Light-Driven Microrobots for Targeted Drug Delivery

As a new micromanipulation tool with the advantages of small size, flexible movement and easy manipulation, light-driven microrobots have a wide range of prospects in biomedical fields such as drug targeting and cell manipulation. Recently, microrobots have been controlled in various ways, and light field has become a research hotspot by its advantages of noncontact manipulation, precise localization, fast response, and biocompatibility. It utilizes the force or deformation generated by the light field to precisely control the microrobot, and combines with the drug release technology to realize the targeted drug application. Therefore, this paper provides an overview of light-driven microrobots with drug targeting to provide new ideas for the manipulation of microrobots. Here, this paper briefly categorizes the driving mechanisms and materials of light-driven microrobots, which mainly include photothermal, photochemical, and biological. Then, typical designs of light-driven microrobots with different driving mechanisms and control strategies for multiple physical fields are summarized. Finally, the applications of microrobots in the fields of drug targeting and bioimaging are presented as well as the future prospects of light-driven microrobots in the biomedical field are demonstrated.

Advanced materials for micro/nanorobotics

Autonomous micro/nanorobots capable of performing programmed missions are at the forefront of next-generation micromachinery. These small robotic systems are predominantly constructed using functional components sourced from micro- and nanoscale materials; therefore, combining them with various advanced materials represents a pivotal direction toward achieving a higher level of intelligence and multifunctionality. This review provides a comprehensive overview of advanced materials for innovative micro/nanorobotics, focusing on the five families of materials that have witnessed the most rapid advancements over the last decade: two-dimensional materials, metal-organic frameworks, semiconductors, polymers, and biological cells. Their unique physicochemical, mechanical, optical, and biological properties have been integrated into micro/nanorobots to achieve greater maneuverability, programmability, intelligence, and multifunctionality in collective behaviors. The design and fabrication methods for hybrid robotic systems are discussed based on the material categories. In addition, their promising potential for powering motion and/or (multi-)functionality is described and the fundamental principles underlying them are explained. Finally, their extensive use in a variety of applications, including environmental remediation, (bio)sensing, therapeutics, etc., and remaining challenges and perspectives for future research are discussed.

Light-driven soft microrobots based on hydrogels and LCEs: development and prospects

In the daily life of mankind, microrobots can respond to stimulations received and perform different functions, which can be used to complete repetitive or dangerous tasks. Magnetic driving works well in robots that are tens or hundreds of microns in size, but there are big challenges in driving microrobots that are just a few microns in size. Therefore, it is impossible to guarantee the precise drive of microrobots to perform tasks. Acoustic driven micro-nano robot can achieve non-invasive and on-demand movement, and the drive has good biological compatibility, but the drive mode has low resolution and requires expensive experimental equipment. Light-driven robots move by converting light energy into other forms of energy. Light is a renewable, powerful energy source that can be used to transmit energy. Due to the gradual maturity of beam modulation and optical microscope technology, the application of light-driven microrobots has gradually become widespread. Light as a kind of electromagnetic wave, we can change the energy of light by controlling the wavelength and intensity of light. Therefore, the light-driven robot has the advantages of programmable, wireless, high resolution and accurate spatio-temporal control. According to the types of robots, light-driven robots are subdivided into three categories, namely light-driven soft microrobots, photochemical microrobots and 3D printed hard polymer microrobots. In this paper, the driving materials, driving mechanisms and application scenarios of light-driven soft microrobots are reviewed, and their advantages and limitations are discussed. Finally, we prospected the field, pointed out the challenges faced by light-driven soft micro robots and proposed corresponding solutions.

Swarm Autonomy: From Agent Functionalization to Machine Intelligence

Swarm behaviors are common in nature, where individual organisms collaborate via perception, communication, and adaptation. Emulating these dynamics, large groups of active agents can self-organize through localized interactions, giving rise to complex swarm behaviors, which exhibit potential for applications across various domains. This review presents a comprehensive summary and perspective of synthetic swarms, to bridge the gap between the microscale individual agents and potential applications of synthetic swarms. It is begun by examining active agents, the fundamental units of synthetic swarms, to understand the origins of their motility and functionality in the presence of external stimuli. Then inter-agent communications and agent-environment communications that contribute to the swarm generation are summarized. Furthermore, the swarm behaviors reported to date and the emergence of machine intelligence within these behaviors are reviewed. Eventually, the applications enabled by distinct synthetic swarms are summarized. By discussing the emergent machine intelligence in swarm behaviors, insights are offered into the design and deployment of autonomous synthetic swarms for real-world applications.

Dynamic Clustering and Scaling Behavior of Active Particles under Confinement

A systematic investigation of the dynamic clustering behavior of active particles under confinement, including the effects of both particle density and active driving force, is presented based on a hybrid coarse-grained molecular dynamics simulation. First, a series of scaling laws are derived with power relationships for the dynamic clustering time as a function of both particle density and active driving force. Notably, the average number of clusters N¯ assembled from active particles in the simulation system exhibits a scaling relationship with clustering time t described by N¯∝t-m. Simultaneously, the scaling behavior of the average cluster size S¯ is characterized by S¯∝tm. Our findings reveal the presence of up to four distinct dynamic regions concerning clustering over time, with transitions contingent upon the particle density within the system. Furthermore, as the active driving force increases, the aggregation behavior also accelerates, while an increase in density of active particles induces alterations in the dynamic procession of the system.

