Glucose-Fueled Micromotors with Highly Efficient Visible-Light Photocatalytic Propulsion

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.

Micro/Nanomotor-Driven Intelligent Targeted Delivery Systems: Dynamics Sources and Frontier Applications

Micro/nanomotors represent a promising class of drug delivery carriers capable of converting surrounding chemical or external energy into mechanical power, enabling autonomous movement. Their distinct autonomous propulsive force distinguishes them from other carriers, offering significant potential for enhancing drug penetration across cellular and tissue barriers. A comprehensive understanding of micro/nanomotor dynamics with various power sources is crucial to facilitate their transition from proof-of-concept to clinical application. In this review, micro/nanomotors are categorized into three classes based on their energy sources: endogenously stimulated, exogenously stimulated, and live cell-driven. The review summarizes the mechanisms governing micro/nanomotor movements under these energy sources and explores factors influencing autonomous motion. Furthermore, it discusses methods for controlling micro/nanomotor movement, encompassing aspects related to their structure, composition, and environmental factors. The remarkable propulsive force exhibited by micro/nanomotors makes them valuable for significant biomedical applications, including tumor therapy, bio-detection, bacterial infection therapy, inflammation therapy, gastrointestinal disease therapy, and environmental remediation. Finally, the review addresses the challenges and prospects for the application of micro/nanomotors. Overall, this review emphasizes the transformative potential of micro/nanomotors in overcoming biological barriers and enhancing therapeutic efficacy, highlighting their promising clinical applications across various biomedical fields.

Autonomous engulfment of active colloids by giant lipid vesicles

Our ability to design artificial micro/nanomachines able to perform sophisticated tasks crucially depends on our understanding of their interaction with biosystems and their compatibility with the biological environment. Here, we design Janus colloids fuelled only by glucose and light, which can autonomously interact with cell-like compartments and trigger endocytosis. We evidence the crucial role played by the far-field hydrodynamic interaction arising from the puller/pusher swimming mode and adhesion. We show that a large contact time between the active particle and the lipid membrane is required to observe the engulfment of a particle inside a floppy giant lipid vesicle. Active Janus colloids showing relatively small velocities and a puller type swimming mode are able to target giant vesicles, deform their membranes and subsequently get stably engulfed. An instability arising from the unbound membrane segment is responsible for the transition between partial and complete stable engulfment. These experiments shed light on the physical criteria required for autonomous active particle engulfment in giant vesicles, which can serve as general principles in disciplines ranging from drug delivery and microbial infection to nanomedicine.

Light-driven rGO/Cu2 + 1O tubular nanomotor with active targeted drug delivery for combination treatment of cancer cells

The unprecedented navigation ability in micro/nanoscale and tailored functionality tunes micro/nanomotors as new target drug delivery systems, open up new horizons for biomedical applications. Herein, we designed a light-driven rGO/Cu2 + 1O tubular nanomotor for active targeting of cancer cells as a drug delivery system. The propulsion performance is greatly enhanced in real cell media (5% glucose cells isotonic solution), attributing to the introduction of oxygen vacancy and reduced graphene oxide (rGO) layer for separating photo-induced electron-hole pairs. The motion speed and direction can be readily modulated. Meanwhile, doxorubicin (DOX) can be loaded quickly on the rGO layer because of π-π bonding effect. The Cu2 + 1O matrix in the tiny robots not only serves as a photocatalyst to generate a chemical concentration gradient as the driving force but also acts as a nanomedicine to kill cancer cells as well. The strong propulsion of light-driven rGO/Cu2 + 1O nanomotors coupled with tiny size endow them with active transmembrane transport, assisting DOX and Cu2 + 1O breaking through the barrier of the cell membrane. Compared with non-powered nanocarrier and free DOX, light-propelled rGO/Cu2 + 1O nanomotors exhibit greater transmembrane transport efficiency and significant therapeutic efficacy. This proof-of-concept nanomotor design presents an innovative approach against tumor, enlarging the list of biomedical applications of light-driven micro/nanomotors to the superficial tissue treatment.

Boosting the Efficiency of Photoactive Rod-Shaped Nanomotors via Magnetic Field-Induced Charge Separation

Photocatalytic nanomotors have attracted a lot of attention because of their unique capacity to simultaneously convert light and chemical energy into mechanical motion with a fast photoresponse. Recent discoveries demonstrate that the integration of optical and magnetic components within a single nanomotor platform offers novel advantages for precise motion control and enhanced photocatalytic performance. Despite these advancements, the impact of magnetic fields on energy transfer dynamics in photocatalytic nanomotors remains unexplored. Here, we introduce dual-responsive rod-like nanomotors, made of a TiO2/NiFe heterojunction, able to (i) self-propel upon irradiation, (ii) align with the direction of an external magnetic field, and (iii) exhibit enhanced photocatalytic performance. Consequently, when combining light irradiation with a homogeneous magnetic field, these nanomotors exhibit increased velocities attributed to their improved photoactivity. As a proof-of-concept, we investigated the ability of these nanomotors to generate phenol, a valuable chemical feedstock, from benzene under combined optical and magnetic fields. Remarkably, the application of an external magnetic field led to a 100% increase in the photocatalytic phenol generation in comparison with light activation alone. By using various state-of-the-art techniques such as photoelectrochemistry, electrochemical impedance spectroscopy, photoluminescence, and electron paramagnetic resonance, we characterized the charge transfer between the semiconductor and the alloy component, revealing that the magnetic field significantly improved charge pair separation and enhanced hydroxyl radical generation. Consequently, our work provides valuable insights into the role of magnetic fields in the mechanisms of light-driven photocatalytic nanomotors for designing more effective light-driven nanodevices for selective oxidations.

