Visible Light-Driven Micromotor with Incident-Angle-Controlled Motion and Dynamic Collective Behavior

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.

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.

Dynamically reversible cooperation and interaction of multiple rotating micromotors

Micromotors have been shown to have great potential in various fields (e.g., targeted therapeutics, self-organizing systems), and research on the cooperative and interactive behaviours of multiple micromotors could potentially revolutionize many fields in terms of performing multiple or complex tasks to compensate for the limitations of individual micromotors; however, dynamically reversible transitions among diverse behaviours remain much less explored, and such dynamic transformations are advantageous for achieving complex tasks. Here, we present a microsystem consisting of multiple disk-like micromotors capable of performing reversible transformations between cooperative and interactive behaviours at the liquid surface. The micromotors with aligned magnetic particles in our system have great magnet properties, which provides a strong magnetic interaction with each other and is vital for the whole microsystem. We offer and analyse the physical models among multiple micromotors concerning the cooperative and interactive modes in the lower and higher frequency ranges, respectively, between which the state transformation can reversibly occur. Furthermore, based on the proposed reversible microsystem, the feasibility of the application of self-organization is verified by demonstrating three different dynamic self-organizing behaviours. Our proposed dynamically reversible system has great potential to serve as a paradigm for studying cooperative and interactive behaviours among multiple micromotors in the future.

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

Through experiments and simulations, we show that fuel-free photoactive TiO₂ microparticles can form mobile, coherent swarms in the presence of UV light, which track the subsequent movement of an irradiated spot in a fluid-filled microchamber. Multiple concurrent propulsion mechanisms (electrolyte diffusioosmotic swarming, photocatalytic expansion, and photothermal migration) control the rich collective behavior of the swarms, which provide a strategy to reversely manipulate cargo. The active swarms can autonomously pick up groups of inert particles, sort them by size, and sequentially release the sorted particles at particular locations in the microchamber. Hence, these swarms overcome three obstacles, limiting the utility of self-propelled particles. Namely, they can (1) undergo directed, long-range migration without the addition of a chemical fuel, (2) perform diverse collective behavior not possible with a single active particle, and (3) repeatedly and controllably isolate and deliver specific components of a multiparticle "cargo". Since light sources are easily fabricated, transported, and controlled, the results can facilitate the development of portable devices, providing broader access to the diagnostic and manufacturing advances enabled by microfluidics.

Facilitated dynamics of an active polymer in 2D crowded environments with obstacles

Understanding the behaviors of a single active chain in complex environments is not only an interesting topic in non-equilibrium physics but also has applicative implications in biological/medical engineering. In this work, by using molecular simulations, we systematically study the dynamical and conformational behaviors of an active polymer in crowded environments, i.e., a single active chain confined in 2D space with randomly arranged obstacles. We found that the competition between the chain's activity and rigidity in the presence of obstacles leads to many interesting dynamical and conformational states, such as the diffusive expanded state, the diffusive collapsed state, and the localized collapsed state. Importantly, we found a counter-intuitive phenomenon, i.e., crowded environments facilitate the diffusion of the active polymer within a large parameter space. As the crowdedness (packing fraction of obstacles) increases, the parameter space in which crowding-enhanced diffusion occurs still remains. This abnormal dynamics is attributed to a structural reason that the obstacles prevent active chains from collapsing. Our findings capture some generic features of active polymers in complex environments and provide insights into the design of novel drug delivery systems.

Controlled propulsion of micro/nanomotors: operational mechanisms, motion manipulation and potential biomedical applications

Inspired by natural mobile microorganisms, researchers have developed micro/nanomotors (MNMs) that can autonomously move by transducing different kinds of energies into kinetic energy. The rapid development of MNMs has created tremendous opportunities for biomedical fields including diagnostics, therapeutics, and theranostics. Although the great progress has been made in MNM research, at a fundamental level, the accepted propulsion mechanisms are still a controversial matter. In practical applications such as precision nanomedicine, the precise control of the motion, including the speed and directionality, of MNMs is also important, which makes advanced motion manipulation desirable. Very recently, diverse MNMs with different propulsion strategies, morphologies, sizes, porosities and chemical structures have been fabricated and applied for various uses. Herein, we thoroughly summarize the physical principles behind propulsion strategies, as well as the recent advances in motion manipulation methods and relevant biomedical applications of these MNMs. The current challenges in MNM research are also discussed. We hope this review can provide a bird's eye overview of the MNM research and inspire researchers to create novel and more powerful MNMs.

