Self-Propelled PLGA Micromotor with Chemotactic Response to Inflammation

Biocompatible polymer-based micro/nanorobots for theranostic translational applications

Recently, micro/nanorobots (MNRs) with self-propulsion have emerged as a promising smart platform for diagnostic, therapeutic and theranostic applications. Especially, polymer-based MNRs have attracted huge attention due to their inherent biocompatibility and versatility, making them actively explored for various medical applications. As the translation of MNRs from laboratory to clinical settings is imperative, the use of appropriate polymers for MNRs is a key strategy, which can prompt the advancement of MNRs to the next phase. In this review, we describe the multifunctional versatile polymers in MNRs, and their biodegradability, motion control, cargo loading and release, adhesion, and other characteristics. After that, we review the theranostic applications of polymer-based MNRs to bioimaging, biosensing, drug delivery, and tissue engineering. Furthermore, we address the challenges that must be overcome to facilitate the translational development of polymeric MNRs with future perspectives. This review would provide valuable insights into the state-of-the-art technologies associated with polymeric MNRs and contribute to their progression for further clinical development.

A Cautionary Perspective on Hydrogel-Induced Concentration Gradient Generation for Studying Chemotaxis

The achievement of consistent and static chemical gradients is critically important in the study of diffusion and chemotaxis at the micro- and nanoscales. In this context, a number of groups have reported on hydrogel-based systems for generating concentration gradients. Here, we analyze the behavior of agarose and gelatin-based hydrogels in hybridization chambers of different heights. Our focus is on the issues that are caused by the presence of robust bulk fluid flows in such systems due to the solutes present in the hydrogel and/or the surrounding fluid. We describe the key insights derived from these experiments, offering practical guidelines for establishing gradients using hydrogel-based systems and make the community aware of different variables that can make the experiments nonreproducible and prone to misinterpretations.

A new direction in periodontitis treatment: biomaterial-mediated macrophage immunotherapy

Periodontitis is a chronic inflammation caused by a bacterial infection and is intimately associated with an overactive immune response. Biomaterials are being utilized more frequently in periodontal therapy due to their designability and unique drug delivery system. However, local and systemic immune response reactions driven by the implantation of biomaterials could result in inflammation, tissue damage, and fibrosis, which could end up with the failure of the implantation. Therefore, immunological adjustment of biomaterials through precise design can reduce the host reaction while eliminating the periodontal tissue's long-term chronic inflammation response. It is important to note that macrophages are an active immune system component that can participate in the progression of periodontal disease through intricate polarization mechanisms. And modulating macrophage polarization by designing biomaterials has emerged as a new periodontal therapy technique. In this review, we discuss the role of macrophages in periodontitis and typical strategies for polarizing macrophages with biomaterials. Subsequently, we discuss the challenges and potential opportunities of using biomaterials to manipulate periodontal macrophages to facilitate periodontal regeneration.

Swarm Autonomy: From Agent Functionalization to Machine Intelligence

Swarm behaviors are common in nature, where individual organisms collaborate via perception, communication, and adaptation. Emulating these dynamics, large groups of active agents can self-organize through localized interactions, giving rise to complex swarm behaviors, which exhibit potential for applications across various domains. This review presents a comprehensive summary and perspective of synthetic swarms, to bridge the gap between the microscale individual agents and potential applications of synthetic swarms. It is begun by examining active agents, the fundamental units of synthetic swarms, to understand the origins of their motility and functionality in the presence of external stimuli. Then inter-agent communications and agent-environment communications that contribute to the swarm generation are summarized. Furthermore, the swarm behaviors reported to date and the emergence of machine intelligence within these behaviors are reviewed. Eventually, the applications enabled by distinct synthetic swarms are summarized. By discussing the emergent machine intelligence in swarm behaviors, insights are offered into the design and deployment of autonomous synthetic swarms for real-world applications.

