Chemotaxis of bio-hybrid multiple bacteria-driven microswimmers

Microfluidic Design of Streamlined Alginate Hydrogel Micromotors with Run and Tumble Motion Patterns

Autonomous micromotors demonstrate remarkable advancements in biomedical applications. A noteworthy example is streamlined motors, which display enhanced movement efficiency with low fluid-resistance. However, existing streamlined motors, primarily constructed from inorganic materials, present challenges due to their complex fabrication procedures and lack of a soft interface for interaction with biological systems. Herein, a novel design of biodegradable streamlined alginate hydrogel micromotors with a teardrop shape by microfluidics is introduced. The platform enables the high-throughput fabrication of monodisperse micromotors with varied dimensions. By incorporating Pt-coated Fe3 O4 nanoparticles, micromotors are equipped with dual capabilities of catalytic propulsion and accurate magnetic guidance. Through precisely tuning the localization regions of catalysts within the micromotors, the streamlined hydrogel micromotors not only exhibit enhanced propelling efficiency, but also accomplish distinct motion patterns of run and tumble. The design provides insights for developing advanced micromotors capable of executing intricate tasks across diverse application scenarios.

Fugetaxis of Cell-in-Catalytic-Coat Nanobiohybrids in Glucose Gradients

Manipulation and control of cell chemotaxis remain an underexplored territory despite vast potential in various fields, such as cytotherapeutics, sensors, and even cell robots. Herein is achieved the chemical control over chemotactic movement and direction of Jurkat T cells, as a representative model, by the construction of cell-in-catalytic-coat structures in single-cell nanoencapsulation. Armed with the catalytic power of glucose oxidase (GOx) in the artificial coat, the nanobiohybrid cytostructures, denoted as Jurkat[Lipo_GOx] , exhibit controllable, redirected chemotactic movement in response to d-glucose gradients, in the opposite direction to the positive-chemotaxis direction of naïve, uncoated Jurkat cells in the same gradients. The chemically endowed, reaction-based fugetaxis of Jurkat[Lipo_GOx] operates orthogonally and complementarily to the endogenous, binding/recognition-based chemotaxis that remains intact after the formation of a GOx coat. For instance, the chemotactic velocity of Jurkat[Lipo_GOx] can be adjusted by varying the combination of d-glucose and natural chemokines (CXCL12 and CCL19) in the gradient. This work offers an innovative chemical tool for bioaugmenting living cells at the single-cell level through the use of catalytic cell-in-coat structures.

Construction of intelligent moving micro/nanomotors and their applications in biosensing and disease treatment

Micro/nanomotors are containers that pass through liquid media and carry cargo. Because they are tiny, micro/nanomotors exhibit excellent potential for biosensing and disease treatment applications. However, their size also makes overcoming random Brownian forces very challenging for micro/nanomotors moving on targets. Additionally, to achieve desired practical applications, the expensive materials, short lifetimes, poor biocompatibility, complex preparation methods, and side effects of micro/nanomotors must be addressed, and potential adverse effects must be evaluated both in vivo and in practical applications. This has led to the continuous development of key materials for driving micro/nanomotors. In this work, we review the working principles of micro/nanomotors. Metallic and nonmetallic nanocomplexes, enzymes, and living cells are explored as key materials for driving micro/nanomotors. We also consider the effects of exogenous stimulations and endogenous substance conditions on micro/nanomotor motions. The discussion focuses on micro/nanomotor applications in biosensing, treating cancer and gynecological diseases, and assisted fertilization. By addressing micro/nanomotor shortcomings, we propose directions for further developing and applying micro/nanomotors.

Fabrication, control, and modeling of robots inspired by flagella and cilia

Flagella and cilia are slender structures that serve important functionalities in the microscopic world through their locomotion induced by fluid and structure interaction. With recent developments in microscopy, fabrication, biology, and modeling capability, robots inspired by the locomotion of these organelles in low Reynolds number flow have been manufactured and tested on the micro-and macro-scale, ranging from medicalin vivomicrobots, microfluidics to macro prototypes. We present a collection of modeling theories, control principles, and fabrication methods for flagellated and ciliary robots.

Multifunctional 4D-Printed Sperm-Hybrid Microcarriers for Assisted Reproduction

Remotely controllable microrobots are appealing for various biomedical in vivo applications. In particular, in recent years, our group has focused on developing sperm-microcarriers to assist sperm cells with motion deficiencies or low sperm count (two of the most prominent male infertility problems) to reach the oocyte toward in-vivo-assisted fertilization. Different sperm carriers, considering their motion in realistic media and confined environments, have been optimized. However, the already-reported sperm carriers have been mainly designed to transport single sperm cell, with limited functionality. Thus, to take a step forward, here, the development of a 4D-printed multifunctional microcarrier containing soft and smart materials is reported. These microcarriers can not only transport and deliver multiple motile sperm cells, but also release heparin and mediate local enzymatic reactions by hyaluronidase-loaded polymersomes (HYAL-Psomes). These multifunctional facets enable in situ sperm capacitation/hyperactivation, and the local degradation of the cumulus complex that surrounds the oocyte, both to facilitate the sperm-oocyte interaction for the ultimate goal of in vivo assisted fertilization.