Collagenase motors in gelatine-based hydrogels

Nano/micromotors outperform Brownian motion due to their self-propulsive capabilities and hold promise as carriers for drug delivery across biological barriers such as the extracellular matrix. This study employs poly(2-(diethylamino)ethyl methacrylate) polymer brushes to enhance the collagenase-loading capacity of silica particle-based motors with the aim to systematically investigate the impact of gelatine viscosity, motors' size, and morphology on their propulsion velocity. Notably, 500 nm and 1 μm motors achieve similar speeds as high as ∼15 μm s-1 in stiff gelatine-based hydrogels when triggered with calcium. Taken together, our findings highlight the potential of collagenase-based motors for navigating the extracellular matrix, positioning them as promising candidates for efficient drug delivery.

NIR-Mediated Cu2O/Au Nanomotors for Synergistically Treating Hepatoma Carcinoma Cells

We presented a NIR-driven Janus Cu2O/Au nanomotor. The nanomotor has a truncated octahedral structure. By asymmetric Au evaporation, the light response range of Cu2O nanomotor is extended to near-infrared range, and the speed of Cu2O/Au nanomotors under NIR is significantly increased. In promoting apoptosis of hepatocellular carcinoma, except the nanotoxicity of Cu2O itself, the Au layer enhances the photothermal properties, allowing Cu2O/Au nanomotors to induce apoptosis in hepatocellular carcinoma cells by heating them. On the other hand, a Schottky barrier formed at the interface of Cu2O and Au, preventing the recombination of electrons, which makes more electrons react with biomolecules to produce toxic ROS to kill hepatocellular cells. The killing rate of hepatocellular carcinoma cells reached 87 % by the combined effect of nanotoxicity inhibition of proliferation and photothermal & photodynamic therapy (PTT & PDT). Nanomotors in combination with multiple approaches are explored as a new treatment to tumor in this article.

A portable microfluidic device for thermally controlled granular sample manipulation

Effective granular sample manipulation with a portable and visualizable microfluidic device is significant for lots of applications, such as point-of-care testing and cargo delivery. Herein, we report a portable microfluidic device for controlled particle focusing, migration and double-emulsion droplet release via thermal fields. The device mainly contains a microfluidic chip, a microcontroller with a DC voltage control unit, a built-in microscope with a video transmission unit and a smartphone. Five microheaters located at the bottom of the microfluidic chip are used to unevenly heat fluids and then induce thermal buoyancy flow and a thermocapillary effect, and the experiments can be conveniently visualized through a smartphone, which provides convenient sample detection in outdoor environments. To demonstrate the feasibility and multifunctionality of this device, the focusing manipulation of multiple particles is first analyzed by using silica particles and yeast cells as experimental samples. We can directly observe the particle focusing states on the screen of a smartphone, and the particle focusing efficiency can be flexibly tuned by changing the control voltage of the microheater. Then the study focus is transferred to single-particle migration. By changing the voltage combinations applied on four strip microheaters, the single particle can migrate at predetermined trajectory and speed, showing attractiveness for those applications needing sample transportation. Finally, we manipulate the special three-phase flow system of double-emulsion drops in thermal fields. Under the combined effect of the thermocapillary effect and increased instability, the shell of double-emulsion droplets gradually thins and finally breaks, resulting in the release of samples in inner cores. The core release speed can also be flexibly adjusted by changing the control voltage of the microheater. These three experiments successfully demonstrate the effectiveness and multifunctionality of this thermally actuated microfluidic device on granular manipulation. Therefore, this portable microfluidic device will be promising for lots of applications, such as analytical detection, microrobot actuation and cargo release.

Ultrasound-propelled nanomotors for efficient cancer cell ferroptosis

Ferroptosis is a non-apoptotic form of cell death that is dependent on the accumulation of intracellular iron that causes elevation of toxic lipid peroxides. Therefore, it is crucial to improve the levels of intracellular iron and reactive oxygen species (ROS) in a short time. Here, we first propose ultrasound (US)-propelled Janus nanomotors (Au-FeOx/PEI/ICG, AFPI NMs) to accelerate cellular internalization and induce cancer cell ferroptosis. This nanomotor consists of a gold-iron oxide rod-like Janus nanomotor (Au-FeOx, AF NMs) and a photoactive indocyanine green (ICG) dye on the surface. It not only exhibits accelerating cellular internalization (∼4-fold) caused by its attractive US-driven propulsion but also shows good intracellular motion behavior. In addition, this Janus nanomotor shows excellent intracellular ROS generation performance due to the synergistic effect of the "Fenton or Fenton-like reaction" and the "photochemical reaction". As a result, the killing efficiency of actively moving nanomotors on cancer cells is 88% higher than that of stationary nanomotors. Unlike previous passive strategies, this work is a significant step toward accelerating cellular internalization and inducing cancer-cell ferroptosis in an active way. These novel US-propelled Janus nanomotors with strong propulsion, efficient cellular internalization and excellent ROS generation are suitable as a novel cell biology research tool.