Insights into characteristic motions and negative chemotaxis of the inanimate motor sensitive to sodium chloride

Inanimate motors, driven by the difference in surface tension, provide platforms for studying the physics of characteristic motion and mimicking the complex behaviors of biological systems. However, it is challenging to endow inanimate motors with high autonomy, with an emphasis on simulating the behavior of living organisms in response to external stimuli. Herein, by applying sodium chloride (NaCl) as an external stimulus, we achieve the regulation of motion mode and chemotaxis in a self-propelled camphor system. We present a comprehensive surface/interface understanding of motion bifurcation with the increase of concentration NaCl, i.e., continuous motion to no motion via oscillatory motion. The features of motions (the speed and frequency) and the mechanisms are elucidated depending on the concentrations of NaCl and sodium dodecyl sulfate (SDS). Furthermore, the characteristic motion and chemotaxis to the salt stimulus are correlated to the dynamic breaking/reforming of the surface tension balance and gradient-type distribution phenomenon triggered by dynamic camphor dissolution, surfactant adsorption /diffusion and camphor-surfactant interaction. This work sheds light on the typical motions of inanimate motors and scrutinizes the synergy between dual additives, which will boost the design of advanced self-propelled systems with nonlinear characteristic motion.

Exploring Emergent Properties in Enzymatic Reaction Networks: Design and Control of Dynamic Functional Systems

The intricate and complex features of enzymatic reaction networks (ERNs) play a key role in the emergence and sustenance of life. Constructing such networks in vitro enables stepwise build up in complexity and introduces the opportunity to control enzymatic activity using physicochemical stimuli. Rational design and modulation of network motifs enable the engineering of artificial systems with emergent functionalities. Such functional systems are useful for a variety of reasons such as creating new-to-nature dynamic materials, producing value-added chemicals, constructing metabolic modules for synthetic cells, and even enabling molecular computation. In this review, we offer insights into the chemical characteristics of ERNs while also delving into their potential applications and associated challenges.

Urease-Powered Black TiO2 Micromotors for Photothermal Therapy of Bladder Cancer

Urease-powered nano/micromotors can move at physiological urea concentrations, making them useful for biomedical applications, such as treating bladder cancer. However, their movement in biological environments is still challenging. Herein, Janus micromotors based on black TiO2 with urease asymmetric catalytic coating were designed to take benefit of the optical properties of black TiO2 under near-infrared light and the movement capability in simulated bladder environments (urea). The black TiO2 microspheres were half-coated with a thin layer of Au, and l-Cysteine was utilized to attach the urease enzyme to the Au surface using its thiol group. Biocatalytic hydrolysis of urea through urease at biologically relevant concentrations provided the driving force for micromotors. A variety of parameters, such as urea fuel concentration, viscosity, and ionic character of the environment, were used to investigate how micromotors moved in different concentrations of urea in water, PBS, NaCl, and urine. The results indicate that micromotors are propelled through ionic self-diffusiophoresis caused by urea enzymatic catalysis. Due to their low toxicity and in vitro anticancer effect, micromotors are effective agents for photothermal therapy, which can help kill bladder cancer cells. These promising results suggest that biocompatible micromotors hold great potential for improving cancer treatment and facilitating diagnosis.

Comprehensive Understanding of Self-Propelled Janus Pt/Fe2O3 Micromotor Dynamics: Impact of Size, Morphology, and Surface Structure

The increasing use of plastics has led to the accumulation of plastic waste in the oceans, resulting in significant global environmental challenges associated with microplastic pollution. Micromotors, capable of capturing and removing microplastics from aquatic systems, have emerged as a promising solution to addressing this problem. This research aims to analyze the factors affecting the speed of micromotors, including size, morphology, and surface structure, while elucidating the underlying mechanisms governing micromotor propulsion to develop efficient and ecofriendly micromotors. In this study, we systematically manipulate various parameters by modifying the synthesis method of hematite-based micromotors, subsequently comparing their propulsion speeds and uncovering the precise role of these parameters in determining the micromotor performance. Furthermore, we shed light on the intricate interplay between drag force and propulsive force, demonstrating how these forces vary under different H2O2 conditions. These findings provide valuable insights into the design of efficient micromotors tailored for dynamic aquatic environments.

