Nano/micromotors in (bio)chemical science applications

Janus Nanoparticles: From Fabrication to (Bio)Applications

Janus nanoparticles (JNPs) refer to the integration of two or more chemically discrepant composites into one structure system. Studies into JNPs have been of significant interest due to their interesting characteristics stemming from their asymmetric structures, which can integrate different functional properties and perform more synergetic functions simultaneously. Herein, we present recent progress of Janus particles, comprehensively detailing fabrication strategies and applications. First, the classification of JNPs is divided into three blocks, consisting of polymeric composites, inorganic composites, and hybrid polymeric/inorganic JNPs composites. Then, the fabrication strategies are alternately summarized, examining self-assembly strategy, phase separation strategy, seed-mediated polymerization, microfluidic preparation strategy, nucleation growth methods, and masking methods. Finally, various intriguing applications of JNPs are presented, including solid surfactants agents, micro/nanomotors, and biomedical applications such as biosensing, controlled drug delivery, bioimaging, cancer therapy, and combined theranostics. Furthermore, challenges and future works in this field are provided.

Research progress of using micro/nanomotors in the detection and therapy of diseases related to the blood environment

Micro/nanomotors bring new possibilities for the detection and therapy of diseases related to the blood environment with their unique motion effect. This work reviews the research progress of using micro/nanomotors in the detection and therapy of diseases related to the blood environment. First, we outline the advantages of using micro/nanomotors in blood-related disease detection. To be specific, the motion capability of micro/nanomotors can increase plasma or blood fluid convection and accelerate the interaction between the sample and the capture probe. This allows the effective reduction of the amount of reagents and treatment steps. Therefore, the application of micro/nanomotors significantly improves the analytical performance. Second, we discuss the key challenges and future prospects of micro/nanomotors in the treatment of blood-environment related diseases. It is very important to design a unique treatment plan according to the etiology and specific microenvironment of the disease. The next generation of micro/nanomotors is expected to bring exciting progress to the detection and therapy of blood-environment related diseases.

Bubbles in microfluidics: an all-purpose tool for micromanipulation

In recent decades, the integration of microfluidic devices and multiple actuation technologies at the microscale has greatly contributed to the progress of related fields. In particular, microbubbles are playing an increasingly important role in microfluidics because of their unique characteristics that lead to specific responses to different energy sources and gas-liquid interactions. Many effective and functional bubble-based micromanipulation strategies have been developed and improved, enabling various non-invasive, selective, and precise operations at the microscale. This review begins with a brief introduction of the morphological characteristics and formation of microbubbles. The theoretical foundations and working mechanisms of typical micromanipulations based on acoustic, thermodynamic, and chemical microbubbles in fluids are described. We critically review the extensive applications and the frontline advances of bubbles in microfluidics, including microflow patterns, position and orientation control, biomedical applications, and development of bubble-based microrobots. We lastly present an outlook to provide directions for the design and application of microbubble-based micromanipulation tools and attract the attention of relevant researchers to the enormous potential of microbubbles in microfluidics.

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.

Oscillatory Motion of a Camphor Object on a Surfactant Solution

The mode bifurcation between oscillatory motion and no motion of a camphor object floating on surfactant aqueous solution was investigated. The camphor and surfactant were used as the sources of driving and suppressing forces, respectively. A circular plastic sheet was attached to the center of the bottom of a camphor disk, which served as the self-propelled object; further, the contact area between the disk and the solution (S) was varied to control the amount of camphor molecules from the disk that dissolved in the solution. Motion features, that is, the maximum speed, frequency of oscillatory motion, and mode bifurcation, varied depending on S. We discussed the relationship between these motion features and the kinetics of camphor and surfactant molecules around the object. The results suggest that the nature of the oscillatory motion depends on the amount of camphor molecules accumulated at the base of the self-propelled object.

Biomedical Micro-/Nanomotors: From Overcoming Biological Barriers to In Vivo Imaging

Self-propelled micro- and nanomotors (MNMs) have shown great potential for applications in the biomedical field, such as active targeted delivery, detoxification, minimally invasive diagnostics, and nanosurgery, owing to their tiny size, autonomous motion, and navigation capacities. To enter the clinic, biomedical MNMs request the biodegradability of their manufacturing materials, the biocompatibility of chemical fuels or externally physical fields, the capability of overcoming various biological barriers (e.g., biofouling, blood flow, blood-brain barrier, cell membrane), and the in vivo visual positioning for autonomous navigation. Herein, the recent advances of synthetic MNMs in overcoming biological barriers and in vivo motion-tracking imaging techniques are highlighted. The challenges and future research priorities are also addressed. With continued attention and innovation, it is believed that, in the future, biomedical MNMs will pave the way to improve the targeted drug delivery efficiency.

