Thermal activation of catalytic microjets in blood samples using microfluidic chips

Propulsion mechanisms of micro/nanorobots: a review

Micro/nanomotors (MNMs) are intelligent, efficient and promising micro/nanorobots (MNR) that can respond to external stimuli (e.g., chemical energy, temperature, light, pH, ultrasound, magnetic, biosignals, ions) and perform specific tasks. The MNR can adapt to different external stimuli and transform into various functional forms to match different application scenarios. So far, MNR have found extensive application in targeted therapy, drug delivery, tissue engineering, environmental remediation, and other fields. Despite the promise of MNR, there are few reviews that focus on them. To shed new light on the further development of the field, it is necessary to provide an overview of the current state of development of these MNR. Therefore, this paper reviews the research progress of MNR in terms of propulsion mechanisms, and points out the pros and cons of different stimulus types. Finally, this paper highlights the current challenges faced by MNR and proposes possible solutions to facilitate the practical application of MNR.

Strategies in design of self-propelling hybrid micro/nanobots for bioengineering applications

Micro/nanobots are integrated devices developed from engineered nanomaterials that have evolved significantly over the past decades. They can potentially be pre-programmed to operate robustly at numerous hard-to-reach organ/tissues/cellular sites for multiple bioengineering applications such as early disease diagnosis, precision surgeries, targeted drug delivery, cancer therapeutics, bio-imaging, biomolecules isolation, detoxification, bio-sensing, and clearing up clogged arteries with high soaring effectiveness and minimal exhaustion of power. Several techniques have been introduced in recent years to develop programmable, biocompatible, and energy-efficient micro/nanobots. Therefore, the primary focus of most of these techniques is to develop hybrid micro/nanobots that are an optimized combination of purely synthetic or biodegradable bots suitable for the execution of user-defined tasks more precisely and efficiently. Recent progress has been illustrated here as an overview of a few of the achievable construction principles to be used to make biomedical micro/nanobots and explores the pivotal ventures of nanotechnology-moderated development of catalytic autonomous bots. Furthermore, it is also foregrounding their advancement offering an insight into the recent trends and subsequent prospects, opportunities, and challenges involved in the accomplishments of the effective multifarious bioengineering applications.

Five in One: Multi-Engine Highly Integrated Microrobot

A multi-engine highly integrated microrobot, which is a Janus hemispherical shell structure composed of Pt and α-Fe2 O3 , is successfully developed. The microrobot can be efficiently driven and flexibly regulated by five stimuli, including an optical field, an acoustic field, magnetic field, an electric field, and chemical fuel. In addition, no matter which way it is driven by, the direction can be effectively controlled through the magnetic field regulation. Furthermore, this microrobot can also utilize magnetic or acoustic fields to achieve excellent aggregation control and swarm movement. Finally, this study demonstrates that the microrobots' propulsion can be effectively synergistically enhanced through the simultaneous action of two driving mechanisms, which can greatly improve the performance of the motor in applications, such as pollutant degradation. This multi-engine, highly integrated microrobot not only can adapt to more complex environments and has a wider application range, better application prospects, but also provides important ideas for designing future advanced micro/nanorobots.

Strain-induced self-rolled-up microtubes for multifunctional on-chip microfluidic applications

On-chip microfluidics are characterized as miniaturized devices that can be either integrated with other components on-chip or can individually serve as a standalone lab-on-a-chip system for a variety of applications ranging from biochemical sensing to macromolecular manipulation. Heterogenous integration with various materials and form factors is, therefore, key to enhancing the performance of such microfluidic systems. The fabrication of complex three-dimensional (3D) microfluidic components that can be easily integrated with other material systems and existing state-of-the-art microfluidics is of rising importance. Research on producing self-assembled 3D architectures by the emerging self-rolled-up membrane (S-RuM) technology may hold the key to such integration. S-RuM technology relies on a strain-induced deformation mechanism to spontaneously transform stacked thin-film materials into 3D cylindrical hollow structures virtually on any kind of substrate. Besides serving as a compact microfluidic chamber, the S-RuM-based on-chip microtubular architecture exhibits several other advantages for microfluidic applications including customizable geometry, biocompatibility, chemical stability, ease of integration, uniform field distributions, and increased surface area to volume ratio. In this Review, we will highlight some of the applications related to molecule/particle sensing, particle delivery, and manipulation that utilized S-RuM technology to their advantage.