Magnetite-based Janus nanoparticles, their synthesis and biomedical applications

The advent of Janus nanoparticles has been a great breakthrough in the emerging field of nanomaterials. Janus nanoparticles refer to a single structure with two distinct chemical functions on either side. Owing to their asymmetric structures, they can be utilized in a variety of applications where monomorphic particles are insufficient. In the last decade, a wide variety of materials have been employed to fabricate Janus nanoparticles, and due to the great advantages of magnetite (Iron-oxide) NPs, they have been considered as one of the best candidates. With the main benefit of magnetic controlling, magnetite Janus nanoparticles fulfill great promises, especially in biomedical areas such as bioimaging, cancer therapies, theranostics, and biosensing. The intrinsic characteristics of magnetite Janus nanoparticles (MJNPs) even hold great potential in magnetite Janus forms of micro-/nanomotors. Despite the great interest and potential in magnetic Janus NPs, the need for a comprehensive review on MJNPs with a concentration on magnetite NPs has been overlooked. Herein, we present recent advancements in the magnetite-based Janus nanoparticles in the flourishing field of biomedicine. First, the synthesis and fabrication methods of Janus nanoparticles are discussed. Then we will delve into their intriguing biomedical applications, with a separate section for magnetite Janus micro-/nanomotors in biomedicine. And finally, the challenges and future outlook are provided. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > In Vitro Nanoparticle-Based Sensing.

Dual enzyme-powered chemotactic cross β amyloid based functional nanomotors

Nanomotor chassis constructed from biological precursors and powered by biocatalytic transformations can offer important applications in the future, specifically in emergent biomedical techniques. Herein, cross β amyloid peptide-based nanomotors (amylobots) were prepared from short amyloid peptides. Owing to their remarkable binding capabilities, these soft constructs are able to host dedicated enzymes to catalyze orthogonal substrates for motility and navigation. Urease helps in powering the self-diffusiophoretic motion, while cytochrome C helps in providing navigation control. Supported by the simulation model, the design principle demonstrates the utilization of two distinct transport behaviours for two different types of enzymes, firstly enhanced diffusivity of urease with increasing fuel (urea) concentration and secondly, chemotactic motility of cytochrome C towards its substrate (pyrogallol). Dual catalytic engines allow the amylobots to be utilized for enhanced catalysis in organic solvent and can thus complement the technological applications of enzymes.

Biodegradable Microrobots and Their Biomedical Applications: A Review

During recent years, microrobots have drawn extensive attention owing to their good controllability and great potential in biomedicine. Powered by external physical fields or chemical reactions, these untethered microdevices are promising candidates for in vivo complex tasks, such as targeted delivery, imaging and sensing, tissue engineering, hyperthermia, and assisted fertilization, among others. However, in clinical use, the biodegradability of microrobots is significant for avoiding toxic residue in the human body. The selection of biodegradable materials and the corresponding in vivo environment needed for degradation are increasingly receiving attention in this regard. This review aims at analyzing different types of biodegradable microrobots by critically discussing their advantages and limitations. The chemical degradation mechanisms behind biodegradable microrobots and their typical applications are also thoroughly investigated. Furthermore, we examine their feasibility and deal with the in vivo suitability of different biodegradable microrobots in terms of their degradation mechanisms; pathological environments; and corresponding biomedical applications, especially targeted delivery. Ultimately, we highlight the prevailing obstacles and perspective solutions, ranging from their manufacturing methods, control of movement, and degradation rate to insufficient and limited in vivo tests, that could be of benefit to forthcoming clinical applications.

