Dye-Enhanced Self-Electrophoretic Propulsion of Light-Driven TiO_2–Au Janus Micromotors

Light-driven synthetic micro-/nanomotors have attracted considerable attention in recent years due to their unique performances and potential applications. We herein demonstrate the dye-enhanced self-electrophoretic propulsion of light-driven Ti O_2–Au Janus micromotors in aqueous dye solutions. Compared to the velocities of these micromotors in pure water, 1.7, 1.5, and 1.4 times accelerated motions were observed for them in aqueous solutions of methyl blue(10-5g L^(-1)), cresol red(10^(-4)g L^(-1)),and methyl orange(10^(-4)g L^(-1)), respectively. We determined that the micromotor speed changes depending on thetype of dyes, due to variations in their photodegradation rates. In addition, following the deposition of a paramagnetic Ni layer between the Au and Ti O_2 layers, the micromotor can be precisely navigated under an external magnetic field. Such magnetic micromotors not only facilitate the recycling of micromotors, but also allow reusability in the context of dye detection and degradation.In general, such photocatalytic micro-/nanomotors provide considerable potential for the rapid detection and ‘‘on-thefly'' degradation of dye pollutants in aqueous environments.

Recent progress on the design and fabrication of micromotors and their biomedical applications

The advancement in the micro-/nanofabrication techniques has greatly facilitated the development of micromotors.A variety of micromotors have been invented with powerful functions,which have attracted a broad range of interests from chemistry, physics,mechanics,biology and medicine.In this paper,we reviewed recent progress in micromotors and highlighted representative works.The mechanisms of micromotors by internal and external energy sources were described.We described general fabrication strategies of the popular micromotors (wire,tubular,helical and Janus)including bottom-up and top-down approaches.In the application section,we primarily focused on the biological applications,such as biological cargo delivery, biosensing and surgery.At last,we discussed the current challenges and provided future prospects.

Chinese herb pollen derived micromotors as active oral drug delivery system for gastric ulcer treatment

Considerable efforts have been devoted to treating gastric ulcers.Attempts in this field tend to develop drug delivery systems with prolonged gastric retention time.Herein,we develop novel Chinese herb pollen-derived micromotors as active oral drug delivery system for treating gastric ulcer.Such Chinese herb pollen-derived micromotors are simply produced by asymmetrically sputtering Mg layer onto one side of pollen grains.When exposed to gastric juice,the Mg layer can react with the hydrogen ions,resulting in intensive generation of hydrogen bubbles to propel the micromotors.Benefiting from the autonomous motion and unique spiny structure,our micromotors can move actively in the stomach and adhere to the surrounding tissues.Besides,their special architecture endows the micromotors with salient capacity of drug loading and releasing.Based on these features,we have demonstrated that our Chinese herb pollen-derived micromotors could effective deliver berberine hydrochloride and show desirable curative effect on the gastric ulcer model of mice.Therefore,these Chinese herb pollen-derived micromotors are anticipated to serve as promising oral drug delivery carriers for clinical applications.

Motion-based pH sensing using spindle-like micromotors

In this study, we report a spindle-like micromotor. This device, which is fabricated using a one-step electrospinning method, consists of biodegradable polycaprolactone and an anionic surfactant. Intriguingly, not only can the resulting micromotor move autonomously on the surface of water for a long period of time （-40 min） due to the Marangoni effect, but it also exhibits a pH sensing behavior due to variations in the surface tension caused by the release of surfactant under different pH conditions. More interestingly, we reveal that the motion-based pH sensing property is size-dependent, with smaller structures exhibiting a higher sensitivity. In addition, since polycaprolactone is a biode- gradable material, the micromotor described in this study can be easily degraded in solution. Hence, features such as one-step fabrication, motion readout, and biodegradability render this micromotor an attractive candidate for sensing algplications.