Magnetic nanoparticle swarm with upstream motility and peritumor blood vessel crossing ability

Micro-nano-robots show great potential and value for applications in targeted drug delivery; however, very few current studies have enabled micro-nano-robots to move against blood flow, and in addition, how micro-nano-robots can penetrate endothelial cells and enter tissues via vascular permeation remains unclear. Inspired by the bionics of dynamic aggregation in wild herring schools and transvascular permeation of leukocytes, we propose a novel drug delivery strategy where thousands of magnetic nanoparticles (MNPs) can be assembled into swarms under the guidance of a specially designed electromagnetic field. The vortex-like swarms of magnetic nanoparticles exhibit excellent stability, allowing them to withstand the impact of high-speed flow and move upstream along the vessel wall, stopping at the target location. When the vortex-like swarms encounter a tumor periphery without a continuous vessel wall, their rheological properties actively adhere them to the edges of the vascular endothelial gap, using their deformability to crawl through narrow intercellular gaps, enabling large-scale targeted drug delivery. This cluster of miniature nanorobots can be reshaped and reconfigured to perform a variety of tasks according to the environmental demands of the circulatory system, providing new solutions for a variety of biomedical field applications.

“Motile-targeting” drug delivery platforms based on micro/nanorobots for tumor therapy

Traditional drug delivery systems opened the gate for tumor-targeted therapy, but they generally took advantage of enhanced permeability and retention or ligand-receptor mediated interaction, and thus suffered from limited recognition range (<0.5 nm) and low targeting efficiency (0.7%, median). Alternatively, micro/nanorobots (MNRs) may act as emerging “motile-targeting” drug delivery platforms to deliver therapeutic payloads, thereby making a giant step toward effective and safe cancer treatment due to their autonomous movement and navigation in biological media. This review focuses on the most recent developments of MNRs in “motile-targeting” drug delivery. After a brief introduction to traditional tumor-targeted drug delivery strategies and various MNRs, the representative applications of MNRs in “motile-targeting” drug delivery are systematically streamlined in terms of the propelling mechanisms. Following a discussion of the current challenges of each type of MNR in biomedical applications, as well as future prospects, several promising designs for MNRs that could benefit in “motile-targeting” drug delivery are proposed. This work is expected to attract and motivate researchers from different communities to advance the creation and practical application of the “motile-targeting” drug delivery platforms.

High shear rate propulsion of acoustic microrobots in complex biological fluids

Untethered microrobots offer a great promise for localized targeted therapy in hard-to-access spaces in our body. Despite recent advancements, most microrobot propulsion capabilities have been limited to homogenous Newtonian fluids. However, the biological fluids present in our body are heterogeneous and have shear rate-dependent rheological properties, which limit the propulsion of microrobots using conventional designs and actuation methods. We propose an acoustically powered microrobotic system, consisting of a three-dimensionally printed 30-micrometer-diameter hollow body with an oscillatory microbubble, to generate high shear rate fluidic flow for propulsion in complex biofluids. The acoustically induced microstreaming flow leads to distinct surface-slipping and puller-type propulsion modes in Newtonian and non-Newtonian fluids, respectively. We demonstrate efficient propulsion of the microrobots in diverse biological fluids, including in vitro navigation through mucus layers on biologically relevant three-dimensional surfaces. The microrobot design and high shear rate propulsion mechanism discussed herein could open new possibilities to deploy microrobots in complex biofluids toward minimally invasive targeted therapy.

Untethered Microrobots for Active Drug Delivery: From Rational Design to Clinical Settings

Recent advances of untethered microrobots, which navigate the complex regions in vivo for therapeutics, have presented promising multiple applications on future healthcare. Microrobots used for active drug delivery system (DDS) have been demonstrated for advanced targeting distribution, improved delivery efficiency, and reduced systemic side effects. In this review, the therapeutic benefits of active DDS are presented compared to the traditional passive DDS, which illustrate the historical reasons for choosing active DDS. An integrated 5D radar chart analysis model containing the core capabilities of the active DDS is innovatively proposed. It would be a practical tool for measurement and mapping of the field of active delivery, followed by the evolutions and bottlenecks of each technical module. The comprehensive consideration of microrobots before clinical application is also discussed from the aspects of robot ethics, dosage, quality control and stability control in actual production. Gastrointestinal and blood administration, as two major clinical scenes of drug delivery, are discussed in detail as examples of the potential bedside applications of active DDS. Finally, combined with the reported analysis model, the current status and future outlook from the translation prospect to the clinical scenes of microrobots are provided.