Kinetic Asymmetry versus Dissipation in the Evolution of Chemical Systems as Exemplified by Single Enzyme Chemotaxis

Single enzyme chemotaxis is a phenomenon by which a nonequilibrium spatial distribution of an enzyme is created and maintained by concentration gradients of the substrate and product of the catalyzed reaction. These gradients can arise either naturally through metabolism or experimentally, e.g., by flow of materials through microfluidic channels or by use of diffusion chambers with semipermeable membranes. Numerous hypotheses regarding the mechanism of this phenomenon have been proposed. Here, we discuss a mechanism based solely on diffusion and chemical reaction and show that kinetic asymmetry, a difference in the transition state energies for dissociation/association of substrate and product, and diffusion asymmetry, a difference in the diffusivities of the bound and free forms of the enzyme, are the determinates of the direction of chemotaxis and can result in either positive or negative chemotaxis, both of which have been demonstrated experimentally. Exploration of these fundamental symmetries that govern nonequilibrium behavior helps to distinguish between possible mechanisms for the evolution of a chemical system from initial to the steady state and whether the principle that determines the direction a system shifts when exposed to an external energy source is based on thermodynamics or on kinetics with the latter being supported by the results of the present paper. Our results show that, while dissipation ineluctably accompanies nonequilibrium phenomena, including chemotaxis, systems do not evolve to maximize or minimize dissipation but rather to attain greater kinetic stability and accumulate in regions where their effective diffusion coefficient is as small as possible. The chemotactic response to the chemical gradients formed by other enzymes participating in a catalytic cascade provides a mechanism for forming loose associations known as metabolons. Significantly, the direction of the effective force due to these gradients depends on the kinetic asymmetry of the enzyme and so can be nonreciprocal, where one enzyme is attracted to another enzyme, but the other enzyme is repelled by the one, in seeming contradiction to Newtons third law. This nonreciprocity is an important ingredient in the behavior of active matter.

Spatial Control over Catalyst Positioning for Increased Micromotor Efficiency

Motion is influenced by many different aspects of a micromotor's design, such as shape, roughness and the type of materials used. When designing a motor, asymmetry is the main requirement to take into account, either in shape or in catalyst distribution. It influences both speed and directionality since it dictates the location of propulsion force. Here, we combine asymmetry in shape and asymmetry in catalyst distribution to study the motion of soft micromotors. A microfluidic method is utilized to generate aqueous double emulsions, which upon UV-exposure form asymmetric microgels. Taking advantage of the flexibility of this method, we fabricated micromotors with homogeneous catalyst distribution throughout the microbead and micromotors with different degrees of catalyst localization within the active site. Spatial control over catalyst positioning is advantageous since less enzyme is needed for the same propulsion speed as the homogeneous system and it provides further confinement and compartmentalization of the catalyst. This proof-of-concept of our new design will make the use of enzymes as driving forces for motors more accessible, as well as providing a new route for compartmentalizing enzymes at interfaces without the need for catalyst-specific functionalization.

Medical micro- and nanomotors in the body

Attributed to the miniaturized body size and active mobility, micro- and nanomotors (MNMs) have demonstrated tremendous potential for medical applications. However, from bench to bedside, massive efforts are needed to address critical issues, such as cost-effective fabrication, on-demand integration of multiple functions, biocompatibility, biodegradability, controlled propulsion and in vivo navigation. Herein, we summarize the advances of biomedical MNMs reported in the past two decades, with particular emphasis on the design, fabrication, propulsion, navigation, and the abilities of biological barriers penetration, biosensing, diagnosis, minimally invasive surgery and targeted cargo delivery. Future perspectives and challenges are discussed as well. This review can lay the foundation for the future direction of medical MNMs, pushing one step forward on the road to achieving practical theranostics using MNMs.

Hydrogel-Based Stimuli-Responsive Micromotors for Biomedicine

The rapid development of medical micromotors draws a beautiful blueprint for the noninvasive or minimally invasive diagnosis and therapy. By combining stimuli-sensitive hydrogel materials, micromotors are bestowed with new characteristics such as stimuli-responsive shape transformation/morphing, excellent biocompatibility and biodegradability, and drug loading ability. Actuated by chemical fuels or external fields (e.g., magnetic field, ultrasound, light, and electric field), hydrogel-based stimuli-responsive (HBSR) micromotors can be utilized to load therapeutic agents into the hydrogel networks or directly grip the target cargos (e.g., drug-loaded particles, cells, and thrombus), transport them to sites of interest (e.g., tumor area and diseased tissues), and unload the cargos or execute a specific task (e.g., cell capture, targeted sampling, and removal of blood clots) in response to a stimulus (e.g., change of temperature, pH, ion strength, and chemicals) in the physiological environment. The high flexibility, adaptive capacity, and shape morphing property enable the HBSR micromotors to complete specific medical tasks in complex physiological scenarios, especially in confined, hard-to-reach tissues, and vessels of the body. Herein, this review summarizes the current progress in hydrogel-based medical micromotors with stimuli responsiveness. The thermo-responsive, photothermal-responsive, magnetocaloric-responsive, pH-responsive, ionic-strength-responsive, and chemoresponsive micromotors are discussed in detail. Finally, current challenges and future perspectives for the development of HBSR micromotors in the biomedical field are discussed.

