Micro/nanomachines: what is needed for them to become a real force in cancer therapy?

Intelligent Micro-/Nanorobots for Cancer Theragnostic

Cancer is one of the most intractable diseases owing to its high mortality rate and lack of effective diagnostic and treatment tools. Advancements in micro-/nanorobot (MNR)-assisted sensing, imaging, and therapeutics offer unprecedented opportunities to develop MNR-based cancer theragnostic platforms. Unlike ordinary nanoparticles, which exhibit Brownian motion in biofluids, MNRs overcome viscous resistance in an ultralow Reynolds number (Re << 1) environment by effective self-propulsion. This unique locomotion property has motivated the advanced design and functionalization of MNRs as a basis for next-generation cancer-therapy platforms, which offer the potential for precise distribution and improved permeation of therapeutic agents. Enhanced barrier penetration, imaging-guided operation, and biosensing are additionally studied to enable the promising cancer-related applications of MNRs. Herein, the recent advances in MNR-based cancer therapy are comprehensively addresses, including actuation engines, diagnostics, medical imaging, and targeted drug delivery; promising research opportunities that can have a profound impact on cancer therapy over the next decade is highlighted.

Optically Triggered Nanoscale Plasmonic Dynamite

Photons as energy carriers are clean and abundant, which can be conveniently applied for nanoactuation but the response is usually slow with very low energy efficiency/density. Here, we underpin the concept of robust nanoscale plasmonic dynamite by incorporating fullerene (C₆₀). The Au@C₆₀ core-shell nanoparticles can be triggered to explode in nanoscale with synergy of plasmon-enhanced photochemical and photothermal effects. It is suggested that a sensible amount of CO₂ was generated and pressurized in nanometric volume in an extremely short time scale (∼ns), which triggers the nanoexplosion, rendering the ejection of Au NPs at the speed over 300 m/s. The ejection generates extremely large local forces (∼1 μN) with thermomechanical energy efficiency up to ∼30%, which is demonstrated as a powerful nanoengine for controlled mobilization of micro-objects on solid surfaces. Such nanoscale plasmonic dynamite is highly exploitable for different types of nanomachines, which provides a powerful energy source for nanoactuation and nanomigration.

Active colloidal molecules in activity gradients

We consider a rigid assembly of two active Brownian particles, forming an active colloidal dimer, in a gradient of activity. We show analytically that depending on the relative orientation of the two particles the active dimer accumulates in regions of either high or low activity, corresponding to, respectively, chemotaxis and antichemotaxis. Certain active dimers show both chemotactic and antichemotactic behavior, depending on the strength of the activity. Our coarse-grained Fokker-Planck approach yields an effective potential, which we use to construct a nonequilibrium phase diagram that classifies the dimers according to their tactic behavior. Moreover, we show that for certain dimers a higher persistence of the motion is achieved similar to the effect of a steering wheel in macroscopic devices. This work could be useful for designing autonomous active colloidal structures which adjust their motion depending on the local activity gradients.

Collective guiding of acoustically propelled nano- and microparticles

One of the most important potential applications of motile nano- and microdevices is targeted drug delivery. To realize this, biocompatible particles that can be guided collectively towards a target inside a patient's body are required. Acoustically propelled nano- and microparticles constitute a promising candidate for such biocompatible, artificial motile particles. The main remaining obstacle to targeted drug delivery by motile nano- and microdevices is to also achieve a reliable and biocompatible method for guiding them collectively to their target. Here, we propose such a method. As we confirm by computer simulations, it allows for the remote guiding of large numbers of acoustically propelled particles to a prescribed target by combining a space- and time-dependent acoustic field and a time-dependent magnetic field. The method works without detailed knowledge about the particle positions and for arbitrary initial particle distributions. With these features, it paves the way for the future application of motile particles as vehicles for targeted drug delivery in nanomedicine.

