartment and leading to axonal swellings in neurons. We then use a co-culture system to demonstrate astrocyte-to- motor neuron DPR propagation, showing that astrocytes may internalise and release aberrant peptides in disease pathogenesis. Overall, our results shed light on the mechanisms of poly- GA cellular uptake and cell-to-cell propagation, suggesting lysosomal impairment as a possible feature underlying the cellular pathogenicity of these DPR species.

Dipeptide repeat (DPR) proteins are aggregation-prone polypeptides encoded by the pathogenic GGGGCC repeat expansion in the C9ORF72 gene, the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. In this study, we focus on the role of poly-GA DPRs in disease spread. We demonstrate that recombinant poly-GA oligomers can directly convert into solid-like aggregates and form characteristic β-sheet fibrils in vitro. To dissect the process of cell-to-cell DPR transmission, we closely follow the fate of poly-GA DPRs in either their oligomeric or fibrillized form after administration in the cell culture medium. We observe that poly-GA DPRs are taken up via dynamin-dependent and -independent endocytosis, eventually converging at the lysosomal compartment and leading to axonal swellings in neurons. We then use a co-culture system to demonstrate astrocyte-to-motor neuron DPR propagation, showing that astrocytes may internalise and release aberrant peptides in disease pathogenesis. Overall, our results shed light on the mechanisms of poly-GA cellular uptake and propagation, suggesting lysosomal impairment as a possible feature underlying the cellular pathogenicity of these DPR species.

Although much attention has focused on self-motile asymmetrical catalytically active "Janus" colloids as a route to enable new fluidic transport applications, the motion of symmetrical catalytically active colloids is less investigated. This is despite isotropically active colloids being more accessible and commonly used as supports for heterogeneous catalysis. Here, we addressed this by systematically investigating the motion of platinum-coated colloids capable of isotropically decomposing hydrogen peroxide. We observed the onset of collective convective flow as the colloidal volume fraction increased above a threshold. The ballistic velocities induced by the collective flow were quantified by particle tracking and were found to increase with the volume fraction. We also determined the associated increase in the Péclet number as evidence of the potential to use convection as a simple method to enhance mass transport rates. By determining the persistence lengths, we were able to correlate the magnitude of convective flow with the overall catalytic activity per unit volume. This suggests that the mechanism for the collective flow is driven by chemical activity-induced local density differences. Finally, we discussed these results in the context of potential new fluidic applications and highlighted the role that activity-induced convection may play in experiments designed to investigate self-motile catalytic systems.

The phoretic mechanisms at stake in the propulsion of asymmetric colloids have been the subject of debates during the past years. In particular, the importance of electrokinetic effects on the motility of Pt-PS Janus sphere was recently discussed. Here, we probe the hydrodynamic flow field around a catalytically active colloid using particle tracking velocimetry both in the freely swimming state and when kept stationary with an external force. Our measurements provide information about the fluid velocity in the vicinity of the surface of the colloid, and confirm a mechanism for propulsion that was proposed recently. In addition to offering a unified understanding of the nonequilibrium interfacial transport processes at stake, our results open the way to a thorough description of the hydrodynamic interactions between such active particles and understanding their collective dynamics., In order to predict the behaviours of self-propelled particles it is important to understand the fluid disturbances they generate. Here the authors measure the flow-fields around active particles and show that they are in agreement with theoretical predictions which take into account electrokinetic effects.

Active colloids are small scale materials capable of producing enhanced motion within fluid environments. The field of active colloids has grown rapidly over the last ten years and is approaching maturity where viable applications are within reach. In this review, recent advances are surveyed with a strong emphasis on developments that can enable autonomous applications, where colloids execute useful tasks without external interventions. These applications are likely to prove transformative as the resulting technologies will be significantly less complex than current methods. A survey of the requirements to achieve autonomous applications is provided, considering guidance, solution compatibilities, manufacture and function; with reference to recent developments in these capacities. Following on from this, progress towards applications in environmental remediation, lab-on-a-chip microfluidics and in vivo drug delivery is highlighted.

