Light-Powered, Fuel-Free Oscillation, Migration, and Reversible Manipulation of Multiple Cargo Types by Micromotor Swarms

Phoresis kernel theory for passive and active spheres with nonuniform phoretic mobility

By introducing geometry-based phoresis kernels, we establish a direct connection between the translational and rotational velocities of a phoretic sphere and the distributions of the driving fields or fluxes. The kernels quantify the local contribution of the field or flux to the particle dynamics. The field kernels for both passive and active particles share the same functional form, depending on the position-dependent surface phoretic mobility. For uniform phoretic mobility, the translational field kernel is proportional to the surface normal vector, while the rotational field kernel is zero; thus, a phoretic sphere with uniform phoretic mobility does not rotate. As case studies, we discuss examples of a self-phoretic axisymmetric particle influenced by a globally-driven field gradient, a general scenario for axisymmetric self-phoretic particle and two of its special cases, and a non-axisymmetric active particle.

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.

Weak Ion-Exchange Based Magnetic Swarm for Targeted Drug Delivery and Chemotherapy

Swimming microrobots that are actuated by multiple stimuli/fields display various intriguing collective behaviors, ranging from phase separation to clustering and giant number fluctuation; however, it is still chanllenging to achieve multiple responses and functionalities within one colloidal system to emulate high environmental adaptability and improved tasking capability of natural swarms. In this work, a weak ion-exchange based swarm is presented that can self-organize and reconfigure by chemical, light, and magnetic fields, showing living crystal, amorphous glass, liquid, chain, and wheel-like structures. By changing the frequency and strength of the rotating magnetic field, various well-controlled and fast transformations are obtained. Experiments show the high adaptability and functionality of the microrobot swarm in delivering drugs in confined spaces, such as narrow channels with turns or obstacles. The drug-carrying swarm exhibits excellent chemtherapy for Hela and CT26 cells due to the pH-enhanced drug release and locomotion. This reconfigurable microswarm provides a new platform for biomedical and environmental applications.

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.

Multifunctional Chiral Chemically-Powered Micropropellers for Cargo Transport and Manipulation

Practical applications of synthetic self-propelled nano and microparticles for microrobotics, targeted drug delivery, and manipulation at the nanoscale are rapidly expanding. However, fabrication limitations often hinder progress, resulting in relatively simple shapes and limited functionality. Here, taking advantage of 3D nanoscale printing, chiral micropropellers powered by the hydrogen peroxide reduction reaction are fabricated. Due to their chirality, the propellers exhibit multifunctional behavior controlled by an applied magnetic field: spinning in place (loitering), directed migration in the prescribed direction, capture, and transport of polymer cargo particles. Design parameters of the propellers are optimized by computation modeling based on mesoscale molecular dynamics. It is predicted by computer simulations, and confirmed experimentally, that clockwise rotating propellers attract each other and counterclockwise repel. These results shed light on how chirality and shape optimization enhance the functionality of synthetic autonomous micromachines.

Magnetic Lateral Ladder for Unidirectional Transport of Microrobots: Design Principles and Potential Applications of Cells-on-Chip

Functionalized microrobots, which are directionally manipulated in a controlled and precise manner for specific tasks, face challenges. However, magnetic field-based controls constrain all microrobots to move in a coordinated manner, limiting their functions and independent behaviors. This article presents a design principle for achieving unidirectional microrobot transport using an asymmetric magnetic texture in the shape of a lateral ladder, which the authors call the "railway track." An asymmetric magnetic energy distribution along the axis allows for the continuous movement of microrobots in a fixed direction regardless of the direction of the magnetic field rotation. The authors demonstrated precise control and simple utilization of this method. Specifically, by placing magnetic textures with different directionalities, an integrated cell/particle collector can collect microrobots distributed in a large area and move them along a complex trajectory to a predetermined location.  The authors can leverage the versatile capabilities offered by this texture concept, including hierarchical isolation, switchable collection, programmable pairing, selective drug-response test, and local fluid mixing for target objects. The results demonstrate the importance of microrobot directionality in achieving complex individual control. This novel concept represents significant advancement over conventional magnetic field-based control technology and paves the way for further research in biofunctionalized microrobotics.

