Stabilization of CsPbBr3 Nanowires Through SU-8 Encapsulation for the Fabrication of Bilayer Microswimmers with Magnetic and Fluorescence Properties.

All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals have drawn great interest because of their excellent photophysical properties and potential applications. However, their poor stability in water greatly limited their use in applications that require stable structures. In this work, a facile approach to stabilize CsPbBr3 nanowires is developed by using SU-8 as a protection medium; thereby creating stable CsPbBr3/SU-8 microstructures. Through photolithography and layer-by-layer deposition, CsPbBr3/SU-8 is used to fabricate bilayer achiral microswimmers (BAMs), which consist of a top CsPbBr3/SU-8 layer and a bottom Fe3O4 magnetic layer. Compared to pure CsPbBr3 nanowires, the CsPbBr3/SU-8 shows long-term structural and fluorescence stability in water against ultrasonication treatment. Due to the magnetic layer, the motion of the microswimmers can be controlled precisely under a rotating magnetic field, allowing them to swim at low Reynolds number and tumble or roll on surfaces. Furthermore, CsPbBr3/SU-8 can be used to fabricate various types of planar microstructures with high throughput, high consistency, and fluorescence properties. This work provides a method for the stabilization of CsPbBr3 and demonstrates the potential to mass fabricate planar microstructures with various shapes, which can be used in different applications such as microrobotics.

Propulsion mechanism of artificial flagellated micro-swimmers actuated by acoustic waves - theory and experimental verification.

This work examines the acoustically actuated motions of artificial flagellated micro-swimmers (AFMSs) and compares the motility of these micro-swimmers with the predictions based on the corrected resistive force theory (CRFT) and the bar-joint model proposed in our previous work. The key ingredient in the theory is the introduction of a correction factorKin drag coefficients to correct the conventional resistive force theory so that the dynamics of an acoustically actuated AFMS with rectangular cross-sections can be accurately modeled. Experimentally, such AFMSs can be easily manufactured based on digital light processing (DLP) of UV-curable resins. We first determined the viscoelastic properties of a UV-cured resin through dynamic mechanical analysis (DMA). In particular, the high-frequency storage moduli and loss factors were obtained based on the assumption of time-temperature superposition (TTS), which were then applied in theoretical calculations. Though the extrapolation based on the TTS implied the uncertainty of high-frequency material response and there is limited accuracy in determining head oscillation amplitude, the differences between the measured terminal velocities of the AFMSs and the predicted ones are less than 50%, which, to us, is well acceptable. These results indicate that the motions of acoustic AFMS can be predicted, and thus, designed, which pave the way for their long-awaited applications in targeted therapy.

Synthesis of Multi-Functional Graphene Monolayers via Bipolar Electrochemistry.

Graphene has gained substantial research interest in many fields due to its remarkable properties among many other two-dimensional materials. In this study, we propose a wireless electrochemical approach, bipolar electrochemistry, for the precise modification of single layers of graphene at predefined locations, such as distinct edges or corners, with a variety of metals or polymers, thus enabling the elaboration of multi-functional monolayer graphene sheets. We illustrate the concept e. g. by depositing multiple metals, or platinum and a catalyst-containing porous polymer on the same graphene sheet, but at separate corners. This configuration allows activating chemiluminescence on the polymer spot, and simultaneously generates the driving force for autonomous motion on the Pt side through the catalytic decomposition of hydrogen peroxide into oxygen bubbles. This integration of different chemical features on the same object, exemplified by these proof-of-principle experiments, enhances the functionality of two-dimensional materials, paving the way for the use of these hybrid materials for a variety of applications, ranging from sensing and catalysis to targeted delivery.

Integrated Microfluidic-Electromagnetic System to Probe Single-Cell Magnetotaxis in Microconfinement.