From radial to unidirectional water pumping in zeta-potential modulated Nafion nanostructures

Chemically propelled micropumps are promising wireless systems to autonomously drive fluid flows for many applications. However, many of these systems are activated by nocuous chemical fuels, cannot operate at high salt concentrations, or have difficulty for controlling flow directionality. In this work we report on a self-driven polymer micropump fueled by salt which can trigger both radial and unidirectional fluid flows. The micropump is based on the cation-exchanger Nafion, which produces chemical gradients and local electric fields capable to trigger interfacial electroosmotic flows. Unidirectional pumping is predicted by simulations and achieved experimentally by nanostructuring Nafion into microarrays with a fine tune modulation of surrounding surface zeta potentials. Nafion micropumps work in a wide range of salt concentrations, are reusable, and can be fueled by different salt cations. We demonstrate that they work with the common water-contaminant cadmium, using the own capture of this ion as fuel to drive fluid pumping. Thus, this system has potential for efficient and fast water purification strategies for environmental remediation. Unidirectional Nafion pumps also hold promise for effective analyte delivery or preconcentration for (bio)sensing assays.

Chemically propelled micropumps are wireless fluid flow driving systems with many potential applications. Here, the authors report a self-driven reusable Nafion micropump fueled by different salt cations in a wide range of concentrations that triggers both radial and unidirectional flows, showing efficient water remediation capabilities.

Halloysite-Based Nanorockets with Light-Enhanced Self-Propulsion for Efficient Water Remediation

Halloysite-based tubular nanorockets with chemical-/light-controlled self-propulsion and on-demand acceleration in velocity are reported. The nanorockets are fabricated by modifying halloysite nanotubes with nanoparticles of silver (Ag) and light-responsive α-Fe₂O₃ to prepare a composite of Ag-Fe₂O₃/HNTs. Compared to the traditional fabrication of tubular micro-/nanomotors, this strategy has merits in employing natural clay as substrates of an asymmetric tubular structure, of abundance, and of no complex instruments required. The velocity of self-propelled Ag-Fe₂O₃/HNTs nanorockets in fuel (3.0% H₂O₂) was ca. 1.7 times higher under the irradiation of visible light than that in darkness. Such light-enhanced propulsion can be wirelessly modulated by adjusting light intensity and H₂O₂ concentration. The highly repeatable and controlled "weak/strong" propulsion can be implemented by turning a light on and off. With the synergistic coupling of the photocatalysis of the Ag-Fe₂O₃ heterostructure and advanced oxidation in H₂O₂/visible light conditions, the Ag-Fe₂O₃/HNTs nanorockets achieve an enhanced performance of wastewater remediation. A test was done by the catalytic degradation of tetracycline hydrochloride. The light-enhanced propulsion is demonstrated to accelerate the degradation kinetics dramatically. All of these results illustrated that such motors can achieve efficient water remediation and open a new path for the photodegradation of organic pollutions.

Acoustically propelled nano- and microcones: fast forward and backward motion

We focus on cone-shaped nano- and microparticles, which have recently been found to show particularly strong propulsion when they are exposed to a traveling ultrasound wave, and study based on direct acoustofluidic computer simulations how their propulsion depends on the cones' aspect ratio. The simulations reveal that the propulsion velocity and even its sign are very sensitive to the aspect ratio, where short particles move forward whereas elongated particles move backward. Furthermore, we identify a cone shape that allows for a particularly large propulsion speed. Our results contribute to the understanding of the propulsion of ultrasound-propelled colloidal particles, suggest a method for separation and sorting of nano- and microcones concerning their aspect ratio, and provide useful guidance for future experiments and applications.