Motor-like microlasers functioning in biological fluids

Microlasers integrated with biological systems have received tremendous attention for their intense light intensity and narrow linewidth recently, serving as a powerful tool for studying complex dynamics and interactions in scattered biological micro-environments. However, manipulation of microlasers with controllable motions and versatile functions remains elusive. Herein, we introduce the concept of motor-like microlasers formed by magnetic-doped liquid crystal droplets, in which the direction and velocity could be controlled by altering internal magnetic nanoparticles or external magnetic fields. Both translational and rotatory motions of the lasing resonator could be continually changed in real-time. Lasing-encoded motors carrying different functions and lasing wavelengths were also achieved. Finally, we demonstrate the potential of motor-like microlasers by functioning as a localized stimulation emission light source to stimulate or illuminate living cells, providing a novel approach for switching on/off light emissions and subcellular imaging. Laser emitting micromotors offer a facile system for precise manipulation of microlasers in biological fluids, providing new insight into the development of programmable on-chip laser devices and laser-emitting intelligent systems.

Fluorescent Microswimmers Based on Cross-β Amyloid Nanotubes and Divergent Cascade Networks

Shaped through millions of years of evolution, the spatial localization of multiple enzymes in living cells employs extensive cascade reactions to enable highly coordinated multimodal functions. Herein, by utilizing a complex divergent cascade, we exploit the catalytic potential as well as templating abilities of streamlined cross-β amyloid nanotubes to yield two orthogonal roles simultaneously. The short peptide based paracrystalline nanotube surfaces demonstrated the generation of fluorescence signals within entangled networks loaded with alcohol dehydrogenase (ADH). The nanotubular morphologies were further used to generate cascade-driven microscopic motility through surface entrapment of sarcosine oxidase (SOX) and catalase (Cat). Moreover, a divergent cascade network was initiated by upstream catalysis of the substrate molecules through the surface mutation of catalytic moieties. Notably, the resultant downstream products led to the generation of motile fluorescent microswimmers by utilizing the two sets of orthogonal properties and, thus, mimicked the complex cascade-mediated functionalities of extant biology.

Enzyme Cascade Reaction Propelled Multicompartmental Colloidal Motors

Compartmentalization is a crucial natural methodology to enable multiple biocatalytic transformations to proceed efficiently. Herein, we report a biocompatible multicompartmental colloidal motor that can achieve autonomous movement in the biological environment through two-enzyme cascade reactions of immobilized enzymes. The colloidal motors with the heterogeneous multicompartment structure were prepared in one step by microfluidic technology, and the compartmentalized encapsulation of glucose oxidase (GOD) and catalase (CAT) was realized. The fabricated colloidal motor was size controllable by tuning the flow rates of the microfluidic system, and its autonomous movement can be triggered by good responsiveness to the alkaline environment. In glucose medium of pH 7.5, the pH-responsive alginate cores of the colloidal motor swell to facilitate fuel penetration and enzyme-catalyzed reactions. The enzyme cascade between GOD and CAT immobilized in the colloidal motor chamber results in the self-propulsion of the colloid motor in glucose medium. The compartmentalized encapsulation of immobilized enzyme improves the stability of the enzyme and enables multicompartmental colloidal motors to self-propel in an alkaline intestinal environment through an enzyme cascade reaction. These features indicate that such multicompartmental colloidal motors actuated by enzyme cascade reaction in biocompatible fuel have great potential for co-encapsulation and autonomous movement in different applications.