A Journey of Nanomotors for Targeted Cancer Therapy: Principles, Challenges, and a Critical Review of the State-of-the-Art

A nanomotor is a miniaturized device that converts energy stored in the environment into mechanical motion. The last two decades have witnessed a surge of research interests in the biomedical applications of nanomotors, but little clinical translation. To accelerate this process, targeted cancer therapy is used as an example to describe a "survive, locate, operate, and terminate" (SLOT) mission of a nanomotor, where it must 1) survive in the unfriendly in vivo environment, 2) locate its target as well as be located by human operators, 3) carry out specific operations, and 4) terminate after the mission is completed. Along this journey, the challenges presented to a nanomotor, including to power, navigate, steer, target, release, control, image, and communicate are discussed, and how state-of-the-art nanomotors meet or fall short of these requirements is critically reviewed. These discussions are then condensed into a table for easy reference. In particular, it is argued that chemically powered nanomotors are intrinsically ill-positioned for targeted cancer therapy, while nanomotors powered by magnetic fields or ultrasound show more promises. Following this argument, a tentative nanomotor design is then presented in the end to conform to the SLOT guideline, and to inspire practical, functional nanorobots that are yet to come.

Progress in Nanorobotics for Advancing Biomedicine

Nanorobotics, which has long been a fantasy in the realm of science fiction, is now a reality due to the considerable developments in diverse fields including chemistry, materials, physics, information and nanotechnology in the past decades. Not only different prototypes of nanorobots whose sizes are nanoscale are invented for various biomedical applications, but also robotic nanomanipulators which are able to handle nano-objects obtain substantial achievements for applications in biomedicine. The outstanding achievements in nanorobotics have significantly expanded the field of medical robotics and yielded novel insights into the underlying mechanisms guiding life activities, remarkably showing an emerging and promising way for advancing the diagnosis & treatment level in the coming era of personalized precision medicine. In this review, the recent advances in nanorobotics (nanorobots, nanorobotic manipulations) for biomedical applications are summarized from several facets (including molecular machines, nanomotors, DNA nanorobotics, and robotic nanomanipulators), and the future perspectives are also presented.

Advanced materials-integrated electrochemical sensors as promising medical diagnostics tools: A review

Electrochemical sensors have increasingly been linked with terms as modern biomedically effective highly selective and sensitive devices, wearable and wireless technology, portable electronics, smart textiles, energy storage, communication and user-friendly operating systems. The work brings the overview of the current advanced materials and their application strategies for improving performance, miniaturization and portability of sensing devices. It provides the extensive information on recently developed (bio)sensing platforms based on voltammetric, amperometric, potentiometric and impedimetric detection modes including portable, non-invasive, wireless, and self-driven miniaturized devices for monitoring human and animal health. Diagnostics of selected free radical precursors, low molecular biomarkers, nucleic acids and protein-based biomarkers, bacteria and viruses of today's interest is demonstrated.

Controllable and Continuous Hollow Fiber Swimmers Based on the Marangoni Effect

The rapid response movement caused by the Marangoni effect, a surface tension gradient-induced mass transfer behavior, has spurred considerable promise for diverse applications from microrobots and microreactors to smart drug delivery. Herein, we fabricated an aligned hollow fiber swimmer that showed self-propel movement on a water surface based on the Marangoni effect. By rational designing of an aligned hollow microstructure and an optimized geometrical shape, this swimmer can move continuously for more than 600 s and the maximum angular velocity can reach 22 rad·s^-1. The movement process of the swimmer is clearly monitored by infrared imaging and the process fluid migration. Moreover, this swimmer exhibited a highly controllable motion mode induced by a magnetic field and a concentration gradient. We designed a novel continuous motion system under the heat conversion from solar energy illumination into mechanical energy. This swimmer shows potential application prospects in controlled cargo transportation and convenient energy conversion systems.

Recent Developments in Bacterial Chemotaxis Analysis Based on the Microfluidic System

Bacterial motility in response to chemicals, also called bacterial chemotaxis, is a critical ability to search for the optimal environment to ensure the survival of bacterial species. Recent advances in microbiology have allowed the engineering of bacterial chemotactic properties. Conventional methods for characterizing bacterial motility are not able to fully monitor chemotactic behavior. Developments in microfluidic technology have enabled the designing of new experimental protocols in which spatiotemporal control of the cellular microenvironment can be achieved, and in which bacterial motility can be precisely and quantitatively measured and compared. This review provides an overview of recent developments of and new insights into microfluidic systems for chemotaxis assay.

Medical Micro/Nanorobots in Precision Medicine

Advances in medical robots promise to improve modern medicine and the quality of life. Miniaturization of these robotic platforms has led to numerous applications that leverages precision medicine. In this review, the current trends of medical micro and nanorobotics for therapy, surgery, diagnosis, and medical imaging are discussed. The use of micro and nanorobots in precision medicine still faces technical, regulatory, and market challenges for their widespread use in clinical settings. Nevertheless, recent translations from proof of concept to in vivo studies demonstrate their potential toward precision medicine.