3D Motion Manipulation for Micro- and Nanomachines: Progress and Future Directions

In the past decade, micro- and nanomachines (MNMs) have made outstanding achievements in the fields of targeted drug delivery, tumor therapy, microsurgery, biological detection, and environmental monitoring and remediation. Researchers have made significant efforts to accelerate the rapid development of MNMs capable of moving through fluids by means of different energy sources (chemical reactions, ultrasound, light, electricity, magnetism, heat, or their combinations). However, the motion of MNMs is primarily investigated in confined two-dimensional (2D) horizontal setups. Furthermore, three-dimensional (3D) motion control remains challenging, especially for vertical movement and control, significantly limiting its potential applications in cargo transportation, environmental remediation, and biotherapy. Hence, an urgent need is to develop MNMs that can overcome self-gravity and controllably move in 3D spaces. This review delves into the latest progress made in MNMs with 3D motion capabilities under different manipulation approaches, discusses the underlying motion mechanisms, explores potential design concepts inspired by nature for controllable 3D motion in MNMs, and presents the available 3D observation and tracking systems.

Light-driven micro/nanomotors in biomedical applications

Nanotechnology brings hope for targeted drug delivery. However, most current drug delivery systems use passive delivery strategies with limited therapeutic efficiency. Over the past two decades, research on micro/nanomotors (MNMs) has flourished in the biomedical field. Compared with other driven methods, light-driven MNMs have the advantages of being reversible, simple to control, clean, and efficient. Under light irradiation, the MNMs can overcome several barriers in the body and show great potential in the treatment of various diseases, such as tumors, and gastrointestinal, cardiovascular and cerebrovascular diseases. Herein, the classification and mechanism of light-driven MNMs are introduced briefly. Subsequently, the applications of light-driven MNMs in overcoming physiological and pathological barriers in the past five years are highlighted. Finally, the future prospects and challenges of light-driven MNMs are discussed as well. This review will provide inspiration and direction for light-driven MNMs to overcome biological barriers in vivo and promote the clinical application of light-driven MNMs in the biomedical field.

Micro-/nanoscale robotics for chemical and biological sensing

The field of micro-/nanorobotics has attracted extensive interest from a variety of research communities and witnessed enormous progress in a broad array of applications ranging from basic research to global healthcare and to environmental remediation and protection. In particular, micro-/nanoscale robots provide an enabling platform for the development of next-generation chemical and biological sensing modalities, owing to their unique advantages as programmable, self-sustainable, and/or autonomous mobile carriers to accommodate and promote physical and chemical processes. In this review, we intend to provide an overview of the state-of-the-art development in this area and share our perspective in the future trend. This review starts with a general introduction of micro-/nanorobotics and the commonly used methods for propulsion of micro-/nanorobots in solution, along with the commonly used methods in their fabrication. Next, we comprehensively summarize the current status of the micro/nanorobotic research in relevance to chemical and biological sensing (e.g., motion-based sensing, optical sensing, and electrochemical sensing). Following that, we provide an overview of the primary challenges currently faced in the micro-/nanorobotic research. Finally, we conclude this review by providing our perspective detailing the future application of soft robotics in chemical and biological sensing.

Biomimetic thermoresponsive superstructures by colloidal soft-and-hard co-assembly

Soft-and-hard hybrid structures are ubiquitous in biological systems and have inspired the design of man-made mechanical devices, actuators, and robots. The realization of these structures, however, has been challenging at microscale, where material integration and actuation become exceedingly less practical. Here, through simple colloidal assembly, we create microscale superstructures consisting of soft and hard materials, which, serving as microactuators, have thermoresponsive shape-transforming properties. In this case, anisotropic metal-organic framework (MOF) particles as the hard components are integrated with liquid droplets, forming spine-mimicking colloidal chains via valence-limited assembly. The chains, with alternating soft and hard segments, are referred to as MicroSpine and can reversibly change shape, switching between straight and curved states through a thermoresponsive swelling/deswelling mechanism. By solidification of the liquid parts within a chain with prescribed patterns, we design various chain morphologies, such as "colloidal arms," with controlled actuating behaviors. The chains are further used to build colloidal capsules, which encapsulate and release guests by the temperature-programmed actuation.

Molecular Imprinted BiVO4 Microswimmers for Selective Target Recognition and Removal

Analogous to photosynthetic systems, photoactive semiconductor-based micro/nanoswimmers display biomimetic features that enable unique light harvesting and energy conversion functions and interactions with their surroundings. However, these artificial swimmers are usually non-selective and provide ineffective target recognition, resulting in poor surface analyte binding that affects the overall reactivity and motion efficiency. Here, the surface engineering of light-driven BiVO₄ microswimmers by molecular imprinting polymerization is presented. After embedding surface recognition sites, the modified microswimmers can self-propel in a solution of a target molecule, without requiring toxic fuels, and degrade the target selectively in a pollutant mixture. These findings show that optimizing the design of semiconductor-based microswimmers with specific target recognition cavities on their surface is a promising strategy to achieve selective capture and degradation of organic pollutants, which is otherwise impossible because of the non-selective behavior of photogenerated reactive radicals. Moreover, this study provides a unique strategy to enhance the motion capabilities of single-component photocatalytic microswimmers in a specific chemical environment.

