Engineering Contactless Particle-Particle Interactions in Active Microswimmers

Non-Reciprocity, Metastability, and Dynamic Reconfiguration in Co-Assembly of Active and Passive Particles

Living organisms often exhibit non-reciprocal interactions where the forces acting on the objects are not equal in magnitude or opposite in direction. The combination of reciprocal and non-reciprocal interactions between synthetic building blocks remains largely unexplored. Here, out-of-equilibrium assemblies of non-motile isotropic passive and metal-patched motile active particles are formed by overlapping bulk interactions with directed self-propulsion. An external alternating current (AC) electric field generates concurrent dipolar and induced-charge electrophoretic forces between the particles which are evaluated using microscopy. The interaction force measurements allow to determine the degree of reciprocity in interactions, which is tunable by designing the active particle and its trajectory. While linearly-propelled active particles evade assembly with passive particles, helically propelled active particles form active-passive clusters with dynamic reconfiguration and long-lived metastability. Large clusters display programmable fluctuations and reconfigurability by controlling the fraction of active particles. The study establishes principles of integrating reciprocal and non-reciprocal interactions in guided colloidal assembly of reconfigurable metastable structures.

Multimode microdimer robot for crossing tissue morphological barrier

Swimming microrobot energized by magnetic fields exhibits remotely propulsion and modulation in complex biological experiment with high precision. However, achieving high environment adaptability and multiple tasking capability in one configuration is still challenging. Here, we present a strategy that use oriented magnetized Janus spheres to assemble the microdimer robots with two magnetic distribution configurations of head-to-side configuration (HTS-config) and head-to-head configuration (HTH-config), achieving performance of multiple tasks through multimode transformation and locomotion. Modulating the magnetic frequency enables multimode motion transformation between tumbling, rolling, and swing motion with different velocities. The dual-asynchronization mechanisms of HTS-config and HTH-config robot dependent on magnetic dipole-dipole angle are investigated by molecular dynamic simulation. In addition, the microdimer robot can transport cell crossing morphological rugae or complete drug delivery on tissues by switching motion modes. This microdimer robot can provide versatile motion modes to address environmental variations or multitasking requirements.

A Versatile Method for Synthesis of Light-Activated, Magnet-Steerable Organic–Inorganic Hybrid Active Colloids

The immense potential of active colloids in practical applications and fundamental research calls for an efficient method to synthesize active colloids of high uniformity. Herein, a facile method is reported to synthesize uniform organic–inorganic hybrid active colloids based on the wetting effect of polystyrene (PS) with photoresponsive inorganic nanoparticles in a tetrahydrofuran/water mixture. The results show that a range of dimer active colloids can be produced by using different inorganic components, such as AgCl, ZnO, TiO2, and Fe2O3 nanoparticles. Moreover, the strategy provides a simple way to prepare dual-drive active colloids by a rational selection of the starting organic materials, such as magnetic PS particles that result in light and magnet dual-drive active colloids. The motions of these active colloids are quantified, and well-controlled movements are demonstrated.

Improving disease prevention, diagnosis, and treatment using novel bionic technologies

Increased human life expectancy, due in part to improvements in infant and childhood survival, more active lifestyles, in combination with higher patient expectations for better health outcomes, is leading to an extensive change in the number, type and manner in which health conditions are treated. Over the next decades as the global population rapidly progresses toward a super-aging society, meeting the long-term quality of care needs is forecast to present a major healthcare challenge. The goal is to ensure longer periods of good health, a sustained sense of well-being, with extended periods of activity, social engagement, and productivity. To accomplish these goals, multifunctionalized interfaces are an indispensable component of next generation medical technologies. The development of more sophisticated materials and devices as well as an improved understanding of human disease is forecast to revolutionize the diagnosis and treatment of conditions ranging from osteoarthritis to Alzheimer's disease and will impact disease prevention. This review examines emerging cutting-edge bionic materials, devices and technologies developed to advance disease prevention, and medical care and treatment in our elderly population including developments in smart bandages, cochlear implants, and the increasing role of artificial intelligence and nanorobotics in medicine.

