Recent Progress on Man-Made Inorganic Nanomachines

Autonomous Nanorobots as Miniaturized Surgeons for Intracellular Applications

Artificial nanorobots have emerged as promising tools for a wide range of biomedical applications, including biosensing, detoxification, and drug delivery. Their unique ability to navigate confined spaces with precise control extends their operational scope to the cellular or subcellular level. By combining tailored surface functionality and propulsion mechanisms, nanorobots demonstrate rapid penetration of cell membranes and efficient internalization, enhancing intracellular delivery capabilities. Moreover, their robust motion within cells enables targeted interactions with intracellular components, such as proteins, molecules, and organelles, leading to superior performance in intracellular biosensing and organelle-targeted cargo delivery. Consequently, nanorobots hold significant potential as miniaturized surgeons capable of directly modulating cellular dynamics and combating metastasis, thereby maximizing therapeutic outcomes for precision therapy. In this review, we provide an overview of the propulsion modes of nanorobots and discuss essential factors to harness propulsive energy from the local environment or external power sources, including structure, material, and engine selection. We then discuss key advancements in nanorobot technology for various intracellular applications. Finally, we address important considerations for future nanorobot design to facilitate their translation into clinical practice and unlock their full potential in biomedical research and healthcare.

Organic nanomotors: emerging versatile nanobots

Artificial nanomotors are self-propelled nanometer-scaled machines that are capable of converting external energy into mechanical motion. A significant progress on artificial nanomotors over the last decades has unlocked the potential of carrying out manipulatable transport and cargo delivery missions with enhanced efficiencies owing to their stimulus-responsive autonomous movement in various complex environments, allowing for future advances in a large range of applications. Emergent kinetic systems with programmable energy-converting mechanisms that are capable of powering the nanomotors are attracting increasing attention. This review highlights the most-recent representative examples of synthetic organic nanomotors having self-propelled motion exclusively powered by organic molecule- or their aggregate-based kinetic systems. The stimulus-responsive propulsion mechanism, motion behaviors, and performance in antitumor therapy of organic nanomotors developed so far are illustrated. A future perspective on the development of organic nanomotors is also proposed. With continuous innovation, it is believed that the scope and possible achievements in practical applications of organic nanomotors with diversified organic kinetic systems will expand.

Programming crack patterns with light in colloidal plasmonic films

Crack formation observed across diverse fields like geology, nanotechnology, arts, structural engineering or surface science, is a chaotic and undesirable phenomenon, resulting in random patterns of cracks generally leading to material failure. Limiting the formation of cracks or "programming" the path of cracks is a great technological challenge since it holds promise to enhance material durability or even to develop low cost patterning methods. Drawing inspiration from negative phototropism in plants, we demonstrate the capability to organize, guide, replicate, or arrest crack propagation in colloidal films through remote light manipulation. The key consists in using plasmonic photothermal absorbers to generate "virtual" defects enabling controlled deviation of cracks. We engineer a dip-coating process coupled with selective light irradiation enabling simultaneous deposition and light-directed crack patterning. This approach represents a rare example of a robust self-assembly process with long-range order that can be programmed in both space and time.

Gallium-Based Liquid Metal Materials for Antimicrobial Applications

The hazards caused by drug-resistant bacteria are rocketing along with the indiscriminate use of antibiotics. The development of new non-antibiotic antibacterial drugs is urgent. The excellent biocompatibility and diverse multifunctionalities of liquid metal have stimulated the studies of antibacterial application. Several gallium-based antimicrobial agents have been developed based on the mechanism that gallium (a type of liquid metal) ions disorder the normal metabolism of iron ions. Other emerging strategies, such as physical sterilization by directly using LM microparticles to destroy the biofilm of bacteria or thermal destruction via infrared laser irradiation, are gaining increasing attention. Different from traditional antibacterial agents of gallium compounds, the pronounced property of gallium-based liquid metal materials would bring innovation to the antibacterial field. Here, LM-based antimicrobial mechanisms, including iron metabolism disorder, production of reactive oxygen species, thermal injury, and mechanical destruction, are highlighted. Antimicrobial applications of LM-based materials are summarized and divided into five categories, including liquid metal motors, antibacterial fabrics, magnetic field-responsive microparticles, liquid metal films, and liquid metal polymer composites. In addition, future opportunities and challenges towards the development and application of LM-based antimicrobial materials are presented.

Polyhedral Micromotors of Metal-Organic Frameworks: Symmetry Breaking and Propulsion

Colloidal micromotors can autonomously propel due to their broken symmetry that leads to unbalanced local mechanical forces. Most commonly, micromotors are synthesized to possess a Janus structure or its variants, having two components distinct in shape, composition, or surface joined together on opposite sides. Here, we report on an alternative approach for creating micromotors, where microcrystals of metal-organic frameworks (MOFs) with various polyhedral shapes are propelled under an AC electric field. In these cases, symmetry breaking is realized by orienting the polyhedral particles in a unique direction to generate uneven electrohydrodynamic flow. The particle orientations are controlled by a delicate competition between the electric and gravitational forces exerted on the particle, which we rationalize using experiments and a theoretical model. Furthermore, by leveraging the MOF types and shapes, or surface properties, we show that the propulsion of MOF motors can be tuned or reversed. Because of the flexibility in designing MOFs and their one-step scalable synthesis, our strategy is simple yet versatile for making well-defined functional micromotors.

Engineering of stimuli-responsive self-assembled biomimetic nanoparticles

Nanoparticle-based therapeutics have the potential to change the paradigm of how we approach the diagnosis and treatment of human disease. Employing naturally derived cell membranes as a surface coating has created a powerful new approach by which nanoparticles can be functionalized towards a wide range of biomedical applications. By using membranes derived from different cell sources, the resulting nanoparticles inherit properties that can make them well-suited for a variety of tasks. In recent years, stimuli-responsive platforms with the ability to release payloads on demand have received increasing attention due to their improved delivery, reduced side effects, and precision targeting. Nanoformulations have been developed to respond to external stimuli such as magnetic fields, ultrasound, and radiation, as well as local stimuli such as pH gradients, redox potentials, and other chemical conditions. Here, an overview of the novel cell membrane coating platform is provided, followed by a discussion of stimuli-responsive platforms that leverage this technology.