A Review of Single-Cell Microrobots: Classification, Driving Methods and Applications

Single-cell microrobots are new microartificial devices that use a combination of single cells and artificial devices, with the advantages of small size, easy degradation and ease of manufacture. With externally driven strategies such as light fields, sound fields and magnetic fields, microrobots are able to carry out precise micromanipulations and movements in complex microenvironments. Therefore, single-cell microrobots have received more and more attention and have been greatly developed in recent years. In this paper, we review the main classifications, control methods and recent advances in the field of single-cell microrobot applications. First, different types of robots, such as cell-based microrobots, bacteria-based microrobots, algae-based microrobots, etc., and their design strategies and fabrication processes are discussed separately. Next, three types of external field-driven technologies, optical, acoustic and magnetic, are presented and operations realized in vivo and in vitro by applying these three technologies are described. Subsequently, the results achieved by these robots in the fields of precise delivery, minimally invasive therapy are analyzed. Finally, a short summary is given and current challenges and future work on microbial-based robotics are discussed.

Photoactive and Intrinsically Fuel Sensing Metal-Organic Framework Motors for Tailoring Collective Behaviors of Active-Passive Colloids

Microorganisms display nonequilibrium predator-prey behaviors, such as chasing-escaping and schooling via chemotactic interactions. Even though artificial systems have revealed such biomimetic behaviors, switching between them by control over chemotactic interactions is rare. Here, a spindle-like iron-based metal-organic framework (MOF) colloidal motor which self-propels in glucose and H2 O2 , triggered by UV light is reported. These motors display intrinsic UV light-triggered fuel-dependent chemotactic interactions, which are used to tailor the collective dynamics of active-passive colloidal mixtures. In particular, the mixtures of active MOF motors with passive colloids exhibit distinctive "chasing-escaping" or "schooling" behaviors, depending on glucose or hydrogen peroxide being used as the fuel. The transition in the collective behaviors is attributed to an alteration in the sign of ionic diffusiophoretic interactions, resulting from a change in the ionic clouds produced. This study offers a new strategy on tuning the communication between active and passive colloids, which holds substantial potentials for fundamental research in active matter and practical applications in cargo delivery, chemical sensing, and particle segregation.

Magnetic Nanoseparation Technology for Efficient Control of Microorganisms and Toxins in Foods: A Review

Outbreaks of foodborne diseases mediated by food microorganisms and toxins remain one of the leading causes of disease and death worldwide. It not only poses a serious threat to human health and safety but also imposes a huge burden on health care and socioeconomics. Traditional methods for the removal and detection of pathogenic bacteria and toxins in various samples such as food and drinking water have certain limitations, requiring a rapid and sensitive strategy for the enrichment and separation of target analytes. Magnetic nanoparticles (MNPs) exhibit excellent performance in this field due to their fascinating properties. The strategy of combining biorecognition elements with MNPs can be used for fast and efficient enrichment and isolation of pathogens. In this review, we describe new trends and practical applications of magnetic nanoseparation technology in the detection of foodborne microorganisms and toxins. We mainly summarize the biochemical modification and functionalization methods of commonly used magnetic nanomaterial carriers and discuss the application of magnetic separation combined with other instrumental analysis techniques. Combined with various detection techniques, it will increase the efficiency of detection and identification of microorganisms and toxins in rapid assays.

Quantum Dot-Based Micromotors with NIR-I Light Photocatalytic Propulsion and NIR-II Fluorescence

Here, we report the first PbS quantum dot (QD)-based micromotors with NIR-I light-driven photocatalytic propulsion and NIR-II fluorescence. Under the irradiation of NIR-I light (808 nm), PbS QD-doped cuprous oxide (Cu₂O@PbS) micromotors can display efficient propulsion in a variety of biocompatible fuels such as malic acid, glucose, and urea. Among them, the Cu₂O@PbS micromotors exhibit the best propulsion performance in a very low concentration of malic acid, with an average speed as high as 11.86 μm/s. The enhanced NIR-I photocatalytic activity of Cu₂O@PbS micromotors benefits from the doping of NIR-I PbS QDs that can be excited by NIR-I light and exhibit high electron transport efficiency. The doped PbS QDs can effectively increase the absorption efficiency of the micromotors in the NIR-I region while also inhibiting the recombination of photogenerated electron-hole pairs. Interestingly, due to the presence of NIR PbS QDs, the Cu₂O@PbS micromotors demonstrate prominent and stable NIR-II fluorescence (emission wavelength: 1100 nm), which offer promising potential for visualization of their position in vivo. In comparison to other photocatalytic micromotors, the simple fabrication strategy, excellent NIR-II fluorescence, together with the NIR-I light-dependent propulsion behavior of the current Cu₂O@PbS micromotors, thus pave the way for further development of advanced smart "robots" for intelligent biomedical applications.

Micro/nanomotor technology: the new era for food safety control

Food poisoning caused by eating contaminated food remains a threat to global public health. Making the situation even worse is the aggravated global environmental pollution, which poses a major threat to the safety of agricultural resources. Food adulteration has been rampant owing to negligent national food safety regulations. The speed at which contaminated food is detected and disposed of determines the extent to which consumers' lives are safeguarded and agricultural economic losses are prevented. Micro/nanomotors offer a high-speed mobile loading platform that substantially increases the chemical reaction rates and, accordingly, exhibit great potential as alternatives to conventional detection and degradation techniques. This review summarizes the propulsion modes applicable to micro/nanomotors in food systems and the advantages of using micro/nanomotors, highlighting examples of their potential use in recent years for the detection and removal of food contaminants. Micro/nanomotors are an emerging technology for food applications that is moving toward mass production, simple preparation, and important functions.