Actuators for Implantable Devices: A Broad View

The choice of actuators dictates how an implantable biomedical device moves. Specifically, the concept of implantable robots consists of the three pillars: actuators, sensors, and powering. Robotic devices that require active motion are driven by a biocompatible actuator. Depending on the actuating mechanism, different types of actuators vary remarkably in strain/stress output, frequency, power consumption, and durability. Most reviews to date focus on specific type of actuating mechanism (electric, photonic, electrothermal, etc.) for biomedical applications. With a rapidly expanding library of novel actuators, however, the granular boundaries between subcategories turns the selection of actuators a laborious task, which can be particularly time-consuming to those unfamiliar with actuation. To offer a broad view, this study (1) showcases the recent advances in various types of actuating technologies that can be potentially implemented in vivo, (2) outlines technical advantages and the limitations of each type, and (3) provides use-specific suggestions on actuator choice for applications such as drug delivery, cardiovascular, and endoscopy implants.

Advances of bacteria-based delivery systems for modulating tumor microenvironment

The components and hospitable properties of tumor microenvironment (TME) are associated with tumor progression. Recently, TME modulating vectors and strategies have garnished significant attention in cancer therapy. Although a pilot work has reviewed TME regulation via nanoparticle-based delivery systems, there is no systematical review that summarizes the natural bacteria-based anti-tumor system to modulate TME. In this review, we conclude the strategies of bacterial carriers (including whole bacteria, bacterial skeleton and bacterial components) to regulate TME from the perspective of TME components and hospitable properties, and the clinical trials of bacteria-mediated cancer therapy. Current challenges and future prospects for the design of bacteria-based carriers are also proposed that provide critical insights into this natural delivery system and related translation from the bench to the clinic.

Micro-Nano Motors with Taxis Behavior: Principles, Designs, and Biomedical Applications

As a novel mobile nanodevice, micro-nano motors (MNMs) can convert the energy of the surrounding environment into mechanical motion. With this unique ability, they promise revolutionary potential in bio-applications including precise drug delivery, bio-sensing, and noninvasive surgery. Yet for practically reaching the target and fulfilling these tasks in dynamically changing bio-environment, environment adaptivity beyond propulsion is important yet challenging. MNMs with taxis behavior/autonomous target-seeking ability offer a desirable solution. These motors can adaptively move to the target location and complete the task. Thanks to the persistent efforts of researchers, tactic MNMs have shown automatic navigation to target under various energy fields, not only in static environments, but also in shear rheological conditions that simulate blood flow. Therefore, tactic motors with self-targeting capability lay a concrete foundation for targeted drug delivery, cell transplantation, and thrombus ablation. This review systematically presents the moving principle, design, and biological applications of tactic MNMs under different energy fields. Through in-depth analysis of state-of-art progress, the obstacles of the field and possible solutions are discussed. With the continuous innovation and breakthroughs of multi-disciplinary researchers, MNMs with taxis behavior are expected to provide a revolutionary solution for cancer and other major diseases in the biomedical field.

Engineering Bacteria and Bionic Bacterial Derivatives with Nanoparticles for Cancer Therapy

Natural bacteria are interesting subjects for cancer treatments owing to their unique autonomy-driven and hypoxic target properties. Genetically modified bacteria (such as bacteria with msbB gene and aroA gene modifications) can effectively cross sophisticated physiological barriers and transport antitumor agents into deep tumor tissues, and they have good biosafety. Additionally, bacteria can secrete cytokines (such as interleukin-224, interferon-gamma [IFN-γ], and interleukin-1β) and activate antitumor immune responses in the tumor microenvironment, resulting in tumor inhibition. All of these characteristics can be easily utilized to develop synergistic antitumor strategies by combining bacteria-based agents with other therapeutic approaches. Herein, representative studies of bacteria-instructed multimodal synergistic cancer therapy are introduced (e.g., photothermal therapy, chemoimmunotherapy, photodynamic therapy, and photocontrolled bacterial metabolite therapy), and their key advantages are systematically expounded. The current challenges and future prospects in advancing the development of bacteria-based micro/nanomedicines in the field of synthetic biology research are also emphasized, which will hopefully promote the development of related bacteria-based cancer therapies.

Nano/Micromotors in Active Matter

Nano/micromotors (NMMs) are tiny objects capable of converting energy into mechanical motion. Recently, a wealth of active matter including synthetic colloids, cytoskeletons, bacteria, and cells have been used to construct NMMs. The self-sustained motion of active matter drives NMMs out of equilibrium, giving rise to rich dynamics and patterns. Alongside the spontaneous dynamics, external stimuli such as geometric confinements, light, magnetic field, and chemical potential are also harnessed to control the movements of NMMs, yielding new application paradigms of active matter. Here, we review the recent advances, both experimental and theoretical, in exploring biological NMMs. The unique dynamical features of collective NMMs are focused on, along with some possible applications of these intriguing systems.

Multifunctional Metal-Organic Framework Exoskeletons Protect Biohybrid Sperm Microrobots for Active Drug Delivery from the Surrounding Threats

Utilizing spermatozoa as the engine unit of robotic systems at a microscale has brought revolutionized inspirations and strategies to the biomedical community. However, the motility of sperms is impaired by the surrounding threats. For example, the antisperm antibody (AsA) can specifically bind with surface antigens on the sperm membrane and adversely affect their propulsion, hindering the operation of sperm-based microrobots in practical environments. In the present work, we report a biohybrid sperm microrobot by encapsulating sperm cells within metal-organic frameworks (MOFs) and zeolitic imidazolate framework-8 (ZIF-8) nanoparticles (NPs) (ZIFSpermbot), capable of active drug delivery and cytoprotection from the biological threats of AsA. ZIF-8 NPs can be facilely coated on the sperm membrane through complexation with tannic acid. Such cell surface engineering has a negligible impact on sperm motility under optimized conditions. The selective permeability of the resulting porous ZIF-8 wrappings protects ZIFSpermbots from the specific binding of AsA, enabling the preservation of intrinsic propulsion of the sperm engine. Besides, ZIF-8 wrappings sustainably release zinc ions and attenuate the oxidative damage generated in sperm cells, allowing the maintenance of sperm movement. Combining the effective protection of sperm propulsion with the drug-loading capacity of ZIF-8 NPs provides new applicability to ZIFSpermbots in risky surroundings with AsA, exhibiting rapid migration in a microfluidic device for active drug delivery with enhanced therapeutic efficacy due to their retained effective propulsion. Imparting bioengine-based microrobots with multifunctional wrappings holds great promise for designing adaptive cell robots that endure harsh environments toward locally extended and diverse operations, facilitating their use in practical and clinical applications.