Controlling the Collective Behaviors of Ultrasound-Driven Nanomotors via Frequency Regulation

Controlling the collective behavior of micro/nanomotors with ultrasound may enable new functionality in robotics, medicine, and other engineering disciplines. Currently, various collective behaviors of nanomotors, such as assembly, reconfiguration, and disassembly, have been explored by using acoustic fields with a fixed frequency, while regulating their collective behaviors by varying the ultrasound frequency still remains challenging. In this work, we designed an ultrasound manipulation methodology that allows nanomotors to exhibit different collective behaviors by regulating the applied ultrasound frequency. The experimental results and FEM simulations demonstrate that the secondary ultrasonic waves produced from the edge of the sample cell lead to the formation of complex acoustic pressure fields and microfluidic patterns, which causes these collective behaviors. This work has important implications for the design of artificial actuated nanomotors and optimize their performances.

Self-Propelling Macroscale Sheets Powered by Enzyme Pumps

Nanoscale enzymes anchored to surfaces act as chemical pumps by converting chemical energy released from enzymatic reactions into spontaneous fluid flow that propels entrained nano- and microparticles. Enzymatic pumps are biocompatible, highly selective, and display unique substrate specificity. Utilizing these pumps to trigger self-propelled motion on the macroscale has, however, constituted a significant challenge and thus prevented their adaptation in macroscopic fluidic devices and soft robotics. Using experiments and simulations, we herein show that enzymatic pumps can drive centimeter-scale polymer sheets along directed linear paths and rotational trajectories. In these studies, the sheets are confined to the air/water interface. With the addition of appropriate substrate, the asymmetric enzymatic coating on the sheets induces chemically driven, buoyancy flows that controllably propel the sheet's motion on the air/water interface. The directionality and speed of the motion can be tailored by changing the pattern of the enzymatic coating, type of enzyme, and nature and concentration of the substrate. This work highlights the utility of biocompatible enzymes for generating motion in macroscale fluidic devices and robotics and indicates their potential utility for in vivo applications.

Interlinking spatial dimensions and kinetic processes in dissipative materials to create synthetic systems with lifelike functionality

Biological systems spontaneously convert energy input into the actions necessary to survive. Motivated by the efficacy of these processes, researchers aim to forge materials systems that exhibit the self-sustained and autonomous functionality found in nature. Success in this effort will require synthetic analogues of the following: a metabolism to generate energy, a vasculature to transport energy and materials, a nervous system to transmit 'commands', a musculoskeletal system to translate commands into physical action, regulatory networks to monitor the entire enterprise, and a mechanism to convert 'nutrients' into growing materials. Design rules must interconnect the material's structural and kinetic properties over ranges of length (that can vary from the nano- to mesoscale) and timescales to enable local energy dissipations to power global functionality. Moreover, by harnessing dynamic interactions intrinsic to the material, the system itself can perform the work needed for its own functionality. Here, we assess the advances and challenges in dissipative materials design and at the same time aim to spur developments in next-generation functional, 'living' materials.

Janus Micromotors for Photophoretic Motion and Photon Upconversion Applications Using a Single Near-Infrared Wavelength

External stimuli can trigger changes in temperature, concentration, and momentum between micromotors and the medium, causing their propulsion and enabling them to perform different tasks with improved kinetic efficiencies. Light-activated micromotors are attractive systems that achieve improved motion and have the potential for high spatiotemporal control. Photophoretic swarming motion represents an attractive means to induce micromotor movement through the generation of temperature gradients in the medium, enabling the micromotors to move from cold to hot regions. The micromotors studied herein are assembled with Fe3O4 nanoparticles, and NaGdF4:Yb3+,Er3+/NaGdF4:Yb3+ and LiYF4:Yb3+,Tm3+ upconverting nanoparticles. The Fe3O4 nanoparticles were localized to one hemisphere to produce a Janus architecture that facilitates improved upconversion luminescence with the upconverting nanoparticles distributed throughout. Under 976 nm excitation, Fe3O4 nanoparticles generate the temperature gradient, while the upconverting nanoparticles produce visible light that is used for micromotor motion tracking and triggering of reactive oxygen species generation. As such, the motion and application of the micromotors are achieved using a single excitation wavelength. To demonstrate the practicality of this system, curcumin was adsorbed to the micromotor surface and degradation of Rhodamine B was achieved with kinetic rates that were over twice as fast as the static micromotors. The upconversion luminescence was also used to track the motion of the micromotors from a single image frame, providing a convenient means to understand the trajectory of these systems. Together, this system provides a versatile approach to achieving light-driven motion while taking advantage of the potential applications of upconversion luminescence such as tracking and detection, sensing, nanothermometry, particle velocimetry, photodynamic therapy, and pollutant degradation.