Hybrid photoresponsive/biocatalytic micro- and nano-swimmers

Micro/nano biomimetic systems that convert energy from the surroundings into mechanical motion have emerged as promising tools to enhance the efficiencies of different biomedical and environmental processes. The inclusion of multiple engines into the same device has become a promising strategy to achieve dual/triple stimuli responses. Such hybrid micro/nanoswimmers combining different propulsion forces exhibit advanced motion behaviors and different physical features that are interesting not only to achieve strong propulsion capabilities in complex environments but also to modulate their movement according to the intended use. The development of hybrid systems that can be actuated by both light and biocompatible fuels is of particular interest. This minireview covers the main types of photoactive/biocatalytic micro/nanoswimmers developed so far. Their main photoresponsive and enzymatic components are discussed along with the most representative designs. The applicability of such hybrid machines for analyte sensing, antibacterial and therapeutical uses are also described. The remaining challenges and opportunities are then explored.

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.

Shape-Controlled Self-Assembly of Light-Powered Microrobots into Ordered Microchains for Cells Transport and Water Remediation

Nature presents the collective behavior of living organisms aiming to accomplish complex tasks, inspiring the development of cooperative micro/nanorobots. Herein, the spontaneous assembly of hematite-based microrobots with different shapes is presented. Autonomous motile light-driven hematite/Pt microrobots with cubic and walnut-like shapes are prepared by hydrothermal synthesis, followed by the deposition of a Pt layer to design Janus structures. Both microrobots show a fuel-free motion ability under light irradiation. Because of the asymmetric orientation of the magnetic dipole moment in the crystal, cubic hematite/Pt microrobots can self-assemble into ordered microchains, contrary to the random aggregation observed for walnut-like microrobots. The microchains exhibit different synchronized motions under light irradiation depending on the mutual orientation of the individual microrobots during the assembly, which allows them to accomplish multiple tasks, including capturing, picking up, and transporting microscale objects, such as yeast cells and suspended matter in water extracted from personal care products, as well as degrading polymeric materials. Such light-powered self-assembled microchains demonstrate an innovative cooperative behavior for small-scale multitasking artificial robotic systems, holding great potential toward cargo capture, transport, and delivery, and wastewater remediation.

Magnetic Nanomotors in Emulsions for Locomotion of Microdroplets

The locomotion of droplets in emulsions is of practical significance for fields related to medicine and chemical engineering, which can be done with a magnetic field to move droplets containing magnetic materials. Here, we demonstrate a new method of droplet locomotion in the oil-in-water emulsion with the help of a nonuniform magnetic field in the case where magnetic nanoparticles (MNPs) are dispersed in the continuous phase of the emulsion. The paper analyses the motion of the droplets in a liquid film and in a capillary for various diameters of droplets, their number density, and viscosity of the continuous phase of the emulsion. It is established that the mechanism of droplet locomotion in the emulsion largely depends on the wettability of MNPs. Hydrophobic nanoparticles are adsorbed on the droplet surfaces, forming the agglomerates of MNPs with the droplets. Such agglomerates move at much higher velocities than passive droplets. Hydrophilic nanoparticles are not adsorbed at the surfaces of the droplets but form mobile magnetic clusters dispersed in the continuous phase of the emulsion. Mobile magnetic clusters set the surrounding liquid and droplets in motion. The results obtained in this paper can be used in drug delivery.

Light-driven carbon nitride microswimmers with propulsion in biological and ionic media and responsive on-demand drug delivery

We propose two-dimensional poly(heptazine imide) (PHI) carbon nitride microparticles as light-driven microswimmers in various ionic and biological media. Their high-speed (15 to 23 micrometer per second; 9.5 ± 5.4 body lengths per second) swimming in multicomponent ionic solutions with concentrations up to 5 M and without dedicated fuels is demonstrated, overcoming one of the bottlenecks of previous light-driven microswimmers. Such high ion tolerance is attributed to a favorable interplay between the particle's textural and structural nanoporosity and optoionic properties, facilitating ionic interactions in solutions with high salinity. Biocompatibility of these microswimmers is validated by cell viability tests with three different cell lines and primary cells. The nanopores of the swimmers are loaded with a model cancer drug, doxorubicin (DOX), resulting in a high (185%) loading efficiency without passive release. Controlled drug release is reported under different pH conditions and can be triggered on-demand by illumination. Light-triggered, boosted release of DOX and its active degradation products are demonstrated under oxygen-poor conditions using the intrinsic, environmentally sensitive and light-induced charge storage properties of PHI, which could enable future theranostic applications in oxygen-deprived tumor regions. These organic PHI microswimmers simultaneously address the current light-driven microswimmer challenges of high ion tolerance, fuel-free high-speed propulsion in biological media, biocompatibility, and controlled on-demand cargo release toward their biomedical, environmental, and other potential applications.

Powering and Fabrication of Small-Scale Robotics Systems

Purpose of Review

The increasing number of contributions in the field of small-scale robotics is significantly associated with the progress in material science and process engineering during the last half century. With the objective of integrating the most optimal materials for the propulsion of these motile micro- and nanosystems, several manufacturing strategies have been adopted or specifically developed. This brief review covers some recent advances in materials and fabrication of small-scale robots with a focus on the materials serving as components for their motion and actuation.