Cargo Transportation and Methylene Blue Degradation by Using Fuel-Powered Micromotors

In the past two decades, micromotors have experienced rapid development, especially in environmental remediation, the biomedical field, and in cargo delivery. In this study micromotors have been synthesized from a variety of materials. Different functional layers and catalytic layers are formed through template electrodeposition (the bottom-up method). At the same time, the article analyzes the influence of hydrogen peroxide concentration, surfactant type and concentration on the speed of the micromotors. Cargo transportation through tubular micromotors has always been a problem that people are eager to solve. In this article, we electrodeposit a layer of Ni in the microtubes, which effectively guides the microtubular motors to complete the cargo transportation. The potential applications of micromotors are also being explored. We added the prepared micromotors to the methylene blue solution to effectively enhance the degradation.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

3D printing of functional microrobots

3D printing (also called "additive manufacturing" or "rapid prototyping") is able to translate computer-aided and designed virtual 3D models into 3D tangible constructs/objects through a layer-by-layer deposition approach. Since its introduction, 3D printing has aroused enormous interest among researchers and engineers to understand the fabrication process and composition-structure-property correlation of printed 3D objects and unleash its great potential for application in a variety of industrial sectors. Because of its unique technological advantages, 3D printing can definitely benefit the field of microrobotics and advance the design and development of functional microrobots in a customized manner. This review aims to present a generic overview of 3D printing for functional microrobots. The most applicable 3D printing techniques, with a focus on laser-based printing, are introduced for the 3D microfabrication of microrobots. 3D-printable materials for fabricating microrobots are reviewed in detail, including photopolymers, photo-crosslinkable hydrogels, and cell-laden hydrogels. The representative applications of 3D-printed microrobots with rational designs heretofore give evidence of how these printed microrobots are being exploited in the medical, environmental, and other relevant fields. A future outlook on the 3D printing of microrobots is also provided.

Investigating the Dynamics of the Magnetic Micromotors in Human Blood

The field of micromotors has been growing exponentially with increased emphasis on biomedical applications, with various in vivo demonstrations of targeted drug delivery, biosensing, and gene delivery, among others. In parallel, these micromotors have been recently used for probing the rheological properties of both intra- and extracellular environments. Here, we demonstrate the application of magnetic micromotors for investigation of rheological properties of human blood. While there are several techniques to sense mechanical properties of blood, such as deformability of the red blood cells, this is the first experimental observation of using micromotors for these biophysical investigations. We hope that this will lead to a better understanding of the nature of interactions of micromotors with biological systems and expand the scope of micromotors for probing other related systems, such as interstitial fluids and other complex biological fluids.

Engineering the Interaction Dynamics between Nano-Topographical Immunocyte-Templated Micromotors across Scales from Ions to Cells

Manufacturing mobile artificial micromotors with structural design factors, such as morphology nanoroughness and surface chemistry, can improve the capture efficiency through enhancing contact interactions with their surrounding targets. Understanding the interplay of such parameters targeting high locomotion performance and high capture efficiency at the same time is of paramount importance, yet, has so far been overlooked. Here, an immunocyte-templated nano-topographical micromotor is engineered and their interactions with various targets across multiple scales, from ions to cells are investigated. The macrophage templated nanorough micromotor demonstrates significantly increased surface interactions and significantly improved and highly efficient removal of targets from complex aqueous solutions, including in plasma and diluted blood, when compared to smooth synthetic material templated micromotors with the same size and surface chemistry. These results suggest that the surface nanoroughness of the micromotors for the locomotion performance and interactions with the multiscale targets should be considered simultaneously, for they are highly interconnected in design considerations impacting applications across scales.

Biocatalytic Micro- and Nanomotors

Enzyme-powered micro- and nanomotors are tiny devices inspired by nature that utilize enzyme-triggered chemical conversion to release energy stored in the chemical bonds of a substrate (fuel) to actuate it into active motion. Compared with conventional chemical micro-/nanomotors, these devices are particularly attractive because they self-propel by utilizing biocompatible fuels, such as glucose, urea, glycerides, and peptides. They have been designed with functional material constituents to efficiently perform tasks related to active targeting, drug delivery and release, biosensing, water remediation, and environmental monitoring. Because only a small number of enzymes have been exploited as bioengines to date, a new generation of multifunctional, enzyme-powered nanorobots will emerge in the near future to selectively search for and utilize water contaminants or disease-related metabolites as fuels. This Minireview highlights recent progress in enzyme-powered micro- and nanomachines.