The role played by bacterial infections in the onset and metastasis of cancer

Understanding various responses of cells towards change in their external environment, presence of other species and is important in identifying and correlating the mechanisms leading to malignant transformations and cancer development. Although uncovering and comprehending the association between bacteria and cancer is highly challenging, it promises excellent perspectives and approaches for successful cancer therapy. This review introduces various bacterial species, their virulence factors, and their role in cell transformations leading to cancer (particularly gastric, oral, colon, and breast cancer). Bacterial dysbiosis permutates host cells, causes inflammation, and results in tumorigenesis. This review explored bacterial-mediated host cell transformation causing chronic inflammation, immune receptor hyperactivation/absconding immune recognition, and genomic instability. Bacterial infections downregulate E-cadherin, leading to loosening of epithelial tight junction polarity and triggers metastasis. In addition to understanding the role of bacterial infections in cancer development, we have also reviewed the application of bacteria for cancer therapy. The emergence of bacteriotherapy combined with conventional therapies led to new and effective ways of overcoming challenges associated with available treatments. This review discusses the application of bacterial minicells, microswimmers, and outer cell membrane vesicles (OMV) for drug delivery applications.

Biohybrid microswimmers against bacterial infections

Biohybrid microswimmers exploit the natural abilities of motile microorganisms e.g. in releasing cargo on-demand. However, using such engineered swarms to release antibiotics addressing bacterial infections has not yet been realized. Herein, a design strategy for biohybrid microswimmers is reported, which features the covalent attachment of antibiotics with a photo-cleavable linker to the algae Chlamydomonas reinhardtii via two synthetic steps. This surface engineering does not rely on genetic manipulations, proceeds with high efficiency, and retains the viability or phototaxis of microalgae. Two different antibiotics have been separately utilized, which result in activity against both gram-positive and gram-negative strains. Guiding the biohybrid microswimmers by an external beacon, and on-demand delivery of the drugs by light with high spatial and temporal control, allowed for strong inhibition of bacterial growth. This efficient strategy could potentially allow for the selective treatment of bacterial infections by engineered algal microrobots with high precision in space and time. STATEMENT OF SIGNIFICANCE: Biological swimmers with innate sensing and actuation capabilities and integrated components have been widely investigated to create autonomous microsystems. The use of natural swimmers as cargo delivery systems presents an alternative strategy to transport therapeutics to the required locations with the difficult access by traditional strategies. Although the transfer of various therapeutic cargo has shown promising results, the utilization of microswimmers for the delivery of antimicrobials was barely covered. Therefore, we present biohybrid microalga-powered swimmers designed and engineered to carry antibiotic cargo against both Gram-positive and Gram-negative bacteria. Guided by an external beacon, these microhybrids deliver the antibiotic payload to the site of bacterial infection, with high spatial and temporal precision, released on-demand by an external trigger to inhibit bacterial growth.

Microfluidics for Microswimmers: Engineering Novel Swimmers and Constructing Swimming Lanes on the Microscale, a Tutorial Review

This paper provides an updated review of recent advances in microfluidics applied to artificial and biohybrid microswimmers. Sharing the common regime of low Reynolds number, the two fields have been brought together to take advantage of the fluid characteristics at the microscale, benefitting microswimmer research multifold. First, microfluidics offer simple and relatively low-cost devices for high-fidelity production of microswimmers made of organic and inorganic materials in a variety of shapes and sizes. Microscale confinement and the corresponding fluid properties have demonstrated differential microswimmer behaviors in microchannels or in the presence of various types of physical or chemical stimuli. Custom environments to study these behaviors have been designed in large part with the help of microfluidics. Evaluating microswimmers in increasingly complex lab environments such as microfluidic systems can ensure more effective implementation for in-field applications. The benefits of microfluidics for the fabrication and evaluation of microswimmers are balanced by the potential use of microswimmers for sample manipulation and processing in microfluidic systems, a large obstacle in diagnostic and other testing platforms. In this review various ways in which these two complementary technology fields will enhance microswimmer development and implementation in various fields are introduced.

The emerging technology of biohybrid micro-robots: a review

Abstract

In the past few decades, robotics research has witnessed an increasingly high interest in miniaturized, intelligent, and integrated robots. The imperative component of a robot is the actuator that determines its performance. Although traditional rigid drives such as motors and gas engines have shown great prevalence in most macroscale circumstances, the reduction of these drives to the millimeter or even lower scale results in a significant increase in manufacturing difficulty accompanied by a remarkable performance decline. Biohybrid robots driven by living cells can be a potential solution to overcome these drawbacks by benefiting from the intrinsic microscale self-assembly of living tissues and high energy efficiency, which, among other unprecedented properties, also feature flexibility, self-repair, and even multiple degrees of freedom. This paper systematically reviews the development of biohybrid robots. First, the development of biological flexible drivers is introduced while emphasizing on their advantages over traditional drivers. Second, up-to-date works regarding biohybrid robots are reviewed in detail from three aspects: biological driving sources, actuator materials, and structures with associated control methodologies. Finally, the potential future applications and major challenges of biohybrid robots are explored.

