Thermal Modulation of Nanomotor Movement

Motion control is essential for various applications of man‐made nanomachines. The ability to control and regulate the movement of catalytic nanowire motors is illustrated by applying short heat pulses that allow the motors to be accelerated or slowed down. The accelerated motion observed during the heat pulses is attributed primarily to the thermal activation of the redox reactions of the H₂O₂ fuel at the Pt and Au segments and to the decreased viscosity of the aqueous medium at elevated temperatures. The thermally modulated motion during repetitive temperature on/off cycles is highly reversible and fast, with speeds of 14 and 45 µm s^?1 at 25 and 65 °C, respectively. A wide range of speeds can be generated by tailoring the temperature to yield a linear speed–temperature dependence. Through the use of nickel‐containing nanomotors, the ability to combine the thermally regulated motion of catalytic nanomotors with magnetic guidance is also demonstrated. Such on‐demand control of the movement of nanowire motors holds great promise for complex operations of future manmade nanomachines and for creating more sophisticated nanomotors.     

Recent Progress on Bioinspired Self‐Propelled Micro/Nanomotors via Controlled Molecular Self‐Assembly

The combination of bottom‐up controllable self‐assembly technique with bioinspired design has opened new horizons in the development of self‐propelled synthetic micro/nanomotors. Over the past five years, a significant advances toward the construction of bioinspired self‐propelled micro/nanomotors has been witnessed based on the controlled self‐assembly technique. Such a strategy permits the realization of autonomously synthetic motors with engineering features, such as sizes, shapes, composition, propulsion mechanism, and function. The construction, propulsion mechanism, and movement control of synthetic micro/nanomotors in connection with controlled self‐assembly in recent research activities are summarized. These assembled nanomotors are expected to have a tremendous impact on current artificial nanomachines in future and hold potential promise for biomedical applications including drug targeted delivery, photothermal cancer therapy, biodetoxification, treatment of atherosclerosis, artificial insemination, crushing kidney stones, cleaning wounds, and removing blood clots and parasites.     

Fuel‐Free Synthetic Micro‐/Nanomachines

Inspired by the swimming of natural microorganisms, synthetic micro‐/nanomachines, which convert energy into movement, are able to mimic the function of these amazing natural systems and help humanity by completing environmental and biological tasks. While offering autonomous propulsion, conventional micro‐/nanomachines usually rely on the decomposition of external chemical fuels (e.g., H₂O₂), which greatly hinders their applications in biologically relevant media. Recent developments have resulted in various micro‐/nanomotors that can be powered by biocompatible fuels. Fuel‐free synthetic micro‐/nanomotors, which can move without external chemical fuels, represent another attractive solution for practical applications owing to their biocompatibility and sustainability. Here, recent developments on fuel‐free micro‐/nanomotors (powered by various external stimuli such as light, magnetic, electric, or ultrasonic fields) are summarized, ranging from fabrication to propulsion mechanisms. The applications of these fuel‐free micro‐/nanomotors are also discussed, including nanopatterning, targeted drug/gene delivery, cell manipulation, and precision nanosurgery. With continuous innovation, future autonomous, intelligent and multifunctional fuel‐free micro‐/nanomachines are expected to have a profound impact upon diverse biomedical applications, providing unlimited opportunities beyond one's imagination.     

Catalytic nanomotors: fabrication,mechanism, and applications

Catalytic nanomotors are nano-to-micrometer-sized actuators that carry an on-board catalyst and convert local chemical fuel in solution into mechanical work. The location of this catalyst as well as the geometry of the structure dictate the swimming behaviors exhibited. The nanomotors can occur naturally in organic molecules, combine natural and artificial parts to form hybrid nanomotors or be purely artificial. Fabrication techniques consist of template directed electroplating, lithography, physical vapor deposition, and other advanced growth methods. Various physical and chemical propulsion mechanisms have been proposed to explain the motion behaviors including diffusiophoresis, bubble propulsion, interfacial tension gradients, and self-electropho-resis. The control and manipulation based upon external fields, catalytic alloys, and motion control through thermal modulation are discussed as well. Catalytic nanomotors represent an exciting technological challenge with the end goal being practical functional nanomachines that can perform a variety of tasks at the nanoscale.     