NEW ROD-SHAPED ULTRASONIC MICROMOTOR AND ITS DRIVING PRINCIPLE

A new rod-shaped traveling wave ultrasonic micromotor is developed. In the micromotor, five pieces of piezoelectric ceramics clamped by two metal cylinders are used as its stator. The driving principle of the rodshaped ultrasonic motor is simulated. The stator structure and the position to lay these piezoelectric ceramics are calculated to improve the electro mechanical conversion efficiency. A flexible rotor is designed to reduce the radial slip between the stator and the rotor, and to improve the motor efficiency. The prototype motor and its micror driver are tested. The motor is 9 mm in out-diameter, 15 mm in length and 3.2 g in weight. When the motor operates with the first bending frequency （72 kHz） of the stator, its maximal rotational speed and the torque reach 520 r/rain and 4.5 mN · m. Results show that the motor has good stability. The speed fluctuation is controlled within 3% by the frequency automatic tracking technique.

Modeling and Analysis of a Micromotor with an Electrostatically Levitated Rotor

The modeling and evaluation of a prototype rotary micromotor where the annular rotor is supported electrostatically in five degrees of freedom is presented in order to study the behavior of this levitated micromotor and further optimize the device geometry. The analytical torque model is obtained based on the principle of a planar variable-capacitance electrostatic motor while the viscous damping caused by air film between the stator and rotor is derived using laminar Couette flow model. Simulation results of the closed-loop drive motor, based on the developed dynamic model after eliminating mechanical friction torque via electrostatic suspension, are presented. The effects of the high-voltage drive, required for rotation of the rotor, on overload capacity and suspension stiffness of the electrostatic bearing system are also analytically evaluated in an effort to determine allowable drive voltage and attainable rotor speed in operation. The analytical results show that maximum speed of the micromotor is limited mainly by viscous drag torque and stiffness of the bearing system. Therefore, it is expected to operate the device in vacuum so as to increase the rotor speed significantly, especially for those electrostatically levitated micromotors to be used as an angular rate micro-gyroscope.

Dual-Fuel-Driven Bactericidal Micromotor

In this paper, we report fabrication of the bimetallic Janus microsphere, a magnesium microsphere with a silver surface coating, through thermal evaporation technique. Because of the Janus structure, this micromotor can be propelled in two different directions by the surface silver or magnesium ‘engine' and hydrogen peroxide or water fuel. In addition, due to the bactericidal property of silver, this autonomous micromotor is capable of killing bacteria in solution. As compared to the static one, the micromotor is able to kill the bacteria at a much faster rate(about nine times of that of the static one),demonstrating the superiority of the motion one. We thus believe that the micromotor shown in the current study is potentially attractive for the environmental hygiene applications.

Micromotor-derived composites for biomedicine delivery and other related purposes

Biocompatible designed micromotor has attracted more and more concerns in the field of biomedicine due to their self-propulsion and delivery abilities.Such micromotors,mostly consisting of alkali earth metals,hydrogels,or other motile biomaterials,can effectively transform chemical energy into mechanical or kinetic energy to achieve the expected delivery of cargos to the sites of action.Except for conveying power,the modifiable surface and inner cavity of micromotors guarantee that their potential as versatile delivery systems for therapeutic agents.Here,this review generalizes the propelling mechanisms,composites,and shapes of micromotors.Besides,the application of micromotor-derived composites for biomedicine delivery and other versatile purposes are also discussed.

Optimization of the Drag Forces of Shell Janus Micromotor:A Study Based on Hydrodynamical Analysis and Numerical Simulation

Micromotors are widely used in cell operation,drug delivery and environmental decontamination due to their small size,low energy consumption and large propelling power.Compared to traditional Janus micromotor,the shell Janusmicromotor has better motion performance.However,the structural optimization of itsmotion performance is still unclear.The main factor restricting the motion performance of shell Janus micromotors is the drag forces.In the current work,theoretical analysis and numerical simulation were applied to analyze the drag forces of shell Janus micromotors.This study aims to design the optimum structure of shell Janus micromotors with minimum drag forces and obtain the magnitude of drag forces considering both the internal and external fluids of the shell Janus micromotors.Moreover,the influence of the motor geometry and Reynolds number on the drag coefficient was analyzed using numerical simulations.The results provide guidance for the optimum flow velocity,opening diameter and shell thickness to achieve minimum drag force.