Magnetotactic bacteria have great potential for use in biomedical and environmental applications due to the ability to direct their navigation with a magnetic field. Applying and accurately controlling a magnetic field within a microscopic region during bacterial magnetotaxis studies at the single-cell level is challenging due to bulky microscope components and the inherent curvilinear field lines produced by commonly used bar magnets. In this paper, a system that integrates microfluidics and electromagnetic coils is presented for generating a linear magnetic field within a microenvironment compatible with microfluidics, enabling magnetotaxis analysis of groups or single microorganisms on-chip. The platform, designed and optimised via finite element analysis, is integrated into an inverted fluorescent microscope, enabling visualisation of bacteria at the single-cell level in microfluidic devices. The electromagnetic coils produce a linear magnetic field throughout a central volume where the microfluidic device containing the magnetotactic bacteria is located. The magnetic field, at this central position, can be accurately controlled from 1 to 10 mT, which is suitable for directing the navigation of magnetotactic bacteria. Potential heating of the microfluidic device from the operating coils was evaluated up to 2.5 A, corresponding to a magnetic field of 7.8 mT, for 10 min. The maximum measured heating was 8.4 °C, which enables analysis without altering the magnetotaxis behaviour or the average swimming speed of the bacteria. Altogether, this work provides a design, characterisation and experimental test of an integrated platform that enables the study of individual bacteria confined in microfluidics, under linear and predictable magnetic fields that can be easily and accurately applied and controlled.

Writing Into Water.

Writing is an ancient communication technique dating back at least 30 000 years. While even sophisticated contemporary writing techniques hinge on solid surfaces for engraving or the deposition of ink, writing within a liquid medium requires a fundamentally different approach. The study here demonstrates the writing of lines, letters, and complex patterns in water by assembling lines of colloidal particles. Unlike established techniques for underwater writing on solid substrates, these lines are fully reconfigurable and do not require any fixation onto the substrate. Exploiting gravity, an ion-exchange bead (pen) is rolled across a layer of sedimented colloidal particles (ink). The pen evokes a hydrodynamic flow collecting ink-particles into a durable, high-contrast line along its trajectory. Deliberate substrate-tilting sequences facilitate pen-steering and thus drawing and writing. The experiments are complemented with a minimal model that quantitatively predicts the observed parameter dependence for writing in fluids and highlights the generic character of writing by line-assembly. Overall, the approach opens a versatile route for writing, drawing, and patterning fluids-even at the micro-scale.

Spatial Distribution of Flagellated Microalgae Chlamydomonas reinhardtii in a Quasi-Two-Dimensional Space.

Although the phenomenon of collective order formation by cell-cell interactions in motile cells, microswimmers, has been a topic of interest, most studies have been conducted under conditions of high cell density, where the space occupancy of a cell population relative to the space size ϕ>0.1 (ϕ is the area fraction). We experimentally determined the spatial distribution (SD) of the flagellated unicellular green alga Chlamydomonas reinhardtii at a low cell density (ϕ≈0.01) in a quasi-two-dimensional (thickness equal to cell diameter) restricted space and used the variance-to-mean ratio to investigate the deviation from the random distribution of cells, that is, do cells tend to cluster together or avoid each other? The experimental SD is consistent with that obtained by Monte Carlo simulation, in which only the excluded volume effect (EV effect) due to the finite size of cells is taken into account, indicating that there is no interaction between cells other than the EV effect at a low cell density of ϕ≈0.01. A simple method for fabricating a quasi-two-dimensional space using shim rings was also proposed.

Interaction of the mechanosensitive microswimmer Paramecium with obstacles.

In this work, we report investigations of the swimming behaviour of Paramecium tetraurelia, a unicellular microorganism, in micro-engineered pools that are decorated with thousands of cylindrical pillars. Two types of contact interactions are measured, either passive scattering of Paramecium along the obstacle or avoiding reactions (ARs), characterized by an initial backward swimming upon contact, followed by a reorientation before resuming forward motion. We find that ARs are only mechanically triggered approximately 10% of the time. In addition, we observe that only a third of all ARs triggered by contact are instantaneous while two-thirds are delayed by approximately 150 ms. These measurements are consistent with a simple electrophysiological model of mechanotransduction composed of a strong transient current followed by a persistent one upon prolonged contact. This is in apparent contrast with previous electrophysiological measurements where immobilized cells were stimulated with thin probes, which showed instantaneous behavioural responses and no persistent current. Our findings highlight the importance of ecologically relevant approaches to unravel the motility of mechanosensitive microorganisms in complex environments.

Prussian blue composite microswimmer based on alginate-chitosan for biofilm removal.