Potential of the Coordinated Actions of Multiple Protein-Based Micromachines for Medical Applications

Untethered mobile micromachines hold great promise in the development of effective and minimally invasive therapies. Although diverse medical micromachines for specific applications have been developed over the past few decades, the coordinated action of multiple machines with different functions remains largely unexplored. In this study, we created three types of biocompatible micromachines using proteins and demonstrated the potential of their coordinated action for medical applications. As a proof of concept, we demonstrated neural replacement therapy, in which neuroblastomas were killed by using an anticancer prodrug and the first machine that contains enzymes, enabling the conversion of the prodrug into a cytotoxic drug. Subsequently, a second machine composed of extracellular matrix was placed on the dead cancer cells to provide a suitable environment for cell adhesion, on which embryonic stem (ES) cells and stromal cells that promote neural differentiation of stem cells were attached by using third machines capable of delivering cells to target positions with desired patterns. As a result, neuroblastomas were replaced with novel healthy neurons derived from ES cells by teaming multiple protein-based machines. We believe that this work highlights the potential of heterogeneous machine groups for medical treatment and the utility of highly biocompatible and functional micromachines made from proteins, representing an important step forward in building more sophisticated micromachine-based therapies.

Asymmetric colloidal motors: from dissymmetric nanoarchitectural fabrication to efficient propulsion strategy

Janus colloidal motors (JCMs) are versatile anisotropic particles that can effectively move autonomously based on their asymmetric structures, providing unlimited possibilities for various tasks. Developing novel JCMs with controllable size, engineered nanostructure and functionalized surface properties has always been a challenge for chemists. This review summarizes the recent progress in synthesized JCMs in terms of their fabrication method, propulsion strategy, and biomedical applications. The design options, construction methods, and typical examples of JCMs are presented. Common propulsion mechanisms of JCMs are reviewed, as well as the approaches to control their motion under complex microscopic conditions based on symmetry-breaking strategies. The precisely controlled motion enables JCMs to be used in biomedicine, environmental remediation, analytical sensing and nanoengineering. Finally, perspectives on future research and development are presented. Through ingenious design and multi-functionality, new JCM-based technologies could address more and more special needs in complex environments.

Modulating drift dynamics of circle swimmers by periodic potentials

We propose a method to modulate the drifting motion of overdamped circle swimmers in steady fluid flows by means of static sinusoidal potentials. Using Langevin formalism, we study drift velocity as a function of potential strength and wavelength with and without diffusional motion. Drift velocity is essentially quantized without diffusion, but in the presence of noise, the displacement per cycle has a continuous range. As a function of dimensionless potential wave number, domains of damped oscillatory and plateau regimes are observed in the drift velocity diagram. At weak potential and fluid velocity less than powered velocity, there is also a regime where drift velocity exceeds the fluid velocity. Methods based on these results can be used to separate biological and artificial circle swimmers based on their dynamical properties.

Superfast Active Droplets as Micromotors for Locomotion of Passive Droplets and Intensification of Mixing

Micromotors are fascinating objects that are able to move autonomously and perform various complex tasks related to drug delivery, chemical processes, and environmental remediation. Among the types of micromotors, droplet-based micromotors are characterized by a wide range of functional properties related to the capability of encapsulation and deformation and the possibility of using them as microreactors. Relevant problems of micromotor utilization in the chemical processes include intensification of mixing and locomotion of passive objects. In this paper, the technique for preparation of superfast active droplets, which can be used as micromotors for effective locomotion of passive droplets in the oil-in-water emulsion, is demonstrated. The possibility of passive droplet locomotion in the emulsion is determined by a relation between the diameters of active and passive droplets. If the diameter of active droplets is larger than the diameter of passive droplets, the agglomerates form spontaneously in the emulsion and move in a straight line. In the case of the opposite relation between diameters, the agglomerates consisting of active and passive droplets rotate intensively. This makes it impossible to move the passive droplets to a given distance. Such micromotors can achieve unprecedentedly high velocities of motion and can be used to intensify mixing on the microscales.