KEY TECHNOLOGY FOR PRACTICAL 1-mm-DIAMETER ELECTROMAGNETIC MICROMOTOR

A 1 mm diameter electromagnetic micromotor was developed as a crux component for MEMS application. The motor has a novel layer structure with a 1 mm diameter rotor in the middle of two stators with the same size. The stator uses multiple layers, slotless and concentrated planar winding. The rotor adopts multipolar permanent magnet with high performance. Ruby bearing is used to prolong operating lifetime of the micromotor. The stator winding, consisting of 6 layer coils, 42 turns, and 9 pairs, is fabricated with microprocessing techniques. The micromotor has long operation lifetime, its running speed is stable and controllable, and rotational direction can be easily reversed. Maximum achieved rotational speed of 18000 r/min with maximum output torque of 1.5 μ N·m has been obtained. This paper presented the key technology for developing this kind of micromotor including the design of structure, magnetic circuit, heat problem, friction improvement, microprocessing techniques, and so on.

Multi-try Counter-meshing Gears Discrimination Device Based on MEMS Technology

A multi-try counter-meshing gears (CMG) discrimination device based on micro electromechani-cal system (MEMS) technology was designed for some specified information fields. The discrimination device consists of two groups of metal CMG, two pawl/ratchet mechanisms, two driving micromotors and two re-setting micromotors, which make the CMG withdraw by raising the pawls. The energy-coupling element is a photoelectric sensor with a circular plate which is notched. Micromotor is fabricated using the ultraviolet LiGA (UV-LiGA) fabrication process and precision mechanical engineering. The discrimination device has the function which can automatically reset, with the correct resetting code, it can be tried another times.

Vibration-Driven Microrobot Positioning Methodologies for Nonholonomic Constraint Compensation

This paper presents the formulation and practical implementation of positioning methodologies that compensate for the nonholonomic constraints of a mobile microrobot that is driven by two vibrating direct current(DC) micromotors. The open-loop and closed-loop approaches described here add the capability for net sidewise displacements of the microrobotic platform. A displacement is achieved by the execution of a number of repeating steps that depend on the desired displacement, the speed of the micromotors, and the elapsed time. Simulation and experimental results verified the performance of the proposed methodologies.

Recent advances in self-propelled particles

"Active" components can be introduced into a passive system to completely change its physical behavior from its typical behavior at thermodynamic equilibrium. To reveal the interaction mechanisms between individuals, researchers have designed unique self-propelled particles to mimic the collective behavior of biological systems. This review focuses on recent theoretical and experimental advances in the study of self-propelled particle systems and their individual and collective behaviors. The potential applications of active particles in chemical, biological and environmental sensing and single particle imaging are discussed.

Biomembrane-inspired design of medical micro/nanorobots:From cytomembrane stealth cloaks to cellularized Trojan horses

Micro/nanorobots are promising for a wide range of biomedical applications(such as targeted tumor,thrombus,and infection therapies in hard-to-reach body sites)because of their tiny size and high maneuverability through the actuation of external fields(e.g.,magnetic field,light,ultrasound,electric field,and/or heat).However,fully synthetic micro/nanorobots as foreign objects are susceptible to phagocytosis and clearance by diverse phagocytes.To address this issue,researchers have attempted to develop various cytomembrane-camouflaged micro/nanorobots by two means:(1)direct coating of micro/nanorobots with cytomembranes derived from living cells and(2)the swallowing of micro/nanorobots by living immunocytes via phagocytosis.The camouflaging with cytomembranes or living immunocytes not only protects micro/nanorobots from phagocytosis,but also endows them with new characteristics or functionalities,such as prolonging propulsion in biofluids,targeting diseased areas,or neutralizing bacterial toxins.In this review,we comprehensively summarize the recent advances and developments of cytomembrane-camouflaged medical micro/nanorobots.We first discuss how cytomembrane coating nanotechnology has been employed to engineer synthetic nanomaterials,and then we review in detail how cytomembrane camouflage tactic can be exploited to functionalize micro/nanorobots.We aim to bridge the gap between cytomembrane-cloaked micro/nanorobots and nanomaterials and to provide design guidance for developing cytomembrane-camouflaged micro/nanorobots.