Nanolocomotion—Catalytic Nanomotors and Nanorotors

Nature's nanomachines, built of dynamically integrated biochemical components, powered by energy‐rich biochemical processes, and designed to perform a useful task, have evolved over millions of years. They provide the foundation of all living systems on our planet today. Yet synthetic nanomotors, driven by simple chemical reactions and which could function as building blocks for synthetic nanomachines that can perform useful tasks, have been discovered only in the last few years. Why did it take so long to power‐up a myriad of synthetic nanostructures from their well‐known static states to new and exciting dynamic ones of the kind that abound in nature? This article will delve into this disconnect between the world of biological and abiological nanomotors, then take a look at some recent developments involving chemically powered nanoscale motors and rotors, and finally try to imagine: what's next for nanolocomotion?     

Motion Manipulation of Micro‐ and Nanomotors

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

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 Sb₂Se₃ nanowires is preferentially absorbed. The core–shell Sb₂Se₃/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.     

Recent Progress on Man‐Made Inorganic Nanomachines

The successful development of nanoscale machinery, which can operate with high controllability, high precision, long lifetimes, and tunable driving powers, is pivotal for the realization of future intelligent nanorobots, nanofactories, and advanced biomedical devices. However, the development of nanomachines remains one of the most difficult research areas, largely due to the grand challenges in fabrication of devices with complex components and actuation with desired efficiency, precision, lifetime, and/or environmental friendliness. In this work, the cutting‐edge efforts toward fabricating and actuating various types of nanomachines and their applications are reviewed, with a special focus on nanomotors made from inorganic nanoscale building blocks, which are introduced according to the employed actuation mechanism. The unique characteristics and obstacles for each type of nanomachine are discussed, and perspectives and challenges of this exciting field are presented.     

Self‐Propelled Micro/Nanomotors for Sensing and Environmental Remediation

Self‐propelled micro/nanomotors have gained attention for successful application in cargo delivery, therapeutic treatments, sensing, and environmental remediation. Unique characteristics such as high speed, motion control, selectivity, and functionability promote the application of micro/nanomotors in analytical sciences. Here, the recent advancements and main challenges regarding the application of self‐propelled micro/nanomotors in sensing and environmental remediation are discussed. The current state of micro/nanomotors is reviewed, emphasizing the period of the last five years, then their developments into the future applications for enhanced sensing and efficient purification of water resources are extrapolated.     

Programming Motions of DNA Origami Nanomachines

DNA nanotechnology enables the precise fabrication of DNA‐based machines with nanoscale dimensions. A wide range of DNA nanomachines are designed, which can be activated by specific inputs to perform various movement and functions. The excellent rigidity and unprecedented addressability of DNA origami have made it an excellent platform for manipulating and investigating the motion behaviors of DNA machines at single‐molecule level. In this Concept, power supply, machine actuation, and motion behavior of DNA machines on origami platforms are summarized and classified. The strategies utilized for programming motion behavior of DNA machines on DNA origami are also discussed with representative examples. The challenges and outlook for future development of manipulating DNA nanomachines at the single molecule level are presented and discussed.     

Micro‐/Nanomotors toward Biomedical Applications: The Recent Progress in Biocompatibility

Inspired by the highly versatile natural motors, artificial micro‐/nanomotors that can convert surrounding energies into mechanical motion and accomplish multiple tasks are devised. In the past few years, micro‐/nanomotors have demonstrated significant potential in biomedicine. However, the practical biomedical applications of these small‐scale devices are still at an infant stage. For successful bench‐to‐bed translation, biocompatibility of micro‐/nanomotor systems is the central issue to be considered. Herein, the recent progress in micro‐/nanomotors in biocompatibility is reviewed, with a special focus on their biomedical applications. Through close collaboration between researches in the nanoengineering, material chemistry, and biomedical fields, it is expected that a promising real‐world application platform based on micro‐/nanomotors will emerge in the near future.     

Nanoparticle‐Regulated Semiartificial Magnetotactic Bacteria with Tunable Magnetic Moment and Magnetic Sensitivity

Micro‐/nanomotors are widely used in micro‐/nanoprocessing, cargo transportation, and other microscale tasks because of their ability to move independently. Many biological hybrid motors based on bacteria have been developed. Magnetotactic bacteria (MTB) have been employed as motors in biological systems because of their good biocompatibility and magnetotactic motion in magnetic fields. However, the magnetotaxis of MTB is difficult to control due to the lack of effective methods. Herein, a strategy that enables control over the motion of MTB is presented. By depositing synthetic Fe₃O₄ magnetic nanoparticles on the surface of MTB, semiartificial magnetotactic bacteria (SAMTB) are produced. The overall magnetic properties of SAMTB, including saturation magnetization, residual magnetization, and blocking temperature, are regulated in a multivariate and multilevel fashion, thus regulating the magnetic sensitivity of SAMTB. This strategy provides a feasible method to manoeuvre MTB for applications in complex fluid environments, such as magnetic drug release systems and real‐time tracking systems. Furthermore, this concept and methodology provide a paradigm for controlling the mobility of micro‐/nanomotors based on natural small organisms.     