The fabrication of multifunctional polymeric Janus colloids that display catalytically driven propulsion, change their size in response to local variations in pH, and vary cargo release rate is demonstrated. Systematic investigation of the colloidal trajectories reveals that in acidic environments the propulsion velocity reduces dramatically due to colloid swelling. This leads to a chemotaxis‐like accumulation for ensembles of these responsive particles in low‐pH regions. In synergy with this chemically defined accumulation, the colloids also show an enhancement in the release rate of an encapsulated cargo molecule. Together, these effects result in a strategy to harness catalytic propulsion for combined autonomous transport and cargo release directed by a chemical stimulus, displaying a greater than 30 times local cargo‐accumulation enhancement. Lactic acid can be used as the stimulus for this behavior, an acid produced by some tumors, suggesting possible eventual utility as a drug‐delivery method. Applications for microfluidic transport are also discussed.

Spherical colloids decorated with a surface coating of catalytically active material are capable of producing autonomous motion in fluids by decomposing dissolved fuel molecules to generate a gaseous product, resulting in momentum generation by bubble growth and release. Such colloids are attractive as they are relatively simple to manufacture compared to more complex tubular devices and have the potential to be used for applications such as environmental remediation. However, despite this interest, little effort has been devoted to understanding the link between the catalyst distribution at the colloid surface and the resulting propulsive trajectories. Here we address this by producing colloids with well-defined distributions of catalytic activity, which can produce motion without the requirement for the addition of surfactant, and measure and analyze the resulting trajectories. By applying analysis including fractal dimension and persistence length calculations, we show that spatially confining catalyti...

The ability to control the degree of spin, or rotational velocity, for catalytic swimming devices opens up the potential to access well defined spiralling trajectories, enhance cargo binding rate, and realise theoretically proposed behaviour such as chiral diffusion. Here we assess the potential to impart a well-defined spin to individual catalytic Janus swimmers by using glancing angle metal evaporation onto a colloidal crystal to break the symmetry of the catalytic patch due to shadowing by neighbouring colloids. Using this approach we demonstrate a well-defined relationship between the glancing angle and the ratio of rotational to translational velocity. This allows batches of colloids with well-defined spin rates in the range 0.25 to 2.5 Hz to be produced. With reference to the shape and thickness variations across the catalytically active shapes, and their propulsion mechanism we discuss the factors that can lead to the observed variations in rotational propulsion.

The self-propulsion mechanism of active colloidal particles often generates not only translational but also rotational motion. For particles with an anisotropic mass density under gravity, the motion is usually influenced by a downwards oriented force and an aligning torque. Here we study the trajectories of self-propelled bottom-heavy Janus particles in three spatial dimensions both in experiments and by theory. For a sufficiently large mass anisotropy, the particles typically move along helical trajectories whose axis is oriented either parallel or antiparallel to the direction of gravity (i.e., they show gravitaxis). In contrast, if the mass anisotropy is small and rotational diffusion is dominant, gravitational alignment of the trajectories is not possible. Furthermore, the trajectories depend on the angular self-propulsion velocity of the particles. If this component of the active motion is strong and rotates the direction of translational self-propulsion of the particles, their trajectories have many loops, whereas elongated swimming paths occur if the angular self-propulsion is weak. We show that the observed gravitational alignment mechanism and the dependence of the trajectory shape on the angular self-propulsion can be used to separate active colloidal particles with respect to their mass anisotropy and angular self-propulsion, respectively., Comment: 12 pages, 6 figures, 2 tables