Expanding the bio-catalysis scope and applied perspectives of nanocarrier immobilized asparaginases

l-asparaginase is an essential enzyme in medicine and a well-known chemotherapeutic agent. This enzyme's importance is not limited to its use as an anti-cancer agent; it also has a wide variety of medicinal applications. Antimicrobial properties, prevention of infectious disorders, autoimmune diseases, and canine and feline cancer are among the applications. Apart from the healthcare industry, its importance has been identified in the food industry as a food manufacturing agent to lower acrylamide levels. When isolated from their natural habitats, they are especially susceptible to different denaturing conditions due to their protein composition. The use of an immobilization technique is one of the most common approaches suggested to address these limitations. Immobilization is a technique that involves fixing enzymes to or inside stable supports, resulting in a heterogeneous immobilized enzyme framework. Strong support structures usually stabilize the enzymes' configuration, and their functions are maintained as a result. In recent years, there has been a lot of curiosity and focus on the ability of immobilized enzymes. The nanomaterials with ideal properties can be used to immobilize enzymes to regulate key factors that determine the efficacy of bio-catalysis. With applications in biotechnology, immunosensing, biomedicine, and nanotechnology sectors have opened a realm of opportunities for enzyme immobilization.

Alkaline-Driven Liquid Metal Janus Micromotor with a Coating Material-Dependent Propulsion Mechanism

Micro/nanomotors have achieved huge progress in driving power divergence and accurate maneuver manipulations in the last two decades. However, there are still several obstacles to the potential biomedical applications, with respect to their biotoxicity and biocompatibility. Gallium- and indium-based liquid metal (LM) alloys are outstanding candidates for solving these issues due to their good biocompatibility and low biotoxicity. Hereby, we fabricate LM Janus micromotors (LMJMs) through ultrasonically dispersing GaInSn LM into microparticles and sputtering different materials as demanded to tune their moving performance. These LMJMs can move in alkaline solution due to the reaction between Ga and NaOH. There are two driving mechanisms when sputtering materials are metallic or nonmetallic. One is self-electrophoresis when sputtering materials are metallic, and the other one is self-diffusiophoresis when sputtering materials are nonmetallic. Our LMJMs can flip between those two modes by varying the deposited materials. The self-electrophoresis-driven LMJMs' moving speed is much faster than the self-diffusiophoresis-driven LMJMs' speed. The reason is that the former occurs galvanic corrosion reaction, while the latter is correlated to chemical corrosion reaction. The switching of the driving mechanism of the LMJMs can be used to fit into different biochemical application scenarios.

The ameliorating approach of nanorobotics in the novel drug delivery systems: a mechanistic review

Nanoscale robotics have the ability that it can productively transform multiple energy sources into motion and strength which reflects an expeditiously appearing and captivating area for research of robotics. In today's plethora, biomedical nanorobotics played an intricate character with numerous units of robots working at the pathological site in a coordinated manner. The synergistic action of the several nanorobotics has been employed for the fulfilment of the task such as large-scale detoxification, delivery of the large pharmacological/therapeutic efficacious payloads, etc. that is nearly unfeasible or unalterable practically by using single nanorobot. The collective intelligence of the nanorobot is advancing progressively at the nanoscale to reinforce their precision treatment potentially. Conclusively, after obtaining certain consideration regarding the nanorobotics sciences, many professionals are compendiously involving in the emerging highly efficacious therapeutic technology that encourages the scientist or designing of the tissues specific for the site-specific nanorobotic diagnostic devices. As a result, the closed and professional type between the field of Nanotechnology and Medical Sciences will provide another new highly oriented level to the domain of nanorobotics.

On the chaotic and hyper-chaotic dynamics of nanobeams with low shear stiffness

We construct a mathematical model of non-linear vibration of a beam nanostructure with low shear stiffness subjected to uniformly distributed harmonic transversal load. The following hypotheses are employed: the nanobeams made from transversal isotropic and elastic material obey the Hooke law and are governed by the kinematic third-order approximation (Sheremetev-Pelekh-Reddy model). The von Kármán geometric non-linear relation between deformations and displacements is taken into account. In order to describe the size-dependent coefficients, the modified couple stress theory is employed. The Hamilton functional yields the governing partial differential equations, as well as the initial and boundary conditions. A solution to the dynamical problem is found via the finite difference method of the second order of accuracy, and next via the Runge-Kutta method of orders from two to eight, as well as the Newmark method. Investigations of the non-linear nanobeam vibrations are carried out with a help of signals (time histories), phase portraits, as well as through the Fourier and wavelet-based analyses. The strength of the nanobeam chaotic vibrations is quantified through the Lyapunov exponents computed based on the Sano-Sawada, Kantz, Wolf, and Rosenstein methods. The application of a few numerical methods on each stage of the modeling procedure allowed us to achieve reliable results. In particular, we have detected chaotic and hyper-chaotic vibrations of the studied nanobeam, and our results are authentic, reliable, and accurate.

Light programmable micro/nanomotors with optically tunable in-phase electric polarization

To develop active nanomaterials that can instantly respond to external stimuli with designed mechanical motions is an important step towards the realization of nanorobots. Herein, we present our finding of a versatile working mechanism that allows instantaneous change of alignment direction and speed of semiconductor nanowires in an external electric field with simple visible-light exposure. The light induced alignment switch can be cycled over hundreds of times and programmed to express words in Morse code. With theoretical analysis and simulation, the working principle can be attributed to the optically tuned real-part (in-phase) electrical polarization of a semiconductor nanowire in aqueous suspension. The manipulation principle is exploited to create a new type of microscale stepper motor that can readily switch between in-phase and out-phase modes, and agilely operate independent of neighboring motors with patterned light. This work could inspire the development of new types of micro/nanomachines with individual and reconfigurable maneuverability for many applications.