Enhanced Light-Harvesting Efficiency and Adaptation: A Review on Visible-Light-Driven Micro/Nanomotors

As visible light accounts for a larger proportion of solar energy and is harmless to living organisms, it has the potential to be the energy source of micro/nanomotors, which transform visible-light energy into mechanical motion, for different applications, especially in environmental remediation. However, how to precisely control the motion of visible-light-driven micro/nanomotors (VLD-MNMs) and efficiently utilize the weak visible-light photon energy to acquire rapid motion are significant challenges. This review summarizes the most critical aspects, involving photoactive materials, propulsion mechanisms, control methods, and applications of VLD-MNMs, and discusses strategies to systematically enhance the energy-harvesting efficiency and adaptation. At first, the photoactive materials have been divided into inorganic and organic photoactive materials and comprehensively discussed. Then, different propulsion mechanisms of the current VLD-MNMs are presented to explain the improvement in the actuation force, speed, and environmental adaptability. In addition, considering the characteristics of easy control of VLD-MNMs, we summarized the direction, speed, and cluster control methods of VLD-MNMs for different application requirements. Subsequently, the potential applications of VLD-MNMs, e.g., in environmental remediation, micropumps, cargo delivery, and sensing in microscale, are presented. Finally, discussions and suggestions for future directions to enhance the energy-harvesting efficiency and adaptation of VLD-MNMs are provided.

Cancer Cells Microsurgery via Asymmetric Bent Surface Au/Ag/Ni Microrobotic Scalpels Through a Transversal Rotating Magnetic Field

The actuation of micro/nanomachines by means of a magnetic field is a promising fuel-free way to transport cargo in microscale dimensions. This type of movement has been extensively studied for a variety of micro/nanomachine designs, and a special magnetic field configuration results in a near-surface walking. We developed "walking" micromachines which transversally move in a magnetic field, and we used them as microrobotic scalpels to enter and exit an individual cancer cell and cut a small cellular fragment. In these microscalpels, the center of mass lies approximately in the middle of their length. The microrobotic scalpels show good propulsion efficiency and high step-out frequencies of the magnetic field. Au/Ag/Ni microrobotic scalpels controlled by a transversal rotating magnetic field can enter the cytoplasm of cancer cells and also are able to remove a piece of the cytosol while leaving the cytoplasmic membrane intact in a microsurgery-like manner. We believe that this concept can be further developed for potential biological or medical applications.

Photocatalytic Micromotors Activated by UV to Visible Light for Environmental Remediation, Micropumps, Reversible Assembly, Transportation, and Biomimicry

Photocatalytic micromotors are light-induced, chemically powered micromachines based on photocatalytic materials, activated by light illumination, and have redox reactions with environmental solutions to produce chemical gradients and bubbles that propel the micromachines through self-diffusiophoresis, self-electrophoresis, and bubble recoil. Due to the fact that excitation light relates largely to the bandgaps of selected materials, the development of photocatalytic micromotors has experienced an evolution from ultraviolet-light-activated to visible-light-activated and potentially biocompatible systems. Furthermore, due to the strong redox capacity and physical effects caused by the products or product gradients, photocatalytic micromotors have applications in environmental remediation, micropumps, reversible assembly, transportation, and biomimicry.

Autonomous Motion of Bubble-Powered Carbonaceous Nanoflask Motors

We report a carbonaceous nanomotor with a characteristic flask-like hollow structure that can autonomously move under the propulsion of oxygen bubbles. The carbonaceous nanoflask (CNF) motor was fabricated by encapsulating platinum nanoparticles (Pt NPs) into the hollow cavity of the CNF. The internally encapsulated Pt NPs act as catalysts to decompose hydrogen peroxide (H₂O₂) fuel into oxygen bubbles. The generated oxygen bubbles recoil the motion of the CNF motors. Besides, the velocity of CNF motors can be controlled by adjusting the concentration of the H₂O₂ solution. The motion velocity increases with the increase of H₂O₂ concentration, up to 109.25 μm s^-1 at 10% H₂O₂. This study provides important implications for understanding the motion behaviors of nanomotors with an internal cavity, and the self-propelled CNF motors as smart carrier systems have potential applications in the future.

Anisotropic Exclusion Effect between Photocatalytic Ag/AgCl Janus Particles and Passive Beads in a Dense Colloidal Matrix

Synthetic nano- and micromotors interact with each other and their surroundings in a complex manner. Here, we report on the anisotropy of active-passive particle interaction in a soft matter system containing an immobile yet photochemical Ag/AgCl-based Janus particle embedded in a dense matrix of passive beads in pure water. The asymmetry in the chemical gradient around the Janus particle, triggered upon visible light illumination, distorts the isotropy of the surrounding electric potential and results in the repulsion of adjacent passive beads to a certain distance away from the Janus particle. This exclusion effect is found to be anisotropic with larger distances to passive beads in front of the Ag/AgCl cap of the Janus particle. We provide insight into this phenomenon by performing the angular analysis of the radii of exclusion and tracking their time evolution at the level of a single bead. Our study provides a novel fundamental insight into the collective behavior of a complex mixture of active and passive particles and is relevant for various application scenarios, e.g., particle transport at micro- and nanoscale and local chemical sensing.

Active Reversible Swimming of Magnetically Assembled "Microscallops" in Non-Newtonian Fluids

Miniaturized devices capable of active swimming at low Reynolds numbers are of fundamental importance and possess potential biomedical utility. The design of colloidal microswimmers requires not only miniaturizing reconfigurable structures but also understanding their interactions with media at low Reynolds numbers. We investigate the dynamics of "microscallops" made of asymmetric magnetic cubes, which are assembled and actuated using magnetic fields. One approach to achieving directional propulsion is to break the symmetry of the viscous forces by coupling the reciprocal motions of such microswimmers with the nonlinear rheology inherent in non-Newtonian fluids. When placed in shear-thinning fluids, the local viscosity gradient resulting from nonuniform shear stresses exerted by time-asymmetric strokes of the microscallops generates propulsive thrust through an effect we term "self-viscophoresis". Surprisingly, we found that the direction of propulsion changes with the size and structure of these assemblies. We analyze the origins of their directional propulsion and explain the variable propulsion direction in terms of multiple counterbalancing domains of shear dissipation around the microscale structures. The principles governing the locomotion of these microswimmers may be extended to other reconfigurable microbots assembled from colloidal-scale units.