Graphic abstract

Biohybrid robotics: From the nanoscale to the macroscale

Biohybrid robotics is a field in which biological entities are combined with artificial materials in order to obtain improved performance or features that are difficult to mimic with hand-made materials. Three main level of integration can be envisioned depending on the complexity of the biological entity, ranging from the nanoscale to the macroscale. At the nanoscale, enzymes that catalyze biocompatible reactions can be used as power sources for self-propelled nanoparticles of different geometries and compositions, obtaining rather interesting active matter systems that acquire importance in the biomedical field as drug delivery systems. At the microscale, single enzymes are substituted by complete cells, such as bacteria or spermatozoa, whose self-propelling capabilities can be used to transport cargo and can also be used as drug delivery systems, for in vitro fertilization practices or for biofilm removal. Finally, at the macroscale, the combinations of millions of cells forming tissues can be used to power biorobotic devices or bioactuators by using muscle cells. Both cardiac and skeletal muscle tissue have been part of remarkable examples of untethered biorobots that can crawl or swim due to the contractions of the tissue and current developments aim at the integration of several types of tissue to obtain more realistic biomimetic devices, which could lead to the next generation of hybrid robotics. Tethered bioactuators, however, result in excellent candidates for tissue models for drug screening purposes or the study of muscle myopathies due to their three-dimensional architecture. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Soft erythrocyte-based bacterial microswimmers for cargo delivery

Bacteria-propelled biohybrid microswimmers have recently shown to be able to actively transport and deliver cargos encapsulated into their synthetic constructs to specific regions locally. However, usage of synthetic materials as cargo carriers can result in inferior performance in load-carrying efficiency, biocompatibility, and biodegradability, impeding clinical translation of biohybrid microswimmers. Here, we report construction and external guidance of bacteria-driven microswimmers using red blood cells (RBCs; erythrocytes) as autologous cargo carriers for active and guided drug delivery. Multifunctional biohybrid microswimmers were fabricated by attachment of RBCs [loaded with anticancer doxorubicin drug molecules and superparamagnetic iron oxide nanoparticles (SPIONs)] to bioengineered motile bacteria, Escherichia coli MG1655, via biotin-avidin-biotin binding complex. Autonomous and on-board propulsion of biohybrid microswimmers was provided by bacteria, and their external magnetic guidance was enabled by SPIONs loaded into the RBCs. Furthermore, bacteria-driven RBC microswimmers displayed preserved deformability and attachment stability even after squeezing in microchannels smaller than their sizes, as in the case of bare RBCs. In addition, an on-demand light-activated hyperthermia termination switch was engineered for RBC microswimmers to control bacteria population after operations. RBCs, as biological and autologous cargo carriers in the biohybrid microswimmers, offer notable advantages in stability, deformability, biocompatibility, and biodegradability over synthetic cargo-carrier materials. The biohybrid microswimmer design presented here transforms RBCs from passive cargo carriers into active and guidable cargo carriers toward targeted drug and other cargo delivery applications in medicine.

High-Yield Production of Biohybrid Microalgae for On-Demand Cargo Delivery

Biohybrid microswimmers exploit the swimming and navigation of a motile microorganism to target and deliver cargo molecules in a wide range of biomedical applications. Medical biohybrid microswimmers suffer from low manufacturing yields, which would significantly limit their potential applications. In the present study, a biohybrid design strategy is reported, where a thin and soft uniform coating layer is noncovalently assembled around a motile microorganism. Chlamydomonas reinhardtii (a single-cell green alga) is used in the design as a biological model microorganism along with polymer-nanoparticle matrix as the synthetic component, reaching a manufacturing efficiency of ≈90%. Natural biopolymer chitosan is used as a binder to efficiently coat the cell wall of the microalgae with nanoparticles. The soft surface coating does not impair the viability and phototactic ability of the microalgae, and allows further engineering to accommodate biomedical cargo molecules. Furthermore, by conjugating the nanoparticles embedded in the thin coating with chemotherapeutic doxorubicin by a photocleavable linker, on-demand delivery of drugs to tumor cells is reported as a proof-of-concept biomedical demonstration. The high-throughput strategy can pave the way for the next-generation generation microrobotic swarms for future medical active cargo delivery tasks.

Bacteria and bacterial derivatives as drug carriers for cancer therapy

The application of bacteria and bacteria-derived membrane vesicles (MVs) has promising potential to make a great impact on the development of controllable targeted drug delivery for combatting cancer. Comparing to most other traditional drug delivery systems, bacteria and their MVs have unique capabilities as drug carriers for cancer treatment. They can overcome physical barriers to target and accumulate in tumor tissues and initiate antitumor immune responses. Furtherly, they are able to be modified both genetically and chemically, to produce and transport anticancer agents into tumor tissues with improved safety and efficacy of cancer treatment but decreased cytotoxic effects to normal cells. In this review, we present some examples of tumor-targeting bacteria and bacteria-derived MVs for the delivery of anticancer drugs, including chemo-therapeutic, radio-therapeutic, photothermal-therapeutic, and immuno-therapeutic agents. We also discuss the advantages as well as the limitations of these tumor-targeting bacteria and their MVs used as platforms for controlled delivery of anticancer therapeutic agents, and further highlight their great potential on clinical translation.