Synthetic cells in biomedical applications

Synthetic cells are engineered vesicles that can mimic one or more salient features of life. These features include directed localization, sense-and-respond behavior, gene expression, metabolism, and high stability. In nanomedicine, many of these features are desirable capabilities of drug delivery vehicles but are difficult to engineer. In this focus article, we discuss where synthetic cells offer unique advantages over nanoparticle and living cell therapies. We review progress in the engineering of the above life-like behaviors and how they are deployed in nanomedicine. Finally, we assess key challenges synthetic cells face before being deployed as drugs and suggest ways to overcome these challenges. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Biology-Inspired Nanomaterials > Lipid-Based Structures.

Micro-/Nanorobots in Antimicrobial Applications: Recent Progress, Challenges, and Opportunities

The evolution of drug-resistant pathogenic bacteria remains one of the most urgent threats to public health worldwide. Even worse, the bacterial cells commonly form biofilms through aggregation and adhesion, preventing antibiotic penetration and resisting environmental stress. Moreover, biofilms tend to grow in some hard-to-reach regions, bringing difficulty for antibiotic delivery at the infected site. The drug-resistant pathogenic bacteria and intractable biofilm give rise to chronic and recurrent infections, exacerbating the challenge in combating bacterial infections. Micro/nanorobots (MNRs) are capable of active cargo delivery, targeted treatment with high precision, and motion-assisted mechanical force, which enable transport and enhance penetration of antibacterial agents into the targeted site, thus showing great promise in emerging as an attractive alternative to conventional antibacterial therapies. This review summarizes the recent advances in micro-/nanorobots for antibacterial applications, with emphasis on those novel strategies for drug-resistance bacterium and stubborn biofilm infections. Insights on the future development of MNRs with good functionality and biosafety offer promising approaches to address infections in the clinic setting.

Sustained release of chlorogenic acid-loaded nanomicelles alleviates bone loss in mouse periodontitis

Abstract Periodontitis is a prevalent chronic inflammatory disease that destroys the periodontal supporting tissues, impinges on oral health, and is correlative with an increased risk of systemic disease. Currently, the...

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.

Enzymatic/Magnetic Hybrid Micromotors for Synergistic Anticancer Therapy

Micro/nanomotors (MNMs), which propel by transforming various forms of energy into kinetic energy, have emerged as promising therapeutic nanosystems in biomedical applications. However, most MNMs used for anticancer treatment are only powered by one engine or provide a single therapeutic strategy. Although double-engined micromotors for synergistic anticancer therapy can achieve more flexible movement and efficient treatment efficacy, their design remains challenging. In this study, we used a facile preparation method to develop enzymatic/magnetic micromotors for synergetic cancer treatment via chemotherapy and starvation therapy (ST), and the size of micromotors can be easily regulated during the synthetic process. The enzymatic reaction of glucose oxidase, which served as the chemical engine, led to self-propulsion using glucose as a fuel and ST via a reduction in the energy available to cancer cells. Moreover, the incorporation of Fe₃O₄ nanoparticles as a magnetic engine enhanced the kinetic power and provided control over the direction of movement. Inherent pH-responsive drug release behavior was observed owing to the acidic decomposition of drug carriers in the intracellular microenvironment of cancer cells. This system displayed enhanced anticancer efficacy owing to the synergetic therapeutic strategies and increased cellular uptake in a targeted area because of the improved motion behavior provided by the double engines. Therefore, the demonstrated micromotors are promising candidates for anticancer biomedical microsystems.

Biodegradability of Micro/Nanomotors: Challenges and Opportunities

Micro/nanomotors (MNMs) are miniature machines that can convert chemical or external energy into their own mechanical motions. In previous decades, significant efforts have been made to improve the performance of MNMs. For practical applications, the biodegradability of MNMs is an important aspect that must be considered, particularly in the biomedical field. In this review, recent progress in the biodegradability of MNMs and their potential applications are summarized. Different biodegradable materials, including metals and polymers, or other strategies for the fabrication of MNMs, are presented. Current challenges and future perspectives are also discussed.