Bacterial infections pose a serious threat to public health, causing worldwide morbidity and about 80 % of bacterial infections are related to biofilm. Removing biofilm without antibiotics remains an interdisciplinary challenge. To solve this problem, we presented a dual-power driven antibiofilm system Prussian blue composite microswimmers based on alginate-chitosan, which designed into an asymmetric structure to achieve self-driven in the fuel solution and magnetic field. Prussian blue embedded in the microswimmers given it the ability to convert light and heat, catalyze Fenton reaction, and produce bubbles and reactive oxygen species. Moreover, with the addition of Fe3O4, the microswimmers could move in group under external magnetic field. The composite microswimmers displayed excellent antibacterial activity against S. aureus biofilm with an efficiency as high as 86.94 %. It is worth mentioning that the microswimmers were fabricated with device-simple and low-cost gas-shearing method. This system integrating physical destruction, chemical damage such chemodynamic therapy and photothermal therapy, and finally kill the plankton bacteria embedded in biofilm. This approach may cause an autonomous, multifunctional antibiofilm platform to promote the present most areas with harmful biofilm difficult to locate the surface for removal.

Typing of cancer cells by microswimmer based on Co-Fe-MOF for one-step simultaneously detect multiple biomarkers.

Capturing, identifying, and counting CTCs cancer cells that have escaped from the tumor and wandered into the bloodstream is a major challenge. We proposed a noval microswimmer dual-mode aptamer (electrochemical and fluorescent)-(Mapt-EF) homogeneous sensor with active capture/controlled release double signaling molecule/separation and release cell based on the Co-Fe-MOF nanomaterial for simultaneous one-step detection of multiple biomarkers protein tyrosine kinase-7 (PTK7), Epithelial cell adhesion molecule (EpCAM), and mucin-1 (MUC1) for diagnosis of multiple cancer cell types. The Co-Fe-MOF is a nano-enzyme capable of catalyzing the decomposition of hydrogen peroxide to release bubbles of oxygen, producing a driving force to conduct hydrogen peroxide through the liquid, and has the capacity to self-decompose during the catalytic process. Phosphoric acid is present in the aptamer chains of PTK7, EpCAM, and MUC1, and the aptamer chains are adsorbed to the surface of the Mapt-EF homogeneous sensor in the form of a gated switch to inhibit the catalytic decomposition activity of hydrogen peroxide. The Mapt-EF homogeneous sensor has the capability to actively target biomarkers that can be entrained by oxygen bubbles without being degraded. The detection time of the sensor was 20 min, the detection limits were 9.6 fg/mL, 8.4 fg/mL and 7.7 fg/mL with the linear range was 0-20 pg/mL, respectively. The Mapt-EF homogeneous sensor has high detection sensitivity, and its detection limit can reach the level of single cell at the lowest. The Mapt-EF homogeneous sensor has great application potential in clinical detection and analysis of tumor cells.

A bar-joint model based on the corrected resistive force theory for artificial flagellated micro-swimmers propelled by acoustic waves.

In this work, we proposed a bar-joint model based on the corrected resistive force theory (CRFT) for studying artificial flagellated micro-swimmers (AFMSs) propelled by acoustic waves in a two-dimensional (2D) flow field or with a rectangular cross-section. Note that the classical resistive-force theory for 3D cylindrical flagellum leads to over 90% deviation in terminal velocity from those of 2D fluid-structure interaction (FSI) simulations, while the proposed CRFT bar-joint model can reduce the deviation to below 5%; hence, it enables a reliable prediction of the 2D locomotion of an acoustically actuated AFMS with a rectangular cross-section, which is the case in some experiments. Introduced in the CRFT is a single correction factorKdetermined by comparing the linear terminal velocities under acoustic actuation obtained from the CRFT with those from simulations. After the determination ofK, detailed comparisons of trajectories between the CRFT-based bar-joint AFMS model and the FSI simulation were presented, exhibiting an excellent consistency. Finally, a numerical demonstration of the purely acoustic or magneto-acoustic steering of an AFMS based on the CRFT was presented, which can be one of the choices for future AFMS-based precision therapy.

Phenotyping single-cell motility in microfluidic confinement.