Recent Findings

Integration of a wealth of materials is now possible in several micro- and nanorobotic designs owing to the advances in micro- and nanofabrication and chemical synthesis. Regarding light-driven swimmers, novel photocatalytic materials and deformable liquid crystal elastomers have been recently reported. Acoustic swimmers are also gaining attention, with several prominent examples of acoustic bubble-based 3D swimmers being recently reported. Magnetic micro- and nanorobots are increasingly investigated for their prospective use in biomedical applications. The adoption of different materials and novel fabrication strategies based on 3D printing, template-assisted electrodeposition, or electrospinning is briefly discussed.

Summary

A brief review on fabrication and powering of small-scale robotics is presented. First, a concise introduction to the world of small-scale robotics and their propulsion by means of magnetic fields, ultrasound, and light is provided. Recent examples of materials and fabrication methodologies for the realization of these devices follow thereafter.

Active matter dynamics in confined microfluidic environments

The field of active matter is a nascent area of research in soft condensed matter physics, which is drawing on the expertise of researchers from diverse disciplines. Small scale active particles-both inorganic and biological-display non-trivial emergent dynamics and interactions that could help us understand complex biological processes and phenomena. Recently, using microfluidic technologies, several research groups have performed important experimental and theoretical studies to understand the behavior of self-propelled particles and molecular active matter within confined environments-to glean a fundamental understanding of the cellular processes occurring under ultra-low Reynolds number conditions. In this chapter, we would like to review applications of microfluidics in active matter research, highlighting a few important theoretical and experimental investigations. We will conclude the discussion with a note on the future of this field mentioning a few open questions that are at the forefront of our minds.

Nano/Microrobots Line Up for Gastrointestinal Tract Diseases: Targeted Delivery, Therapy, and Prevention

Nano/microrobots (NMRs) are tiny devices that can convert energy into motion and operate at nano/microscales.54 Especially in biomedical research, NMRs have received much attention over the past twenty years because of their excellent capabilities and great potential in various applications, including on-demand drug delivery, gene and cell transport, and precise microsurgery. Reports published in recent years show that synthetic nano/microrobots have promising potential to function in the gastrointestinal (GI) region, particularly in terms of drug delivery. These tiny robots were able to be designed in such a way that they propel in their surroundings (biological media) with high speed, load cargo (drug) efficiently, transport it safely, and release upon request successfully. Their propulsion, retention, distribution, and toxicity in the GI tract of mice has been evaluated. The results envisage that such nano/microrobots can be further modified and developed as a new-generation treatment of GI tract diseases. In this minireview, we focus on the functionality of micro/nanorobots as a biomedical treatment system for stomach/intestinal diseases. We review the research progress from the first in vivo report in December 2014 to the latest in August 2021. Then, we discuss the treatment difficulties and challenges in vivo application (in general) and possible future development routes.

Light-Activated Fuel-Free Janus Metal Organic Framework Colloidal Motors for the Removal of Heavy Metal Ions

Light-powered fuel-free colloidal motors possess significant potential for practical applications ranging from nanomedicine to environmental remediation. However, current light-powered colloidal motors often require the incorporation of expensive metals or high concentrations of toxic chemical fuels, which is a severe limitation for their practical applications. Integrating highly ordered and porous materials with a large surface area into colloidal motors is a promising strategy for upsurging their self-propelled speed and adsorption, which will benefit many applications. Here, highly efficient, fuel-free, and light-activated metal organic framework (MOF)-3-trimethoxysilyl propyl methacrylate Janus colloidal motors with a hierarchical morphology are reported. These colloidal motors can be driven by UV or visible light, with a self-propelled speed tuned by the light intensity. The speed can be further enhanced by morphology optimization or by the addition of H₂O₂ as a fuel. The colloidal motors display a superior efficiency in removing heavy metal ions of Hg, which is up to ∼90% within 40 min from the contaminated water, attributed to their high surface area, hierarchical morphology, large number of active sites, and high mobility. This work not only offers a facile approach to incorporate a versatile MOF family into the design of fuel-free and light-powered Janus colloidal motors, but also demonstrates their potential for real-life applications such as environmental remediation.

Functional coatings enable navigation of light-propelled micromotors in blood for effective biodetoxification

Herein we report the coating of visible light-driven polycaprolactone (PCL) based micromotors with an anti-biofouling poly lactic-co-glycolic acid (PLGA) layer for effective navigation and detoxification in blood samples. The micromotors encapsulate CdSe@ZnS quantum dots as photoresponsive materials and a Fe₃O₄ nanoparticle patch to promote electron transfer and reaction with glucose present in the media for diffusiophoretic propulsion in diluted blood. The coating of the micromotor with the PLGA layer prevents red blood cell adhesion and protein adsorption due to the creation of a highly efficient hydration layer. This results in an enhanced speed and efficient operation for enhanced toxin removal as compared with the bare PCL micromotors. Hemolysis and MTT assays along with no platelets aggregation revealed the high biocompatibility of the micromotors with living cells. Effective adsorptive removal of two relevant toxins, sepsis associated Escherichia coli O111:B4 toxin and snake venom α-bungarotoxin from blood is achieved with the PLGA micromotors. The new developments illustrated here represent one step forward in the use of light-driven micromotors for biomedical applications.

Better fuels for photocatalytic micromotors: a case study of triethanolamine

Efficient fuels are critical for designing photocatalytic micromotors with high performance. We discover that 0.5 mM of triethanolamine can power TiO₂-Pt motors at 35 μm s^-1 without producing bubbles, a significant improvement over conventional fuels such as water, H₂O₂ or hydroquinone. The effectiveness of hole scavengers such as triethanolamine can be generalized to other photocatalytic micromotors containing a heterojunction with an n-type (but not a p-type) semiconductor.