Self-Propelled Micro/Nanomotors for On-Demand Biomedical Cargo Transportation

Micro/nanomotors (MNMs) are miniaturized machines that can perform assigned tasks at the micro/nanoscale. Over the past decade, significant progress has been made in the design, preparation, and applications of MNMs that are powered by converting different sources of energy into mechanical force, to realize active movement and fulfill on-demand tasks. MNMs can be navigated to desired locations with precise controllability based on different guidance mechanisms. A considerable research effort has gone into demonstrating that MNMs possess the potential of biomedical cargo loading, transportation, and targeted release to achieve therapeutic functions. Herein, the recent advances of self-propelled MNMs for on-demand biomedical cargo transportation, including their self-propulsion mechanisms, guidance strategies, as well as proof-of-concept studies for biological applications are presented. In addition, some of the major challenges and possible opportunities of MNMs are identified for future biomedical applications in the hope that it may inspire future research.

Medical micro/nanorobots in complex media

Medical micro/nanorobots have received tremendous attention over the past decades owing to their potential to be navigated into hard-to-reach tissues for a number of biomedical applications ranging from targeted drug/gene delivery, bio-isolation, detoxification, to nanosurgery. Despite the great promise, the majority of the past demonstrations are primarily under benchtop or in vitro conditions. Many developed micro/nanoscale propulsion mechanisms are based on the assumption of a homogeneous, Newtonian environment, while realistic biological environments are substantially more complex. Moving toward practical medical use, the field of micro/nanorobotics must overcome several major challenges including propulsion through complex media (such as blood, mucus, and vitreous) as well as deep tissue imaging and control in vivo. In this review article, we summarize the recent research efforts on investigating how various complexities in biological environments impact the propulsion of micro/nanoswimmers. We also highlight the emerging technological approaches to enhance the locomotion of micro/nanorobots in complex environments. The recent demonstrations of in vivo imaging, control and therapeutic medical applications of such micro/nanorobots are introduced. We envision that continuing materials and technological innovations through interdisciplinary collaborative efforts can bring us steps closer to the fantasy of "swallowing a surgeon".

Nanoerythrosome-functionalized biohybrid microswimmers

Biohybrid microswimmers, which are realized through the integration of motile microscopic organisms with artificial cargo carriers, have a significant potential to revolutionize autonomous targeted cargo delivery applications in medicine. Nonetheless, there are many open challenges, such as motility performance and immunogenicity of the biological segment of the microswimmers, which should be overcome before their successful transition to the clinic. Here, we present the design and characterization of a biohybrid microswimmer, which is composed of a genetically engineered peritrichously flagellated Escherichia coli species integrated with red blood cell-derived nanoliposomes, also known as nanoerythrosomes. Initially, we demonstrated nanoerythrosome fabrication using the cell extrusion technique and characterization of their size and functional cell membrane proteins with dynamic light scattering and flow cytometry analyses, respectively. Then, we showed the construction of biohybrid microswimmers through the conjugation of streptavidin-modified bacteria with biotin-modified nanoerythrosomes by using non-covalent streptavidin interaction. Finally, we investigated the motility performance of the nanoerythrosome-functionalized biohybrid microswimmers and compared it with the free-swimming bacteria. The microswimmer design approach presented here could lead to the fabrication of personalized biohybrid microswimmers from patients' own cells with high fabrication efficiencies and motility performances.

Red/Far-Red Light Switchable Cargo Attachment and Release in Bacteria-Driven Microswimmers

In bacteria-driven microswimmers, i.e., bacteriabots, artificial cargos are attached to flagellated chemotactic bacteria for active delivery with potential applications in biomedical technology. Controlling when and where bacteria bind and release their cargo is a critical step for bacteriabot fabrication and efficient cargo delivery/deposition at the target site. Toward this goal, photoregulating the cargo integration and release in bacteriabots using red and far-red light, which are noninvasive stimuli with good tissue penetration and provide high spatiotemporal control, is proposed. In the bacteriabot design, the surfaces of E. coli and microsized model cargo particles with the proteins PhyB and PIF6, which bind to each other under red light and dissociate from each other under far-red light are functionalized. Consequently, the engineered bacteria adhere and transport the model cargo under red light and release it on-demand upon far-red light illumination due to the photoswitchable PhyB-PIF6 protein interaction. Overall, the proof-of-concept for red/far-red light switchable bacteriabots, which opens new possibilities in the photoregulation in biohybrid systems for bioengineering, targeted drug delivery, and lab-on-a-chip devices, is demonstrated.

Biohybrid robotics with living cell actuation

This review comprehensively discusses recent advances in the basic components, controlling methods and especially in the applications of biohybrid robots.

Living Materials Herald a New Era in Soft Robotics

Living beings have an unsurpassed range of ways to manipulate objects and interact with them. They can make autonomous decisions and can heal themselves. So far, a conventional robot cannot mimic this complexity even remotely. Classical robots are often used to help with lifting and gripping and thus to alleviate the effects of menial tasks. Sensors can render robots responsive, and artificial intelligence aims at enabling autonomous responses. Inanimate soft robots are a step in this direction, but it will only be in combination with living systems that full complexity will be achievable. The field of biohybrid soft robotics provides entirely new concepts to address current challenges, for example the ability to self-heal, enable a soft touch, or to show situational versatility. Therefore, "living materials" are at the heart of this review. Similarly to biological taxonomy, there is a recent effort for taxonomy of biohybrid soft robotics. Here, an expansion is proposed to take into account not only function and origin of biohybrid soft robotic components, but also the materials. This materials taxonomy key demonstrates visually that materials science will drive the development of the field of soft biohybrid robotics.