Active colloidal molecules assembled via selective and directional bonds

The assembly of active and self-propelled particles is an emerging strategy to create dynamic materials otherwise impossible. However, control of the complex particle interactions remains challenging. Here, we show that various dynamic interactions of active patchy particles can be orchestrated by tuning the particle size, shape, composition, etc. This capability is manifested in establishing dynamic colloidal bonds that are highly selective and directional, which greatly expands the spectrum of colloidal structures and dynamics by assembly. For example, we demonstrate the formation of colloidal molecules with tunable bond angles and orientations. They exhibit controllable propulsion, steering, reconfiguration as well as other dynamic behaviors that collectively reflect the bond properties. The working principle is further extended to the co-assembly of synthetic particles with biological entities including living cells, giving rise to hybrid colloidal molecules of various types, for example, a colloidal carrousel structure. Our strategy should enable active systems to perform sophisticated tasks in future such as selective cell treatment.

The assembly of active and self-propelled particles is an emerging strategy to create dynamic materials otherwise impossible. Here, the authors show the assembly of active colloidal molecules with a wide spectrum of new structures and dynamics, conferred to them by highly selective and directional interactions.

Polymer-Based Micromotor Fluorescence Immunoassay for On-the-Move Sensitive Procalcitonin Determination in Very Low Birth Weight Infants' Plasma

A new fluorescence micromotor-based immunoassay (FMIm) has been developed for procalcitonin (PCT) determination as an early sepsis diagnostic analytical tool. The micromotors combine the high binding capacity of the specific antibodies onto their polymeric polypyrrole outer layer (PPy layer), with their magnetic guidance (Ni layer) and self-propulsion by catalytic generation of oxygen bubbles (PtNP inner layer) to actively recognize the PCT antigen. This FMIm allowed a sensitive (LOD = 0.07 ng mL^-1) and direct PCT determination in clinical samples from very low-birth-weight infants (VLBWI) with sepsis suspicion, using small volumes of sample (25 μL) in a clinically relevant range of concentrations (0.5-150 ng mL^-1). The good agreement between PCT levels obtained by our micromotor-based method and routine immunofluorescence hospital determination demonstrates the feasibility for the analysis in VLBWI samples and its potential as a point-of-care diagnostic tool for sepsis.

Scale-free, programmable design of morphable chain loops of kilobots and colloidal motors

Micron-scale robots require systems that can morph into arbitrary target configurations controlled by external agents such as heat, light, electricity, and chemical environment. Achieving this behavior using conventional approaches is challenging because the available materials at these scales are not programmable like their macroscopic counterparts. To overcome this challenge, we propose a design strategy to make a robotic machine that is both programmable and compatible with colloidal-scale physics. Our strategy uses motors in the form of active colloidal particles that constantly propel forward. We sequence these motors end-to-end in a closed chain forming a two-dimensional loop that folds under its mechanical constraints. We encode the target loop shape and its motion by regulating six design parameters, each scale-invariant and achievable at the colloidal scale. We demonstrate the plausibility of our design strategy using centimeter-scale robots called kilobots We use Brownian dynamics simulation to explore the large design space beyond that possible with kilobots, and present an analytical theory to aid the design process. Multiple loops can also be fused together to achieve several complex shapes and robotic behaviors, demonstrated by folding a letter shape "M," a dynamic gripper, and a dynamic pacman The material-agnostic, scale-free, and programmable nature of our design enables building a variety of reconfigurable and autonomous robots at both colloidal scales and macroscales.

Integrating modification and detection in acoustic microchip for in-situ analysis

Ultrasound as a biocompatible and powerful approach has been advanced in biotechnology. Here we present an acoustic microchip integrating modification and detection for in-situ analysis. Such microchip employs two pairs of piezoelectric transducers (PZTs) for acoustic field generation and a polydimethylsiloxane (PDMS) microcavity on a polyethylene terephthalate (PET) substrate for producing microparticle array. The applying of acoustic field results in rapidly forming microparticle array by adjusting the inputting frequency and voltage. In-situ modification and detection are accelerated due to the dynamic ultrasonic streaming around the ultrasound induced microparticle array. Such array also benefits from reducing the detection errors by coupling of multiple points. With this strategy, biomarkers (e.g. miRNA) can be enriched, and achieve in-situ modification and detection via simple two steps with excellent specificity. After the detection, samples are regained from the output channel by releasing the acoustic field, which is benefit for further analysis. Such integrated modification and detection acoustic microchip shows great potential in visual in-situ analysis and enriching ultratrace biomarkers for clinical diagnosis.