Active, Yet Little Mobility: Asymmetric Decomposition of H2O2 Is Not Sufficient in Propelling Catalytic Micromotors

A popular principle in designing chemical micromachines is to take advantage of asymmetric chemical reactions such as the catalytic decomposition of H₂O₂. Contrary to intuition, we use Janus micromotors half-coated with platinum (Pt) or catalase as an example to show that this ingredient is not sufficient in powering a micromotor into self-propulsion. In particular, by annealing a thin Pt film on a SiO₂ microsphere, the resulting microsphere half-decorated with discrete Pt nanoparticles swims ∼80% more slowly than its unannealed counterpart in H₂O₂, even though they both catalytically produce comparable amounts of oxygen. Similarly, SiO₂ microspheres half-functionalized with the enzyme catalase show negligible self-propulsion despite high catalytic activity toward decomposing H₂O₂. In addition to highlighting how surface morphology of a catalytic cap enables/disables a chemical micromotor, this study offers a refreshed perspective in understanding how chemistry powers nano- and microscopic objects (or not): our results are consistent with a self-electrophoresis mechanism that emphasizes the electrochemical decomposition of H₂O₂ over nonelectrochemical pathways. More broadly, our finding is a critical piece of the puzzle in understanding and designing nano- and micromachines, in developing capable model systems of active colloids, and in relating enzymes to active matter.

Spontaneous directional flow of active magnetic particles

We predict the emergence of large-scale polar order and spontaneous directional flows in a class of self-propelled autonomous particles that interact via passive repulsion between off-center sites. The coupling of active motion with the passive torque acting about the particle centers results in hybrid active-passive interactions responsible for a macroscopic phase transition from an isotropic state to a polar-aligned state in systems of particles with front interaction sites. We employ a continuum kinetic theory to explain that the emergence of long-ranged orientational order, which occurs in unbounded domains at finite densities, can be externally activated independently of the self-propulsion mechanism and drives a macroscopic particle flow in a direction selected by symmetry breaking.

Shape-Tunable Janus Micromotors via Surfactant-Induced Dewetting

The ability to tune shapes of micromotors is challenging yet crucial for creating intelligent and functional micromachines with shape-dependent dynamics. Here, we demonstrate a facile strategy to synthesize Janus micromotors in large quantity whose shapes can be precisely tuned by a surfactant-induced dewetting strategy. The Janus micromotor is composed of a TiO₂ microparticle partially encapsulated within a polysiloxane microsphere. A range of particle shapes, from approximately spherical to snowman, is achieved, and the shape-tunable dynamics of the micromotors are quantified. Our strategy is versatile and can be applicable to other photoactive materials, such as ZnO and Fe₂O₃ nanoparticles, demonstrating a general approach to synthesize Janus micromotors with controllable shapes. Such shape-tunable micromotors provide colloidal model systems for fundamental research on active matter, as well as building blocks for the fabrication of micromachines.

Active Assembly of Spheroidal Photocatalytic BiVO4 Microswimmers

We create single-component photocatalytic bismuth vanadate (BiVO₄) microswimmers with a spheroidal shape that move individually upon irradiation without any asymmetrization step. These particles form active assemblies which we investigate combining an experimental approach with numerical simulations and analytical calculations. We systematically explore the speed and assembly of the swimmers into clusters of up to four particles and find excellent agreement between experiment and theory, which helps us to understand motion patterns and speed trends. Moreover, different batches of particles can be functionalized separately, making them ideal candidates to fulfill a multitude of tasks, such as sensing or environmental remediation. To exemplify this, we coat our swimmers with silica (SiO₂) and selectively couple some of their modules to fluorophores in a way which does not inhibit self-propulsion. The present work establishes spheroidal BiVO₄ microswimmers as a versatile platform to design multifunctional microswimmers.

Controlling the Speed of Light-Activated Colloids with a Constant, Uniform Magnetic Field

It is demonstrated how the strength of activation for photocatalytic, self-propelled colloids can be enhanced with a constant, uniform magnetic field. When exposed to ultraviolet light and hydrogen peroxide, the titanium dioxide-based colloids become actively propelled. Due to the iron oxide core, a uniform field oriented perpendicular to the surface where motion takes place causes the asymmetrically shaped particles to rotate, which consequently leads to an increase in activity. The field-dependent dynamics of self-propulsion is quantified, and a qualitative description of how this effect arises is proposed. Since the application of the field is easily reversible, modulating the field on-and-off serves as a de facto "switch" that controls particle behavior.