Nano/Micromotors for Cancer Diagnosis and Therapy: Innovative Designs to Improve Biocompatibility

Nano/micromotors are artificial robots at the nano/microscale that are capable of transforming energy into mechanical movement. In cancer diagnosis or therapy, such "tiny robots" show great promise for targeted drug delivery, cell removal/killing, and even related biomarker sensing. Yet biocompatibility is still the most critical challenge that restricts such techniques from transitioning from the laboratory to clinical applications. In this review, we emphasize the biocompatibility aspect of nano/micromotors to show the great efforts made by researchers to promote their clinical application, mainly including non-toxic fuel propulsion (inorganic catalysts, enzyme, etc.), bio-hybrid designs, ultrasound propulsion, light-triggered propulsion, magnetic propulsion, dual propulsion, and, in particular, the cooperative swarm-based strategy for increasing therapeutic effects. Future challenges in translating nano/micromotors into real applications and the potential directions for increasing biocompatibility are also described.

Collective Behaviors of Isotropic Micromotors: From Assembly to Reconstruction and Motion Control under External Fields

Swarms of self-propelled micromotors can mimic the processes of natural systems and construct artificial intelligent materials to perform complex collective behaviors. Compared to self-propelled Janus micromotors, the isotropic colloid motors, also called micromotors or microswimmers, have advantages in self-assembly to form micromotor swarms, which are efficient in resistance to external disturbance and the delivery of large quantity of cargos. In this minireview, we summarize the fundamental principles and interactions for the assembly of isotropic active particles to generate micromotor swarms. Recent discoveries based on either catalytic or external physical field-stimulated micromotor swarms are also presented. Then, the strategy for the reconstruction and motion control of micromotor swarms in complex environments, including narrow channels, maze, raised obstacles, and high steps/low gaps, is summarized. Finally, we outline the future directions of micromotor swarms and the remaining challenges and opportunities.

Light-stimulated micromotor swarms in an electric field with accurate spatial, temporal, and mode control

Swarming, a phenomenon widely present in nature, is a hallmark of nonequilibrium living systems that harness external energy into collective locomotion. The creation and study of manmade swarms may provide insights into their biological counterparts and shed light to the rules of life. Here, we propose an innovative mechanism for rationally creating multimodal swarms with unprecedented spatial, temporal, and mode control. The research is realized in a system made of optoelectric semiconductor nanorods that can rapidly morph into three distinct modes, i.e., network formation, collectively enhanced rotation, and droplet-like clustering, pattern, and switch in-between under light stimulation in an electric field. Theoretical analysis and semiquantitative modeling well explain the observation by understanding the competition between two countering effects: the electrostatic assembly for orderliness and electrospinning-induced disassembly for disorderliness. This work could inspire the rational creation of new classes of reconfigurable swarms for both fundamental research and emerging applications.

Visible-light-induced self-propelled nanobots against nanoplastics

The accumulation of plastic debris in aquatic organisms has raised serious concerns about the potential health implications of their incorporation into the food chain. However, conventional water remediation techniques are incapable of effectively removing nanoplastics (NPs) smaller than 200 nm, which can have harmful effect on animal and human health. Herein, we demonstrate the "on-the-fly" capture of NPs through their enlargement (approximately 4,100 times) using self-propelled nanobots composed of a metal-organic framework. Under visible-light irradiation, the iron hexacyanoferrate (FeHCF) nanobot exhibits fuel-free motion by electrostatically adsorbing NPs. This strategy can contribute to reducing plastic pollution in the environment, which is a significant environmental challenge. Light-induced intervalence charge transfer in the FeHCF nanobot lattice induces bipolarity on the nanobot surface, leading to the binding of negatively charged NPs. The local electron density in the lattice then triggers self-propulsion, thereby inducing agglomeration of FeHCF@NP complexes to stabilize their metastable state. The FeHCF nanobot exhibits a maximum removal capacity of 3,060 mg∙g-1 and rate constant of 0.69 min-1, which are higher than those recorded for materials reported in the literature.

