Shape-Dependent Motion of Structured Photoactive Microswimmers

Recent Advances in Light-Driven Semiconductor-Based Micro/Nanomotors: Optimization Strategies and Emerging Applications

Micro/nanomotors represent a burgeoning field of research featuring small devices capable of autonomous movement in liquid environments through catalytic reactions and/or external stimuli. This review delves into recent advancements in light-driven semiconductor-based micro/nanomotors (LDSM), focusing on optimized syntheses, enhanced motion mechanisms, and emerging applications in the environmental and biomedical domains. The survey commences with a theoretical introduction to micromotors and their propulsion mechanisms, followed by an exploration of commonly studied LDSM, emphasizing their advantages. Critical properties affecting propulsion, such as surface features, morphology, and size, are presented alongside discussions on external conditions related to light sources and intensity, which are crucial for optimizing the propulsion speed. Each property is accompanied by a theoretical background and conclusions drawn up to 2018. The review further investigates recent adaptations of LDSM, uncovering underlying mechanisms and associated benefits. A brief discussion is included on potential synergistic effects between different external conditions, aiming to enhance efficiency-a relatively underexplored topic. In conclusion, the review outlines emerging applications in biomedicine and environmental monitoring/remediation resulting from recent LDSM research, highlighting the growing significance of this field. The comprehensive exploration of LDSM advancements provides valuable insights for researchers and practitioners seeking to leverage these innovative micro/nanomotors in diverse applications.

pH-Responsive swimming behavior of light-powered rod-shaped micromotors

Micromotors have emerged as promising devices for a wide range of applications e.g., microfluidics, lab-on-a-chip devices, active matter, environmental monitoring, etc. The control over the activity of micromotors with the ability to exhibit multimode swimming is one of the most desirable features for many of the applications. Here, we demonstrate a rod-shaped light-driven micromotor whose activity and swimming behavior can easily be controlled. The rod-shaped micromotors are fabricated through the dynamic shadowing growth (DSG) technique, where a 2 μm long arm of titanium dioxide (TiO2) is grown over spherical silica (SiO2) particles (1 μm diameter). Under low-intensity UV light exposure, the micromotors exhibit self-propulsion in an aqueous peroxide medium. When activated, the swimming behavior of micromotors greatly depends on the pH of the medium. The swimming direction, i.e., forward or backward movement, as well as swimming modes like translational or rotational motion, can be controlled by changing the pH values. The observed dynamics has been rationalized using a theoretical model incorporating chemical activity, hydrodynamic flow, and the effect of gravity for a rod-shaped active particle near a planar wall. The pH-dependent translational and rotational dynamics of micromotors provide a versatile platform for achieving controlled and responsive behaviors. Continued research and development in this area hold great promise for advancing micromotors and enabling novel applications in microfluidics, micromachining, environmental sciences, and biomedicine.

Shape-Dictated Self-Assembly of Photoresponsive Hybrid Colloids

The shape-dictated self-assembly of hybrid colloids induced by chemical concentration gradients generated by photocatalytic reactions of the colloids is studied. Different shapes enable the formation of assemblies with distinct lattice structures including hexagons, distorted hexagons, and squares, which are corroborated by computer simulations. Furthermore, assemblies change from lattices to chains when increasing the attraction between the colloids. The results show that photoresponsive hybrid colloids possess a unique capability for shape-dependent self-assembly, offering a practical and versatile approach to manipulate self-assembly at the microscale.

2D-Material-Integrated Micromachines: Competing Propulsion Strategy and Enhanced Bacterial Disinfection

2D transition-metal-dichalcogenide materials, such as molybdenum disulfide (MoS₂ ) have received immense interest owing to their remarkable structure-endowed electronic, catalytic, and mechanical properties for applications in optoelectronics, energy storage, and wearable devices. However, 2D materials have been rarely explored in the field of micro/nanomachines, motors, and robots. Here, MoS₂  with anatase TiO₂  is successfully integrated into an original one-side-open hollow micromachine, which demonstrates increased light absorption of TiO₂ -based micromachines to the visible region and the first observed motion acceleration in response to ionic media. Both experimentation and theoretical analysis suggest the unique type-II bandgap alignment of MoS₂ /TiO₂  heterojunction that accounts for the observed unique locomotion owing to a competing propulsion mechanism. Furthermore, by leveraging the chemical properties of MoS₂ /TiO₂ , the micromachines achieve sunlight-powered water disinfection with 99.999% Escherichia coli lysed in an hour. This research suggests abundant opportunities offered by 2D materials in the creation of a new class of micro/nanomachines and robots.

Doxorubicin-Loaded Walnut-Shaped Polydopamine Nanomotor for Photothermal-Chemotherapy of Cancer

The combination of photothermal therapy and chemical drug therapy shows good prospects in cancer treatment, but there are also some limitations such as low permeability of therapeutic agents and uneven photothermal therapy. Here, we synthesized a walnut-shaped polydopamine (PDA) nanomotor driven by near infrared (NIR) light. The nanomotor was modified by methoxy polyethylene glycol amine (mPEG-NH₂) for improving water solubility. PDA-PEG loaded adriamycin through π-π accumulation and hydrogen bonding. The experimental results showed that the PDA nanomotors had good biocompatibility and photothermal effect. Further, the NIR light irradiation and tumor cell microenvironment are conducive to drug release. In addition, under the irradiation of an NIR laser, the asymmetry of walnut-shaped nanoparticles makes the particles obtain the ability of autonomous movement, which can improve the permeability of particles in 3D tumor balls, which can provide support for drug penetration and heat dispersion. This strategy offers potential innovative materials for photothermal/chemotherapy synergistic therapy of tumors.