Confined 1D Propulsion of Metallodielectric Janus Micromotors on Microelectrodes under Alternating Current Electric Fields

There is mounting interest in synthetic microswimmers ("micromotors") as microrobots as well as a model system for the study of active matters, and spatial navigation is critical for their success. Current navigational technologies mostly rely on magnetic steering or guiding with physical boundaries, yet limitations with these strategies are plenty. Inspired by an earlier work with magnetic domains on a garnet film as predefined tracks, we present an interdigitated microelectrodes (IDE) system where, upon the application of AC electric fields, metallodielectric (e.g., SiO₂-Ti) Janus particles are hydrodynamically confined and electrokinetically propelled in one dimension along the electrode center lines with tunable speeds. In addition, comoving micromotors moved in single files, while those moving in opposite directions primarily reoriented and moved past each other. At high particle densities, turbulence-like aggregates formed as many-body interactions became complicated. Furthermore, a micromotor made U-turns when approaching an electrode closure, while it gradually slowed down at the electrode opening and was collected in large piles. Labyrinth patterns made of serpentine chains of Janus particles emerged by modifying the electrode configuration. Most of these observations can be qualitatively understood by a combination of electroosmotic flows pointing inward to the electrodes, and asymmetric electrical polarization of the Janus particles under an AC electric field. Emerging from these observations is a strategy that not only powers and confines micromotors on prefabricated tracks in a contactless, on-demand manner, but is also capable of concentrating active particles at predefined locations. These features could prove useful for designing tunable tracks that steer synthetic microrobots, as well as to enable the study of single file diffusion, active turbulence, and other collective behaviors of active matters.

How non-bonding domains affect the active assembly of microtubule spools

Structural defects can determine and influence various properties of materials, and many technologies rely on the manipulation of defects (e.g., semiconductor industries). In biological systems, management of defects/errors (e.g. DNA repair) is critical to an organism's survival, which has inspired the design of artificial nanomachines that mimic nature's ability to detect defects and repair damage. Biological motors have captured considerable attention in developing such capabilities due to their ability to convert energy into directed motion in response to environmental stimuli, which maximizes their ability for detection and repair. The objective of the present study was to develop an understanding of how the presence of non-bonding domains, here considered as a "defect", in microtubule (MT) building blocks affect the kinesin-driven, active assembly of MT spools. The assembly/joining of micron-scale bonding (i.e., biotin-containing) and non-bonding (i.e., no biotin) MTs resulted in segmented MT building blocks consisting of alternating bonding and non-bonding domains. Here, the introduction of these MT building blocks into a kinesin gliding motility assay along with streptavidin-coated quantum dots resulted in the active assembly of spools with altered morphology but retained functionality. Moreover, it was noted that non-bonding domains were autonomously and preferentially released from the spools over time, representing a mechanism by which defects may be removed from these structures. Overall, our findings demonstrate that this active assembly system has an intrinsic ability for quality control, which can be potentially expanded to a wide range of applications such as self-regulation and healing of active materials.

A Review of Micromotors in Confinements: Pores, Channels, Grooves, Steps, Interfaces, Chains, and Swimming in the Bulk

One of the recent frontiers of nanotechnology research involves machines that operate at nano- and microscales, also known as nano/micromotors. Their potential applications in biomedicine, environmental sciences and engineering, military and defense industries, self-assembly, and many other areas have fueled an intense interest in this topic over the last 15 years. Despite deepened understanding of their propulsion mechanisms, we are still in the early days of exploring the dynamics of micromotors in complex and more realistic environments. Confinements, as a typical example of complex environments, are extremely relevant to the applications of micromotors, which are expected to travel in mucus gels, blood vessels, reproductive and digestive tracts, microfluidic chips, and capillary tubes. In this review, we summarize and critically examine recent studies (mostly experimental ones) of micromotor dynamics in confinements in 3D (spheres and porous network, channels, grooves, steps, and obstacles), 2D (liquid-liquid, liquid-solid, and liquid-air interfaces), and 1D (chains). In addition, studies of micromotors moving in the bulk solution and the usefulness of acoustic levitation is discussed. At the end of this article, we summarize how confinements can affect micromotors and offer our insights on future research directions. This review article is relevant to readers who are interested in the interactions of materials with interfaces and structures at the microscale and helpful for the design of smart and multifunctional materials for various applications.

Paper Origami-Inspired Design and Actuation of DNA Nanomachines with Complex Motions

Significant progress in DNA nanotechnology has accelerated the development of molecular machines with functions like macroscale machines. However, the mobility of DNA self-assembled nanorobots is still dramatically limited due to challenges with designing and controlling nanoscale systems with many degrees of freedom. Here, an origami-inspired method to design transformable DNA nanomachines is presented. This approach integrates stiff panels formed by bundles of double-stranded DNA connected with foldable creases formed by single-stranded DNA. To demonstrate the method, a DNA version of the paper origami mechanism called a waterbomb base (WBB) consisting of six panels connected by six joints is constructed. This nanoscale WBB can follow four distinct motion paths to transform between five distinct configurations including a flat square, two triangles, a rectangle, and a fully compacted trapezoidal shape. To achieve this, the sequence specificity of DNA base-pairing is leveraged for the selective actuation of joints and the ion-sensitivity of base-stacking interactions is employed for the flattening of joints. In addition, higher-order assembly of DNA WBBs into reconfigurable arrays is achieved. This work establishes a foundation for origami-inspired design for next generation synthetic molecular robots and reconfigurable nanomaterials enabling more complex and controllable motion.