Nano-architectural design of TiO2 for high performance photocatalytic degradation of organic pollutant: A review

In the past several decades, significant efforts have been paid toward photocatalytic degradation of organic pollutants in environmental research. During the past years, titanium dioxide nano-architectures (TiO₂ NAs) have been widely used in water purification applications with photocatalytic degradation processes under Uv/Vis light illumination. Photocatalysis process with nano-architectural design of TiO₂ is viewed as an efficient procedure for directly channeling solar energy into water treatment reactions. The considerable band-gap values and the subsequent short life time of photo-generated charge carriers are showed among the limitations of this approach. One of these effective efforts is the using of oxidation processes with advance semiconductor photocatalyst NAs for degradation the organic pollutants under UV/Vis irradiation. Among them, nano-architectural design of TiO₂ photocatalyst (such as Janus, yolk-shell (Y@S), hollow microspheres (HMSs) and nano-belt) is an effective way to improve oxidation processes for increasing photocatalytic activity in water treatment applications. In the light of the above issues, this study tends to provide a critical overview of the used strategies for preparing TiO₂ photocatalysts with desirable physicochemical properties like enhanced absorption of light, low density, high surface area, photo-stability, and charge-carrier behavior. Among the various nanoarchitectural design of TiO₂, the Y@S and HMSs have created a great appeal given their considerable large surface area, low density, homogeneous catalytic environment, favorable light harvesting properties, and enhanced molecular diffusion kinetics of the particles. In this review was summarized the developments that have been made for nano-architectural design of TiO2 photocatalyst. Additional focus is placed on the realization of interfacial charge and the possibility of achieving charge carriers separation for these NAs as electron migration is the extremely important factor for increasing the photocatalytic activity.

Enzyme-Photocatalyst Tandem Microrobot Powered by Urea for Escherichia coli Biofilm Eradication

Urinary-based infections affect millions of people worldwide. Such bacterial infections are mainly caused by Escherichia coli (E. coli) biofilm formation in the bladder and/or urinary catheters. Herein, the authors present a hybrid enzyme/photocatalytic microrobot, based on urease-immobilized TiO₂ /CdS nanotube bundles, that can swim in urea as a biocompatible fuel and respond to visible light. Upon illumination for 2 h, these microrobots are able to remove almost 90% of bacterial biofilm, due to the generation of reactive radicals, while bare TiO₂ /CdS photocatalysts (non-motile) or urease-coated microrobots in the dark do not show any toxic effect. These results indicate a synergistic effect between the self-propulsion provided by the enzyme and the photocatalytic activity induced under light stimuli. This work provides a photo-biocatalytic approach for the design of efficient light-driven microrobots with promising applications in microbiology and biomedicine.

Microswimming by oxidation of dibenzylamine

Chemiophoretic nano- and micromotors require a constant flow of product molecules to maintain a gradient that enables their propulsion. Apart from a smaller number of redox reactions that have been used, catalytic reactions are the main source of energy with the obvious benefit of making on-board fuel storage obsolete. However, the decomposition of H₂O₂ seems to strongly dominate the literature and although motion in H₂O through water splitting is becoming more popular, so far only a few different reactions have been used for propulsion of photocatalytic microswimmers. Here, we investigate the possibility of extending the range of possible fuelling reactions to organic reactions with high significance in organic synthesis - the oxidation of amines to imines. Herein, motion of the microswimmers is analysed at different amine concentrations and light intensities. The findings thereof are correlated with the reaction products identified and quantified by gas chromatography (GC).

Powering bioanalytical applications in biomedicine with light-responsive Janus micro-/nanomotors

Possessing both unique asymmetric structures and remote-controlled active movement, light-responsive Janus micro-/nanomotors offer the possibility of breaking through the limitations of traditional biomedicine, and have fascinated and inspired researchers. Despite many obstacles toward the clinical application, impressive progress of light-responsive Janus micro-/nanomotors for bioanalytical applications has been made over the past decades. In this review, we first briefly introduced several main light-driven Janus micro-/nanomotors, then focused on their typical bioanalytical applications such as biosensing, bioimaging, and theranostic. In the end, we summarized the remaining challenges of light-responsive Janus micro-/nanomotors in the practical application and also proposed potential solutions in the future.

Heat-Mediated Optical Manipulation

Progress in optical manipulation has stimulated remarkable advances in a wide range of fields, including materials science, robotics, medical engineering, and nanotechnology. This Review focuses on an emerging class of optical manipulation techniques, termed heat-mediated optical manipulation. In comparison to conventional optical tweezers that rely on a tightly focused laser beam to trap objects, heat-mediated optical manipulation techniques exploit tailorable optothermo-matter interactions and rich mass transport dynamics to enable versatile control of matter of various compositions, shapes, and sizes. In addition to conventional tweezing, more distinct manipulation modes, including optothermal pulling, nudging, rotating, swimming, oscillating, and walking, have been demonstrated to enhance the functionalities using simple and low-power optics. We start with an introduction to basic physics involved in heat-mediated optical manipulation, highlighting major working mechanisms underpinning a variety of manipulation techniques. Next, we categorize the heat-mediated optical manipulation techniques based on different working mechanisms and discuss working modes, capabilities, and applications for each technique. We conclude this Review with our outlook on current challenges and future opportunities in this rapidly evolving field of heat-mediated optical manipulation.