Magnesium‐Based Micromotors: Water‐Powered Propulsion,Multifunctionality, and Biomedical and Environmental Applications

The new capabilities and functionalities of synthetic micro/nanomotors open up considerable opportunities for diverse environmental and biomedical applications. Water‐powered micromachines are particularly attractive for realizing many of these applications. Magnesium‐based motors directly use water as fuel to generate hydrogen bubbles for their propulsion, eliminating the requirement of common toxic fuels. This Review highlights the development of new Mg‐based micromotors and discusses the chemistry that makes it extremely attractive for micromotor applications. Understanding these Mg properties and its transient nature is essential for controlling the propulsion efficiency, lifetime, and overall performance. The unique and attractive behavior of Mg offers significant advantages, including efficient water‐powered movement, remarkable biocompatibility, controlled degradation, convenient functionalization, and built‐in acid neutralization ability, and has paved the way for multifunctional micromachines for diverse real‐life applications, including operation in living animals. A wide range of such Mg motor‐based applications, including the detection and destruction of environmental threats, effective in‐vivo cargo delivery, and autonomous release, have been demonstrated. In conclusion, the current challenges, future opportunities, and performance improvements of the Mg‐based micromotors are discussed. With continuous innovation and attention to key challenges, it is expected that Mg‐based motors will have a profound impact on diverse biomedical and environmental applications.     

Self‐Propelled Micro/Nanomotors for On‐Demand Biomedical Cargo Transportation

Micro/nanomotors (MNMs) are miniaturized machines that can perform assigned tasks at the micro/nanoscale. Over the past decade, significant progress has been made in the design, preparation, and applications of MNMs that are powered by converting different sources of energy into mechanical force, to realize active movement and fulfill on‐demand tasks. MNMs can be navigated to desired locations with precise controllability based on different guidance mechanisms. A considerable research effort has gone into demonstrating that MNMs possess the potential of biomedical cargo loading, transportation, and targeted release to achieve therapeutic functions. Herein, the recent advances of self‐propelled MNMs for on‐demand biomedical cargo transportation, including their self‐propulsion mechanisms, guidance strategies, as well as proof‐of‐concept studies for biological applications are presented. In addition, some of the major challenges and possible opportunities of MNMs are identified for future biomedical applications in the hope that it may inspire future research.     

Utilizing Iron's Attractive Chemical and Magnetic Properties in Microrocket Design,Extended Motion,and Unique Performance

All‐in‐one material for microrocket propulsion featuring acid‐based bubble generation and magnetic guidance is presented. Electrochemically deposited iron serves as both a propellant, toward highly efficient self‐propulsion in acidic environments, and as a magnetic component enabling complete motion control. The new microrockets display longer lifetime and higher propulsion efficiency compared to previously reported active metal zinc‐based microrockets due to the chemical properties of iron and the unique structure of the microrockets. These iron‐based microrockets also demonstrate unique and attractive cargo towing and autonomous release capabilities. The latter is realized upon loss of the magnetic properties due to acid‐driven iron dissolution. More interestingly, these bubble‐propelled microrockets assemble via magnetic interactions into a variety of complex configurations and train structures, which enrich the behavior of micromachines. Modeling of the magnetic forces during the microrocket assembly and cargo capture confirms these unique experimentally observed assembly and cargo‐towing behaviors. These findings provide a new concept of blending propellant and magnetic components into one, toward simplifying the design and fabrication of artificial micro/nanomachines, realizing new functions and capabilities for a variety of future applications.     