Cancer Cells Microsurgery via Asymmetric Bent Surface Au/Ag/Ni Microrobotic Scalpels Through a Transversal Rotating Magnetic Field

The actuation of micro/nanomachines by means of a magnetic field is a promising fuel-free way to transport cargo in microscale dimensions. This type of movement has been extensively studied for a variety of micro/nanomachine designs, and a special magnetic field configuration results in a near-surface walking. We developed "walking" micromachines which transversally move in a magnetic field, and we used them as microrobotic scalpels to enter and exit an individual cancer cell and cut a small cellular fragment. In these microscalpels, the center of mass lies approximately in the middle of their length. The microrobotic scalpels show good propulsion efficiency and high step-out frequencies of the magnetic field. Au/Ag/Ni microrobotic scalpels controlled by a transversal rotating magnetic field can enter the cytoplasm of cancer cells and also are able to remove a piece of the cytosol while leaving the cytoplasmic membrane intact in a microsurgery-like manner. We believe that this concept can be further developed for potential biological or medical applications.

Anisotropic Exclusion Effect between Photocatalytic Ag/AgCl Janus Particles and Passive Beads in a Dense Colloidal Matrix

Synthetic nano- and micromotors interact with each other and their surroundings in a complex manner. Here, we report on the anisotropy of active-passive particle interaction in a soft matter system containing an immobile yet photochemical Ag/AgCl-based Janus particle embedded in a dense matrix of passive beads in pure water. The asymmetry in the chemical gradient around the Janus particle, triggered upon visible light illumination, distorts the isotropy of the surrounding electric potential and results in the repulsion of adjacent passive beads to a certain distance away from the Janus particle. This exclusion effect is found to be anisotropic with larger distances to passive beads in front of the Ag/AgCl cap of the Janus particle. We provide insight into this phenomenon by performing the angular analysis of the radii of exclusion and tracking their time evolution at the level of a single bead. Our study provides a novel fundamental insight into the collective behavior of a complex mixture of active and passive particles and is relevant for various application scenarios, e.g., particle transport at micro- and nanoscale and local chemical sensing.

Shape- and Material-Dependent Self-Propulsion of Photocatalytic Active Colloids, Interfacial Effects, and Dynamic Interparticle Interactions

Active colloids powered by self-generated, local chemical concentration gradients exhibit dynamics that are a function of the particles' morphology and material properties. These characteristics also govern how the active colloids interact with surfaces, including other particles and nearby walls. Thus, by targeted design, the dynamic behavior, on average, can be engineered, despite a lack of "external" control such as an applied magnetic field. This allows for the development of new applications and the investigation of novel effects that arise when self-propelled active colloids have complex shapes and material composition. Here, we explore some of our recent work on this topic including the dynamics and interactions of photoactivated, self-propelled colloids with such multifaceted properties. We also delve into some special cases, such as a new variety of active particle-particle interaction that we recently developed, in which direct contact between the active colloids is forbidden, and the direction of propulsion for pairs of particles is correlated. The unifying theme of the work highlighted herein is the relationship between the physical, chemical, and material properties of active colloids and their motive behavior, the understanding of which opens up a wide range of new possibilities as we move toward the ultimate goal of realizing functional, man-made micro- and nanomachinery.

Shape-programmed 3D printed swimming microtori for the transport of passive and active agents

Through billions of years of evolution, microorganisms mastered unique swimming behaviors to thrive in complex fluid environments. Limitations in nanofabrication have thus far hindered the ability to design and program synthetic swimmers with the same abilities. Here we encode multi-behavioral responses in microscopic self-propelled tori using nanoscale 3D printing. We show experimentally and theoretically that the tori continuously transition between two primary swimming modes in response to a magnetic field. The tori also manipulated and transported other artificial swimmers, bimetallic nanorods, as well as passive colloidal particles. In the first behavioral mode, the tori accumulated and transported nanorods; in the second mode, nanorods aligned along the toriʼs self-generated streamlines. Our results indicate that such shape-programmed microswimmers have a potential to manipulate biological active matter, e.g. bacteria or cells.

While there are many demonstrations of self-propelled synthetic particles, there are fewer realisations of multimode swimming for the same particle. Here the authors demonstrate two swimming behaviours in magnetically manipulated microtori and show that these can manipulate other active particles.