Engineering Active Micro and Nanomotors

Micro- and nanomotors (MNMs) are micro/nanoparticles that can perform autonomous motion in complex fluids driven by different power sources. They have been attracting increasing attention due to their great potential in a variety of applications ranging from environmental science to biomedical engineering. Over the past decades, this field has evolved rapidly, with many significant innovations contributed by global researchers. In this review, we first briefly overview the methods used to propel motors and then present the main strategies used to design proper MNMs. Next, we highlight recent fascinating applications of MNMs in two examplary fields, water remediation and biomedical microrobots, and conclude this review with a brief discussion of challenges in the field.

Visible-Light-Driven Asymmetric TiO2-Based Photocatalytic Micromotor Hybridized with a Conjugated Polyelectrolyte and Glucose Oxidase

We fabricated a TiO₂-based micromotor that was asymmetrically decorated with a water-soluble conjugated polymer (WSP) on one hemisphere and glucose oxidase (GO_x) on the opposite hemisphere. The WSP, which had photocatalytic activity for H₂O₂ decomposition, enabled motion of the micromotor under visible light. The GO_x on the other hemisphere of the micromotor decomposed glucose to produce H₂O₂ and enabled motion of the micromotor without light irradiation. In addition, WSP and GO_x were attached to TiO₂ by chemical bonds, providing stability during use. As a result, the micromotor could move by self-generating H₂O₂ for its own fuel by consuming glucose even without photoirradiation. The micromotor could move faster than without visible light irradiation through the synergistic decomposition of glucose and H₂O₂ under visible light by the diffusiophoretic mechanism with a speed of 7.49 μm/s.

Plasmon Induced Photocatalysts for Light-Driven Nanomotors

Micro/nanomachines (MNMs) correspond to human-made devices with motion in aqueous solutions. There are different routes for powering these devices. Light-driven MNMs are gaining increasing attention as fuel-free devices. On the other hand, Plasmonic nanoparticles (NPs) and their photocatalytic activity have shown great potential for photochemistry reactions. Here we review several photocatalyst nanosystems, with a special emphasis in Plasmon induced photocatalytic reactions, as a novel proposal to be explored by the MNMs community in order to extend the light-driven motion of MNMs harnessing the visible and near-infrared (NIR) light spectrum.

Carbon-nitride-based micromotor driven by chromate-hydrogen peroxide redox system: Application for removal of sulfamethaxazole

In this study, a Janus Fe/C₃N₄ micromotor driven by a chromate-hydrogen peroxide (Cr(VI)/H₂O₂) redox system was developed and its movement was analyzed. The motion of the micromotor was tracked via nanoparticle tracking analysis (NTA) and the corresponding diffusion coefficients (D) were determined. The NTA results revealed that D = 0 in water in the absence of additives (Cr(VI) or H₂O₂). The addition of H₂O₂ resulted in an increase in D from 0 to 12 × 10⁶ nm² s^-1, which further increased to 20 × 10⁶, 26.5 × 10⁶, 29 × 10⁶, and 44 × 10⁶ nm² s^-1 with the addition of 0.5, 1, 2, and 5 ppm of Cr(VI), respectively. Cr(VI) alone did not efficiently propel the Fe/C₃N₄-based micromotor. Therefore, it was proposed that the Cr(VI)/H₂O₂ redox system generates O₂, which plays a major role in the movement of the C₃N₄-based micromotor. In addition, the formation of reactive species, such as OH and ¹O₂, was confirmed through electron spin resonance experiments. The reactive species efficiently degraded sulfamethaxazole (SMX), an organic pollutant, as demonstrated through degradation studies and product analyses. The effects of various parameters, such as H₂O₂ concentration, Cr(VI) concentration, and initial pH on the movement of micromotor and degradation of SMX were also documented.

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.

Microchannels with Self-Pumping Walls

When asymmetric Janus micromotors are immobilized on a surface, they act as chemically powered micropumps, turning chemical energy from the fluid into a bulk flow. However, such pumps have previously produced only localized recirculating flows, which cannot be used to pump fluid in one direction. Here, we demonstrate that an array of three-dimensional, photochemically active Au/TiO₂ Janus pillars can pump water. Upon UV illumination, a water-splitting reaction rapidly creates a directional bulk flow above the active surface. By lining a 2D microchannel with such active surfaces, various flow profiles are created within the channels. Analytical and numerical models of a channel with active surfaces predict flow profiles that agree very well with the experimental results. The light-driven active surfaces provide a way to wirelessly pump fluids at small scales and could be used for real-time, localized flow control in complex microfluidic networks.