General Thermodynamic-Controlled Coating Method to Prepare Janus Mesoporous Nanomotors for Improving Tumor Penetration

Artificial nanomotors are undergoing significant developments in several biomedical applications. However, current experimental strategies for producing nanomotors still have inherent drawbacks such as the requirement for expensive equipment, strict controlling of experimental conditions, and strenuous processes with several complex procedures. In this study, we describe for the first time a facile single-step thermodynamic-controlled coating method to prepare Janus mesoporous organosilica nanomotors. By controlling the total free energy of organosilica oligomers (G) from a low development level to a high level in the reaction system, the nonspontaneous nucleation on the platinum (Pt) nanosurface and the spontaneous nucleation in a solvent can be controlled, respectively. More importantly, we reveal that the molecular arrangement and contact angle of deposited organosilica on Pt cores vary with the total free energy of organosilica oligomers (G). Different values of θ would change the trend of detachment from Pt for organosilica nucleated cores and carry out diverse coating modes. These are indicated by the morphology evolution of platinum/organosilica hybrids, from naked platinum nanoparticles, evenly distributed organosilica shell/core, nonconcentric to typical Janus nanomotor. The prepared Janus mesoporous nanomotor (JMN) showed typical mesopore structures and active propelling behaviors under H₂O₂ stimulation. In addition, the JMN modified with hyaluronic acid exhibited excellent biocompatibility and improved tumor penetration under H₂O₂ stimulation. The successful construction of other nanomotor frameworks based on a gold-templated core proves the perfect applicability of the thermodynamic-coating method for the production of nanomotors. In conclusion, this work establishes a manufacturing methodology for nanomotors and drives nanomotors for promising biomedical applications.

Relative Diffusivities of Bound and Unbound Protein Can Control Chemotactic Directionality

Enzyme-based systems have been shown to undergo chemotactic motion in response to their substrate gradient. This phenomenon has been exploited to direct the motion of enzymes and enzyme-attached particles to specific locations in space. Here, we propose a new kinetic model to analyze the directional movement of an ensemble of protein molecules in response to a gradient of the ligand. We also formulate a separate model to probe the motion of enzyme molecules in response to a gradient of the substrate under catalytic conditions. The only input for the new enzymatic model is the Michaelis-Menten constant which is the relevant measurable constant for enzymatic reactions. We show how our model differs from previously proposed models in a significant manner. For both binding and catalytic reactions, a net movement up the ligand/substrate gradient is predicted when the diffusivity of the ligand/substrate-bound protein is lower than that of the unbound protein (positive chemotaxis). Conversely, movement down the ligand/substrate gradient is expected when the diffusivity of the ligand/substrate-bound protein is higher than that of the unbound protein (negative chemotaxis). However, there is no net movement of protein/enzyme when the diffusivities of the bound and free species are equal. The work underscores the critical importance of measuring the diffusivity of the bound protein and comparing it with that of the free protein.

Self-propelled nanomotor reconstructs tumor microenvironment through synergistic hypoxia alleviation and glycolysis inhibition for promoted anti-metastasis

Solid tumors always exhibit local hypoxia, resulting in the high metastasis and inertness to chemotherapy. Reconstruction of hypoxic tumor microenvironment (TME) is considered a potential therapy compared to directly killing tumor cells. However, the insufficient oxygen delivery to deep tumor and the confronting “Warburg effect” compromise the efficacy of hypoxia alleviation. Herein, we construct a cascade enzyme-powered nanomotor (NM-si), which can simultaneously provide sufficient oxygen in deep tumor and inhibit the aerobic glycolysis to potentiate anti-metastasis in chemotherapy. Catalase (Cat) and glucose oxidase (GOx) are co-adsorbed on our previously reported CAuNCs@HA to form self-propelled nanomotor (NM), with hexokinase-2 (HK-2) siRNA further condensed (NM-si). The persistent production of oxygen bubbles from the cascade enzymatic reaction propels NM-si to move forward autonomously and in a controllable direction along H₂O₂ gradient towards deep tumor, with hypoxia successfully alleviated in the meantime. The autonomous movement also facilitates NM-si with lysosome escaping for efficient HK-2 knockdown to inhibit glycolysis. In vivo results demonstrated a promising anti-metastasis effect of commercially available albumin-bound paclitaxel (PTX@HSA) after pre-treated with NM-si for TME reconstruction. This cascade enzyme-powered nanomotor provides a potential prospect in reversing the hypoxic TME and metabolic pathway for reinforced anti-metastasis of chemotherapy.