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.

Dipole-Moment Induced Phototaxis and Fuel-Free Propulsion of ZnO/Pt Janus Micromotors

Light-driven micromotors have stimulated considerate interests due to their potentials in biomedicine, environmental remediation, or serving as the model system for non-equilibrium physics of active matter. Simultaneous control over the motion direction and speed of micro/nanomotors is crucial for their functionality but still difficult since Brownian motion always randomizes the orientations. Here, a highly efficient light-driven ZnO/Pt Janus micromotor capable of aligning itself to illumination direction and exhibiting negative phototaxis at high speeds (up to 32 µm s^-1 ) without the addition of any chemical fuels is developed. A light-triggered self-built electric field parallel to the light illumination exists due to asymmetrical surface chemical reactions induced by the limited penetration depth of light along the illumination. The phototactic motion of the motor is achieved through electrophoretic rotation induced by the asymmetrical distribution of zeta potential on the two hemispheres of the Janus micromotor, into alignment with the electric field. Notably, similar phototactic propulsion is also achieved on TiO₂ /Pt and CdS/Pt micromotors, which presents explicit proof of extending the mechanism of dipole-moment induced phototactic propulsion in other light-driven Janus micromotors. Finally, active transportation of yeast cells are achieved by the motor, proving its capability in performing complex tasks.

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.

Polypyrrole-Based Nanorobots Powered by Light and Glucose for Pollutant Degradation in Water

Novel photoactive and enzymatically active nanomotors were developed for efficient organic pollutant degradation. The developed preparation route is simple and scalable. Light-absorbing polypyrrole nanoparticles were equipped with a bi-enzyme [glucose oxidase/catalase (GOx/Cat)] system enabling the simultaneous utilization of light and glucose as energy sources for jet-induced nanoparticle movement and active radical production. The GOx utilizes glucose to produce hydrogen peroxide, which is subsequently degraded by Cat, resulting in the generation of active radicals and/or oxygen bubbles that propel the particles. Uneven grafting of GOx/Cat molecules on the nanoparticle surface ensures inhomogeneity of peroxide creation/degradation, providing the nanomotor random propelling. The nanomotors were tested for their ability to degrade chlorophenol, under various experimental conditions, that is, with and without simulated sunlight illumination or glucose addition. In all cases, degradation was accelerated by the presence of the self-propelled nanoparticles or light illumination. Light-induced heating also positively affects enzymatic activity, further accelerating nanomotor diffusion and pollutant degradation. In fact, the chemical and photoactivities of the nanoparticles led to more than 95% removal of chlorophenol in 1 h, without any external stirring. Finally, the quality of the purified water and the extent of pollutant removal were checked using an eco-toxicological assay, with demonstrated significant synergy between glucose pumping and sunlight illumination.

Visible-light-driven cuprous oxide nanomotors with surface-heterojunction-induced propulsion

The controllable synthesis and customized design of micro/nanomotors represents a highly desired paradigm in the field of intelligent nanovehicles. Exploiting asymmetrical structures and geometry-dependent propulsion are the two main strategies for achieving light-driven micro/nanomotors. However, inherent crystal-structure differences in a single colloidal motor have rarely been explored. Here, we propose the first surface-heterojunction-induced propulsion methodology for cuprous oxide (Cu₂O) nanomotors, by tailoring the crystal morphology of a Cu₂O crystalloid from a sphere into a truncated octahedron and preserving the controllable-index crystal facets of {100} and {111} in a single colloid. Due to the high crystallinity and distinct activity of the exposed crystal facets, a surface heterojunction between the {100} and {111} facets is formed to enhance electron-hole separation, as confirmed by density functional theory (DFT) calculations, thus endowing the truncated octahedral Cu₂O nanomotors with autonomous and vigorous movement in biocompatible fuels under visible light. These Cu₂O nanomotors can reach a propulsion speed in water of over two times faster than that of polycrystalline spherical motors with low crystallinity. The efficient Cu₂O nanomotors offer a promising guideline not only for the synthesis of novel light-driven motors with desired structures, but also for potential applications in biocompatible environments.

Light-Responsive Nanofibrous Motor with Simultaneously Precise Locomotion and Reversible Deformation

Light-powered micromotors have drawn enormous attention because of their potential applications in cargo delivery, environmental monitoring, and noninvasive surgery. However, the existing micromotors still suffer from some challenges, including slow speed, poor controllability, single locomotion mode, and no deformation during movement. Herein, we employ a combined electrospinning with brushing of Chinese ink to simply fabricate a light-responsive gradient-structured poly(vinyl alcohol)/carbon (PVA/carbon) composite motor. Because of the surface deposition and ultrahigh loading amount of carbon nanoparticles (ca. 43%), the motor exhibits rapid (39 mm/s), direction-controlled, and multimodal locomotion (vertical movement, horizontal motion, rotation) under light irradiation. Simultaneously, gradient alignment structure of the PVA nanofibrous matrix endows the motor with controllable and reversible deformation during locomotion. We finally demonstrate the potential applications of the motors in leakage monitoring, object salvage, smart access, and intelligent assembly. The present work will inspire the design of novel photosensitive motors for applications in various fields, such as microrobots, environmental monitoring, and biomedicine.