Capture and transport of rod-shaped cargo via programmable active particles

We study the influence of the cargo shape on the capture and transport process of colloidal rods via swarms of active particles using Brownian dynamics simulations. Starting at random initial conditions, active particles that interact via the Lennard-Jones potential and possess a tuneable speed are utilised to capture passive rods inside a hexagonal cage of individually addressable units. By adjusting the velocity of the individual active particles, the rod can then be transported. To guarantee a successful capture process (with a strong localisation), we find that specific geometric and energetic constraints have to be met; i.e., the length of the rod must approximately be in the vicinity of an odd multiple of the lattice constant of the hexagonal cage, and the Lennard-Jones interaction strength must be in the range of [Formula: see text] to [Formula: see text]. If the cargo aspect ratio gets too large, the subsequent transport of successfully captured rods can fail. For systems where transport is possible, an increase in the cargo aspect ratio decreases the achievable transport velocity. Our work shows that the particle shape must be considered while designing interaction rules to accomplish specific tasks via groups of controllable units.

Self-Solidifying Active Droplets Showing Memory-Induced Chirality

Most synthetic microswimmers do not reach the autonomy of their biological counterparts in terms of energy supply and diversity of motions. Here, this work reports the first all-aqueous droplet swimmer powered by self-generated polyelectrolyte gradients, which shows memory-induced chirality while self-solidifying. An aqueous solution of surface tension-lowering polyelectrolytes self-solidifies on the surface of acidic water, during which polyelectrolytes are gradually emitted into the surrounding water and induce linear self-propulsion via spontaneous symmetry breaking. The low diffusion coefficient of the polyelectrolytes leads to long-lived chemical trails which cause memory effects that drive a transition from linear to chiral motion without requiring any imposed symmetry breaking. The droplet swimmer is capable of highly efficient removal (up to 85%) of uranium from aqueous solutions within 90 min, benefiting from self-propulsion and flow-induced mixing. These results provide a route to fueling self-propelled agents which can autonomously perform chiral motion and collect toxins.

Autonomous hydrogel locomotion regulated by light and electric fields

Autonomous robotic functions in materials beyond simple stimulus-response actuation require the development of functional soft matter that can complete well-organized tasks without step-by-step control. We report the design of photo- and electroactivated hydrogels that can capture and deliver cargo, avoid obstacles, and return without external, stepwise control. By incorporating two spiropyran monomers with different chemical substituents in the hydrogel, we created chemically random networks that enabled photoregulated charge reversal and autonomous behaviors under a constant electric field. In addition, using perturbations in the electric field induced by a dielectric inhomogeneity, the hydrogel could be attracted to high dielectric constant materials and autonomously bypasses the low dielectric constant materials under the guidance of the electric field vector. The photo- and electroactive hydrogels investigated here can autonomously perform tasks using constant external stimuli, an encouraging observation for the potential development of molecularly designed intelligent robotic materials.

Light-powered swarming phoretic antimony chalcogenide-based microrobots with "on-the-fly" photodegradation abilities

Microrobots are at the forefront of research for biomedical and environmental applications. Whereas a single microrobot exhibits quite low performance in the large-scale environment, swarms of microrobots are representing a powerful tool in biomedical and environmental applications. Here, we fabricated phoretic Sb₂S₃-based microrobots that exhibited swarming behavior under light illumination without any addition of chemical fuel. The microrobots were prepared in an environmentally friendly way by reacting the precursors with bio-originated templates in aqueous solution in a microwave reactor. The crystalline Sb₂S₃ material provided the microrobots with interesting optical and semiconductive properties. Because of the formation of reactive oxygen species (ROS) upon light illumination, the microrobots possessed photocatalytic properties. To demonstrate the photocatalytic abilities, industrially used dyes, quinoline yellow and tartrazine were degraded using microrobots in the "on-the-fly" mode. Overall, this proof-of-concept work showed that Sb₂S₃ photoactive material is suitable for designing swarming microrobots for environmental remediation applications.