Rise of cyborg microrobot: different story for different configuration

By integrating organic parts achieved through evolution and inorganic parts developed by human civilisation, the cyborg microrobot is rising by taking advantage of the high flexibility, outstanding energy efficiency, extremely exquisite structure in the natural components and the fine upgradability, nice controllability in the artefact parts. Compared to the purely synthetic microrobots, the cyborg microrobots, due to the exceptional biocompatibility and biodegradability, have already been utilised in in situ diagnosis, precise therapy and other biomedical applications. In this review, through a thorough summary of recent advances of cyborg microrobots, the authors categorise the cyborg microrobots into four major classes according to the configuration between biomaterials and artefact materials, i.e. microrobots integrated inside living cell, microrobots modified with biological debris, microrobots integrated with single cell and microrobots incorporated with multiple cells. Cyborg microrobots with the four types of configurations are introduced and summarised with the combination approaches, actuation mechanisms, applications and challenges one by one. Moreover, they conduct a comparison among the four different cyborg microrobots to guide the actuation force promotion, locomotion control refinement and future applications. Finally, conclusions and future outlook of the development and potential applications of the cyborg microrobots are discussed.

Dynamics near planar walls for various model self-phoretic particles

For chemically active particles suspended in a liquid solution and moving by self-phoresis, the dynamics near chemically inert, planar walls is studied theoretically by employing various choices for the activity function, i.e., the spatial distribution of the sites where various chemical reactions take place. We focus on the case of solutions composed of electrically neutral species. This analysis extends previous studies of the case that the chemical activity can be modeled effectively as the release of a "product" molecular species from parts of the surface of the particle by accounting for annihilation of the product molecules by chemical reactions, either on the rest of the surface of the particle or in the volume of the surrounding solution. We show that, for the models considered here, the emergence of "sliding" and "hovering" wall-bound states is a generic, robust feature. However, the details of these states, such as the range of parameters within which they occur, depend on the specific model for the activity function. Additionally, in certain cases there is a reversal of the direction of the motion compared to the one observed if the particle is far away from the wall. We have also studied the changes of the dynamics induced by a direct interaction between the particle and the wall by including a short-ranged repulsive component to the interaction in addition to the steric one (a procedure often employed in numerical simulations of active colloids). Upon increasing the strength of this additional component, while keeping its range fixed, significant qualitative changes occur in the phase portraits of the dynamics near the wall: for sufficiently strong short-ranged repulsion, the sliding steady states of the dynamics are transformed into hovering states. Furthermore, our studies provide evidence for an additional "oscillatory" wall-bound steady state of motion for chemically active particles due to a strong, short-ranged, and direct repulsion. This kind of particle translates along the wall at a distance from it which oscillates between a minimum and a maximum.

A stochastic model for bacteria-driven micro-swimmers

Experiments have recently shown the feasibility of utilising bacteria as micro-scale robotic devices, with special attention paid to the development of bacteria-driven micro-swimmers taking advantage of built-in actuation and sensing mechanisms of cells. Here we propose a stochastic fluid dynamic model to describe analytically and computationally the dynamics of microscopic particles driven by the motion of surface-attached bacteria undergoing run-and-tumble motion. We compute analytical expressions for the rotational diffusion coefficient, the swimming speed and the effective diffusion coefficient. At short times, the mean squared displacement (MSD) is proportional to the square of the swimming speed, which is independent of the particle size (for fixed density of attached bacteria) and scales linearly with the number of attached bacteria; in contrast, at long times the MSD scales quadratically with the size of the swimmer and is independent of the number of bacteria. We then extend our result to the situation where the surface-attached bacteria undergo chemotaxis within the linear response regime. We demonstrate that bacteria-driven particles are capable of performing artificial chemotaxis, with a chemotactic drift velocity linear in the chemical concentration gradient and independent of the size of the particle. Our results are validated against numerical simulations in the Brownian dynamics limit and will be relevant to the optimal design of micro-swimmers for biomedical applications.

Cell-Like Micromotors

In the past decade, versatile micro- and nanosized machines have emerged as active agents for large-scale detoxification, sensing, microfabrication, and many other promising applications. Micromachines have also been envisioned as the next advancement in dynamic therapy with numerous proof-of-concept studies in drug delivery, microsurgery, and detoxification. However, the practical use of synthetic micromotors in the body requires the development of fully biocompatible designs facilitating micromotor movement in biological fluids of diverse composition and displaying desired functions in specific locations. The combination of the efficient movement of synthetic micromotors with the biological functions of natural cells has resulted in cell-like micromotors with expanded therapeutic and toxin-removing capabilities toward different biological applications. Thus, these biocompatible and biomimetic cell-like micromotors can provide efficient movement in complex biofluids and mimic the functionalities of natural cells. This Account highlights a variety of recent proof-of-concept examples of cell-like micromotors, based on different designs and actuation mechanisms, which perform diverse in vivo tasks. The cell-like micromotors are divided into two groups: (i) cell membrane-coated micromotors, which use natural cell membranes derived from red blood cells, platelets, or a combination of different cells to cloak and functionalize synthetic motors, and (ii) cell-based micromotors, which directly use entire cells such as blood cells, spermatozoa, and bacteria as the micromotor engine. Cell-like micromotors, composed of different cellular components and actuated by different mechanisms, have shown unique advantages for operation in complex biofluids such as blood. Due to the inherent biocompatibility of cell-derived materials, these cell-like micromotors do not provoke an immune response while utilizing useful secondary functions of the blood cells such as strong ability to soak up foreign agents or bind toxins. Additionally, the utilization of autonomously motile cells (e.g., bacteria) allows for built-in chemotactic motion, which eliminates the need for harmful fuels or complex actuation equipment. Furthermore, a broad range of cells, both passive and motile, can be incorporated into micromachine designs constituting a large library of functional components depending on the limits of the desired application. The coupling of cellular and artificial components has led to active biohybrid swimming microsystems with greatly enhanced capabilities and functionalities compared to the individual biological or synthetic components. These characteristics have positioned these cell-like micromotors as promising biomimetic dynamic tools for potential actuation in vivo. Finally, the key challenges and limitations of cell-like micromotors are discussed in the context of expanded future clinical uses and translation to human trials.