Active matter therapeutics

Nanotherapies based on micelles, liposomes, polymersomes, nanocapsules, magnetic nanoparticles, and noble metal nanoparticles have been at the forefront of drug delivery in the past few decades. Some of these nanopharmaceuticals have been commercially applied to treat a wide range of diseases, from dry eye syndrome to cancer. However, the majority involve particles that are passive, meaning that they do not change shape, and they lack motility; the static features can limit their therapeutic efficacy. In this review, we take a critical look at an emerging field that seeks to utilize active matter for therapeutics. In this context, active matter can be broadly referred to as micro or nanosized constructs that energetically react with their environment or external fields and translate, rotate, vibrate or change shape. Essentially, the recent literature suggests that such particles could significantly augment present-day drug delivery, by enhancing transport and increasing permeability across anatomical barriers by transporting drugs within solid tumor microenvironments or disrupting cardiovascular plaque. We discuss examples of such particles and link the transport and permeability properties of active matter to potential therapeutic applications in the context of two major diseases, namely cancer and heart disease. We also discuss potential challenges, opportunities, and translational hurdles.

Photoacoustic Imaging-Trackable Magnetic Microswimmers for Pathogenic Bacterial Infection Treatment

Micro/nanorobots have been extensively explored as a tetherless small-scale robotic biodevice to perform minimally invasive interventions in hard-to-reach regions. Despite the emergence of versatile micro/nanorobots in recent years, matched in vivo development remains challenging, limited by unsatisfactory integration of core functions. Herein, we report a polydopamine (PDA)-coated magnetic microswimmer consisting of a magnetized Spirulina (MSP) matrix and PDA surface. Apart from the properties of the existing MSP (e.g., robust propulsion, natural fluorescence, tailored biodegradation, and selective cytotoxicity), the introduced PDA coating enhances the photoacoustic (PA) signal and photothermal effect of the MSP, thus making PA image tracking and photothermal therapy possible. Meanwhile, the PDA's innate fluorescence quenching and diverse surface reactivity allows an off-on fluorescence diagnosis with fluorescence probes (e.g., coumarin 7). As a proof of concept, real-time image tracking (by PA imaging) and desired theranostic capabilities of PDA-MSP microswimmer swarms are demonstrated for the treatment of pathogenic bacterial infection. Our study suggests a feasible antibacterial microrobot for in vivo development and a facile yet versatile functionalization strategy of micro/nanorobots.

Multimodal chemo-/magneto-/phototaxis of 3G CNT-bots to power fuel cells

We report the development of a 3G microswimmer, namely, CNT-bot, capable of undergoing acid-, alkali-, magneto- and phototaxis inside acidic or alkaline baths of peroxide fuel and/or water. The use of carboxyl-functionalised multi-walled carbon nanotubes (MWCNTs) facilitated the propulsion of CNT-bots in an alkaline-water solution by ejecting carbon-dioxide bubbles. Furthermore, doping of magnetite nanoparticles (FeONPs), ferrous ions (Fe²⁺) and titanium dioxide nanoparticles (TiONPs) induces magnetic, chemical and photonic modes of propulsion. While FeONPs stimulated magnetotaxis at a rate of up to ~10 body lengths per second under the influence of a bar magnet, chemotaxis of a similar speed in a peroxide fuel was achieved by bubble-propulsion of oxygen gas originating from the Fenton reaction. In addition, the light-stimulated photo-Fenton reaction led to phototaxis of CNT-bots. A thin coating of magnesium imparted a half-faced Janus appearance to the CNT-bots, which facilitated motion in normal or acidic water media through the ejection of hydrogen gas bubbles. This chemotaxis could be transformed into pH-stimulated directional motion by establishing an acid or alkali concentration gradient across the peroxide and/or water baths. The capacity of CNT-bots to produce oxygen (hydrogen) bubbles in peroxide (acidic water) fuel was exploited to power a PEM fuel cell to generate electricity. The pure oxygen and hydrogen gases generated by CNT-bots in separate chambers were fed directly into the fuel cell in which the incessant motions of the particle facilitated the creation and release of the pure gases to achieve on-demand electricity generation. The motor could also induce dye degradation through advanced oxidation owing to the production of intermediate hydroxyl radicals during the Fenton reaction.

Simple Universal Strategy for Quantification of Carboxyl Groups on Carbon Nanomaterials: Carbon Dioxide Vapor Generation Coupled to Microplasma for Optical Emission Spectrometric Detection

The physicochemical properties and applications of carbon nanomaterials are remarkably dependent on the amount of carboxyl group on their surfaces. Unfortunately, it is challenging to determine the carboxyl group on carbon nanomaterials at an ultralow density not only due to the low sensitivities of conventional techniques, but also because there are no matrix-matched certified reference materials available. In this work, a novel strategy comprising coupling carbon dioxide vapor generation to a microplasma optical emission spectrometer was developed for the sensitive and accurate quantification of surface carboxyl groups on carbon nanomaterials. The carboxyl group on multiwall carbon nanotubes (MWCNTs), graphene (G), or its oxide (GO) was converted to carboxylic acid using concentrated hydrochloric acid prior to quantification. The generated carboxylic acid was purified and then reacted with sodium bicarbonate to generate CO₂, which was swept into a miniaturized point discharge optical emission spectrometer (μPD-OES) for the detection of carbon atomic emission lines. Potassium hydrogen phthalate (KHP) served as a calibration standard for quantification of the carboxyl group on G/GO/MWCNTs, thus, overcoming the lack of CRMs. Owing to the high sensitivity of μPD-OES for the detection of CO₂, a limit of detection of 0.1 μmol g^-1 (1 nmol) was obtained for the carboxyl group based on a sample mass of 10 mg G/GO/MWCNTs, superior to that obtained using conventional methods. Moreover, the proposed method not only retains several unique advantages of good accuracy and elimination of the use of complicated, expensive, and high power-consumption instruments, but was also applicable to the quantification of the carboxyl group on other nanomaterials such as carboxylated magnetic microspheres.