Sperm-Driven Micromotors Moving in Oviduct Fluid and Viscoelastic Media

Biohybrid micromotors propelled by motile cells are fascinating entities for autonomous biomedical operations on the microscale. Their operation under physiological conditions, including highly viscous environments, is an essential prerequisite to be translated to in vivo settings. In this work, a sperm-driven microswimmer, referred to as a spermbot, is demonstrated to operate in oviduct fluid in vitro. The viscoelastic properties of bovine oviduct fluid (BOF), one of the fluids that sperm cells encounter on their way to the oocyte, are first characterized using passive microrheology. This allows to design an artificial oviduct fluid to match the rheological properties of oviduct fluid for further experiments. Sperm motion is analyzed and it is confirmed that kinetic parameters match in real and artificial oviduct fluids, respectively. It is demonstrated that sperm cells can efficiently couple to magnetic microtubes and propel them forward in media of different viscosities and in BOF. The flagellar beat pattern of coupled as well as of free sperm cells is investigated, revealing an alteration on the regular flagellar beat, presenting an on-off behavior caused by the additional load of the microtube. Finally, a new microcap design is proposed to improve the overall performance of the spermbot in complex biofluids.

Multifunctional surface microrollers for targeted cargo delivery in physiological blood flow

Mobile microrobots offer great promise for minimally invasive targeted medical theranostic applications at hard-to-access regions inside the human body. The circulatory system represents the ideal route for navigation; however, blood flow impairs propulsion of microrobots especially for the ones with overall sizes less than 10 micrometers. Moreover, cell- and tissue-specific targeting is required for efficient recognition of disease sites and long-term preservation of microrobots under dynamic flow conditions. Here, we report cell-sized multifunctional surface microrollers with ~3.0 and ~7.8-micrometer diameters, inspired by leukocytes in the circulatory system, for targeted drug delivery into specific cells and controlled navigation inside blood flow. The leukocyte-inspired spherical microrollers are composed of magnetically responsive Janus microparticles functionalized with targeting antibodies against cancer cells (anti-HER2) and light-cleavable cancer drug molecules (doxorubicin). Magnetic propulsion and steering of the microrollers resulted in translational motion speeds up to 600 micrometers per second, around 76 body lengths per second. Targeting cancer cells among a heterogeneous cell population was demonstrated by active propulsion and steering of the microrollers over the cell monolayers. The multifunctional microrollers were propelled against physiologically relevant blood flow (up to 2.5 dynes per square centimeter) on planar and endothelialized microchannels. Furthermore, the microrollers generated sufficient upstream propulsion to locomote on inclined three-dimensional surfaces in physiologically relevant blood flow. The multifunctional microroller platform described here presents a bioinspired approach toward in vivo controlled propulsion, navigation, and targeted active cargo delivery in the circulatory system.

Sperm Micromotors for Cargo Delivery through Flowing Blood

Micromotors are recognized as promising candidates for untethered micromanipulation and targeted cargo delivery in complex biological environments. However, their feasibility in the circulatory system has been limited due to the low thrust force exhibited by many of the reported synthetic micromotors, which is not sufficient to overcome the high flow and complex composition of blood. Here we present a hybrid sperm micromotor that can actively swim against flowing blood (continuous and pulsatile) and perform the function of heparin cargo delivery. In this biohybrid system, the sperm flagellum provides a high propulsion force while the synthetic microstructure serves for magnetic guidance and cargo transport. Moreover, single sperm micromotors can assemble into a train-like carrier after magnetization, allowing the transport of multiple sperm or medical cargoes to the area of interest, serving as potential anticoagulant agents to treat blood clots or other diseases in the circulatory system.