Confined 1D Propulsion of Metallodielectric Janus Micromotors on Microelectrodes under Alternating Current Electric Fields

There is mounting interest in synthetic microswimmers ("micromotors") as microrobots as well as a model system for the study of active matters, and spatial navigation is critical for their success. Current navigational technologies mostly rely on magnetic steering or guiding with physical boundaries, yet limitations with these strategies are plenty. Inspired by an earlier work with magnetic domains on a garnet film as predefined tracks, we present an interdigitated microelectrodes (IDE) system where, upon the application of AC electric fields, metallodielectric (e.g., SiO₂-Ti) Janus particles are hydrodynamically confined and electrokinetically propelled in one dimension along the electrode center lines with tunable speeds. In addition, comoving micromotors moved in single files, while those moving in opposite directions primarily reoriented and moved past each other. At high particle densities, turbulence-like aggregates formed as many-body interactions became complicated. Furthermore, a micromotor made U-turns when approaching an electrode closure, while it gradually slowed down at the electrode opening and was collected in large piles. Labyrinth patterns made of serpentine chains of Janus particles emerged by modifying the electrode configuration. Most of these observations can be qualitatively understood by a combination of electroosmotic flows pointing inward to the electrodes, and asymmetric electrical polarization of the Janus particles under an AC electric field. Emerging from these observations is a strategy that not only powers and confines micromotors on prefabricated tracks in a contactless, on-demand manner, but is also capable of concentrating active particles at predefined locations. These features could prove useful for designing tunable tracks that steer synthetic microrobots, as well as to enable the study of single file diffusion, active turbulence, and other collective behaviors of active matters.

Review: Interactions of Active Colloids with Passive Tracers

Collective phenomena existing universally in both biological systems and artificial active matter are increasingly attracting interest. The interactions can be grouped into active-active and active-passive ones, where the reports on the purely active system are still clearly dominating. Despite the growing interest, summarizing works for active-passive interactions in artificial active matter are still missing. For that reason, we start this review with a general introduction, followed by a short spotlight on theoretical works and then an extensive overview of experimental realizations. We classify the cases according to the active colloids’ mechanisms of motion and discuss the principles of the interactions. A few key applications of the active-passive interaction of current interest are also highlighted (such as cargo transport, flow field mapping, assembly of structures). We expect that this review will help the fundamental understanding and inspire further studies on active matter.

Theory of light-activated catalytic Janus particles

We study the dynamics of active Janus particles that self-propel in solution by light-activated catalytic decomposition of chemical "fuel." We develop an analytical model of a photo-active self-phoretic particle that accounts for "self-shadowing" of the light by the opaque catalytic face of the particle. We find that self-shadowing can drive "phototaxis" (rotation of the catalytic cap toward the light source) or "anti-phototaxis," depending on the properties of the particle. Incorporating the effect of thermal noise, we show that the distribution of particle orientations is captured by a Boltzmann distribution with a nonequilibrium effective potential. Furthermore, the mean vertical velocity of phototactic (anti-phototactic) particles exhibits a superlinear (sublinear) dependence on intensity. Overall, our findings show that photo-active particles exhibit a rich "tactic" response to light, which could be harnessed to program complex three-dimensional trajectories.

Chemical Nanomotors at the Gram Scale Form a Dense Active Optorheological Medium

The rheological properties of a colloidal suspension are a function of the concentration of the colloids and their interactions. While suspensions of passive colloids are well studied and have been shown to form crystals, gels, and glasses, examples of energy-consuming "active" colloidal suspensions are still largely unexplored. Active suspensions of biological matter, such as motile bacteria or dense mixtures of active actin-motor-protein mixtures have, respectively, reveals superfluid-like and gel-like states. Attractive inanimate systems for active matter are chemically self-propelled particles. It has so far been challenging to use these swimming particles at high enough densities to affect the bulk material properties of the suspension. Here, it is shown that light-triggered asymmetric titanium dioxide that self-propel, can be obtained in large quantities, and self-organize to make a gram-scale active medium. The suspension shows an activity-dependent tenfold reversible change in its bulk viscosity.