Furin-responsive triterpenine-based liposomal complex enhances anticervical cancer therapy through size modulation

The accumulation and penetration of antitumor drugs in tumor tissues are directly related to their antitumor effects. The particle size of the nanodrug delivery system is one of the most important factors for the accumulation and penetration of antitumor drugs within tumor tissues. Generally, nanodelivery systems of intermediate size (100–120 nm) are capable of efficient accumulation owing to prolonged circulation and enhanced permeability and retention (EPR) effect; however, smaller ones (20–40 nm) are effective for deep penetration within tumor tissue. Currently a conventional drug delivery system cannot possess two types of optimal sizes, simultaneously. To solve this and to enhance cervical cancer treatment, a furin-responsive triterpenine-based liposomal complex (PEGcleavable Tf-CTM/L), with Tf-CTM (transferrin-modified tripterine-loaded coix seed oil microemulsion) in core, coated with a thermo-sensitive lipid and a kind of PEG shell modified with a furin-cleavable peptide was developed to improve tumor-specific accumulation and penetration. Herein, PEGcleavable Tf-CTM/L was capable of efficient accumulation because of EPR effect. The PEG shells could timely detach under stimulation of overexpressed furin protein to solve the problem of the steric hindrance dilemma. The small-sized Tf-CTM released under stimulation of tumor microthermal environment in cervical cancer, which was efficient with regards to deep penetration at tumor sites. Notably, compared to the use of triterpenine alone, PEGcleavable Tf-CTM/L promoted anticervical efficacy and displayed diminished systemic toxicity by efficient accumulation and deep penetration of antitumor drugs within tumor tissues. Our study provides a new strategy, and holds promising potential for anticervical cancer treatment.

Biosafety, Functionalities, and Applications of Biomedical Micro/nanomotors

Due to their unique ability to actively move, micro/nanomotors offer the possibility of breaking through the limitations of traditional passive drug delivery systems for the treatment of many diseases, and have attracted the increasing attention of researchers. However, at present, the realization of many advantages of micro/nanomotors in disease treatment in vivo is still in its infancy, because of the complexity and particularity of diseases in different parts of human body. In this Minireview, we first focus on the biosafety and functionality of micro/nanomotors as a biomedical treatment system. Then, we address the treatment difficulties of various diseases in vivo (such as ophthalmic disease, orthopedic disease, gastrointestinal disease, cardiovascular disease, and cancer), and then review the research progress of biomedical micro/nanomotors in the past 20 years, Finally, we propose the challenges in this field and possible future development directions.

Enhanced cargo loading of electrically powered metallo-dielectric pollen bearing multiple dielectrophoretic traps

The use of active particles for cargo transport offers unique potential for applications ranging from targeted drug delivery to lab-on-a-particle systems. Previously, deployment of metallo-dielectric Janus spherical particles (JPs) as mobile microelectrodes for transport and dielectrophoretic manipulation of cargo has been shown to be singularly controlled via an applied electric field. Herein, we extended this to a metallo-dielectric pollen featuring multiple dielectrophoretic traps associated with its many spikes, and characterized its loading capacities for various cargo sizes and frequencies. When compared to spherical JPs, the active pollen exhibited a significantly enhanced cargo loading capacity due to its multiple dielectrophoretic traps. These findings open new opportunities for application of bio-hybrid particles with diverse and irregular shapes, such as pollen, as efficient cargo carriers, local electroporation and targeted drug delivery.