We report a method to prepare catalytically active Janus colloids that "swim" in fluids and describe how to determine their 3D motion using\ud fluorescence microscopy. One commonly deployed method for catalytically active colloids to produce enhanced motion is via an asymmetrical\ud distribution of catalyst. Here this is achieved by spin coating a dispersed layer of fluorescent polymeric colloids onto a flat planar substrate,\ud and then using directional platinum vapor deposition to half coat the exposed colloid surface, making a two faced "Janus" structure. The Janus\ud colloids are then re-suspended from the planar substrate into an aqueous solution containing hydrogen peroxide. Hydrogen peroxide serves\ud as a fuel for the platinum catalyst, which is decomposed into water and oxygen, but only on one side of the colloid. The asymmetry results in\ud gradients that produce enhanced motion, or "swimming". A fluorescence microscope, together with a video camera is used to record the motion\ud of individual colloids. The center of the fluorescent emission is found using image analysis to provide an x and y coordinate for each frame of\ud the video. While keeping the microscope focal position fixed, the fluorescence emission from the colloid produces a characteristic concentric\ud ring pattern which is subject to image analysis to determine the particles relative z position. In this way 3D trajectories for the swimming colloid\ud are obtained, allowing swimming velocity to be accurately measured, and physical phenomena such as gravitaxis, which may bias the colloids\ud motion to be detected.\ud

Stirring small volumes of solution can reduce immunoassay readout time, homogenize cell cultures, and increase enzyme reactivity in bioreactors. However, at present many small scale stirring methods require external actuation, which can be cumbersome. To address this, here, reactive inkjet printing is shown to be able to produce autonomously rotating biocompatible silk-based microstirrers that can enhance fluid mixing. Rotary motion is generated either by release of a surface active agent (small molecular polyethylene glycol) resulting in Marangoni effect, or by catalytically powered bubble propulsion. The Marangoni driven devices do not require any chemicals to be added to the fluid as the "fuel," while the catalytically powered devices are powered by decomposing substrate molecules in solution. A comparison of Marangoni effect and enzyme powered stirrers is made. Marangoni effect driven stirrers rotate up to 600 rpm, 75-100-fold faster than enzyme driven microstirrers, however enzyme powered stirrers show increased longevity. Further to stirring applications, the sensitivity of the motion generation mechanisms to fluid properties allows the rotating devices to also be exploited for sensing applications, for example, acting as motion sensors for water pollution.

Model poly(glycerol monomethacrylate)-based macromonomers have been used to prepare sterically stabilized polystyrene latexes by either aqueous emulsion or alcoholic dispersion polymerization, affording mean latex diameters of approximately 107 or 1188 nm respectively as judged by dynamic light scattering. Such PGMA50−PS latexes have appropriate surface wettability to stabilize 25−250 μm oil-in-water Pickering emulsions, depending on the latex concentration and oil type. Colloidosomes were formed by covalent cross-linking of the hydroxyl-functional stabilizer chains from within the oil droplets using a polymeric diisocyanate (tolylene 2,4-diisocyanate-terminated poly(propylene glycol) [PPG-TDI]. This oil-soluble cross-linker was confined within the oil droplets, allowing colloidosomes to be prepared at 50 vol % solids without any aggregation. The resulting microcapsules survive removal of the internal oil phase using excess alcohol, unlike the noncross-linked Pickering emulsion precursor. These observatio...

The ability of alkanethiol monolayers deposited on copper to prevent surface oxidation has suggested their application as preservatives for fluxless soldering. However, the utility of such coatings for this purpose will critically depend on their ability to continue to preserve the substrate during exposure to elevated temperatures throughout the electronics manufacturing process. Consequently, the aim of this paper is to systematically determine the effect of storage temperature and duration on the ability of alkanethiol coated copper samples to undergo fluxless soldering. Similarly, the effect of pre-heating copper immediately prior to soldering is also investigated. The effect of reducing atmospheric oxygen concentration during storage and soldering is also considered as a potential route to improve the thermal resilience of the coatings. Parallel to ascertaining these industrially relevant performance parameters, a quantitative correlation between surface chemistry and solder wetting is established, and the temperature dependence of the kinetics of surface oxidation through an alkanethiol barrier layer is discussed.