Enhanced Light-Harvesting Efficiency and Adaptation: A Review on Visible-Light-Driven Micro/Nanomotors

As visible light accounts for a larger proportion of solar energy and is harmless to living organisms, it has the potential to be the energy source of micro/nanomotors, which transform visible-light energy into mechanical motion, for different applications, especially in environmental remediation. However, how to precisely control the motion of visible-light-driven micro/nanomotors (VLD-MNMs) and efficiently utilize the weak visible-light photon energy to acquire rapid motion are significant challenges. This review summarizes the most critical aspects, involving photoactive materials, propulsion mechanisms, control methods, and applications of VLD-MNMs, and discusses strategies to systematically enhance the energy-harvesting efficiency and adaptation. At first, the photoactive materials have been divided into inorganic and organic photoactive materials and comprehensively discussed. Then, different propulsion mechanisms of the current VLD-MNMs are presented to explain the improvement in the actuation force, speed, and environmental adaptability. In addition, considering the characteristics of easy control of VLD-MNMs, we summarized the direction, speed, and cluster control methods of VLD-MNMs for different application requirements. Subsequently, the potential applications of VLD-MNMs, e.g., in environmental remediation, micropumps, cargo delivery, and sensing in microscale, are presented. Finally, discussions and suggestions for future directions to enhance the energy-harvesting efficiency and adaptation of VLD-MNMs are provided.

Photocatalytic Micromotors Activated by UV to Visible Light for Environmental Remediation, Micropumps, Reversible Assembly, Transportation, and Biomimicry

Photocatalytic micromotors are light-induced, chemically powered micromachines based on photocatalytic materials, activated by light illumination, and have redox reactions with environmental solutions to produce chemical gradients and bubbles that propel the micromachines through self-diffusiophoresis, self-electrophoresis, and bubble recoil. Due to the fact that excitation light relates largely to the bandgaps of selected materials, the development of photocatalytic micromotors has experienced an evolution from ultraviolet-light-activated to visible-light-activated and potentially biocompatible systems. Furthermore, due to the strong redox capacity and physical effects caused by the products or product gradients, photocatalytic micromotors have applications in environmental remediation, micropumps, reversible assembly, transportation, and biomimicry.

Cobalt-platinum nanomotors for local gas generation

Bimetallic Co-Pt nanorods exhibit an enhanced capacity for the production of gas from liquid-phase chemicals. Based on the systematic structural and magnetic characterization we discuss potential applications of these hybrid nanostructures for localized fuel generation in microdevices. Experimental proof of the feasibility for controlling the rate of catalytic reaction via external magnetic stimuli is shown. This unique functionality makes these hybrids promising candidates for optimizing the energy conversion rate in microfluidics fuel cells.

One body, two hands: photocatalytic function- and Fenton effect-integrated light-driven micromotors for pollutant degradation

The threat of water pollution represents a serious global concern and requires rapid and efficient neutralization methods. Herein, we report novel two-in-one light-driven micromotors, i.e., light-driven TiO₂-Fe Janus micromotors with both photocatalysis and photo-Fenton processes, for efficiently degrading organic pollutants in contaminated water. The TiO₂-Fe micromotors moved rapidly by utilizing the photocatalytic H₂O₂ decomposition over TiO₂ under UV irradiation, as well as generating highly reactive oxygen species responsible for the in situ degradation of the organic pollutants into non-harmful products. Notably, such coupling of photocatalysis generated on the TiO₂ sides and the photo-Fenton process generated on the Fe sides, along with the rapid movement of these catalytic Janus micromotors, results in a synergetic effect that can greatly enhance the degradation of organic pollutants. The degradation efficiency of the TiO₂-Fe micromotors is 52-fold that of only Fenton effects, and it is further improved by 40% compared to photocatalytic degradation alone. Considering the excellent advantages of the high efficiency, simple structure, reusability and the bubble-driven property, the new "on-the-fly" TiO₂-Fe micromotor-based method has a promising potential for future water cleaning and waste-water treatments.

Light-controlled two-dimensional TiO2 plate micromotors

In this work, UV light-controlled two-dimensional (2D) TiO2 plate micromotors are demonstrated for the first time. The 2D TiO2 micromotors are produced by the well-known anodic oxidation method in combination with a cracking and separation process. When the motor is placed in H2O2 aqueous solution under UV irradiation, oxygen bubbles are generated in the holes of the TiO2 membrane. The 2D micromotor thus moves upon O2 bubbles under its own weight. In contrast to bubble-propelled micromotors, which require an addition of surfactants to chemical fuels, the 2D micromotor is capable of moving in aqueous H2O2 solution without surfactants. Moreover, speed of the 2D TiO2 micromotor can be controlled by the intensity of the UV light. Such surfactant-free micromotors and their facile fabrication hold considerable promise for diverse practical applications in the biomedical and energy fields, for example, and in new materials.

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.

Metal-Free Visible-Light Photoactivated C3N4 Bubble-Propelled Tubular Micromotors with Inherent Fluorescence and On/Off Capabilities

Photoactivated micromachines are at the forefront of the micro- and nanomotors field, as light is the main power source of many biological systems. Currently, this rapidly developing field is based on metal-containing segments, typically TiO₂ and precious metals. Herein, we present metal-free tubular micromotors solely based on graphitic carbon nitride, as highly scalable and low-cost micromachines that can be actuated by turning on/off the light source. These micromotors are able to move by a photocatalytic-induced bubble-propelled mechanism under visible light irradiation, without any metal-containing part or biochemical molecule on their structure. Furthermore, they exhibit interesting properties, such as a translucent tubular structure that allows the optical visualization of the O₂ bubble formation and migration inside the microtubes, as well as inherent fluorescence and adsorptive capability. Such properties were exploited for the removal of a heavy metal from contaminated water with the concomitant optical monitoring of its adsorption by fluorescence quenching. This multifunctional approach contributes to the development of metal-free bubble-propelled tubular micromotors actuated under visible light irradiation for environmental applications.