Frontiers of Medical Micro/Nanorobotics: in vivo Applications and Commercialization Perspectives Toward Clinical Uses

The field of medical micro/nanorobotics holds considerable promise for advancing medical diagnosis and treatment due to their unique ability to move and perform complex task at small scales. Nevertheless, the grand challenge of the field remains in its successful translation towards widespread patient use. We critically address the frontiers of the current methodologies for in vivo applications and discuss the current and foreseeable perspectives of their commercialization. Although no "killer application" that would catalyze rapid commercialization has yet emerged, recent engineering breakthroughs have led to the successful in vivo operation of medical micro/nanorobots. We also highlight how standardizing report summaries of micro/nanorobotics is essential not only for increasing the quality of research but also for minimizing investment risk in their potential commercialization. We review current patents and commercialization efforts based on emerging proof-of-concept applications. We expect to inspire future research efforts in the field of micro/nanorobotics toward future medical diagnosis and treatment.

Catalytic Janus Colloids: Controlling Trajectories of Chemical Microswimmers

Catalytic Janus colloids produce rapid motion in fluids by decomposing dissolved fuel. There is great potential to exploit these "autonomous chemical swimmers" in applications currently performed by diffusion limited passive colloids. Key application areas for colloids include transporting active ingredients for drug delivery, gathering analytes for medical diagnostics, and self-assembling into regular structures used for photonic materials and lithographic templating. For drug delivery and medical diagnostics, controlling colloidal motion is key in order to target therapies, and transport analytes through lab-on-a-chip devices. Here, the autonomous motion of catalytic Janus colloids can remove the current requirements to induce and control colloid motion using external fields, thereby reducing the technological complexity required for medical therapies and diagnostics. For materials applications exploiting colloidal self-assembly, the additional interactions introduced by catalytic activity and rapid motion are predicted to allow access to new reconfigurable and responsive structures. In order to realize these goals, it is vital to develop methods to control both individual colloidal paths and collective behavior in motile catalytic colloidal systems. However, catalytic Janus colloids' trajectories are randomized by Brownian effects, and so require new strategies in order to be harnessed for transport. This is achievable using a variety of different approaches. For example, self-assembly and control of catalyst geometry can introduce controlled amounts of rotary motion, or "spin" into chemical swimmer trajectories. Furthermore, rotary motion combined with gravity, produces well-defined orientated helical trajectories. In addition, when catalytic colloids interact with topographical features, such as edges and trenches, they are steered. This gives rise to a new approach for autonomous colloidal microfluidic transport that could be deployed in future lab-on-a-chip devices. Chemical gradients can also influence the motion of catalytic Janus colloids, for example, to cause collective accumulations at specific locations. However, at present, the predicted theoretical degree of control over this phenomenon has not been fully verified in experimental systems. Collective behavior control for chemical swimmers is also possible by exploiting the potential for the complex interactions in these systems to allow access to self-assembled, dynamic and reconfigurable ordered structures. Again, current experiments have not yet accessed the breadth of possible behavior. Consequently, continued efforts are required to understand and control these interaction mechanisms in real world systems. Ultimately, this will help realize the use of catalytic Janus colloids for tasks that require well-controlled motion and structural organization, enabling functions such as analyte capture and concentration, or targeted drug delivery.

Self-Phoretic Microswimmers Propel at Speeds Dependent upon an Adjacent Surface's Physicochemical Properties

Self-phoretic colloids are emerging as critical components of programmable nano- and microscale active matter and may usher in a new area of complex, small-scale machinery. To date, most studies have focused upon active particles confined by gravity to a plane located just above a solid/liquid interface. Despite this ubiquity, little attention has been directed at how the physicochemical qualities of this interface might affect motion. Here, we show that both the chemical and physical properties of the solid, above which motion takes place, significantly influence the behavior of particles propelled by self-generated concentration gradients. More specifically, titania/silica (TiO₂/SiO₂) photoactive microswimmers move faster when the local osmotic flow over the stationary solid is diminished, which we demonstrate by reducing the magnitude of the surface's zeta potential or by increasing surface roughness. Our results suggest that consideration of surface properties is crucial for modeling self-phoretic active matter while simultaneously offering a new avenue for engineering the kinematic behavior of such systems.