Isotropic Hedgehog-Shaped-TiO2/Functional-Multiwall-Carbon-Nanotube Micromotors with Phototactic Motility in Fuel-Free Environments

Directional motion in response to specific signals is critically important for micro/nanomotors in precise cargo transport, obstacle avoidance, collective control, and complex maneuvers. In this work, a kind of isotropic light-driven micromotor that is made of hedgehog-shaped TiO₂ and functional multiwall carbon nanotubes (Hs-TiO₂@FCNTs) has been developed. The FCNTs are closely entangled with Hs-TiO₂ and form a close-knit matrix on the surface of Hs-TiO₂, which facilitates the transfer of electrons from Hs-TiO₂ to FCNTs. Due to the high redox potential of Hs-TiO₂, excellent electron-hole separation efficiency by the addition of FCNTs, and isotropic morphology of the micromotor, these Hs-TiO₂@FCNT micromotors show phototactic and fuel-free propulsion under unidirectional irradiation of UV light. It is the first time to demonstrate isotropic micromotors that are propelled by self-electrophoresis. The isotropy of Hs-TiO₂@FCNT micromotors makes them immune to the rotational Brownian diffusion and local flows, exhibiting superior directionality. The motion direction of our micromotors can be precisely tuned by light and a velocity of 8.9 μm/s is achieved under 160 mW/cm² UV light illumination. Photodegradation of methylene blue and active transportation of polystyrene beads are demonstrated for a proof-of-concept application of our micromotors. The isotropic design of the Hs-TiO₂@FCNT micromotors with enhanced photocatalytic properties unfolds a new paradigm for addressing the limitations of directionality control and chemical fuels in the current asymmetric light-driven micromotors.

Enzyme-driven micro/nanomotors: Recent advances and biomedical applications

Micro/nanomotors (MNMs), both self-propelled actuators and external fields-promoted machines, have joined forces in the past decade to accomplish versatile tasks such as precise detection and targeted cargo delivery with adequate propulsion and desirable locomotion. Amongst, enzyme-driven MNMs have been able to differentiate themselves from others owing to their distinct characteristics, such as absence of chemical fuel, enhanced cellular uptake and the possibility to be easily conjugated with many therapeutics, including both small molecules and biologics, displaying superior efficacy, enhanced specificity and diminished side effects. In the present review, we aim to highlight and summarize recent advances in enzyme-driven MNMs, particularly to provide an in-depth discussion focusing on the enzyme linking approaches onto those MNMs and motion control strategies of such MNMs with advantages and limitations thereof. Conclusions and future perspectives are also provided in brief.

Light-driven nanomotors and micromotors: envisioning new analytical possibilities for bio-sensing

The aim of this conceptual review is to cover recent developments of light-propelled micromotors for analytical (bio)-sensing. Challenges of self-propelled light-driven micromotors in complex (biological) media and potential solutions from material aspects and propulsion mechanism to achieve final analytical detection for in vivo and in vitro applications will be comprehensively covered. Graphical abstract.

Smartdust 3D-Printed Graphene-Based Al/Ga Robots for Photocatalytic Degradation of Explosives

Milli/micro/nanorobots are considered smart devices able to convert energy taken from different sources into mechanical movement and accomplish the appointed tasks. Future advances and realization of these tiny devices are mostly limited by the narrow window of material choices, the fuel requirement, multistep surface functionalization, rational structural design, and propulsion ability in complex environments. All these aspects call for intensive improvements that may speed up the real application of such miniaturized robots. 3D-printed graphene-based smartdust robots provided with a magnetic response and filled with aluminum/gallium molten alloy (Al/Ga) for autonomous motion are presented. These robots can swim by reacting with the surrounding environment without adding any fuel. Because their outer surface is coated with a hydrogel/photocatalyst (chitosan/carbon nitride, C₃ N₄ ) layer, these robots are used for the photocatalytic degradation of the picric acid as an explosive model molecule under visible light. The results show a fast and efficient degradation of picric acid that is attributed to a synergistic effect between the adsorption capability of the chitosan and the photocatalytic activity of C₃ N₄ particles. This work provides added insight into the large-scale fabrication, easy functionalization, and propulsion of tiny robots for environmental applications.

Micro/Nanorobot: A Promising Targeted Drug Delivery System

Micro/nanorobot, as a research field, has attracted interest in recent years. It has great potential in medical treatment, as it can be applied in targeted drug delivery, surgical operation, disease diagnosis, etc. Differently from traditional drug delivery, which relies on blood circulation to reach the target, the designed micro/nanorobots can move autonomously, which makes it possible to deliver drugs to the hard-to-reach areas. Micro/nanorobots were driven by exogenous power (magnetic fields, light energy, acoustic fields, electric fields, etc.) or endogenous power (chemical reaction energy). Cell-based micro/nanorobots and DNA origami without autonomous movement ability were also introduced in this article. Although micro/nanorobots have excellent prospects, the current research is mainly based on in vitro experiments; in vivo research is still in its infancy. Further biological experiments are required to verify in vivo drug delivery effects of micro/nanorobots. This paper mainly discusses the research status, challenges, and future development of micro/nanorobots.