Motion Manipulation of Micro- and Nanomotors

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

“Z”-Shaped Rotational Au/Pt Micro-Nanorobot

Drug delivery, minimally-invasive surgery, and a hospital-in-the-body are highly desirable for meeting the rapidly growing needs of nanorobot. This paper reports a Z-shaped gold/platinum (Au/Pt) hybrid nanorobot which realizes the self-rotational movement without an external force field. The Z-shaped Au/Pt hybrid nanorobot was fabricated by focused ion beam (FIB) and plasma sputtering. The purity of the nanorobot was tested by energy dispersive X-ray analysis (EDS). The weight percentage of Pt and Au at the tip were 94.28% and 5.72%, respectively. The weight percentage of Pt and Au at the bottom were 17.39% and 82.75%, respectively. The size of the nanorobot was 2.58 × 10⁻¹⁶ m² and the mass of the nanorobot was 8.768 × 10⁻⁸ kg. The driving force of the nanorobot was 9.76 × 10⁻¹⁴ N at the 6.9% concentration of hydrogen peroxide solution. The rotation speed was 13 rpm, 14 rpm, and 19 rpm at 5.6%, 6.2%, and 7.8% concentrations, respectively.

Hybrid Magnetoelectric Nanowires for Nanorobotic Applications: Fabrication, Magnetoelectric Coupling, and Magnetically Assisted In Vitro Targeted Drug Delivery

An FeGa@P(VDF-TrFE) wire-shaped magnetoelectric nanorobot is designed and fabricated to demonstrate a proof-of-concept integrated device, which features wireless locomotion and on-site triggered therapeutics with a single external power source (i.e., a magnetic field). The device can be precisely steered toward a targeted location wirelessly by rotating magnetic fields and perform on-demand magnetoelectrically assisted drug release to kill cancer cells.

Designing Micro- and Nanoswimmers for Specific Applications

Self-propelled colloids have emerged as a new class of active matter over the past decade. These are micrometer sized colloidal objects that transduce free energy from their surroundings and convert it to directed motion. The self-propelled colloids are in many ways, the synthetic analogues of biological self-propelled units such as algae or bacteria. Although they are propelled by very different mechanisms, biological swimmers are typically powered by flagellar motion and synthetic swimmers are driven by local chemical reactions, they share a number of common features with respect to swimming behavior. They exhibit run-and-tumble like behavior, are responsive to environmental stimuli, and can even chemically interact with nearby swimmers. An understanding of self-propelled colloids could help us in understanding the complex behaviors that emerge in populations of natural microswimmers. Self-propelled colloids also offer some advantages over natural microswimmers, since the surface properties, propulsion mechanisms, and particle geometry can all be easily modified to meet specific needs.

From a more practical perspective, a number of applications, ranging from environmental remediation to targeted drug delivery, have been envisioned for these systems. These applications rely on the basic functionalities of self-propelled colloids: directional motion, sensing of the local environment, and the ability to respond to external signals. Owing to the vastly different nature of each of these applications, it becomes necessary to optimize the design choices in these colloids. There has been a significant effort to develop a range of synthetic self-propelled colloids to meet the specific conditions required for different processes. Tubular self-propelled colloids, for example, are ideal for decontamination processes, owing to their bubble propulsion mechanism, which enhances mixing in systems, but are incompatible with biological systems due to the toxic propulsion fuel and the generation of oxygen bubbles. Spherical swimmers serve as model systems to understand the fundamental aspects of the propulsion mechanism, collective behavior, response to external stimuli, etc. They are also typically the choice of shape at the nanoscale due to their ease of fabrication. More recently biohybrid swimmers have also been developed which attempt to retain the advantages of synthetic colloids while deriving their propulsion from biological swimmers such as sperm and bacteria, offering the means for biocompatible swimming. In this Account, we will summarize our effort and those of other groups, in the design and development of self-propelled colloids of different structural properties and powered by different propulsion mechanisms. We will also briefly address the applications that have been proposed and, to some extent, demonstrated for these swimmer designs.

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.