Autonomous Low-Reynolds-Number Soft Robots with Structurally Encoded Motion and Their Thermodynamic Efficiency

Soft low-Reynolds-number robotics hold the potential to significantly impact numerous fields including drug delivery, sensing, and diagnostics. Realizing this potential is predicated upon the ability to design soft robots tailored to their intended function. In this work, we identify the effect of different geometric and symmetry parameters on the motion of soft, autonomous robots that operate in the low-Reynolds-number regime and use organic fuel. The ability to power low-Reynolds-number soft robots using an organic fuel would provide a new avenue for their potential use in biomedical applications, as is the use of a polymeric biocompatible material as is done here. We introduce a simple and cost-effective 3D-printer-assisted method to fabricate robots of different shapes that is scalable and widely applicable for a variety of materials. The efficiency of chemical energy to mechanical energy conversion is measured in soft low-Reynolds-number robots for the first time, and their mechanism of motion is assessed. Motion is a result of a periodic and oscillatory change in the charge state of the gel. This work lays the groundwork for the structure-function design of soft, chemically operated, and autonomous low-Reynolds-number robots.

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.

The Energy Conversion behind Micro-and Nanomotors

Inspired by the autonomously moving organisms in nature, artificially synthesized micro-nano-scale power devices, also called micro-and nanomotors, are proposed. These micro-and nanomotors that can self-propel have been used for biological sensing, environmental remediation, and targeted drug transportation. In this article, we will systematically overview the conversion of chemical energy or other forms of energy in the external environment (such as electrical energy, light energy, magnetic energy, and ultrasound) into kinetic mechanical energy by micro-and nanomotors. The development and progress of these energy conversion mechanisms in the past ten years are reviewed, and the broad application prospects of micro-and nanomotors in energy conversion are provided.

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.

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.

Clustering and Phase Separation in Mixtures of Dipolar and Active Particles in an External Field

Directing the assembly of colloidal particles through the use of external electric or magnetic fields shows promise for the creation of reconfigurable materials. Self-propelled particles can also be used to dynamically drive colloidal systems to nonequilibrium steady states. We investigate colloidal systems that combine both of these methods of directed assembly, simulating mixtures of passive dipolar colloids and active soft spheres in an external magnetic field using Brownian dynamics in two dimensions. In these systems, the dipolar particles align in the direction of the external field, but the active particles are unaffected by the field. The phase behaviors exhibited included a percolated dipolar network, dipolar string-fluid, isotropic fluid, and phase-separated state. We find that the external field allows the dipolar particles to form a percolated network more easily compared to when no external field is present. Additionally, the mixture phase separates at lower active particle velocity in an external field than when no field is present. Our results suggest that combining multiple methods of directing colloidal assembly could lead to new pathways to fabricate reconfigurable materials.

A practical guide to active colloids: choosing synthetic model systems for soft matter physics research

Synthetic active colloids that harvest energy stored in the environment and swim autonomously are a popular model system for active matter. This emerging field of research sits at the intersection of materials chemistry, soft matter physics, and engineering, and thus cross-talk among researchers from different backgrounds becomes critical yet difficult. To facilitate this interdisciplinary communication, and to help soft matter physicists with choosing the best model system for their research, we here present a tutorial review article that describes, in appropriate detail, six experimental systems of active colloids commonly found in the physics literature. For each type, we introduce their background, material synthesis and operating mechanisms and notable studies from the soft matter community, and comment on their respective advantages and limitations. In addition, the main features of each type of active colloid are summarized into two useful tables. As materials chemists and engineers, we intend for this article to serve as a practical guide, so those who are not familiar with the experimental aspects of active colloids can make more informed decisions and maximize their creativity.