Micro-/Nanorobots Propelled by Oscillating Magnetic Fields

Recent strides in micro- and nanomanufacturing technologies have sparked the development of micro-/nanorobots with enhanced power and functionality. Due to the advantages of on-demand motion control, long lifetime, and great biocompatibility, magnetic propelled micro-/nanorobots have exhibited considerable promise in the fields of drug delivery, biosensing, bioimaging, and environmental remediation. The magnetic fields which provide energy for propulsion can be categorized into rotating and oscillating magnetic fields. In this review, recent developments in oscillating magnetic propelled micro-/nanorobot fabrication techniques (such as electrodeposition, self-assembly, electron beam evaporation, and three-dimensional (3D) direct laser writing) are summarized. The motion mechanism of oscillating magnetic propelled micro-/nanorobots are also discussed, including wagging propulsion, surface walker propulsion, and scallop propulsion. With continuous innovation, micro-/nanorobots can become a promising candidate for future applications in the biomedical field. As a step toward designing and building such micro-/nanorobots, several types of common fabrication techniques are briefly introduced. Then, we focus on three propulsion mechanisms of micro-/nanorobots in oscillation magnetic fields: (1) wagging propulsion; (2) surface walker; and (3) scallop propulsion. Finally, a summary table is provided to compare the abilities of different micro-/nanorobots driven by oscillating magnetic fields.

Shape-Transformable, Fusible Rodlike Swimming Liquid Metal Nanomachine

The T-1000 liquid metal terminator, which can transform and self-repair, represents a dream for decades that robots can fundamentally change our daily life. Until now, some large-scale liquid metal machines have been developed. However, there is no report on nanoscaled liquid metal machines and their biomedical applications. We describe here a shape-transformable and fusible rodlike swimming liquid metal nanomachine, based on the biocompatible and transformable liquid metal gallium. These nanomachines were prepared by a pressure-filter-template technology, and the diameter and length could be controlled by adjusting the nanoporous templates, filter time, and pressure. The as-prepared liquid gallium nanomotors display a core-shell nanorod structure composed of a liquid gallium core and solid gallium oxide shell. Upon exposure to an ultrasound field, the generated acoustic radiation force in the levitation plane can propel them to move autonomously. The liquid metal nanomachine can actively seek cancer cells and transform from a rod to a droplet after drilling into cells owing to the removal of gallium oxide layers in the acidic endosomes. These transformed nanomachines could fuse together inside cells and photothermally kill cancer cells under illumination of near-infrared light. Such acoustically propelled shape-transformable rodlike liquid metal nanomachines have great potential for biomedical applications.

Real-time magnetic actuation of DNA nanodevices via modular integration with stiff micro-levers

DNA nanotechnology has enabled complex nanodevices, but the ability to directly manipulate systems with fast response times remains a key challenge. Current methods of actuation are relatively slow and only direct devices into one or two target configurations. Here we report an approach to control DNA origami assemblies via externally applied magnetic fields using a low-cost platform that enables actuation into many distinct configurations with sub-second response times. The nanodevices in these assemblies are manipulated via mechanically stiff micron-scale lever arms, which rigidly couple movement of a micron size magnetic bead to reconfiguration of the nanodevice while also enabling direct visualization of the conformation. We demonstrate control of three assemblies—a rod, rotor, and hinge—at frequencies up to several Hz and the ability to actuate into many conformations. This level of spatiotemporal control over DNA devices can serve as a foundation for real-time manipulation of molecular and atomic systems.

DNA molecular machines hold promise for biological nanotechnology, but how to actuate them in a fast and programmable manner remains challenging. Here, Lauback et al. demonstrate direct manipulation of DNA origami assemblies via a micrometer-long stiff mechanical lever controlled by a magnetic field.

Micro/nanomotors towards in vivo application: cell, tissue and biofluid

Inspired by highly efficient natural motors, synthetic micro/nanomotors are self-propelled machines capable of converting the supplied fuel into mechanical motion. A significant advance has been made in the construction of diverse motors over the last decade. These synthetic motor systems, with rapid transporting and efficient cargo towing abilities, are expected to open up new horizons for various applications. Utilizing emergent motor platforms for in vivo applications is one important aspect receiving growing interest as conventional therapeutic methodology still remains limited for cancer, heart, or vasculature diseases. In this review we will highlight the recent efforts towards realistic in vivo application of various motor systems. With ever booming research enthusiasm in this field and increasing multidisciplinary cooperation, micro/nanomotors with integrated multifunctionality and selectivity are on their way to revolutionize clinical practice.

Electric-Field-Guided Precision Manipulation of Catalytic Nanomotors for Cargo Delivery and Powering Nanoelectromechanical Devices

We report a controllable and precision approach in manipulating catalytic nanomotors by strategically applied electric (E-) fields in three dimensions (3-D). With the high controllability, the catalytic nanomotors have demonstrated versatility in capturing, delivering, and releasing of cargos to designated locations as well as in situ integration with nanomechanical devices (NEMS) to chemically power the actuation. With combined AC and DC E-fields, catalytic nanomotors can be accurately aligned by the AC E-fields and effectively change their speeds instantly by the DC E-fields. Within the 3-D orthogonal microelectrode sets, the in-plane transport of catalytic nanomotors can be swiftly turned on and off, and these catalytic nanomotors can also move in the vertical direction. The interplaying nanoforces that govern the propulsion and alignment are investigated. The modeling of catalytic nanomotors proposed in previous works has been confirmed quantitatively here. Finally, the prowess of the precision manipulation of catalytic nanomotors by E-fields is demonstrated in two applications: the capture, transport, and release of cargos to prepatterned microdocks, and the assembly of catalytic nanomotors on NEMS to power the continuous rotation. The concepts and approaches reported in this work could further advance applications of catalytic nanomotors, e.g., for assembling and powering nanomachines, nanorobots, and complex NEMS devices.