2-D steering and propelling of acoustic bubble-powered microswimmers

This paper describes bi-directional (linear and rotational) propelling and 2-D steering of acoustic bubble-powered microswimmers that are achieved in a centimeter-scale pool (beyond chip level scale). The core structure of a microswimmer is a microtube with one end open in which a gaseous bubble is trapped. The swimmer is propelled by microstreaming flows that are generated when the trapped bubble is oscillated by an external acoustic wave. The bubble oscillation and thus propelling force are highly dependent on the frequency of the acoustic wave and the bubble length. This dependence is experimentally studied by measuring the resonance behaviors of the testing pool and bubble using a laser Doppler vibrometer (LDV) and by evaluating the generated streaming flows. The key idea in the present 2-D steering is to utilize this dependence. Multiple bubbles with different lengths are mounted on a single microswimmer with a variety of arrangements. By controlling the frequency of the acoustic wave, only frequency-matched bubbles can strongly oscillate and generate strong propulsion. By arranging multiple bubbles of different lengths in parallel but with their openings opposite and switching the frequency of the acoustic wave, bi-directionally linear propelling motions are successfully achieved. The propelling forces are calculated by a CFD analysis using the Ansys Fluent® package. For bi-directional rotations, a similar method but with diagonal arrangement of bubbles on a rectangular swimmer is also applied. The rotation can be easily reversed when the frequency of the acoustic wave is switched. For 2-D steering, short bubbles are aligned perpendicular to long bubbles. It is successfully demonstrated that the microswimmer navigates through a T-junction channel under full control with and without carrying a payload. During the navigation, the frequency is the main control input to select and resonate targeted bubbles. All of these operations are achieved by a single piezoelectric actuator.

Photochromic Spatiotemporal Control of Bubble-Propelled Micromotors by a Spiropyran Molecular Switch

Controlling the environment in which bubble-propelled micromotors operate represents an attractive strategy to influence their motion, especially when the trigger is as simple as light. We demonstrate that spiropyrans, which isomerize to amphiphilic merocyanines under UV irradiation, can act as molecular switches that drastically affect the locomotion of the micrometer-sized engines. The phototrigger could be either a point or a field source, thus allowing different modes of control to be executed. A whole ensemble of micromotors was repeatedly activated and deactivated by just altering the spiropyran-merocyanine ratio with light. Moreover, the velocity of individual micromotors was altered using a point irradiation source that caused only localized changes in the environment. Such selective manipulation, achieved here with an optical microscope and a photochromic additive in the medium, reveals the ease of the methodology, which can allow micro- and nanomotors to reach their full potential of not just stochastic, but directional controlled motion.

Disk-like nanojets with steerable trajectory using platinum nozzle nanoengines

Nanojets with one off-center platinum nozzle nanoengine can propel forward circularly, while the nanojets with two identically and symmetrically distributed platinum nozzle nanoengines are capable of moving forward in a linear way.

Hydrogen-peroxide-fuelled platinum–nickel–SU-8 microrocket with steerable propulsion using an eccentric nanoengine

Microrockets with eccentric nanoengines are able to realize the steerable propulsion in either a clockwise or a counter-clockwise direction.

John AE, Lim EB, Chien D, Lee A, Zhang JQ, Piret JM, Machan LS, Burke TF, White NJ, Kastrup CJ. Self-propelled particles that transport cargo through flowing blood and halt hemorrhage

Delivering therapeutics deep into damaged tissue during bleeding is challenging because of the outward flow of blood. When coagulants cannot reach and clot blood at its source, uncontrolled bleeding can occur and increase surgical complications and fatalities. Self-propelling particles have been proposed as a strategy for transporting agents upstream through blood. Many nanoparticle and microparticle systems exhibiting autonomous or collective movement have been developed, but propulsion has not been used successfully in blood or used in vivo to transport therapeutics. We show that simple gas-generating microparticles consisting of carbonate and tranexamic acid traveled through aqueous solutions at velocities of up to 1.5 cm/s and delivered therapeutics millimeters into the vasculature of wounds. The particles transported themselves through a combination of lateral propulsion, buoyant rise, and convection. When loaded with active thrombin, these particles worked effectively as a hemostatic agent and halted severe hemorrhage in multiple animal models of intraoperative and traumatic bleeding. Many medical applications have been suggested for self-propelling particles, and the findings of this study show that the active self-fueled transport of particles can function in vivo to enhance drug delivery.

Micropropulsion by an acoustic bubble for navigating microfluidic spaces

This paper describes an underwater micropropulsion principle where a gaseous bubble trapped in a suspended microchannel and oscillated by external acoustic excitation generates a propelling force. The propelling swimmer is designed and microfabricated from parylene on the microscale (the equivalent diameter of the cylindrical bubble is around 60 μm) using microphotolithography. The propulsion mechanism is studied and verified by computational fluid dynamics (CFD) simulations as well as experiments. The acoustically excited and thus periodically oscillating bubble generates alternating flows of intake and discharge through an opening of the microchannel. As the Reynolds number of oscillating flow increases, the difference between the intake and discharge flows becomes significant enough to generate a net flow (microstreaming flow) and a propulsion force against the channel. As the size of the device is reduced, however, the Reynolds number is also reduced. To maintain the Reynolds number in a certain range and thus generate a strong propulsion force in the fabricated device, the oscillation amplitude of the bubble is maximized (resonated) and the oscillation frequency is set high (over 10 kHz). Propelling motions by a single bubble as well as an array of bubbles are achieved on the microscale. In addition, the microswimmer demonstrates payload carrying. This propulsion mechanism may be applied to microswimmers that navigate microfluidic environments and possibly narrow passages in human bodies to perform biosensing, drug delivery, imaging, and microsurgery.