Biomimetic Platelet‐Camouflaged Nanorobots for Binding and Isolation of Biological Threats

One emerging and exciting topic in robotics research is the design of micro‐/nanoscale robots for biomedical operations. Unlike industrial robots that are developed primarily to automate routine and dangerous tasks, biomedical nanorobots are designed for complex, physiologically relevant environments, and tasks that involve unanticipated biological events. Here, a biologically interfaced nanorobot is reported, made of magnetic helical nanomotors cloaked with the plasma membrane of human platelets. The resulting biomimetic nanorobots possess a biological membrane coating consisting of diverse functional proteins associated with human platelets. Compared to uncoated nanomotors which experience severe biofouling effects and hence hindered propulsion in whole blood, the platelet‐membrane‐cloaked nanomotors disguise as human platelets and display efficient propulsion in blood over long time periods. The biointerfaced nanorobots display platelet‐mimicking properties, including adhesion and binding to toxins and platelet‐adhering pathogens, such as Shiga toxin and Staphylococcus aureus bacteria. The locomotion capacity and platelet‐mimicking biological function of the biomimetic nanomotors offer efficient binding and isolation of these biological threats. The dynamic biointerfacing platform enabled by platelet‐membrane cloaked nanorobots thus holds considerable promise for diverse biomedical and biodefense applications.     

Cancer Cell Membrane Camouflaged Semi‐Yolk@Spiky‐Shell Nanomotor for Enhanced Cell Adhesion and Synergistic Therapy

Cell adhesion of nanosystems is significant for efficient cellular uptake and drug delivery in cancer therapy. Herein, a near‐infrared (NIR) light‐driven biomimetic nanomotor is reported to achieve the improved cell adhesion and cellular uptake for synergistic photothermal and chemotherapy of breast cancer. The nanomotor is composed of carbon@silica (C@SiO₂) with semi‐yolk@spiky‐shell structure, loaded with the anticancer drug doxorubicin (DOX) and camouflaged with MCF‐7 breast cancer cell membrane (i.e., mC@SiO₂@DOX). Such biomimetic mC@SiO₂@DOX nanomotors display efficient self‐thermophoretic propulsion due to a thermal gradient generated by asymmetrically spatial distribution. Moreover, the MCF‐7 cancer cell membrane coating can remarkably reduce the bioadhesion of nanomotors in biological medium and exhibit highly specific self‐recognition of the source cell line. The combination of effective propulsion and homologous targeting dramatically improves cell adhesion and the resultant cellular uptake efficiency in vitro from 26.2% to 67.5%. Therefore, the biomimetic mC@SiO₂@DOX displays excellent synergistic photothermal and chemotherapy with over 91% MCF‐7 cell growth inhibition rate. Such smart design of the fuel‐free, NIR light‐powered biomimetic nanomotor may pave the way for the application of self‐propelled nanomotors in biomedicine.     

The Brownian and Flow‐Driven Rotational Dynamics of a Multicomponent DNA Origami‐Based Rotor

Nanomechanical devices are becoming increasingly popular due to the very diverse field of potential applications, including nanocomputing, robotics, and drug delivery. DNA is one of the most promising building materials to realize complex 3D structures at the nanoscale level. Several mechanical DNA origami structures have already been designed capable of simple operations such as a DNA box with a controllable lid, bipedal walkers, and cargo sorting robots. However, the nanomechanical properties of mechanically interlinked DNA nanostructures that are in general highly deformable have yet to be extensively experimentally evaluated. In this work, a multicomponent DNA origami‐based rotor is created and fully characterized by electron microscopy under negative stain and cryo preparations. The nanodevice is further immobilized on a microfluidic chamber and its Brownian and flow‐driven rotational behaviors are analyzed in real time by single‐molecule fluorescence microscopy. The rotation in previous DNA rotors based either on strand displacement, electric field or Brownian motion. This study is the first to attempt to manipulate the dynamics of an artificial nanodevice with fluidic flow as a natural force.     

Self‐Organized Multiconstituent Catalytic Nanomotors

Self‐organized catalytic nanomotors consisting of more than one individual component are presented. Tadpole‐like catalytic nanomotors fabricated by dynamic shadowing growth (DSG) self‐organize randomly to form two‐nanomotor clusters (≈1–3% yield) that spin as opposed to circular motion exhibited by the individual structures. By introducing magnetic materials to another system, self‐assembled “helicopter” nanomotors consisting of a V‐shaped nanomotor and a microbead are formed with ≈25% yield, showing a significantly higher yield than the control (0%). A flexible swimmer system that performs complex swimming, such as maneuvering around stationary objects, is also presented. These nanomotor systems are inherently more complex than those previously studied and may be the next step towards building sophisticated multifunctional nanomachinery systems.     

Nanomachines Based on Carbon Nanotubes Walls Motion: Operation Modes and Controlling Forces

Abstract

Possible types of nanomachines based on threadlike relative motion of nanotube walls are considered. The theory for dynamics of such a motion is developed. Types of motion, controlling forces, and operation modes for these nanomachines are analyzed.