Light-Powered Micro/Nanomotors

Designed micro/nanomotors are micro/nanoscale machines capable of autonomous motion in fluids, which have been emerging in recent decades owing to their great potential for biomedical and environmental applications. Among them, light-powered micro/nanomotors, in which motion is driven by light, exhibit various advantages in their precise motion manipulation and thereby a superior scope for application. This review summarizes recent advances in the design, manufacture and motion manipulation of different types of light-powered micro/nanomotors. Their structural features and motion performance are reviewed and compared. The challenges and opportunities of light-powered micro/nanomotors are also discussed. With rapidly increasing innovation, advanced, intelligent and multifunctional light-powered micro/nanomachines will certainly bring profound impacts and changes for human life in the future.

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.

Controllable Swarming and Assembly of Micro/Nanomachines

Motion is a common phenomenon in biological processes. Major advances have been made in designing various self-propelled micromachines that harvest different types of energies into mechanical movement to achieve biomedicine and biological applications. Inspired by fascinating self-organization motion of natural creatures, the swarming or assembly of synthetic micro/nanomachines (often referred to micro/nanoswimmers, micro/nanorobots, micro/nanomachines, or micro/nanomotors), are able to mimic these amazing natural systems to help humanity accomplishing complex biological tasks. This review described the fuel induced methods (enzyme, hydrogen peroxide, hydrazine, et al.) and fuel-free induced approaches (electric, ultrasound, light, and magnetic) that led to control the assembly and swarming of synthetic micro/nanomachines. Such behavior is of fundamental importance in improving our understanding of self-assembly processes that are occurring on molecular to macroscopic length scales.

Twists and Turns of Orbiting and Spinning Metallic Microparticles Powered by Megahertz Ultrasound

Micromotors powered by megahertz ultrasound, first reported about 5 years ago, have lately been considered a promising platform for a wide range of microscale applications, yet we are only at the early stage of understanding their operating mechanisms. Through carefully designed experiments, and by comparing the results to acoustic theories, we present here an in-depth study of the behaviors of particles activated by ultrasound, especially their in-plane orbiting and spinning dynamics. Experiments suggest that metallic microrods orbit in tight circles near the resonance ultrasound frequency, likely driven by localized acoustic streaming due to slightly bent particle shapes. On the other hand, particle spins around their long axes on nodal lines, where phase-mismatched orthogonal sound waves possibly produce a viscous torque. Intriguingly, such a torque spins metal-dielectric Janus microspheres back and forth in an unusual "rocking chair" fashion. Overall, our observations and analysis provide fresh and much needed insights on the interesting particle dynamics in resonating ultrasound and could help with developing more powerful and controllable micromachines with biocompatible energy sources.

Rocket-inspired tubular catalytic microjets with grating-structured walls as guiding empennages

Controllable locomotion in the micro-/nanoscale is challenging and attracts increasing research interest. Tubular microjets self-propelled by microbubbles are intensively investigated due to their high energy conversion efficiency, but the imperfection of the tubular geometry makes it harder to realize linear motion. Inspired by the macro rocket, we designed a tubular microjet with a grating-structured wall which mimics the guiding empennage of the macro rocket, and we found that the fluid can be effectively guided by the grooves. Both theoretical simulation and experimental work have been carried out, and the obtained results demonstrate that the stability margin of the grating-structured microjet can be enhanced. Compared with microjets with smooth walls, the structured microjets show an enhanced ability of moving linearly. In 10% H₂O₂, only 20% of the smooth microjets demonstrate linear trajectories, while 80% of the grating-structured microjets keep moving straight. The grating-structured microjet can maintain linear motion under external disturbance. We further propose to increase the stability by introducing a helical grating structure.

Ultrasound-propelled nanowire motors enhance asparaginase enzymatic activity against cancer cells

Ultrasound-(US) propelled nanowires consisting of Au/Ni/Au/PEDOT-PPy-COOH segments are modified with asparaginase enzyme and applied as an effective anti-cancer agent. After immobilization of asparaginase onto the surface of the nanowire motors, the enzyme displays enhanced thermal and pH stabilities, improved resistance towards protease, and higher affinity for the substrate. The fast motion of the motor-carrying asparaginase leads to greatly accelerated biocatalytic depletion of asparagine and hence to a significantly enhanced inhibition efficacy against El4 lymphoma cancer cells (92%) as compared to free enzyme counterpart (17%) and other control groups. Such enhanced enzymatic activity against cancer cells is attributed to the fast motion of the motors which facilitates the interaction between the enzyme and the cancer cells. While asparaginase and EL4 tumor cells are used as a model system in the present study for cancer cell inhibition, the same mechanism can be expanded to other types of enzymes and biomolecules for the corresponding biofunctions.

Highly Efficient Freestyle Magnetic Nanoswimmer

The unique swimming strategies of natural microorganisms have inspired recent development of magnetic micro/nanorobots powered by artificial helical or flexible flagella. However, as artificial nanoswimmers with unique geometries are being developed, it is critical to explore new potential modes for kinetic optimization. For example, the freestyle stroke is the most efficient of the competitive swimming strokes for humans. Here we report a new type of magnetic nanorobot, a symmetric multilinked two-arm nanoswimmer, capable of efficient "freestyle" swimming at low Reynolds numbers. Excellent agreement between the experimental observations and theoretical predictions indicates that the powerful "freestyle" propulsion of the two-arm nanorobot is attributed to synchronized oscillatory deformations of the nanorobot under the combined action of magnetic field and viscous forces. It is demonstrated for the first time that the nonplanar propulsion gait due to the cooperative "freestyle" stroke of the two magnetic arms can be powered by a plane oscillatory magnetic field. These two-arm nanorobots are capable of a powerful propulsion up to 12 body lengths per second, along with on-demand speed regulation and remote navigation. Furthermore, the nonplanar propulsion gait powered by the consecutive swinging of the achiral magnetic arms is more efficient than that of common chiral nanohelical swimmers. This new swimming mechanism and its attractive performance opens new possibilities in designing remotely actuated nanorobots for biomedical operation at the nanoscale.