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.

Cooperative Chemotaxis of Magnesium Microswimmers for Corrosion Spotting

Numerous artificial micro- and nanomotors, as well as various swimmers have been inspired by living organisms that are able to move in a coordinated manner. Their cooperation has also gained a lot of attention because the resulting clusters are able to adapt to changes in their environment and to perform complex tasks. However, mimicking such a collective behavior remains a challenge. In the present work, magnesium microparticles are used as chemotactic swimmers with pronounced collective features, allowing the gradual formation of macroscopic agglomerates. The formed clusters act like a single swimmer able to follow pH gradients. This dynamic behavior can be used to spot localized corrosion events in a straightforward way. The autonomous docking of the swimmers to the corrosion site leads to the formation of a local protection layer, thus increasing corrosion resistance and triggering partial self-healing.

Swarming behavior and in vivo monitoring of enzymatic nanomotors within the bladder

Enzyme-powered nanomotors are an exciting technology for biomedical applications due to their ability to navigate within biological environments using endogenous fuels. However, limited studies into their collective behavior and demonstrations of tracking enzyme nanomotors in vivo have hindered progress toward their clinical translation. Here, we report the swarming behavior of urease-powered nanomotors and its tracking using positron emission tomography (PET), both in vitro and in vivo. For that, mesoporous silica nanoparticles containing urease enzymes and gold nanoparticles were used as nanomotors. To image them, nanomotors were radiolabeled with either ¹²⁴I on gold nanoparticles or ¹⁸F-labeled prosthetic group to urease. In vitro experiments showed enhanced fluid mixing and collective migration of nanomotors, demonstrating higher capability to swim across complex paths inside microfabricated phantoms, compared with inactive nanomotors. In vivo intravenous administration in mice confirmed their biocompatibility at the administered dose and the suitability of PET to quantitatively track nanomotors in vivo. Furthermore, nanomotors were administered directly into the bladder of mice by intravesical injection. When injected with the fuel, urea, a homogeneous distribution was observed even after the entrance of fresh urine. By contrast, control experiments using nonmotile nanomotors (i.e., without fuel or without urease) resulted in sustained phase separation, indicating that the nanomotors' self-propulsion promotes convection and mixing in living reservoirs. Active collective dynamics, together with the medical imaging tracking, constitute a key milestone and a step forward in the field of biomedical nanorobotics, paving the way toward their use in theranostic applications.

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.

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.

Self-propelling micro-/nano-motors: Mechanisms, applications, and challenges in drug delivery

In recent times, numerous efforts have been put forward to fabricating the self-propelling micro-/nano-motors (MNMs) for various applications, such as drug delivery, environmental remediation, biosensing, and precision surgery at the micro-/nanoscale, among others. Owing to their potential advantages, the application of such innovative architectures has been increasingly recognized towards addressing various challenges in the related fields. Specifically, these MNMs offer enormous potential in nanomedicine in overcoming the significant challenge of low permeation of the biological barriers. Herein, we emphasize the powered mechanism of MNMs, including artificial and natural-based MNMs, and discuss the characteristics, as well as the challenges being faced by MNMs in drug delivery. Further, the research progress of MNMs as drug carriers in different environments (gastrointestinal tract, saliva, urinary bladder, blood, and extracellular matrix, ECM) of the body in recent years is summarized, highlighting the representative works on MNMs towards in vivo applications. Together, we firmly believe that these innovative MNMs-based designs may play a crucial role in the clinical practice in the future.

Enzyme-Powered Porous Micromotors Built from a Hierarchical Micro- and Mesoporous UiO-Type Metal-Organic Framework

Here, we report the design, synthesis, and functional testing of enzyme-powered porous micromotors built from a metal-organic framework (MOF). We began by subjecting a presynthesized microporous UiO-type MOF to ozonolysis, to confer it with mesopores sufficiently large to adsorb and host the enzyme catalase (size: 6-10 nm). We then encapsulated catalase inside the mesopores, observing that they are hosted in those mesopores located at the subsurface of the MOF crystals. In the presence of H₂O₂ fuel, MOF motors (or MOFtors) exhibit jet-like propulsion enabled by enzymatic generation of oxygen bubbles. Moreover, thanks to their hierarchical pore system, the MOFtors retain sufficient free space for adsorption of additional targeted species, which we validated by testing a MOFtor for removal of rhodamine B during self-propulsion.