Insulated conjugated bimetallopolymer with sigmoidal response by dual self-controlling system as a biomimetic material

Biological systems are known to spontaneously adjust the functioning of neurotransmitters, ion channels, and the immune system, being promoted or regulated through allosteric effects or inhibitors, affording non-linear responses to external stimuli. Here we report that an insulated conjugated bimetallopolymer, in which Ru(II) and Pt(II) complexes are mutually connected with insulated conjugations, exhibits phosphorescence in response to CO gas. The net profile corresponds to a sigmoidal response with a dual self-controlling system, where drastic changes were exhibited at two threshold concentrations. The first threshold for activation of the system is triggered by the depolymerization of the non-radiative conjugated polymer to luminescent monomers, while the second one for regulation is triggered by the switch in the rate-determining step of the Ru complex. Such a molecular design with cooperative multiple transition metals would provide routes for the development of higher-ordered artificial molecular systems bearing bioinspired responses with autonomous modulation.

Requirement and Development of Hydrogel Micromotors towards Biomedical Applications

With controllable size, biocompatibility, porosity, injectability, responsivity, diffusion time, reaction, separation, permeation, and release of molecular species, hydrogel microparticles achieve multiple advantages over bulk hydrogels for specific biomedical procedures. Moreover, so far studies mostly concentrate on local responses of hydrogels to chemical and/or external stimuli, which significantly limit the scope of their applications. Tetherless micromotors are autonomous microdevices capable of converting local chemical energy or the energy of external fields into motive forces for self-propelled or externally powered/controlled motion. If hydrogels can be integrated with micromotors, their applicability can be significantly extended and can lead to fully controllable responsive chemomechanical biomicromachines. However, to achieve these challenging goals, biocompatibility, biodegradability, and motive mechanisms of hydrogel micromotors need to be simultaneously integrated. This review summarizes recent achievements in the field of micromotors and hydrogels and proposes next steps required for the development of hydrogel micromotors, which become increasingly important for in vivo and in vitro bioapplications.

Multigear Bubble Propulsion of Transient Micromotors

Transient, chemically powered micromotors are promising biocompatible engines for microrobots. We propose a framework to investigate in detail the dynamics and the underlying mechanisms of bubble propulsion for transient chemically powered micromotors. Our observations on the variations of the micromotor active material and geometry over its lifetime, from initial activation to the final inactive state, indicate different bubble growth and ejection mechanisms that occur stochastically, resulting in time-varying micromotor velocity. We identify three processes of bubble growth and ejection, and in analogy with macroscopic multigear machines, we call each process a gear. Gear 1 refers to bubbles that grow on the micromotor surface before detachment while in Gear 2 bubbles hop out of the micromotor. Gear 3 is similar in nature to Gear 2, but the bubbles are too small to contribute to micromotor motion. We study the characteristics of these gears in terms of bubble size and ejection time, and how they contribute to micromotor displacement. The ability to tailor the shell polarity and hence the bubble growth and ejection and the surrounding fluid flow is demonstrated. Such understanding of the complex multigear bubble propulsion of transient chemical micromotors should guide their future design principles and serve for fine tuning the performance of these micromotors.

Requirement and Development of Hydrogel Micromotors towards Biomedical Applications

With controllable size, biocompatibility, porosity, injectability, responsivity, diffusion time, reaction, separation, permeation, and release of molecular species, hydrogel microparticles achieve multiple advantages over bulk hydrogels for specific biomedical procedures. Moreover, so far studies mostly concentrate on local responses of hydrogels to chemical and/or external stimuli, which significantly limit the scope of their applications. Tetherless micromotors are autonomous microdevices capable of converting local chemical energy or the energy of external fields into motive forces for self-propelled or externally powered/controlled motion. If hydrogels can be integrated with micromotors, their applicability can be significantly extended and can lead to fully controllable responsive chemomechanical biomicromachines. However, to achieve these challenging goals, biocompatibility, biodegradability, and motive mechanisms of hydrogel micromotors need to be simultaneously integrated. This review summarizes recent achievements in the field of micromotors and hydrogels and proposes next steps required for the development of hydrogel micromotors, which become increasingly important for in vivo and in vitro bioapplications.

The light-driven macroscopic directional motion of a water droplet on an azobenzene-calix[4]arene modified surface

A novel photo-responsive surface was constructed via modifying azobenzene-calix[4]arene (ABC4) on a microstructured silicon surface. Asymmetric UV light irradiation could drive the macroscopic directional motion of a water droplet on this photo-responsive surface.