The advent of autonomous self-propulsion has instigated research towards making colloidal machines that can deliver mechanical work in the form of transport, and other functions such as sensing and cleaning. While much progress has been made in the last 10 years on various mechanisms to generate self-propulsion, the ability to steer self-propelled colloidal devices has so far been much more limited. A critical barrier in increasing the impact of such motors is in directing their motion against the Brownian rotation, which randomizes particle orientations. In this context, here we report directed motion of a specific class of catalytic motors when moving in close proximity to solid surfaces. This is achieved through active quenching of their Brownian rotation by constraining it in a rotational well, caused not by equilibrium, but by hydrodynamic effects. We demonstrate how combining these geometric constraints can be utilized to steer these active colloids along arbitrary trajectories., Self-propelled colloidal particles can be potentially used to transport cargoes at the microscale, but it is challenging to prevent randomization of their motion by Brownian rotations. Here, Das et al. quench these rotations by solid walls, which guide in-plane swimming without the need for external fields.

The mode of lysozyme protein adsorption at end-tethered thiol-terminated polyethylene oxide brushes grafted upon gold was determined in situ by neutron reflectivity using the INTER instrument at target station 2, ISIS, RAL, UK. It was found that the most probable position of protein adsorption at these weakly protein resistive brushes was at the gold-brush interface in the so-called primary protein position.

The effect of added salt on the propulsion of Janus platinum-polystyrene colloids in hydrogen peroxide solution is studied experimentally. It is found that micromolar quantities of potassium and silver nitrate salts reduce the swimming velocity by similar amounts, while leading to significantly different effects on the overall rate of catalytic breakdown of hydrogen peroxide. It is argued that the seemingly paradoxical experimental observations could be theoretically explained by using a generalised reaction scheme that involves charged intermediates and has the topology of two nested loops., Two EPS Figures

In this work, we show that the asymmetrical distribution of mass at the surface of catalytic Janus swimmers results in the devices preferentially propelling themselves upward in a gravitational field. We demonstrate the existence of this gravitaxis phenomenon by observing the trajectories of fueled Janus swimmers, which generate thrust along a vector pointing away from their metallically coated half. We report that as the size of the spherical swimmer increases, the propulsive trajectories are no longer isotropic with respect to gravity, and they start to show a pronounced tendency to move in an upward direction. We suggest that this effect is due to the platinum caps asymmetric mass exerting an increasing influence on the azimuthal angle of the Janus sphere with size, biasing its orientation toward a configuration where the heavier propulsion generating surface faces down. This argument is supported by the good agreement we find between the experimentally observed azimuthal angle distribution for the Janus swimmers and predictions made by simple Boltzmann statistics. This gravitaxis phenomenon provides a mechanism to autonomously control and direct the motion of catalytic swimming devices and so enable a route to make autonomous transport devices and develop new separation, sensing, and controlled release applications.

The propulsion velocity of active colloids that asymmetrically catalyze a chemical reaction is probed experimentally as a function of their sizes. It is found that over the experimentally accessible range, the velocity decays as a function of size, with a rate that is compatible with an inverse size dependence. A diffusion-reaction model for the concentrations of the fuel and waste molecules that takes into account a two-step process for the asymmetric catalytic activity on the surface of the colloid is shown to predict a similar behavior for colloids at the large size limit, with a saturation for smaller sizes. © 2012 American Physical Society.

A class of artificial microswimmers with combined translational and rotational self-propulsion is studied experimentally. The chemically fueled microswimmers are made of doublets of Janus colloidal beads with catalytic patches that are positioned at a fixed angle relative to one another. The mean-square displacement and the mean-square angular displacement of the active doublets are analyzed in the context of a simple Langevin description, using which the physical characteristics of the microswimmers such as the spontaneous translational and rotational velocities are extracted. Our work suggests strategies for designing microswimmers that could follow prescribed cycloidal trajectories.