Proliferation-mediated asymmetric nanoencapsulation of singlecell and motility differentiation

The biointerface engineering of living cells by creating an abiotic shell has important implications for endowing cells with exogenous properties with improved cellular behavior,which then boosts the development of the emerging field of living cell hybrid materials.Herein,we develop a way to perform active nanoencapsulation of single cell,which then endows the encapsulated cells with motion ability that they do not inherently possess.The emerging motion characteristics of the encapsulated cells could be self-regulated in terms of both the motion velocity and orbits by different proliferation modes.Accordingly,by taking advantage of the emergence of differentiated moving abilities,we achieve the self-sorting between mother cells and daughter cells in a proliferated Saccharomyces cerevisiae cell community.Therefore,it is anticipated that our highlighted study could not only serve as a new technique in the field of single-cell biology analysis and sorting such as in studying the aging process in Saccharomyces cerevisiae,but also open up opportunities to manipulate cell functionality by creating biohybrid materials to fill the gap between biological systems and engineering abiotic materials.

Near-infrared light-driven Janus capsule motors： Fabrication, propulsion, and simulation

We report a fuel-free, near-infrared （NIR）-driven Janus microcapsule motor. The Janus microcapsule motors were fabricated by template-assisted polyelectrolyte layer-by-layer assembly, followed by spraying of a gold layer on one side. The NIR-powered Janus motors achieved high propulsion with a maximum speed of 42μm.s-1 in water. The propulsion mechanism of the Janus motor was attributed to the self-thermophoresis effect： The asymmetric distribution of the gold layer generated a local thermal gradient, which in turn generated thermophoretic force to propel the Janus motor. Such NIR-propelled Janus capsule motors can move efficiently in cell culture medium and have no obvious effects on the cell at the power of the NIR laser, indicating considerable promise for future biomedical applications.

REVIEW ON PIEZOELECTRIC,ULTRASONIC,AND MAGNETOELECTRIC ACTUATORS

Piezoelectric actuators operating in piezoelectric-induced strain/stress or electromechanical res.onanceinduced vibration or wave-motion friction drive mechanism have shown many advantagesover traditional electromagnetic mot ors,especilly,when miniaturizing into millimeter-scale size,while magnetoelectric act uators operating in magnetostrictive mechanism are capable of piezo-dlectric self-sensing and remote operation under an applied magnetic field.This paper summarizesthe recent progresses in piezoelectric ceramic and single crystal materials based actuators andmicromotors,ferromagnetic/ferroeletric laminated magnetoelectric actuat ors,including rotary,linear,planner,and spherical motion actuators,and bending motion magnetoelectric actuators.Their driving mechanisms,operation propertics,and applications are also explained.

Designing bioactive micro-/nanomotors for engineered regeneration

Micro/nanoscale motors(MNMs)have been regarded as promising tools in the field of engineered regeneration due to unique property of autonomous motion.Herein,a review on the advancements of MNMs in the area of engineered regeneration is presented,covering aspects from their propulsion mechanisms to their frontiers in engineered regeneration,listing the revolutionary applications in biosensing,medical imaging,drug delivery and tissue engineering.Finally,challenges and future directions of MNMs are finally discussed on the basis of the achievements.

One-dimensional micro/nanomotors for biomedicine:delivery,sensing and surgery

The rapid development of artificial micro/nanomachines brings promising strategies to overcome challenges in biomedicine,including delivery,sensing and surgery.One-dimensional(1D)micro/nanomotors are one of the most attractive micro/nanomachines due to their high specific surface area,powerful impetus and weak rotation diffusion.In this review,different propulsion mechanisms and motion control strategies of 1D micro/nanomotors are summarized,and recent efforts towards their fabrication methods and biomedical applications are discussed.We envision the multidisciplinary research efforts in the field of 1D micro/nanomotors will pave their way to practical applications in bioimaging and biomedicine.