Cargo carrying bacteria at interfaces

The displacements of ensembles of colloids at the interface between oil and suspensions of the bacterium Pseudomonas aeruginosa PA14ΔpelA indicate enhanced colloid mobilities and apparently diffusive motion driven by interactions with the bacteria. However, inspection of individual trajectories of ∼500 particles reveals prolonged, directed displacements inconsistent with purely hydrodynamic interactions between swimming bacteria and colloids. Analysis of the properties of colloid paths indicates trajectories can be sorted into four distinct categories, including diffusive, persistent, curly, and mixed trajectory types. Non-diffusive trajectories are the norm, comprising 2/3 of the observed trajectories. Imaging of colloids in the interface reveals anisotropic assemblies formed by colloids decorated with one or more adhered bacteria that drive the colloids along these paths. The trajectories and enhanced transport result from individual colloids being moved as cargo by these adhered bacteria. The implications of these structures and open questions for interfacial transport are discussed and related to the active colloid literature.

Artificial Magnetotaxis of Microbot: Magnetophoresis versus Self-Swimming

An artificial magnetotactic microbot was created by integrating the microalgal cell with magnetic microbead for its potential application as biomotor in microscale environment. Here, we demonstrate the remote magnetotactic control of the microbot under a low gradient magnetic field (<100 T/m). We characterize the kinematic behavior of the microbots carrying magnetic microbeads of two different sizes, with diameter of 2 and 4.5 μm, in the absence and presence of magnetic field. In the absence of magnetic field, we observed the microbot showed a helical motion as a result of the misalignment between the thrust force and the symmetry axis after the attachment. The microbot bound with a larger magnetic microbead moved with higher translational velocity but rotated slower about its axis of rotation. The viscous force was balanced by the thrust force of the microbot, resulting in a randomized swimming behavior of the microbot at its terminal velocity. Meanwhile, under the influence of a low gradient magnetic field, we demonstrated that the directional control of the microbot was based on following principles: (1) magnetophoretic force was insignificant on influencing its perpendicular motion and (2) its parallel motion was dependent on both self-swimming and magnetophoresis, in which this cooperative effect was a function of separation distance from the magnet. As the microbot approached the magnet, the magnetophoretic force suppressed its self-swimming behavior, leading to a positive magnetotaxis of the microbot toward the source of magnetic field. Our experimental results and kinematic analysis revealed the contribution of mass density variation of particle-and-cell system on influencing its dynamical behavior.

Motility and chemotaxis of bacteria-driven microswimmers fabricated using antigen 43-mediated biotin display

Bacteria-driven biohybrid microswimmers (bacteriabots) combine synthetic cargo with motile living bacteria that enable propulsion and steering. Although fabrication and potential use of such bacteriabots have attracted much attention, existing methods of fabrication require an extensive sample preparation that can drastically decrease the viability and motility of bacteria. Moreover, chemotactic behavior of bacteriabots in a liquid medium with chemical gradients has remained largely unclear. To overcome these shortcomings, we designed Escherichia coli to autonomously display biotin on its cell surface via the engineered autotransporter antigen 43 and thus to bind streptavidin-coated cargo. We show that the cargo attachment to these bacteria is greatly enhanced by motility and occurs predominantly at the cell poles, which is greatly beneficial for the fabrication of motile bacteriabots. We further performed a systemic study to understand and optimize the ability of these bacteriabots to follow chemical gradients. We demonstrate that the chemotaxis of bacteriabots is primarily limited by the cargo-dependent reduction of swimming speed and show that the fabrication of bacteriabots using elongated E. coli cells can be used to overcome this limitation.

Micro- and nano-motors: the new generation of drug carriers

Micro- and nano-motors are emerging as novel drug delivery platforms, offering advantages such as rapid drug transport, high tissue penetration and motion controllability. They can be propelled and/or guided by endogenous (i.e., chemotaxis) or exogenous stimuli (e.g., ultrasound, magnetic fields, light) toward the area of interest. Moreover, such stimuli can be used to trigger the release of a therapeutic payload when the motor reaches certain location in order to improve the drug targeting. In this review article, we highlight medically oriented micro-/nano-motors, in particular the ones created for targeted drug delivery, and discuss their current limitations and possibilities toward in vivo applications.

Sperm-Hybrid Micromotor for Targeted Drug Delivery

A sperm-driven micromotor is presented as a targeted drug delivery system, which is appealing to potentially treat diseases in the female reproductive tract. This system is demonstrated to be an efficient drug delivery vehicle by first loading a motile sperm cell with an anticancer drug (doxorubicin hydrochloride), guiding it magnetically, to an in vitro cultured tumor spheroid, and finally freeing the sperm cell to deliver the drug locally. The sperm release mechanism is designed to liberate the sperm when the biohybrid micromotor hits the tumor walls, allowing it to swim into the tumor and deliver the drug through the sperm-cancer cell membrane fusion. In our experiments, the sperm cells exhibited a high drug encapsulation capability and drug carrying stability, conveniently minimizing  toxic side effects and unwanted drug accumulation in healthy tissues. Overall, sperm cells are excellent candidates to operate in physiological environments, as they neither express pathogenic proteins nor proliferate to form undesirable colonies, unlike other cells or microorganisms. This sperm-hybrid micromotor is a biocompatible platform with potential application in gynecological healthcare, treating or detecting cancer or other diseases in the female reproductive system.