Magnetically Driven Micro and Nanorobots

Manipulation and navigation of micro and nanoswimmers in different fluid environments can be achieved by chemicals, external fields, or even motile cells. Many researchers have selected magnetic fields as the active external actuation source based on the advantageous features of this actuation strategy such as remote and spatiotemporal control, fuel-free, high degree of reconfigurability, programmability, recyclability, and versatility. This review introduces fundamental concepts and advantages of magnetic micro/nanorobots (termed here as “MagRobots”) as well as basic knowledge of magnetic fields and magnetic materials, setups for magnetic manipulation, magnetic field configurations, and symmetry-breaking strategies for effective movement. These concepts are discussed to describe the interactions between micro/nanorobots and magnetic fields. Actuation mechanisms of flagella-inspired MagRobots (i.e., corkscrew-like motion and traveling-wave locomotion/ciliary stroke motion) and surface walkers (i.e., surface-assisted motion), applications of magnetic fields in other propulsion approaches, and magnetic stimulation of micro/nanorobots beyond motion are provided followed by fabrication techniques for (quasi-)spherical, helical, flexible, wire-like, and biohybrid MagRobots. Applications of MagRobots in targeted drug/gene delivery, cell manipulation, minimally invasive surgery, biopsy, biofilm disruption/eradication, imaging-guided delivery/therapy/surgery, pollution removal for environmental remediation, and (bio)sensing are also reviewed. Finally, current challenges and future perspectives for the development of magnetically powered miniaturized motors are discussed.

Thermally-driven gold@poly(N-isopropylacrylamide) core-shell nanotransporters for molecular extraction

HYPOTHESIS

Molecular extraction efficiency can be boosted with the assistance of nanoparticles (NPs). It is based on adsorption of the extractants in one phase and desorption in another phase, which requires a reversible phase transfer of the NPs.

EXPERIMENTS

We synthesized the gold@poly(N-isopropylacryamide) (Au@PNIPAM) NPs via an interfacial self-assembly method enhanced by post-polymerization. We adopted Rhodamine 6G (R6G) as the model molecule for the extraction test. In comparison, UV-Vis extinction spectra were recorded to monitor the extraction processes with or without the Au@PNIPAM NPs. We further analyzed theoretically with thermodynamics and first-principle calculations.

FINDINGS

The hybrid Au@PNIPAM NPs show a reversible phase transfer between the interface and chloroform phases. The Au NPs assisted extraction efficiency of R6G shows 5 times higher than that without Au NPs. The thermodynamic analysis of the nanotransportation system agrees well with the ab initio density functional theory calculations. This nanoparticle-assisted molecular transportation modifies the extraction kinetics significantly, which will provide further implications for biphasic catalysis, pollutant treatment and drug delivery.

Chemically Propelled Nano and Micromotors in the Body: Quo Vadis?

The active delivery of drugs to disease sites in response to specific biomarkers is a holy grail in theranostics. If successful, it would greatly diminish the therapeutic dosage and reduce collateral cytotoxicity. In this context, the development of nano and micromotors that are able to harvest local energy to move directionally is an important breakthrough. However, serious hurdles remain before such active systems can be employed in vivo in therapeutic applications. Such motors and their energy sources must be safe and biocompatible, they should be able to move through complex body fluids, and have the ability to reach specific cellular targets. Given the complexity in the design and deployment of nano and micromotors, it is also critically important to show that they are significantly superior to inactive "smart" nanoparticles in theranostics. Furthermore, receiving regulatory approval requires the ability to scale-up the production of nano and micromotors with uniformity in structure, function, and activity. In this essay, the limitations of the current nano and micromotors and the issues that need to be resolved before such motors are likely to find theranostic applications are discussed.

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.

Research Progress of Micro/Nanomotors for Cancer Treatment

Nanomaterials have been widely used in cancer treatment and have achieved remarkable results. However, the specificity of the tumor microenvironment and a series of biological barriers (such as blood flow, cell membrane, dense tissue, etc.) have caused many obstacles faced by nanomaterials after entering the human body, which makes traditional drug delivery vehicles have insurmountable difficulties, such as low delivery efficiency, poor permeability, etc. The micro/nanomotors with autonomous movement capabilities provide the possibility to solve the above problems. Therefore, this review summarizes the current researches of micro/nanomotors strategies to overcome the different biological barriers of nanomaterials in cancer treatment. The advantages and disadvantages of three typical micro/nanomotors (biological, physical and chemical micro/nanomotors) in cancer treatment are summarized separately, and the future design of micro/nanomotors more suitable for tumor environment was discussed.