Chemically powered micro- and nanomotors

Chemically powered micro- and nanomotors are small devices that are self-propelled by catalytic reactions in fluids. Taking inspiration from biomotors, scientists are aiming to find the best architecture for self-propulsion, understand the mechanisms of motion, and develop accurate control over the motion. Remotely guided nanomotors can transport cargo to desired targets, drill into biomaterials, sense their environment, mix or pump fluids, and clean polluted water. This Review summarizes the major advances in the growing field of catalytic nanomotors, which started ten years ago.

Biofunctionalized self-propelled micromotors as an alternative on-chip concentrating system

Sample pre-concentration is crucial to achieve high sensitivity and low detection limits in lab-on-a-chip devices. Here, we present a system in which self-propelled catalytic micromotors are biofunctionalized and trapped acting as an alternative concentrating mechanism. This system requires no external energy source, which facilitates integration and miniaturization.

Catalytic nanomotors for environmental monitoring and water remediation

Self-propelled nanomotors hold considerable promise for developing innovative environmental applications. This review highlights the recent progress in the use of self-propelled nanomotors for water remediation and environmental monitoring applications, as well as the effect of the environmental conditions on the dynamics of nanomotors. Artificial nanomotors can sense different analytes-and therefore pollutants, or "chemical threats"-can be used for testing the quality of water, selective removal of oil, and alteration of their speeds, depending on the presence of some substances in the solution in which they swim. Newly introduced micromotors with double functionality to mix liquids at the microscale and enhance chemical reactions for the degradation of organic pollutants greatly broadens the range of applications to that of environmental. These "self-powered remediation systems" could be seen as a new generation of "smart devices" for cleaning water in small pipes or cavities difficult to reach with traditional methods. With constant improvement and considering the key challenges, we expect that artificial nanomachines could play an important role in environmental applications in the near future.

Effect of surfactants on the performance of tubular and spherical micromotors - a comparative study

The development of artificial micromotors is one of the greatest challenges of modern nanotechnology. Even though many kinds of motors have been published in recent times, systematic studies on the influence of components of the fuel solution are widely missing. Therefore, the autonomous movement of Pt-microtubes and Pt-covered silica particles is comparatively observed in the presence and absence of surfactants in the medium. One representative of each of the three main surfactant classes - anionic (sodium dodecyl sulfate, SDS), cationic (benzalkonium chloride, BACl) and non-ionic (Triton X) - has been chosen and studied.

Conformal cytocompatible ferrite coatings facilitate the realization of a nanovoyager in human blood

Controlled motion of artificial nanomotors in biological environments, such as blood, can lead to fascinating biomedical applications, ranging from targeted drug delivery to microsurgery and many more. In spite of the various strategies used in fabricating and actuating nanomotors, practical issues related to fuel requirement, corrosion, and liquid viscosity have limited the motion of nanomotors to model systems such as water, serum, or biofluids diluted with toxic chemical fuels, such as hydrogen peroxide. As we demonstrate here, integrating conformal ferrite coatings with magnetic nanohelices offer a promising combination of functionalities for having controlled motion in practical biological fluids, such as chemical stability, cytocompatibility, and the generated thrust. These coatings were found to be stable in various biofluids, including human blood, even after overnight incubation, and did not have significant influence on the propulsion efficiency of the magnetically driven nanohelices, thereby facilitating the first successful "voyage" of artificial nanomotors in human blood. The motion of the "nanovoyager" was found to show interesting stick-slip dynamics, an effect originating in the colloidal jamming of blood cells in the plasma. The system of magnetic "nanovoyagers" was found to be cytocompatible with C2C12 mouse myoblast cells, as confirmed using MTT assay and fluorescence microscopy observations of cell morphology. Taken together, the results presented in this work establish the suitability of the "nanovoyager" with conformal ferrite coatings toward biomedical applications.