Light-Driven Micro/Nanomotor for Promising Biomedical Tools: Principle, Challenge, and Prospect

A micro/nanomotor (MNM), as miniaturized machinery, can potentially bridge the application gap between the traditional macroscale motor and the molecular motor to manipulate materials at the cellular scale. The fascinating biomedical potential application for these tiny robots has been long envisioned by science fiction, such as "Fantastic Voyage", where complicated surgery can be performed at single cell precision without any surgical incision. However, to enter the highly conservative biomedical and healthcare industry in practice, the MNM must provide unique advantages over existing technology without introducing additional health risk, which has not been fully materialized. As an emerging approach, light-driven micro/nanomotors (LMNMs) have demonstrated several unique advantages over other MNMs, which will be addressed in this Account. As a control signal, light promises additional degrees of freedom to manipulate MNMs by modulating the light intensity, frequency, polarization, and propagation direction with spatial and temporal precision, which enables excellent controllability and programmability of LMNMs. Additionally, the fruitful knowledge and catalysts from the well-studied photocatalysis can be readily transferred to LMNMs for photoelectrochemical reactions, which provides a rich materials inventory for the development of advanced LMNM systems. A model LMNM in general can be regarded as a miniaturized solar cell combined with electrokinetic propulsion parts, where electric current is provided by the photovoltaic effect and then converted to propulsion thrust through a variety of electrokinetic mechanisms. It can be envisioned that the electric current may be further regulated with the onboard electronic circuit for advanced logic-controlled nanorobots. Finally, because incident photons instead of active chemicals provide the energy for LMNM propulsion, the highly active but toxic chemical fuels can be avoided, which suggested their better biocompatibility. It is essential to emphasize that all of these promises rely on the in-depth understanding of the photoelectrochemical reaction as well as the physics of electrokinetic phenomena, which requires further investigations. As a persistent endeavor, the biomedical application is the most attractive but challenging target for MNMs. Currently, most of the MNMs are demonstrated with in vitro conditions largely deviating from the biological environment, and nontrivial in vivo studies and cytotoxicity experiments are rarely reported. As merits of MNMs, the efficiency, biocompatibility, ion tolerance, and controllability critically determine the future success of MNMs. In this Account, existing and prospective solutions in these aspects are systemically discussed for light-propelled MNMs. We believe that, with a better understanding of the fundamental photoelectrochemical and electrokinetic processes, the development of motor design strategies, and improved fabrication methods, the promised practical biomedical application, such as early disease diagnosis, interventional therapy, targeted therapy, and microsurgery, could be realized in the near future.

Photocatalytic Micro/Nanomotors: From Construction to Applications

Synthetic micro/nanomotors (MNMs) are a particular class of micrometer or nanometer scale devices with controllable motion behavior in solutions by transferring various energies (chemical, optical, acoustic, magnetic, electric, etc.) into mechanical energy. These tiny devices can be functionalized either chemically or physically to accomplish complex tasks in a microcosm. Up to now, MNMs have exhibited great potential in various fields, ranging from environmental remediation, nanofabrication, to biomedical applications. Recently, light-driven MNMs as classic artificial MNMs have attracted much attention. Under wireless remote control, they can perform reversible and repeatable motion behavior with immediate photoresponse. Photocatalytic micro/nanomotors (PMNMs) based on photocatalysts, one of the most important light-driven MNMs, can utilize energy from both the external light source and surrounding chemicals to achieve efficient propulsion. Unlike other kinds of MNMs, the PMNMs have a unique characteristic: photocatalytic property. On one hand, since photocatalysts can convert both optical and chemical energy inputs into mechanical propulsion of PMNMs via photocatalytic reactions, the propulsion generated can be modulated in many ways, such as through chemical concentration or light intensity. In addition, these PMNMs can be operated at low levels of optical and chemical energy input which is highly desired for more practical scenarios. Furthermore, PMNMs can be operated with custom features, including go/stop motion control through regulating an on/off switch, speed modulation through varying light intensities, direction control through adjusting light source position, and so forth. On the other hand, as superoxide radicals can be generated by photocatalytic reactions of activated photocatalysts, the PMNMs show great potential in environment remediation, especially in organic pollutant degradation. In order to construct more practical PMNMs for future applications and further extend their application fields, the ideal PMNMs should be operated in a fully environmentally friendly system with strong propulsion. In the past decade, great progress in the construction, motion regulation, and application of PMNMs has been achieved, but there are still some challenges to realize the perfect system. In this Account, we will summarize our recent efforts and those of other groups in the development toward attractive PMNM systems. First, we will illustrate basic principles about the photocatalytic reactions of photocatalysts and demonstrate how the photocatalytic reactions affect the propulsion of PMNMs. Then, we will illustrate the construction strategies for highly efficient and biocompatible PMNMs from two key aspects: (1) Improvement of energy conversion efficiency to achieve strong propulsion of PMNMs. (2) Expansion of the usable wavelengths of light to operate PMNMs in environment-friendly conditions. Next, potential applications of PMNMs have been described. In particular, environment remediation has taken major attention for the applications of PMNMs due to their photocatalytic properties. Finally, in order to promote the development of PMNMs which can be operated in fully green environments for more practical applications, an outlook of key challenges and opportunities in construction of ideal PMNMs is presented.