Nanomotor-Enabled pH-Responsive Intracellular Delivery of Caspase-3: Toward Rapid Cell Apoptosis

Direct and efficient intracellular delivery of enzymes to cytosol holds tremendous therapeutic potential while remaining an unmet technical challenge. Herein, an ultrasound (US)-propelled nanomotor approach and a high-pH-responsive delivery strategy are reported to overcome this challenge using caspase-3 (CASP-3) as a model enzyme. Consisting of a gold nanowire (AuNW) motor with a pH-responsive polymer coating, in which the CASP-3 is loaded, the resulting nanomotor protects the enzyme from release and deactivation prior to reaching an intracellular environment. However, upon entering a cell and exposure to the higher intracellular pH, the polymer coating is dissolved, thereby directly releasing the active CASP-3 enzyme to the cytosol and causing rapid cell apoptosis. In vitro studies using gastric cancer cells as a model cell line demonstrate that such a motion-based active delivery approach leads to remarkably high apoptosis efficiency within a significantly shorter time and with a lower amount of CASP-3 compared to other control groups not involving US-propelled nanomotors. For instance, the reported nanomotor system can achieve 80% apoptosis of human gastric adenocarcinoma cells within only 5 min, which dramatically outperforms other CASP-3 delivery approaches. These results indicate that the US-propelled nanomotors may act as a powerful vehicle for cytosolic delivery of active therapeutic proteins, which would offer an attractive means to enhance the current landscape of intracellular protein delivery and therapy. While CASP-3 is selected as a model protein in this study, the same nanomotor approach can be readily applied to a variety of different therapeutic proteins.

Micro/Nanorobots for Biomedicine: Delivery, Surgery, Sensing, and Detoxification

Micro- and nanoscale robots that can effectively convert diverse energy sources into movement and force represent a rapidly emerging and fascinating robotics research area. Recent advances in the design, fabrication, and operation of micro/nanorobots have greatly enhanced their power, function, and versatility. The new capabilities of these tiny untethered machines indicate immense potential for a variety of biomedical applications. This article reviews recent progress and future perspectives of micro/nanorobots in biomedicine, with a special focus on their potential advantages and applications for directed drug delivery, precision surgery, medical diagnosis and detoxification. Future success of this technology, to be realized through close collaboration between robotics, medical and nanotechnology experts, should have a major impact on disease diagnosis, treatment, and prevention.

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., H₂ O₂ ), 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.

Biobased High-Performance Rotary Micromotors for Individually Reconfigurable Micromachine Arrays and Microfluidic Applications

In this work, we report an innovative type of rotary biomicromachines by using diatom frustules as integrated active components, including the assembling, operation, and performance characterization. We further investigate and demonstrate unique applications of the biomicromachines in achieving individually reconfigurable micromachine arrays and microfluidic mixing. Diatom frustules are porous cell walls of diatoms made of silica. We assembled rotary micromachines consisting of diatom frustules serving as rotors and patterned magnets serving as bearings in electric fields. Ordered arrays of micromotors can be integrated and rotated with controlled orientation and a speed up to ∼3000 rpm, one of the highest rotational speeds in biomaterial-based rotary micromachines. Moreover, by exploiting the distinct electromechanical properties of diatom frustules and metallic nanowires, we realized the first reconfigurable rotary micro/nanomachine arrays with controllability in individual motors. Finally, the diatom micromachines are successfully integrated in microfluidic channels and operated as mixers. This work demonstrated the high-performance rotary micromachines by using bioinspired diatom frustules and their applications, which are essential for low-cost bio-microelectromechanical system/nanoelectromechanical system (bio-MEMS/NEMS) devices and relevant to microfluidics.

Light-Driven Au-WO3@C Janus Micromotors for Rapid Photodegradation of Dye Pollutants

A novel light-driven Au-WO₃@C Janus micromotor based on colloidal carbon WO₃ nanoparticle composite spheres (WO₃@C) prepared by one-step hydrothermal treatment is described. The Janus micromotors can move in aqueous media at a speed of 16 μm/s under 40 mW/cm² UV light due to diffusiophoretic effects. The propulsion of such Au-WO₃@C Janus micromotors (diameter ∼ 1.0 μm) can be generated by UV light in pure water without any external chemical fuels and readily modulated by light intensity. After depositing a paramagnetic Ni layer between the Au layer and WO₃, the motion direction of the micromotor can be precisely controlled by an external magnetic field. Such magnetic micromotors not only facilitate recycling of motors but also promise more possibility of practical applications in the future. Moreover, the Au-WO₃@C Janus micromotors show high sensitivity toward extremely low concentrations of sodium-2,6-dichloroindophenol (DCIP) and Rhodamine B (RhB). The moving speed of motors can be significantly accelerated to 26 and 29 μm/s in 5 × 10^-4 wt % DCIP and 5 × 10^-7 wt % RhB aqueous solutions, respectively, due to the enhanced diffusiophoretic effect, which results from the rapid photocatalytic degradation of DCIP and RhB by WO₃. This photocatalytic acceleration of the Au-WO₃@C Janus micromotors confirms the self-diffusiophoretic mechanism and opens an opportunity to tune the motility of the motors. This work also offers the light-driven micromotors a considerable potential for detection and rapid photodegradation of dye pollutants in water.

Visible light-driven, magnetically steerable gold/iron oxide nanomotors

Au–Fe₂O₃ nanorods are propelled by visible light and steered magnetically in dilute H₂O₂ solutions.