Nano/Micromotors for Diagnosis and Therapy of Cancer and Infectious Diseases

Micromotors are man-made nano/microscale devices capable of transforming energy into mechanical motion. The accessibility and force offered by micromotors hold great promise to solve complex biomedical challenges. This Review highlights current progress and prospects in the use of nano and micromotors for diagnosis and treatment of infectious diseases and cancer. Motion-based sensing and fluorescence switching detection strategies along with therapeutic approaches based on direct cell capture; killing by direct contact or specific drug delivery to the affected site, will be comprehensively covered. Future challenges to translate the potential of nano/micromotors into practical applications will be described in the conclusions.

Phoretic Liquid Metal Micro/Nanomotors as Intelligent Filler for Targeted Microwelding

Micro/nanomotors (MNMs) have emerged as active micro/nanoplatforms that can move and perform functions at small scales. Much of their success, however, hinges on the use of functional properties of new materials. Liquid metals (LMs), due to their good electrical conductivity, biocompatibility, and flexibility, have attracted considerable attentions in the fields of flexible electronics, biomedicine, and soft robotics. The design and construction of LM-based motors is therefore a research topic with tremendous prospects, however current approaches are mostly limited to macroscales. Here, the fabrication of an LM-MNM (made of Galinstan, a gallium-indium-tin alloy) is reported and its potential application as an on-demand, self-targeting welding filler is demonstrated. These LM-MNMs (as small as a few hundred nanometers) are half-coated with a thin layer of platinum (Pt) and move in H₂ O₂ via self-electrophoresis. In addition, the LM-MNMs roaming in a silver nanowire network can move along the nanowires and accumulate at the contact junctions where they become fluidic and achieve junction microwelding at room temperature by reacting with acid vapor. This work presents an intelligent and soft nanorobot capable of repairing circuits by welding at small scales, thus extending the pool of available self-propelled MNMs and introducing new applications.

Stomatocyte structural color-barcode micromotors for multiplex assays

Artificial micromotors have a demonstrated value in the biomedical area. Attempts to develop this technology tend to impart micromotors with novel functions to improve the values. Herein, we present novel structural color-barcode micromotors for the multiplex assays. We found that, by rapidly extracting solvent and assembling monodispersed nanoparticles in droplets, it could form stomatocyte colloidal crystal clusters, which not only showed striking structural colors and characteristic reflection peaks due to their ordered nanoparticles arrangement, but also provided effective cavities for the integration of functional elements. Thus, the micromotors with catalysts or magnetic elements in their cavities, as well as with the corresponding structural color coding, could be achieved by using the platinum and ferric oxide dispersed pre-gel to fill and duplicate the stomatocyte colloidal crystal clusters. We have demonstrated that the self-movement of these structural color-barcode micromotors could efficiently accelerate the mixing speed of the detection sample and greatly increase the probe–target interactions towards faster and more sensitive single or multiplex detection, and the magnetism of these barcode micromotors enables the flexible collection of the micromotors, which could facilitate the detection processes. These features make the stomatocyte structural color-barcode micromotors ideal for biomedical applications.

Catalyst-Loaded Capsules that Spontaneously Inflate and Violently Eject their Core

We present a design for polymer capsules that exhibit a range of unusual autonomous behaviors when exposed to a chemical fuel. The capsules have a physically gelled core (alginate-Ca²⁺) loaded with catalytic (silver) particles and a shell composed of a chemically cross-linked gel. In the presence of the fuel (H₂O₂), a catalytic reaction occurs, which generates oxygen (O₂) gas. The gas collects in a zone between the core and the shell, and the resulting gas pressure causes the elastic shell to stretch. This makes the capsule inflate in a process reminiscent of a swelling pufferfish. As the capsule inflates, the polymer chains in the shell continue to stretch until a breaking point is reached, whereupon the shell ruptures. Three rupture modes are documented: gentle, moderate, and violent. The latter involves the gelled core being forcefully ejected out of the shell in a manner similar to the ejection of needles out of nematocysts on jellyfish. The extent and duration of inflation can be tuned by altering the core and shell composition; for example, shells that are more densely cross-linked swell less and rupture faster. Also, instead of a catalytic reaction, capsule inflation can be achieved by combining reactants, one in the capsule and the other in the external solution, that together generate a different gas (e.g., CO₂).

Microswimmers with Heat Delivery Capacity for 3D Cell Spheroid Penetration

Micro- and nanoswimmers are a fast emerging concept that changes how colloidal and biological systems interact. They can support drug delivery vehicles, assist in crossing biological barriers, or improve diagnostics. We report microswimmers that employ collagen, a major extracellular matrix (ECM) constituent, as fuel and that have the ability to deliver heat via incorporated magnetic nanoparticles when exposed to an alternating magnetic field (AMF). Their assembly and heating properties are outlined followed by the assessment of their calcium-triggered mobility in aqueous solution and collagen gels. It is illustrated that the swimmers in collagen gel in the presence of a steep calcium gradient exhibit fast and directed mobility. The experimental data are supported with theoretical considerations. Finally, the successful penetration of the swimmers into 3D cell spheroids is shown, and upon exposure to an AMF, the cell viability is impaired due to the locally delivered heat. This report illustrates an opportunity to employ swimmers to enhance tissue penetration for cargo delivery via controlled interaction with the ECM.