Multifunctional and self-propelled spherical Janus nano/micromotors: recent advances

Recent progress in autonomous self-propelled multifunctional Janus nano/micromotors, which are able to convert chemical or light energy into mechanical motion, is presented. This technology of moving micro- and nanodevices is at the forefront of materials research and is a promising and growing technology with the possibility of using these motors in both biomedical and environmental applications. The development of novel multifunctional Janus motors together with their motion mechanisms is discussed. Different preparation and synthesis routes are compared. The effects of the size, interfacial structures and porosity on the directional motion and the speed of Janus micromotors are discussed. For light-derived Janus micromotors, newly developed techniques that are able to observe directly the interfaces' charge distribution on a nanometer scale are presented in order to clarify the underlying electrophoresis motion mechanism. This review aims to encourage further research in the field of micromotors using new and facile methodologies for obtaining novel Janus motors with enhanced motion and activity.

Cu@TiO2 Janus microswimmers with a versatile motion mechanism

We report novel metal-capped TiO2 photochemically-active colloids endowed with a 'hybrid drive': directional motion is achieved in water upon UV illumination, as well as in dilute peroxide solutions upon illumination with UV or visible light. From the different behaviours of nearby particles, we infer that distinct reaction pathways affect the local composition and flow of the solution.

Light-induced propulsion of a giant liposome driven by peptide nanofibre growth

Light-driven nano/micromotors are attracting much attention, not only as molecular devices but also as components of bioinspired robots. In nature, several pathogens such as Listeria use actin polymerisation machinery for their propulsion. Despite the development of various motors, it remains challenging to mimic natural systems to create artificial motors propelled by fibre formation. Herein, we report the propulsion of giant liposomes driven by light-induced peptide nanofibre growth on their surface. Peptide-DNA conjugates connected by a photocleavage unit were asymmetrically introduced onto phase-separated giant liposomes. Ultraviolet (UV) light irradiation cleaved the conjugates and released peptide units, which self-assembled into nanofibres, driving the translational movement of the liposomes. The velocity of the liposomes reflected the rates of the photocleavage reaction and subsequent fibre formation of the peptide-DNA conjugates. These results showed that chemical design of the light-induced peptide nanofibre formation is a useful approach to fabricating bioinspired motors with controllable motility.

Light-driven micro/nanomotors: from fundamentals to applications

Light, as an external stimulus, is capable of driving the motion of micro/nanomotors (MNMs) with the advantages of reversible, wireless and remote manoeuvre on demand with excellent spatial and temporal resolution. This review focuses on the state-of-the-art light-driven MNMs, which are able to move in liquids or on a substrate surface by converting light energy into mechanical work. The general design strategies for constructing asymmetric fields around light-driven MNMs to propel themselves are introduced as well as the photoactive materials for light-driven MNMs, including photocatalytic materials, photothermal materials and photochromic materials. Then, the propulsion mechanisms and motion behaviors of the so far developed light-driven MNMs are illustrated in detail involving light-induced phoretic propulsion, bubble recoil and interfacial tension gradient, followed by recent progress in the light-driven movement of liquid crystalline elastomers based on light-induced deformation. An outlook is further presented on the future development of light-driven MNMs towards overcoming key challenges after summarizing the potential applications in biomedical, environmental and micro/nanoengineering fields.

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.

Light-Guided Nanomotor Systems for Autonomous Photothermal Cancer Therapy

Machines have greatly contributed to the human civilization, enabling tasks beyond our capacities for improved quality of life. Recently, the progress in nanotechnology has triggered to build a miniaturized machine of nanoscale. In this context, synthetic nanomotors have gained considerable interest because of their great promise for diverse applications. Currently, the movement control of these nanomotors has been widely investigated using various stimuli. Here, we demonstrate near-infrared (NIR) light controlled on/off motion of stomatocyte nanomotors powered by the conversion of hydrogen peroxide. The nanomotors encapsulating naphthalocyanine (NC) are aggregated or separated (collective motion) with or without near-IR light illumination, resulting in the well-controlled movement. Remarkably, the nanomotors can move directionally toward hydrogen peroxide released from cancer cells and photothermally ablate the cancer cells. Taken together, our stomatocyte nanomotor systems can be effectively harnessed for autonomous photothermal cancer therapy.

Motion Manipulation of Micro- and Nanomotors

Inspired by the self-migration of microorganisms in nature, artificial micro- and nanomotors can mimic this fantastic behavior by converting chemical fuel or external energy into mechanical motion. These self-propelled micro- and nanomotors, designed either by top-down or bottom-up approaches, are able to achieve different applications, such as environmental remediation, sensing, cargo/sperm transportation, drug delivery, and even precision micro-/nanosurgery. For these various applications, especially biomedical applications, regulating on-demand the motion of micro- and nanomotors is quite essential. However, it remains a continuing challenge to increase the controllability over motors themselves. Here, we will discuss the recent advancements regarding the motion manipulation of micro- and nanomotors by different approaches.