Medical micro/nanorobots in complex media

Medical micro/nanorobots have received tremendous attention over the past decades owing to their potential to be navigated into hard-to-reach tissues for a number of biomedical applications ranging from targeted drug/gene delivery, bio-isolation, detoxification, to nanosurgery. Despite the great promise, the majority of the past demonstrations are primarily under benchtop or in vitro conditions. Many developed micro/nanoscale propulsion mechanisms are based on the assumption of a homogeneous, Newtonian environment, while realistic biological environments are substantially more complex. Moving toward practical medical use, the field of micro/nanorobotics must overcome several major challenges including propulsion through complex media (such as blood, mucus, and vitreous) as well as deep tissue imaging and control in vivo. In this review article, we summarize the recent research efforts on investigating how various complexities in biological environments impact the propulsion of micro/nanoswimmers. We also highlight the emerging technological approaches to enhance the locomotion of micro/nanorobots in complex environments. The recent demonstrations of in vivo imaging, control and therapeutic medical applications of such micro/nanorobots are introduced. We envision that continuing materials and technological innovations through interdisciplinary collaborative efforts can bring us steps closer to the fantasy of "swallowing a surgeon".

Rational Design of Reversible Redox Shuttle for Highly Efficient Light-Driven Microswimmer

The light-driven micro/nanomotor (LMNM) is machinery that harvests photon energy and generates self-propulsion in varieties of liquid media. Though visions are made that these tiny swimming machines can serve future medicine for accurate drug delivery and noninvasive microsurgery, their biomedical application is still impeded by the insufficient propulsion efficiency. Here we provide a holistic model of LMNM by considering (i) photovoltaic, (ii) electrochemical, and (iii) electrokinetic processes therein. Such a quantitative model revealed the pivotal role of reaction kinetics and diffusion properties of shuttle ions in the propulsion efficiency of LMNM. With the guidance of this model, a group of ferrocene-based reversible redox shuttles, which generate slow-diffusion ions, was identified, showcasing a high locomotion velocity of ∼500 μm/s (∼100 body length per second) at an ultralow concentration (70 μM). Owing to the in-depth understanding of the fundamental energy conversion processes in LMNM, we anticipate that the development of other high-performance supporting chemicals and LMNM systems will be greatly motivated, foreseeing the advent of LMNM systems with superior efficiency.

Temporal Light Modulation of Photochemically Active, Oscillating Micromotors: Dark Pulses, Mode Switching, and Controlled Clustering

Photochemically powered micromotors are prototype microrobots, and spatiotemporal control is pivotal for a wide range of potential applications. Although their spatial navigation has been extensively studied, temporal control of photoactive micromotors remains much less explored. Using Ag-based oscillating micromotors as a model system, a strategy is presented for the controlled modulation of their individual and collective dynamics via periodically switching illumination on and off. In particular, such temporal light modulation drives individual oscillating micromotors into a total of six regimes of distinct dynamics, as the light-toggling frequencies vary from 0 to 10³ Hz. On an ensemble level, toggling light at 5 Hz gives rise to controlled, reversible clustering of oscillating micromotors and self-assembly of tracer microspheres into colloidal crystals. A qualitative mechanism based on Ag-catalyzed decomposition of H₂O₂ is given to account for some, but not all, of the above observations. This study might potentially inspire more sophisticated temporal control of micromotors and the development of smart, biomimetic materials that respond to environmental stimuli that not only change in space but also in time.

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.

Bimetallic coatings synergistically enhance the speeds of photocatalytic TiO2 micromotors

A bimetallic cap containing sputtered silver is a better catalyst that significantly improves the performance of catalytically powered micromotors.

From Strong Dichroic Nanomotor to Polarotactic Microswimmer

Light-driven micro/nanomotors are promising candidates for long-envisioned next-generation nanorobotics for targeted drug delivery, noninvasive surgery, nanofabrication, and beyond. To achieve these fantastic applications, effective control of the micro/nanomotor is essential. Light has been proved as the most versatile method for microswimmer manipulation, while the light propagation direction, intensity, and wavelength have been explored as controlling signals for light-responsive nanomotors. Here, the controlling method is expanded to the polarization state of the light, and a nanomotor with a significant dichroic ratio is demonstrated. Due to the anisotropic crystal structure, light polarized parallel to the Sb₂ Se₃ nanowires is preferentially absorbed. The core-shell Sb₂ Se₃ /ZnO nanomotor exhibits strong dichroic swimming behavior: the swimming speed is ≈3 times faster when illuminated with parallel polarized light than perpendicular polarized light. Furthermore, by incorporating two cross-aligned dichroic nanomotors, a polarotactic artificial microswimmer is achieved, which can be navigated by controlling the polarization direction of the incident light. Compared to the well-studied light-driven rotary motors based on optical tweezers, this dichroic microswimmer offers eight orders of magnitude light-intensity reduction, which may enable large-scale nanomanipulation as well as other heat-sensitive applications.

Visible-Light-Driven Janus Microvehicles in Biological Media

A light-driven multifunctional Janus micromotor for the removal of bacterial endotoxins and heavy metals is described. The micromotor was assembled by using the biocompatible polymer polycaprolactone for the encapsulation of CdTe or CdSe@ZnS quantum dots (QDs) as photoactive materials and an asymmetric Fe₃ O₄ patch for propulsion. The micromotors can be activated with visible light (470-490 nm) to propel in peroxide or glucose media by a diffusiophoretic mechanism. Efficient propulsion was observed for the first time in complex samples such as human blood serum. These properties were exploited for efficient endotoxin removal using lipopolysaccharides from Escherichia coli O111:B4 as a model toxin. The micromotors were also used for mercury removal by cationic exchange with the CdSe@ZnS core-shell QDs. Cytotoxicity assays in HeLa cell lines demonstrated the high biocompatibility of the micromotors for future detoxification applications.