Light-Activated, Multi-Semiconductor Hybrid Microswimmers

Using a dynamic fabrication process, hybrid, photoactivated microswimmers made from two different semiconductors, titanium dioxide (TiO₂ ) and cuprous oxide (Cu₂ O) are developed, where each material occupies a distinct portion of the multiconstituent particles. Structured light-activated microswimmers made from only TiO₂ or Cu₂ O are observed to be driven in hydrogen peroxide and water most vigorously under UV or blue light, respectively, whereas hybrid structures made from both of these materials exhibit wavelength-dependent modes of motion due to the disparate responses of each photocatalyst. It is also found that the hybrid particles are activated in water alone, a behavior which is not observed in those made from a single semiconductor, and thus, the system may open up a new class of fuel-free photoactive colloids that take advantage of semiconductor heterojunctions. The TiO₂ /Cu₂ O hybrid microswimmer presented here is but an example of a broader method for inducing different modes of motion in a single light-activated particle, which is not limited to the specific geometries and materials presented in this study.

Shape-Dependent Motion of Structured Photoactive Microswimmers

We investigate the dynamics of structured photoactive microswimmers and show that morphology sensitively determines the swimming behavior. Particular to this study, a major portion of the light-activated particles' underlying structure is built from a photocatalytic material, made possible by dynamic physical vapor deposition (DPVD). We find that swimmers of this type exhibit unique shape-dependent autonomous swimming that is distinct from what is seen in systems with similar structural morphology but not fabricated directly from the catalyst. Notably, the direction of motion is a function of these parameters. Because the swimming behavior is strongly correlated with particle shape and material composition, DPVD allows for engineering small-scale propulsion by adjusting the fabrication parameters to match the desired performance.

Reconfigurable engineered motile semiconductor microparticles

Locally energized particles form the basis for emerging classes of active matter. The design of active particles has led to their controlled locomotion and assembly. The next generation of particles should demonstrate robust control over their active assembly, disassembly, and reconfiguration. Here we introduce a class of semiconductor microparticles that can be comprehensively designed (in size, shape, electric polarizability, and patterned coatings) using standard microfabrication tools. These custom silicon particles draw energy from external electric fields to actively propel, while interacting hydrodynamically, and sequentially assemble and disassemble on demand. We show that a number of electrokinetic effects, such as dielectrophoresis, induced charge electrophoresis, and diode propulsion, can selectively power the microparticle motions and interactions. The ability to achieve on-demand locomotion, tractable fluid flows, synchronized motility, and reversible assembly using engineered silicon microparticles may enable advanced applications that include remotely powered microsensors, artificial muscles, reconfigurable neural networks and computational systems.

Active particles that demonstrate life-like behavior may find use in bio-inspired technologies, but achieving on-demand reconfiguration remains challenging. Here, the authors demonstrate controllable, collective behavior in silicon microparticles, which are fabricated via conventional semiconductor methods.

Artificial molecular and nanostructures for advanced nanomachinery

Artificial nanomachines can be broadly defined as manmade molecular and nanosystems that are capable of performing useful tasks, very often, by means of doing mechanical work at the nanoscale. Recent advances in nanoscience allow these tiny machines to be designed and made with unprecedented sophistication and complexity, showing promise in novel applications, including molecular assemblers, self-propelling nanocarriers and in vivo molecular computation. This Feature Article overviews and compares major types of nanoscale machines, including molecular machines, self-assembled nanomachines and hybrid inorganic nanomachines, to reveal common structural features and operating principles across different length scales and material systems. We will focus on systems with feature size between 1 and 100 nm, where classical laws of physics meet those of quantum mechanics, giving rise to a spectrum of exotic physiochemical properties. Concepts of nanomachines will be illustrated by selected seminal work along with state-of-the-art progress, including our own contribution, across the fields. The Article will conclude with a brief outlook of this exciting research area.

Geometry Design, Principles and Assembly of Micromotors

Discovery of bio-inspired, self-propelled and externally-powered nano-/micro-motors, rotors and engines (micromachines) is considered a potentially revolutionary paradigm in nanoscience. Nature knows how to combine different elements together in a fluidic state for intelligent design of nano-/micro-machines, which operate by pumping, stirring, and diffusion of their internal components. Taking inspirations from nature, scientists endeavor to develop the best materials, geometries, and conditions for self-propelled motion, and to better understand their mechanisms of motion and interactions. Today, microfluidic technology offers considerable advantages for the next generation of biomimetic particles, droplets and capsules. This review summarizes recent achievements in the field of nano-/micromotors, and methods of their external control and collective behaviors, which may stimulate new ideas for a broad range of applications.