Enzyme-driven micro/nanomotors: Recent advances and biomedical applications

Micro/nanomotors (MNMs), both self-propelled actuators and external fields-promoted machines, have joined forces in the past decade to accomplish versatile tasks such as precise detection and targeted cargo delivery with adequate propulsion and desirable locomotion. Amongst, enzyme-driven MNMs have been able to differentiate themselves from others owing to their distinct characteristics, such as absence of chemical fuel, enhanced cellular uptake and the possibility to be easily conjugated with many therapeutics, including both small molecules and biologics, displaying superior efficacy, enhanced specificity and diminished side effects. In the present review, we aim to highlight and summarize recent advances in enzyme-driven MNMs, particularly to provide an in-depth discussion focusing on the enzyme linking approaches onto those MNMs and motion control strategies of such MNMs with advantages and limitations thereof. Conclusions and future perspectives are also provided in brief.

Fundamentals and applications of enzyme powered micro/nano-motors

Micro/nanomotors (MNMs) are miniaturized machines that can convert many kinds of energy into mechanical motion. Over the past decades, a variety of driving mechanisms have been developed, which have greatly extended the application scenarios of MNMs. Enzymes exist in natural organisms which can convert chemical energy into mechanical force. It is an innovative attempt to utilize enzymes as biocatalyst providing driving force for MNMs. The fuels for enzymatic reactions are biofriendly as compared to traditional counterparts, which makes enzyme-powered micro/nanomotors (EMNMs) of great value in biomedical field for their nature of biocompatibility. Until now, EMNMs with various shapes can be propelled by catalase, urease and many others. Also, they can be endowed with multiple functionalities to accomplish on-demand tasks. Herein, combined with the development process of EMNMs, we are committed to present a comprehensive understanding of EMNMs, including their types, propelling principles, and potential applications. In this review, we will introduce single enzyme that can be used as motor, enzyme powered molecule motors and other micro/nano-architectures. The fundamental mechanism of energy conversion process of EMNMs and crucial factors that affect their movement behavior will be discussed. The current progress of proof-of-concept applications of EMNMs will also be elaborated in detail. At last, we will summarize and prospect the opportunities and challenges that EMNMs will face in their future development.

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Clear classification and description of different enzyme-powered micro/nanomotors (EMNMs).

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Discussion of the fundamental mechanism of energy conversion process of EMNMs and their movement influence factors.

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Introduction of the current progress of proof-of-concept applications of EMNMs.

Ionic Species Affect the Self-Propulsion of Urease-Powered Micromotors

Enzyme-powered motors self-propel through the catalysis of in situ bioavailable fuels, which makes them excellent candidates for biomedical applications. However, fundamental issues like their motion in biological fluids and the understanding of the propulsion mechanism are critical aspects to be tackled before a future application in biomedicine. Herein, we investigated the physicochemical effects of ionic species on the self-propulsion of urease-powered micromotors. Results showed that the presence of PBS, NaOH, NaCl, and HEPES reduced self-propulsion of urease-powered micromotors pointing towards ion-dependent mechanisms of motion. We studied the 3D motion of urease micromotors using digital holographic microscopy to rule out any motor-surface interaction as the cause of motion decay when salts are present in the media. In order to protect and minimize the negative effect of ionic species on micromotors' performance, we coated the motors with methoxypolyethylene glycol amine (mPEG) showing higher speed compared to noncoated motors at intermediate ionic concentrations. These results provide new insights into the mechanism of urease-powered micromotors, study the effect of ionic media, and contribute with potential solutions to mitigate the reduction of mobility of enzyme-powered micromotors.

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.

Enzyme catalysis powered micro/nanomotors for biomedical applications

This review provides insights on enzyme powered motors using fuels present in biological environments for biomedical applications.