Biohybrid actuators for robotics: A review of devices actuated by living cells

Actuation is essential for artificial machines to interact with their surrounding environment and to accomplish the functions for which they are designed. Over the past few decades, there has been considerable progress in developing new actuation technologies. However, controlled motion still represents a considerable bottleneck for many applications and hampers the development of advanced robots, especially at small length scales. Nature has solved this problem using molecular motors that, through living cells, are assembled into multiscale ensembles with integrated control systems. These systems can scale force production from piconewtons up to kilonewtons. By leveraging the performance of living cells and tissues and directly interfacing them with artificial components, it should be possible to exploit the intricacy and metabolic efficiency of biological actuation within artificial machines. We provide a survey of important advances in this biohybrid actuation paradigm.

Multifunctional Bacteria-Driven Microswimmers for Targeted Active Drug Delivery

High-performance, multifunctional bacteria-driven microswimmers are introduced using an optimized design and fabrication method for targeted drug delivery applications. These microswimmers are made of mostly single Escherichia coli bacterium attached to the surface of drug-loaded polyelectrolyte multilayer (PEM) microparticles with embedded magnetic nanoparticles. The PEM drug carriers are 1 μm in diameter and are intentionally fabricated with a more viscoelastic material than the particles previously studied in the literature. The resulting stochastic microswimmers are able to swim at mean speeds of up to 22.5 μm/s. They can be guided and targeted to specific cells, because they exhibit biased and directional motion under a chemoattractant gradient and a magnetic field, respectively. Moreover, we demonstrate the microswimmers delivering doxorubicin anticancer drug molecules, encapsulated in the polyelectrolyte multilayers, to 4T1 breast cancer cells under magnetic guidance in vitro. The results reveal the feasibility of using these active multifunctional bacteria-driven microswimmers to perform targeted drug delivery with significantly enhanced drug transfer, when compared with the passive PEM microparticles.

Propulsion and Chemotaxis in Bacteria-Driven Microswimmers

Despite the large body of experimental work recently on biohybrid microsystems, few studies have focused on theoretical modeling of such systems, which is essential to understand their underlying functioning mechanisms and hence design them optimally for a given application task. Therefore, this study focuses on developing a mathematical model to describe the 3D motion and chemotaxis of a type of widely studied biohybrid microswimmer, where spherical microbeads are driven by multiple attached bacteria. The model is developed based on the biophysical observations of the experimental system and is validated by comparing the model simulation with experimental 3D swimming trajectories and other motility characteristics, including mean squared displacement, speed, diffusivity, and turn angle. The chemotaxis modeling results of the microswimmers also agree well with the experiments, where a collective chemotactic behavior among multiple bacteria is observed. The simulation result implies that such collective chemotaxis behavior is due to a synchronized signaling pathway across the bacteria attached to the same microswimmer. Furthermore, the dependencies of the motility and chemotaxis of the microswimmers on certain system parameters, such as the chemoattractant concentration gradient, swimmer body size, and number of attached bacteria, toward an optimized design of such biohybrid system are studied. The optimized microswimmers would be used in targeted cargo, e.g., drug, imaging agent, gene, and RNA, transport and delivery inside the stagnant or low-velocity fluids of the human body as one of their potential biomedical applications.

Cooperative Micromanipulation Using the Independent Actuation of Fifty Microrobots in Parallel

Micromanipulation for applications in areas such as tissue engineering can require mesoscale structures to be assembled with microscale resolution. One method for achieving such manipulation is the parallel actuation of many microrobots in parallel. However, current microrobot systems lack the independent actuation of many entities in parallel. Here, the independent actuation of fifty opto-thermocapillary flow-addressed bubble (OFB) microrobots in parallel is demonstrated. Individual microrobots and groups of microrobots were moved along linear, circular, and arbitrary 2D trajectories. The independent addressing of many microrobots enables higher-throughput microassembly of micro-objects, and cooperative manipulation using multiple microrobots. Demonstrations of manipulation with multiple OFB microrobots include the transportation of microstructures using a pair or team of microrobots, and the cooperative manipulation of multiple micro-objects. The results presented here represent an order of magnitude increase in the number of independently actuated microrobots in parallel as compared to other magnetically or electrostatically actuated microrobots, and a factor of two increase as compared to previous demonstrations of OFB microrobots.

Spermatozoa as Functional Components of Robotic Microswimmers

In recent years, the combination of synthetic micro- and nanomaterials with spermatozoa as functional components has led to the development of tubular and helical spermbots - microrobotic devices with potential applications in the biomedical and nanotechnological field. Here, the initial advances in this field are discussed and the use of spermatozoa as functional parts in microdevices elaborated. Besides the potential uses of these hybrid robotic microswimmers, the obstacles along the way are discussed, with suggestions for solutions of the encountered challenges also given.

Mobile microrobots for bioengineering applications

Untethered micron-scale mobile robots can navigate and non-invasively perform specific tasks inside unprecedented and hard-to-reach inner human body sites and inside enclosed organ-on-a-chip microfluidic devices with live cells. They are aimed to operate robustly and safely in complex physiological environments where they will have a transforming impact in bioengineering and healthcare. Research along this line has already demonstrated significant progress, increasing attention, and high promise over the past several years. The first-generation microrobots, which could deliver therapeutics and other cargo to targeted specific body sites, have just been started to be tested inside small animals toward clinical use. Here, we review frontline advances in design, fabrication, and testing of untethered mobile microrobots for bioengineering applications. We convey the most impactful and recent strategies in actuation, mobility, sensing, and other functional capabilities of mobile microrobots, and discuss their potential advantages and drawbacks to operate inside complex, enclosed and physiologically relevant environments. We lastly draw an outlook to provide directions in the veins of more sophisticated designs and applications, considering biodegradability, immunogenicity, mobility, sensing, and possible medical interventions in complex microenvironments.