Self-propelled round-trip motion of Janus particles in static line optical tweezers

Controlled propulsion of microparticles and micromachines in fluids could revolutionize many aspects of technology, such as biomedicine, microfluidics, micro-mechanics, optomechanics, and cell biology. We report the self-propelled cyclic round-trip motion of metallo-dielectric Janus particles in static line optical tweezers (LOT). The Janus particle is a 5 μm-diameter polystyrene sphere half-coated with 3 nanometer thick gold film. Both experiment and theory show that this cyclic translational and rotational motion is a consequence of the collective and fine action of the gold-face orientation dependent propulsion optical force, the gradient optical force, and the spontaneous symmetry breaking induced optical torque in different regions of the LOT. This study indicates a novel way to propel and manipulate the mechanical motion of microscopic motors and machines wirelessly in fluid, air, or vacuum environments using a static optical field with a smartly designed non-uniform intensity profile allowing fully controlled momentum and angular momentum exchange between light and the particle.

Acoustically propelled nanoshells

Herein we report a new design for acoustic nanoswimmers, making use of a nanoshell geometry that was synthesized using a sphere template process. Such shell-shaped nanomotors display highly efficient acoustic propulsion on the nanoscale by converting energy from the ambient acoustic field into motion. The propulsion mechanism of the nanoshell motors relies on acoustic streaming stress over the asymmetric surface to produce the driving force for motion. The shell-shaped nanomotors offer a high surface area to volume ratio, allow for efficient scalability and provide higher cargo towing capacity (in comparison to acoustically propelled nanowires). Furthermore, a detailed study of the parameters relevant to propulsion performance, including the material density, size and shape of the motors, reveals that the nanoshell motors exhibit a different propulsion behavior from that predicted by recent theoretical and experimental models for acoustically propelled nanomotors. Such findings indicate that further studies are needed to predict the behavior of acoustic nanomotors with different geometry designs. Practical applications of the new nanoshell motors, including "on-the-move" capture and the transport of multiple cargoes and internalization and movement inside live MCF-7 cancer cells, are demonstrated. These capabilities hold considerable promise for designing fuel-free nanoswimmers capable of performing complex tasks for diverse biomedical applications.

Light-driven micro-tool equipped with a syringe function

Leveraging developments in microfabrication open new possibilities for optical manipulation. With the structural design freedom from three-dimensional printing capabilities of two-photon polymerization, we are starting to see the emergence of cleverly shaped 'light robots' or optically actuated micro-tools that closely resemble their macroscopic counterparts in function and sometimes even in form. In this work, we have fabricated a new type of light robot that is capable of loading and unloading cargo using photothermally induced convection currents within the body of the tool. We have demonstrated this using silica and polystyrene beads as cargo. The flow speeds of the cargo during loading and unloading are significantly larger than when using optical forces alone. This new type of light robot presents a mode of material transport that may have a significant impact on targeted drug delivery and nanofluidics injection.

Fabrication and Manipulation of Ciliary Microrobots with Non-reciprocal Magnetic Actuation

Magnetically actuated ciliary microrobots were designed, fabricated, and manipulated to mimic cilia-based microorganisms such as paramecia. Full three-dimensional (3D) microrobot structures were fabricated using 3D laser lithography to form a polymer base structure. A nickel/titanium bilayer was sputtered onto the cilia part of the microrobot to ensure magnetic actuation and biocompatibility. The microrobots were manipulated by an electromagnetic coil system, which generated a stepping magnetic field to actuate the cilia with non-reciprocal motion. The cilia beating motion produced a net propulsive force, resulting in movement of the microrobot. The magnetic forces on individual cilia were calculated with various input parameters including magnetic field strength, cilium length, applied field angle, actual cilium angle, etc., and the translational velocity was measured experimentally. The position and orientation of the ciliary microrobots were precisely controlled, and targeted particle transportation was demonstrated experimentally.

Rocket Science at the Nanoscale

Autonomous propulsion at the nanoscale represents one of the most challenging and demanding goals in nanotechnology. Over the past decade, numerous important advances in nanotechnology and material science have contributed to the creation of powerful self-propelled micro/nanomotors. In particular, micro- and nanoscale rockets (MNRs) offer impressive capabilities, including remarkable speeds, large cargo-towing forces, precise motion controls, and dynamic self-assembly, which have paved the way for designing multifunctional and intelligent nanoscale machines. These multipurpose nanoscale shuttles can propel and function in complex real-life media, actively transporting and releasing therapeutic payloads and remediation agents for diverse biomedical and environmental applications. This review discusses the challenges of designing efficient MNRs and presents an overview of their propulsion behavior, fabrication methods, potential rocket fuels, navigation strategies, practical applications, and the future prospects of rocket science and technology at the nanoscale.

Acoustically Propelled Nanomotors for Intracellular siRNA Delivery

An effective intracellular gene silencing strategy based on acoustically propelled nanowires modified with an interfering RNA's (siRNA) payload is described. The gold nanowires (AuNW) are wrapped with a Rolling Circle Amplification (RCA) DNA strand, which serves to anchor the siRNA therapy. The ultrasound (US)-powered propulsion of the AuNW leads to fast internalization and rapid intracellular movement and hence to an accelerated siRNA delivery and silencing response. To optimize the micromotor gene silencing procedure, the influence of motion, time, and siRNA dosage was investigated, leading up to a 94% silencing after few minutes treatment with US-propelled siRNA-AuNWs, and to a dramatic (∼13-fold) improvement in the silencing response compared to the static modified nanowires. The ability of the nanomotor-based method for gene silencing has been demonstrated by measuring the GFP silencing response in two different cell lines (HEK-293 and MCF-7) and using detailed control experiments. The viability of the cells after the nanomotors treatment was examined using the MCF-7 cancer cell line. The use of DNA structures carried by the US-propelled nanomotors for gene silencing represents an efficient tool that addresses the challenges associated with RNA transportation and intracellular delivery. Future implementation of nanomachines in gene therapy applications can be expanded into a co-delivery platform for therapeutics.