The movement trajectories of organisms serve as dynamic read-outs of their behaviour and physiology. For microorganisms this can be difficult to resolve due to their small size and fast movement. Here, we devise a novel droplet microfluidics assay to encapsulate single micron-sized algae inside closed arenas, enabling ultralong high-speed tracking of the same cell. Comparing two model species - Chlamydomonas reinhardtii (freshwater, 2 cilia), and Pyramimonas octopus (marine, 8 cilia), we detail their highly-stereotyped yet contrasting swimming behaviours and environmental interactions. By measuring the rates and probabilities with which cells transition between a trio of motility states (smooth-forward swimming, quiescence, tumbling or excitable backward swimming), we reconstruct the control network that underlies this gait switching dynamics. A simplified model of cell-roaming in circular confinement reproduces the observed long-term behaviours and spatial fluxes, including novel boundary circulation behaviour. Finally, we establish an assay in which pairs of droplets are fused on demand, one containing a trapped cell with another containing a chemical that perturbs cellular excitability, to reveal how aneural microorganisms adapt their locomotor patterns in real-time.

Improving Swimming Performance of Photolithography-Based Microswimmers Using Curvature Structures.

The emergence of robotic microswimmers and their huge potential in biomedical applications such as drug delivery, non-invasive surgery, and bio-sensing facilitates studies to improve their effectiveness. Recently, achiral microswimmers that have neither flexible nor helical structures have garnered attention because of their simple structures and fabrication process while preserving adequate swimming velocity and controllability. In this paper, the crescent shape was utilized to create photolithography-fabricated crescent-shaped achiral microswimmers. The microswimmers were actuated using rotating magnetic fields at low Reynolds numbers. Compared with the previously reported achiral microswimmers, the crescent-shaped microswimmers showed significant improvement in forward swimming speed. The effects of different curvatures, arm angles, and procession angles on the velocities of microswimmers were investigated. Moreover, the optimal swimming motion was defined by adjusting the field strength of the magnetic field. Finally, the effect of the thickness of the microswimmers on their swimming velocity was investigated.

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.

Cooperative Self-Assembled Magnetic Micropaddles at Liquid Surfaces.

Cooperative controls of magnetic microswimmers are desired for complex micromanipulation and microassembly tasks. Self-assembled magnetic micropaddles as microswimmers that can locomote freely and cooperate at liquid surfaces are proposed inspired by the paddling motion. The micropaddles are self-assembled with metallic disks under a rotating magnetic field, and they are endowed with controlled propulsion in the precessing field. The micropaddles can locomote freely with a maximum speed of approximately 3.3 mm/s and manipulate objects at the liquid surface. It is found that the micropaddles reverse moving directions at high frequencies and that those with different lengths can locomote in opposite directions under the same precessing magnetic field. Based on the distinctive motion properties, not only could several micropaddles combine into the longer ones but a single micropaddle could also be disassembled into two cooperative partners. Assemblies of different parts based on their cooperation are realized in this study, which is challenging for other types of magnetic microswimmers. Micropaddles with adjustable length, flexible locomotion, and cooperative capability present a promising avenue for various micromanipulation applications.

Control and controllability of microswimmers by a shearing flow.

With the continuing rapid development of artificial microrobots and active particles, questions of microswimmer guidance and control are becoming ever more relevant and prevalent. In both the applications and theoretical study of such microscale swimmers, control is often mediated by an engineered property of the swimmer, such as in the case of magnetically propelled microrobots. In this work, we will consider a modality of control that is applicable in more generality, effecting guidance via modulation of a background fluid flow. Here, considering a model swimmer in a commonplace flow and simple geometry, we analyse and subsequently establish the efficacy of flow-mediated microswimmer positional control, later touching upon a question of optimal control. Moving beyond idealized notions of controllability and towards considerations of practical utility, we then evaluate the robustness of this control modality to sources of variation that may be present in applications, examining in particular the effects of measurement inaccuracy and rotational noise. This exploration gives rise to a number of cautionary observations, which, overall, demonstrate the need for the careful assessment of both policy and behavioural robustness when designing control schemes for use in practice.

Geometric Methods for Efficient Planar Swimming of Copepod Nauplii.