Light-driven microrobots: capture and transport of bacteria and microparticles in a fluid medium

The design of simple microrobotic systems with capabilities to address various applications like cargo transportation, as well as biological sample capture and manipulation in an individual unit, provides a novel route for designing advanced multifunctional microscale systems. Here, we demonstrate a facile approach to fabricate such multifunctional and fully controlled light-driven microrobots. The microrobots are titanium dioxide-silica Janus particles that are propelled in aqueous hydroquinone/benzoquinone fuel when illuminated by low-intensity UV light. The application of light provides control over the speed as well as activity of the microrobots. When modified with additional thin film coatings of nickel and gold, the microrobots exhibit the capturing and transportation of silica microparticles and E. coli bacteria. While transporting, they also show guided swimming under an external uniform magnetic field, which is interesting for deciding their moving path or the start/end positions. The fluorescent dye-based live/dead tests confirm that in the microrobot system almost no bacteria were harmed during the capturing or transportation. The simplistic design and steerable swimming with the ability to capture and transport are the important features of the microrobots. These features make them an ideal candidate for in vitro or lab-on-a-chip based studies, e.g., drug delivery, bacterial sensing, cell treatment, etc., where the capturing and transport of microscopic entities play a crucial role.

Enhanced and Robust Directional Propulsion of Light-Activated Janus Micromotors by Magnetic Spinning and the Magnus Effect

Advances in the versatile design and synthesis of nanomaterials have imparted diverse functionalities to Janus micromotors as autonomous vehicles. However, a significant challenge remains in maneuvering Janus micromotors by following desired trajectories for on-demand motility and intelligent control due to the inherent rotational Brownian motion. Here, we present the enhanced and robust directional propulsion of light-activated Fe₃O₄@TiO₂/Pt Janus micromotors by magnetic spinning and the Magnus effect. Once exposed to a low-intensity rotating magnetic field, the micromotors become physically actuated, and their rotational Brownian diffusion is quenched by the magnetic rotation. Photocatalytic propulsion can be triggered by unidirectional irradiation based on a self-electrophoretic mechanism. Thus, a transverse Magnus force can be generated due to the rotational motion and ballistic motion (photocatalytic propulsion) of the micromotors. Both the self-electrophoretic propulsion and the Magnus force are periodically changed due to the magnetic rotation, which results in an overall directed motion moving toward a trajectory with a deflection angle from the direction of incident light with enhanced speed, maneuverability, and steering robustness. Our study illustrates the admirable directional motion capabilities of light-driven Janus micromotors based on magnetic spinning and the Magnus effect, which unfolds a new paradigm for addressing the limitations of directionality control in the current asymmetric micromotors.

Surface Roughening of Pt-Polystyrene Spherical Janus Micromotors for Enhanced Motion Speed

Spherical Janus micromotors (SJMs) have attracted much attention, and their high-speed motion is highly desired due to their various potential applications. However, the conventional template-deposition method often leads to an active Pt coating with a smooth surface, which is unbeneficial to speed enhancement in terms of catalytic reaction. Here, a facile surface roughening method is presented to fabricate the Pt-polystyrene (PS) SJMs with rough Pt surface (or Pt_r-PS SJMs) by plasma-etching the PS colloidal monolayer and then depositing Pt. The Pt_r-PS SJMs can exhibit directional motion pushed by the Pt in the various H₂O₂ solutions, and they show much higher motion speeds than the Pt-PS SJMs with smooth Pt surfaces at the same H₂O₂ concentration. The Pt-pushed motion is related to the locally asymmetric catalytic reaction of the Pt coating on PS. The speed is also associated with the surface roughness of the Pt coating. The Pt film with a rough surface causes enhanced motion speed due to the improvement of reaction catalytic activity. This work presents a new route to enhancing the motor motion speed, which is of significance in designing micromotors with high-speed motion.

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.