Scalable Chemical Synthesis Route to Manufacture pH-Responsive Janus CaCO3 Micromotors

A cost-effective scalable chemical route to produce pH-responsive active colloids (ACs) is developed here. For the first time, calcium carbonate particles are half-coated with a silica layer via Pickering emulsion methodology. This methodology allows to create anisotropy on the particles' surfaces and benefit from the decomposition of the calcium carbonate in acidic media to generate self-propulsion. The coupling between the self-diffusiophoretic motion of these ACs and acid concentrations is experimentally investigated in Newtonian media via optical microscopy. With increasing hydrogen-ion concentrations, the pH-responsive colloids experience higher mean-square displacements because of self-propulsion velocities and enhanced long-time diffusivities. Because they are biocompatible and environmentally friendly, these ACs constitute a platform for advanced diagnostics, targeted drug delivery, and water/soil remediation.

Current status of micro/nanomotors in drug delivery

Synthetic micro/nanomotors (MNMs) are novel, self-propelled nano or microscale devices that are widely used in drug transport, cell stimulation and isolation, bio-imaging, diagnostic and monitoring, sensing, photocatalysis and environmental remediation. Various preparation methods and propulsion mechanisms make MNMs "tailormade" nanosystems for the intended purpose or use. As the one of the newest members of nano carriers, MNMs open a new perspective especially for rapid drug transport and gene delivery. Although there exists limited number of in-vivo studies for drug delivery purposes, existence of in-vitro supportive data strongly encourages researchers to move on in this field and benefit from the manoeuvre capability of these novel systems. In this article, we reviewed the preparation and propulsion mechanisms of nanomotors in various fields with special attention to drug delivery systems.

Thermo-responsive plasmonic systems: old materials with new applications

Thermo-responsive plasmonic systems of gold and poly(N-isopropylacrylamide) have been actively studied for several decades but this system keeps reinventing itself, with new concepts and applications which seed new fields. In this minireview, we show the latest few years development and applications of this intriguing system. We start from the basic working principles of this puzzling system which shows different plasmon shifts for even slightly different chemistries. We then present its applications to colloidal actuation, plasmon/meta-film tuning, and bioimaging and sensing. Finally we briefly summarize and propose several promising applications of the ongoing effort in this field.

Near-Infrared Light-Powered Janus Nanomotor Significantly Facilitates Inhibition of Amyloid-β Fibrillogenesis

Inspired by the natural motors, artificial nanomotors (NMs) have emerged as intelligent, advanced, and multifunctional nanoplatforms that can perform complex tasks in living environments. However, the functionalization of these fantastic materials is in its infancy, hindering the success of this booming field. Herein, an inhibitor-conjugated near-infrared (NIR) laser-propelled Janus nanomotor (JNM-I) was constructed and first applied in the modulation of amyloid-β protein (Aβ) aggregation which is highly associated with Alzheimer's disease (AD). Under NIR light illumination, JNM-I exhibited efficient propulsion through the "self-thermophoresis" effect, and the active motion of JNM-I increased the opportunity of the contacts between the immobilized inhibitors and Aβ species, leading to an intensification of JNM-I on modulating the on-pathway Aβ aggregation, as evidenced by the distinct changes of the amyloid morphology, conformation, and cytotoxicity. For example, with a NIR irradiation, 200 μg/mL of JNM-I increased the cultured SH-SY5Y cell viability from 68% to nearly 100%, but it only protected the cells to 89% viability without an NIR irradiation. Meanwhile, the NIR irradiation effectively improved the blood-brain barrier (BBB) penetration of JNM-I. Such a JNM-I has connected artificial nanomotors with protein aggregation and provided new insight into the potential applications of various nanomotors in the prevention and treatment of AD.