Light-harvesting synthetic nano- and micromotors: a review

Nano- and micromotors are machines that can be made to perform specialized tasks as they propel themselves in response to certain stimuli. While the design of these self-propelling nano- and micromotors remains challenging, they have nevertheless attracted considerable research due to their many promising applications. Most self-propelled nano- and micromotors are based on the conversion of chemical energy into mechanical movement. Recently, however, the development of motors that can be propelled by light as an external stimulus has received much attention. The reason being that light is a renewable energy source that does not require any physical connection to the motor, does not usually lead to any waste products, and is easy to control. This review highlights recent progress in the development of light-harvesting synthetic motors that can be efficiently propelled and accurately controlled by exposure to light, and gives an overview of their fabrication methods, propulsion mechanisms, and practical applications.

Multiwavelength Light-Responsive Au/B-TiO2 Janus Micromotors

Conventional photocatalytic micromotors are limited to the use of specific wavelengths of light due to their narrow light absorption spectrum, which limits their effectiveness for applications in biomedicine and environmental remediation. We present a multiwavelength light-responsive Janus micromotor consisting of a black TiO₂ microsphere asymmetrically coated with a thin Au layer. The black TiO₂ microspheres exhibit absorption ranges between 300 and 800 nm. The Janus micromotors are propelled by light, both in H₂O₂ solutions and in pure H₂O over a broad range of wavelengths including UV, blue, cyan, green, and red light. An analysis of the particles' motion shows that the motor speed decreases with increasing wavelength, which has not been previously realized. A significant increase in motor speed is observed when exploiting the entire visible light spectrum (>400 nm), suggesting a potential use of solar energy, which contains a great portion of visible light. Finally, stop-go motion is also demonstrated by controlling the visible light illumination, a necessary feature for the steerability of micro- and nanomachines.

Emerging materials for the fabrication of micro/nanomotors

Self-propelled autonomous nano and micromotors are devices which in many aspects mimic living organisms: they take chemical energy from the environment and convert it to motion; they are capable of phototaxis, chemotaxis and magnetotaxis, following the gradient of fuel, a magnetic field or light. There is an immense spectrum of possible applications of these devices, ranging from environmental remediation to the biomedical field. All of these developments depend on the materials used and there has been intensive development of materials allowing more efficient propulsion, phototaxis, chemotaxis and enhanced applications of these devices. Here we review the emerging materials employed in the fabrication of nano/micromotors and discuss their applications in the field of nanorobots.

Self-thermophoretic motion of controlled assembled micro-/nanomotors

Controlled assembled micro-/nanomotors are driven in fluid by near infrared light. The behaviour and mechanism of self-thermophoretic motion are reviewed.

UV Light⁻Induced Aggregation of Titania Submicron Particles

In this study, aggregation of TiO₂ (rutile and anatase) submicron particles in deionized (DI) water under ultra-violet (UV) light irradiation was investigated. While no aggregation was observed in the dark, rutile and anatase submicron particles started aggregating upon application of UV light and ceased aggregation in about 2 and 8.4 h, respectively. It has been demonstrated that UV light directly mitigated the particle mobility of TiO₂, resulting in a neutralization effect of the Zeta potential. It was also observed that rutile particles aggregated much faster than anatase particles under UV radiation, indicating that the Zeta potential of as-prepared rutile is less than that of anatase in deionized (DI) water. In addition, the interaction energy of rutile and anatase particles was simulated using the Derjaguin–Landau–Verwey–Overbeek (DLVO) model. The results showed a significant reduction of barrier energy from 118.2 k_BT to 33.6 k_BT for rutile and from 333.5 k_BT to 46.1 k_BT for anatase, respectively, which further validated the remarkable influence of UV irradiation on the aggregation kinetics of rutile and anatase submicron particles. This work presents a further understanding of the aggregation mechanism of light-controlled submicron particles and has a promising potential application in environmental remediation.

Motion Control of Micro-/Nanomotors

As we progress towards employing self-propelled micro-/nanomotors in envisioned applications such as cargo delivery, environmental remediation, and therapeutic treatments, precise control of the micro-/nanomotors direction and their speed is essential. In this Review, major emerging approaches utilized for the motion control of micro-/nanomotors have been discussed, together with the lastest publications describing these approaches. Future studies could incorporate investigations on micro-/nanomotors motion control in a real-world environment in which matrix complexity might disrupt successful manipulation of these small-scale devices.

Progress toward Light-Harvesting Self-Electrophoretic Motors: Highly Efficient Bimetallic Nanomotors and Micropumps in Halogen Media

We have developed a highly efficient, bubble-free autonomous nanomotor based on a nanobattery. Bimetallic silver-platinum nanorods are powered by self-electrophoresis and show speeds much higher than those of other electrophoretic motors at similar fuel concentrations. The fuel (I2) can be regenerated by exposure to ambient light, leading to renewed motion of the motor. This versatile system can also be made into a micropump that transports fluid and particles.

Accelerated microrockets with a biomimetic hydrophobic surface

A biomimetic method was employed to accelerate the velocity and thereby to improve its propulsion efficiency of microrockets.

Autonomous movement in mixed metal based soft-oxometalates induced by CO2evolution and topological effects on their propulsion

Exploiting the intrinsic acidic nature of mixed-metal soft-oxometalates (SOMs) motility is induced using bicarbonate as fuel.