Osmotic-engine-driven liposomes in microfluidic channels

Self-propelled underwater microrobots that locomote without external sources of energy have potential application as drug carriers and probes in narrow spaces. In this study, we focused on an osmotic engine model, which is a migration mechanism, and applied it as a negative chemotaxis mechanism to induce liposome displacement. First, we confirmed the osmotic flow across the lipid bilayer and calculated the osmotic flow velocity to be 8.5 fL min-1 μm-2 when a salt concentration difference was applied to the lipid bilayer. Next, we designed and fabricated a microchannel that can trap a giant liposome and apply a salt concentration difference to the front and rear of the liposome. Then, we demonstrated the movement of the liposome by flowing it to the microchannel. The liposome successfully moved in the direction of the lower ion concentration at a speed of 0.6 μm min-1 owing to the osmotic pressure difference. Finally, we visualized the inner flow in the liposome by encapsulating microbeads in the liposome and observed the movement of the microbeads to verify that an osmotic flow was generated on the liposome. As a result, we observed the circulation of the microbeads in the liposome when the concentration difference was applied to the front and rear of the liposome, suggesting that the movement of the liposome was driven by the osmotic flow generated by the osmotic pressure difference. These results indicate that the osmotic-pressure-based migration mechanism has the potential to be utilized as the actuator of molecular robots.

Phototactic Flocking of Photochemical Micromotors

Inspired by astonishing collective motions and tactic behaviors in nature, here we show phototactic flocking of synthetic photochemical micromotors. When enriched with hydroxyl groups, TiO2 micromotors can spontaneously gather into flocks in aqueous media through electrolyte diffusiophoresis. Under light irradiation, due to the dominant nonelectrolyte diffusiophoretic interaction resulting from the overlap of asymmetric nonelectrolyte clouds around adjacent individuals, these flocks exhibit intriguing collective behaviors, such as dilatational negative phototaxis, high collective velocity, and adaptive group reconfiguration. Consequently, the micromotor flocks can migrate along pre-designed paths and actively bypass obstacles with reversible dilatation (expansion/contraction) under pulsed light navigation. Furthermore, owing to the enhanced driving force and rapid dilatational area covering, they are able to execute cooperative tasks that single micromotors cannot achieve, such as cooperative large-cargo transport and collective microenvironment mapping. Our discovery would promote the creation of reconfigurable microrobots, active materials, and intelligent synthetic systems.

Nano-and Micromotors Designed for Cancer Therapy

Research on nano- and micromotors has evolved into a frequently cited research area with innovative technology envisioned for one of current humanities' most deadly problems: cancer. The development of cancer targeting drug delivery strategies involving nano-and micromotors has been a vibrant field of study over the past few years. This review aims at categorizing recent significant results, classifying them according to the employed propulsion mechanisms starting from chemically driven micromotors, to field driven and biohybrid approaches. In concluding remarks of section 2, we give an insight into shape changing micromotors that are envisioned to have a significant contribution. Finally, we critically discuss which important aspects still have to be addressed and which challenges still lie ahead of us.

ATP-Mediated Transient Behavior of Stomatocyte Nanosystems

In nature, dynamic processes are ubiquitous and often characterized by adaptive, transient behavior. Herein, we present the development of a transient bowl-shaped nanoreactor system, or stomatocyte, the properties of which are mediated by molecular interactions. In a stepwise fashion, we couple motility to a dynamic process, which is maintained by transient events; namely, binding and unbinding of adenosine triphosphate (ATP). The surface of the nanosystem is decorated with polylysine (PLL), and regulation is achieved by addition of ATP. The dynamic interaction between PLL and ATP leads to an increase in the hydrophobicity of the PLL-ATP complex and subsequently to a collapse of the polymer; this causes a narrowing of the opening of the stomatocytes. The presence of the apyrase, which hydrolyzes ATP, leads to a decrease of the ATP concentration, decomplexation of PLL, and reopening of the stomatocyte. The competition between ATP input and consumption gives rise to a transient state that is controlled by the out-of-equilibrium process.

Enhanced ion tolerance of electrokinetic locomotion in polyelectrolyte-coated microswimmer

Over the last decade, researchers have endeavored to mimic the naturally motile microorganisms and develop artificial nano/microswimmers, which propel themselves in aqueous media. However, most of these nano/microswimmers are propelled by the self-electrophoretic mechanism, which has one critical incompetency: the inability to operate in a high concentration electrolyte solution, such as the most important body fluid, blood. This ionic quenching behavior is well backed by the classical Helmholtz-Smoluchowski theory and seems to be an insurmountable challenge which has shadowed the otherwise promising biomedical applications for artificial nano/microswimmers. Here, we propose that the active nano/microswimmer's self-electrophoresis is fundamentally different from the passive nanoparticle electrophoresis. By significantly increasing the Dukhin number with polyelectrolyte coating and geometry optimization, a favorable deviation from the Helmholtz-Smoluchowski behavior can be realized, and ion tolerance is enhanced by over 100 times for a visible light-powered self-electrophoretic microswimmer.