Multimodal chemo-/magneto-/phototaxis of 3G CNT-bots to power fuel cells

We report the development of a 3G microswimmer, namely, CNT-bot, capable of undergoing acid-, alkali-, magneto- and phototaxis inside acidic or alkaline baths of peroxide fuel and/or water. The use of carboxyl-functionalised multi-walled carbon nanotubes (MWCNTs) facilitated the propulsion of CNT-bots in an alkaline-water solution by ejecting carbon-dioxide bubbles. Furthermore, doping of magnetite nanoparticles (FeONPs), ferrous ions (Fe²⁺) and titanium dioxide nanoparticles (TiONPs) induces magnetic, chemical and photonic modes of propulsion. While FeONPs stimulated magnetotaxis at a rate of up to ~10 body lengths per second under the influence of a bar magnet, chemotaxis of a similar speed in a peroxide fuel was achieved by bubble-propulsion of oxygen gas originating from the Fenton reaction. In addition, the light-stimulated photo-Fenton reaction led to phototaxis of CNT-bots. A thin coating of magnesium imparted a half-faced Janus appearance to the CNT-bots, which facilitated motion in normal or acidic water media through the ejection of hydrogen gas bubbles. This chemotaxis could be transformed into pH-stimulated directional motion by establishing an acid or alkali concentration gradient across the peroxide and/or water baths. The capacity of CNT-bots to produce oxygen (hydrogen) bubbles in peroxide (acidic water) fuel was exploited to power a PEM fuel cell to generate electricity. The pure oxygen and hydrogen gases generated by CNT-bots in separate chambers were fed directly into the fuel cell in which the incessant motions of the particle facilitated the creation and release of the pure gases to achieve on-demand electricity generation. The motor could also induce dye degradation through advanced oxidation owing to the production of intermediate hydroxyl radicals during the Fenton reaction.

Modeling of an acoustically actuated artificial micro-swimmer

Some recent achievements in microfabrication have demonstrated ultrasound-actuated artificial micro-swimmers for medical applications. However, the theoretical model of actuation and swimming is still lacking. Here we report a theoretical study of an acoustically actuated sperm-like artificial micro-swimmer which consists of a rigid head and a flexible flagellum. We provide the quantitative relation between head oscillation amplitude and acoustic pressure and frequency, and the theoretical account of how the flagellum is whipped, which brings about propulsion. The resistive force theory is employed in our model to relate the dynamic response of a flagellum and the motility of the swimmer. In order to make our theoretical model applicable in a realistic design of sperm-like micro-swimmer, we have involved the inertia term and material damping in the governing equation and considered the variable cross-section of a flagellum. The numerical results reveal that the micro-swimmer actuated by ultrasound can achieve a perceptible velocity, especially at resonance. Influences of non-dimensional parameters, such as the resonance index, sperm number, and material damping coefficient, are discussed and a comparison with experimental results demonstrates the validity of the proposed model.

Motor and Rotor in One: Light-Active ZnO/Au Twinned Rods of Tunable Motion Modes

Precise control of the motion of micromachines is the key to achieving their functions for practical applications. The main challenge is that a given micromachine can typically exhibit only one motion mode, i.e., translation or rotation, while having multiple modes of motion resulting from a simple actuation is still rare. Here we designed and synthesized photochemically powered zinc oxide/gold (ZnO/Au) rods that exhibit multiple motion modes. Under homogeneous UV irradiation, these ZnO/Au rods undergo a transition from ballistic motion to persistent rotational motion upon increasing the fuel concentration or the light intensity. In addition, the rods can switch modes from a circular motion to a helical motion and then a straight-line motion by tuning the angle of incident light. We envision that such attractive colloidal micromachines with controllable motions hold considerable promise for diverse practical applications.

Material-dependent performance of fuel-free, light-activated, self-propelling colloids

Self-propelling, light-activated colloidal particles can be actuated in water alone, but their performance is material-dependent.

Fuel-Free Micro-/Nanomotors as Intelligent Therapeutic Agents

There are many efficient biological motors in Nature that perform complex functions by converting chemical energy into mechanical motion. Inspired by this, the development of their synthetic counterparts has aroused tremendous research interest in the past decade. Among these man-made motor systems, the fuel-free (or light, magnet, ultrasound, or electric field driven) motors are advantageous in terms of controllability, lifespan, and biocompatibility concerning bioapplications, when compared with their chemically powered counterparts. Therefore, this review will highlight the latest biomedical applications in the versatile field of externally propelled micro-/nanomotors, as well as elucidating their driving mechanisms. A perspective into the future of the micro-/nanomotors field and a discussion of the challenges we need to face along the road towards practical clinical translation of external-field-propelled micro-/nanomotors will be provided.

A Review of Micromotors in Confinements: Pores, Channels, Grooves, Steps, Interfaces, Chains, and Swimming in the Bulk

One of the recent frontiers of nanotechnology research involves machines that operate at nano- and microscales, also known as nano/micromotors. Their potential applications in biomedicine, environmental sciences and engineering, military and defense industries, self-assembly, and many other areas have fueled an intense interest in this topic over the last 15 years. Despite deepened understanding of their propulsion mechanisms, we are still in the early days of exploring the dynamics of micromotors in complex and more realistic environments. Confinements, as a typical example of complex environments, are extremely relevant to the applications of micromotors, which are expected to travel in mucus gels, blood vessels, reproductive and digestive tracts, microfluidic chips, and capillary tubes. In this review, we summarize and critically examine recent studies (mostly experimental ones) of micromotor dynamics in confinements in 3D (spheres and porous network, channels, grooves, steps, and obstacles), 2D (liquid-liquid, liquid-solid, and liquid-air interfaces), and 1D (chains). In addition, studies of micromotors moving in the bulk solution and the usefulness of acoustic levitation is discussed. At the end of this article, we summarize how confinements can affect micromotors and offer our insights on future research directions. This review article is relevant to readers who are interested in the interactions of materials with interfaces and structures at the microscale and helpful for the design of smart and multifunctional materials for various applications.

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.