Diffusiophoresis of active colloids in viscoelastic media

Self-diffusiophoresis of synthetic Janus (Si/Pt) microspheres in the presence of hydrogen peroxide in complex environments is here investigated. We aim to address the single particle dynamics of these active colloids in different viscoelastic fluids. Experimentally, the Janus colloids were dispersed in a dilute polyvinylpyrrolidone (PVP) solution and in a polyacrylamide (PAM) solution in semi-dilute and semi-dilute entangled regime to analyze their Brownian and active motion. These two systems were chosen to probe different relaxation times from relatively short (∼5 ms) for PVP to large (∼14.5 s) for PAM but always smaller than the rotary Brownian motion time scale. Within this regime, we investigate the coupling between the self-propulsion velocity and the medium rheology. Janus particles are found to get physically confined by polymeric entanglements but surprisingly they are able to escape the physical cage in a time scale much shorter than the relaxation time of the polymer solution. This is particularly relevant for application of self-propelling particles in biomedicine, water and soil remediation where complex environments are naturally present.

Tadpole-like Unimolecular Nanomotor with Sub-100 nm Size Swims in a Tumor Microenvironment Model

Inspired by the natural motors capable of performing multiple tasks in complex living environments, synthetic nanomotors emerge as a potential vehicle for revolutionizing biomedical processes. Yet current motors suffer from decreased and even completely hindered motion in a complex physiological environment, shadowing the future of this booming field. To address this problem, a unimolecular nanomotor based on molecular bottlebrush (MBB) of sub-100 nm size is reported. This motor is constructed precisely via controlled radical polymerization and click chemistry and propelled with biocompatible catalase. Such a molecular nanomotor possesses tadpole-like asymmetry and is able to overcome Brownian motion, and demonstrates strong directional propulsion (linear and coiled cyclic trajectories) in a viscous tumor microenvironment gel model at an ultralow hydrogen peroxide level of 2 mM (0.006%). In addition, the molecular nanomotor exhibits superior stability in serum containing cell medium and good biocompatibility in blood. Such molecular bottlebrush based nanomotors may represent a unique platform for overcoming the tissue penetration barrier.

Self-Propelled Janus Microdimer Swimmers under a Rotating Magnetic Field

Recent strides in micro- and nanofabrication technology have enabled researchers to design and develop new micro- and nanorobots for biomedicine and environmental monitoring. Due to its non-invasive remote actuation and convenient navigation abilities, magnetic propulsion has been widely used in micro- and nanoscale robotic systems. In this article, a highly efficient Janus microdimer swimmer propelled by a rotating uniform magnetic field was investigated experimentally and numerically. The velocity of the Janus microdimer swimmer can be modulated by adjusting the magnetic field frequency with a maximum speed of 133 μm·s-1 (≈13.3 body length s-1) at the frequency of 32 Hz. Fast and accurate navigation of these Janus microdimer swimmers in complex environments and near obstacles was also demonstrated. This efficient propulsion behavior of the new Janus microdimer swimmer holds considerable promise for diverse future practical applications ranging from nanoscale manipulation and assembly to nanomedicine.

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.

Biocompatibility of artificial micro/nanomotors for use in biomedicine

The advent of micro/nanomotors (MNMs) has shed light on the innovation of active biomedical systems or devices that might bring revolutionary solutions to traditional biomedical strategies. In spite of development beyond expectation over the last decade with a fair number of proof-of-concept demonstrations, the in vivo practical application of MNMs for clinical use is still in its infancy. The biocompatibility of MNMs is the first consideration before realizing practicality, taking into account the complicated interactions between the self-propelled MNMs and biological systems. Therefore, in this review, we focused on the biocompatibility of MNMs with regard to the fabrication materials and propulsion mechanisms by means of in-depth discussions on the advantages and limitations of MNMs for operating under physiological conditions. The future prospective and suggestions on the development of MNMs toward practical biomedical applications will also be proposed.