Copepod nauplii are larval crustaceans with important ecological functions. Due to their small size, they experience an environment of low Reynolds number within their aquatic habitat. Here we provide a mathematical model of a swimming copepod nauplius with two legs moving in a plane. This model allows for both rotation and two-dimensional displacement by the periodic deformation of the swimmer's body. The system is studied from the framework of optimal control theory, with a simple cost function designed to approximate the mechanical energy expended by the copepod. We find that this model is sufficiently realistic to recreate behavior similar to those of observed copepod nauplii, yet much of the mathematical analysis is tractable. In particular, we show that the system is controllable, but there exist singular configurations where the degree of non-holonomy is non-generic. We also partially characterize the abnormal extremals and provide explicit examples of families of abnormal curves. Finally, we numerically simulate normal extremals and observe some interesting and surprising phenomena.

Circular swimming motility and disordered hyperuniform state in an algae system.

Active matter comprises individually driven units that convert locally stored energy into mechanical motion. Interactions between driven units lead to a variety of nonequilibrium collective phenomena in active matter. One of such phenomena is anomalously large density fluctuations, which have been observed in both experiments and theories. Here we show that, on the contrary, density fluctuations in active matter can also be greatly suppressed. Our experiments are carried out with marine algae ([Formula: see text]), which swim in circles at the air-liquid interfaces with two different eukaryotic flagella. Cell swimming generates fluid flow that leads to effective repulsions between cells in the far field. The long-range nature of such repulsive interactions suppresses density fluctuations and generates disordered hyperuniform states under a wide range of density conditions. Emergence of hyperuniformity and associated scaling exponent are quantitatively reproduced in a numerical model whose main ingredients are effective hydrodynamic interactions and uncorrelated random cell motion. Our results demonstrate the existence of disordered hyperuniform states in active matter and suggest the possibility of using hydrodynamic flow for self-assembly in active matter.

A Bayesian Framework to Estimate Fluid and Material Parameters in Micro-swimmer Models.

To advance our understanding of the movement of elastic microstructures in a viscous fluid, techniques that utilize available data to estimate model parameters are necessary. Here, we describe a Bayesian uncertainty quantification framework that is highly parallelizable, making parameter estimation tractable for complex fluid-structure interaction models. Using noisy in silico data for swimmers, we demonstrate the methodology's robustness in estimating fluid and elastic swimmer parameters, along with their uncertainties. We identify correlations between model parameters and gain insight into emergent swimming trajectories of a single swimmer or a pair of swimmers. Our proposed framework can handle data with a spatiotemporal resolution representative of experiments, showing that this framework can be used to aid in the development of artificial micro-swimmers for biomedical applications, as well as gain a fundamental understanding of the range of parameters that allow for certain motility patterns.

Biomimetic soft micro-swimmers: from actuation mechanisms to applications.

Underwater robot designs inspired by the behavior and morphological characteristics of aquatic animals can provide reinforced mobility and energy efficiency. In the past two decades, the emerging materials science and integrated circuit technology have been combined and applied to various types of bionic soft underwater miniaturized robots by researchers around the world. Further, the potential applications of biomimetic soft micro-swimmers in the biological and medical fields have been explored. Here, this paper reviews the development of biomimetic soft tiny swimmers, which are designed based on a variety of intelligent materials and control strategies. This review focuses on the various actuation mechanisms of soft tiny swimmers reported in the past two decades and classifies these robots into four categories: fish-like, snake-like, jellyfish-like and microbial-inspired ones. Besides, this review considers the practical challenges faced by actuation mechanisms of each type of robot, and summarizes and prospects how these challenges affect the potential applications of robots in real environments.

Recent Advances in Microswimmers for Biomedical Applications.

Microswimmers are a rapidly developing research area attracting enormous attention because of their many potential applications with high societal value. A particularly promising target for cleverly engineered microswimmers is the field of biomedical applications, where many interesting examples have already been reported for e.g., cargo transport and drug delivery, artificial insemination, sensing, indirect manipulation of cells and other microscopic objects, imaging, and microsurgery. Pioneered only two decades ago, research studies on the use of microswimmers in biomedical applications are currently progressing at an incredibly fast pace. Given the recent nature of the research, there are currently no clinically approved microswimmer uses, and it is likely that several years will yet pass before any clinical uses can become a reality. Nevertheless, current research is laying the foundation for clinical translation, as more and more studies explore various strategies for developing biocompatible and biodegradable microswimmers fueled by in vivo-friendly means. The aim of this review is to provide a summary of the reported biomedical applications of microswimmers, with focus on the most recent advances. Finally, the main considerations and challenges for clinical translation and commercialization are discussed.