Man-made rotary nanomotors: a review of recent developments

The development of rotary nanomotors is an essential step towards intelligent nanomachines and nanorobots. In this article, we review the concept, design, working mechanisms, and applications of state-of-the-art rotary nanomotors made from synthetic nanoentities. The rotary nanomotors are categorized according to the energy sources employed to drive the rotary motion, including biochemical, optical, magnetic, and electric fields. The unique advantages and limitations for each type of rotary nanomachines are discussed. The advances of rotary nanomotors is pivotal for realizing dream nanomachines for myriad applications including microfluidics, biodiagnosis, nano-surgery, and biosubstance delivery.

Converting Chemical Energy to Electricity through a Three-Jaw Mini-Generator Driven by the Decomposition of Hydrogen Peroxide

Energy conversion from a mechanical form to electricity is one of the most important research advancements to come from the horizontal locomotion of small objects. Until now, the Marangoni effect has been the only propulsion method to produce the horizontal locomotion to induce an electromotive force, which is limited to a short duration because of the specific property of surfactants. To solve this issue, in this article we utilized the decomposition of hydrogen peroxide to provide the propulsion for a sustainable energy conversion from a mechanical form to electricity. We fabricated a mini-generator consisting of three parts: a superhydrophobic rotator with three jaws, three motors to produce a jet of oxygen bubbles to propel the rotation of the rotator, and three magnets integrated into the upper surface of the rotator to produce the magnet flux. Once the mini-generator was placed on the solution surface, the motor catalyzed the decomposition of hydrogen peroxide. This generated a large amount of oxygen bubbles that caused the generator and integrated magnets to rotate at the air/water interface. Thus, the magnets passed under the coil area and induced a change in the magnet flux, thus generating electromotive forces. We also investigated experimental factors, that is, the concentration of hydrogen peroxide and the turns of the solenoid coil, and found that the mini-generator gave the highest output in a hydrogen peroxide solution with a concentration of 10 wt % and under a coil with 9000 turns. Through combining the stable superhydrophobicity and catalyst, we realized electricity generation for a long duration, which could last for 26 000 s after adding H2O2 only once. We believe this work provides a simple process for the development of horizontal motion and provides a new path for energy reutilization.

Cracking-assisted fabrication of nanoscale patterns for micro/nanotechnological applications

Cracks are frequently observed in daily life, but they are rarely welcome and are considered as a material failure mode. Interestingly, cracks cause critical problems in various micro/nanofabrication processes such as colloidal assembly, thin film deposition, and even standard photolithography because they are hard to avoid or control. However, increasing attention has been given recently to control and use cracks as a facile, low-cost strategy for producing highly ordered nanopatterns. Specifically, cracking is the breakage of molecular bonds and occurs simultaneously over a large area, enabling fabrication of nanoscale patterns at both high resolution and high throughput, which are difficult to obtain simultaneously using conventional nanofabrication techniques. In this review, we discuss various cracking-assisted nanofabrication techniques, referred to as crack lithography, and summarize the fabrication principles, procedures, and characteristics of the crack patterns such as their position, direction, and dimensions. First, we categorize crack lithography techniques into three technical development levels according to the directional freedom of the crack patterns: randomly oriented, unidirectional, or multidirectional. Then, we describe a wide range of novel practical devices fabricated by crack lithography, including bioassay platforms, nanofluidic devices, nanowire sensors, and even biomimetic mechanosensors.

Nanocommunication via FRET With DyLight Dyes Using Multiple Donors and Acceptors

The phenomenon of Förster Resonance Energy Transfer, commonly used to measure the distances between fluorophore molecules and to study interactions between fluorescent-tagged proteins in life sciences, can also be applied in nanocommunication networks to transfer information bits. The mechanism offers a relatively large throughput and very small delays, but at the same time the channel bit error rate is too high and the transmission ranges are too limited for communication purposes. In this paper, multiple donors at the transmitter side and multiple acceptors at the receiver side are considered to decrease the bit error rate. As nanoantennas, the DyLight fluorescent dyes, which are very well suited to long range nanocommunication due to their large Förster distances and high degrees of labeling, are proposed. The reported results of the recent laboratory experiments confirm efficient communication on distances over 10 nm.

Acoustic Microcannons: Toward Advanced Microballistics

Acoustically triggered microcannons, capable of loading and firing nanobullets (Nbs), are presented as powerful microballistic tools. Hollow conically shaped microcannon structures have been synthesized electrochemically and fully loaded with nanobullets made of silica or fluorescent microspheres, and perfluorocarbon emulsions, embedded in a gel matrix stabilizer. Application of a focused ultrasound pulse leads to the spontaneous vaporization of the perfluorocarbon emulsions within the microcannon and results in the rapid ejection of the nanobullets. Such Nbs "firing" at remarkably high speeds (on the magnitude of meters per second) has been modeled theoretically and demonstrated experimentally. Arrays of microcannons anchored in a template membrane were used to demonstrate the efficient Nbs loading and the high penetration capabilities of the ejected Nbs in a tissue phantom gel. This acoustic-microcannon approach could be translated into advanced microscale ballistic tools, capable of efficient loading and firing of multiple cargoes, and offer improved accessibility to target locations and enhanced tissue penetration properties.

Delayed ignition and propulsion of catalytic microrockets based on fuel-induced chemical dealloying of the inner alloy layer

The delayed ignition of catalytic microrockets based on chemical dealloying of an inner alloy layer is demonstrated.

Facile synthesis of highly uniform selenium nanoparticles using glucose as the reductant and surface decorator to induce cancer cell apoptosis

Herein, we demonstrated a new and facial synthesis of highly uniformed SeNPs using glucose as the reductant and surface decorator. Glu–SeNPs induced cancer cell apoptosisviainduction of apoptosis by triggering intracellular ROS overproduction and mitochondrial dysfunction.