On-chip Microfluidic Multimodal Swimmer toward 3D Navigation

3D-printed microrobots for biomedical applications

Microrobots, which can perform tasks in difficult-to-reach parts of the human body under their own or external power supply, are potential tools for biomedical applications, such as drug delivery, microsurgery, imaging and monitoring, tissue engineering, and sensors and actuators. Compared with traditional fabrication methods for microrobots, recent improvements in 3D printers enable them to print high-precision microrobots, breaking through the limitations of traditional micromanufacturing technologies that require high skills for operators and greatly shortening the design-to-production cycle. Here, this review first introduces typical 3D printing technologies used in microrobot manufacturing. Then, the structures of microrobots with different functions and application scenarios are discussed. Next, we summarize the materials (body materials, propulsion materials and intelligent materials) used in 3D microrobot manufacturing to complete body construction and realize biomedical applications (e.g., drug delivery, imaging and monitoring). Finally, the challenges and future prospects of 3D printed microrobots in biomedical applications are discussed in terms of materials, manufacturing and advancement.

Assessing the Challenges of Nanotechnology-Driven Targeted Therapies: Development of Magnetically Directed Vectors for Targeted Cancer Therapies and Beyond

Targeted delivery, in which therapeutic agents are preferentially concentrated at the diseased site, has the potential to improve therapeutic outcomes by minimizing off-target interactions in healthy tissue. Both passive and active methods of targeting delivery have been proposed, often with particular emphasis on cancer treatment. Passive methods rely on the overexpression of a biomarker in diseased tissue that can then be used to target the therapy. Active techniques involve physically guiding therapeutic agents toward the target region. Since the motion of magnetic particles can be remotely controlled by external magnetic fields, magnetic technologies have the potential to drive and hold drugs or other cargo at the required therapeutic site, increasing the localized dose while minimizing overall exposure. Directed motion may be generated either by simple magnetic attraction or by causing the particles to perform swimming strokes to produce propulsion. This chapter will compare the different strategies using magnetic nanotechnology to produce directed motion compatible with that required for targeted cargo delivery and magnetically assisted therapies and assess their potential to meet the challenges of operating within the human body.

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.

Artificial Intelligence in Elite Sports—A Narrative Review of Success Stories and Challenges

This paper explores the role of artificial intelligence (AI) in elite sports. We approach the topic from two perspectives. Firstly, we provide a literature based overview of AI success stories in areas other than sports. We identified multiple approaches in the area of Machine Perception, Machine Learning and Modeling, Planning and Optimization as well as Interaction and Intervention, holding a potential for improving training and competition. Secondly, we discover the present status of AI use in elite sports. Therefore, in addition to another literature review, we interviewed leading sports scientist, which are closely connected to the main national service institute for elite sports in their countries. The analysis of this literature review and the interviews show that the most activity is carried out in the methodical categories of signal and image processing. However, projects in the field of modeling & planning have become increasingly popular within the last years. Based on these two perspectives, we extract deficits, issues and opportunities and summarize them in six key challenges faced by the sports analytics community. These challenges include data collection, controllability of an AI by the practitioners and explainability of AI results.

Dynamic tracking of a magnetic micro-roller using ultrasound phase analysis

Microrobots (MRs) have attracted significant interest for their potentialities in diagnosis and non-invasive intervention in hard-to-reach body areas. Fine control of biomedical MRs requires real-time feedback on their position and configuration. Ultrasound (US) imaging stands as a mature and advantageous technology for MRs tracking, but it suffers from disturbances due to low contrast resolution. To overcome these limitations and make US imaging suitable for monitoring and tracking MRs, we propose a US contrast enhancement mechanism for MR visualization in echogenic backgrounds (e.g., tissue). Our technique exploits the specific acoustic phase modulation produced by the MR characteristic motions. By applying this principle, we performed real-time visualization and position tracking of a magnetic MR rolling on a lumen boundary, both in static flow and opposing flow conditions, with an average error of 0.25 body-lengths. Overall, the reported results unveil countless possibilities to exploit the proposed approach as a robust feedback strategy for monitoring and tracking biomedical MRs in-vivo.

Liquid Crystal Elastomer-Based Magnetic Composite Films for Reconfigurable Shape-Morphing Soft Miniature Machines

Stimuli-responsive and active materials promise radical advances for many applications. In particular, soft magnetic materials offer precise, fast, and wireless actuation together with versatile functionality, while liquid crystal elastomers (LCEs) are capable of large reversible and programmable shape-morphing with high work densities in response to various environmental stimuli, e.g., temperature, light, and chemical solutions. Integrating the orthogonal stimuli-responsiveness of these two kinds of active materials could potentially enable new functionalities and future applications. Here, magnetic microparticles (MMPs) are embedded into an LCE film to take the respective advantages of both materials without compromising their independent stimuli-responsiveness. This composite material enables reconfigurable magnetic soft miniature machines that can self-adapt to a changing environment. In particular, a miniature soft robot that can autonomously alter its locomotion mode when it moves from air to hot liquid, a vine-like filament that can sense and twine around a support, and a light-switchable magnetic spring are demonstrated. The integration of LCEs and MMPs into monolithic structures introduces a new dimension in the design of soft machines and thus greatly enhances their use in applications in complex environments, especially for miniature soft robots, which are self-adaptable to environmental changes while being remotely controllable.

3-D swimming microdrone powered by acoustic bubbles

Mobile microrobots that maneuver in liquid environments and navigate inside the human body have drawn a great interest due to their possibility for medical uses serving as an in vivo cargo. For this system, the effective self-propelling method, which should be powered wirelessly and controllable in 3-D space, is of paramount importance. This article describes a bubble-powered swimming microdrone that can navigate in 3-D space in a controlled manner. To enable 3-D propulsion with steering capability, air bubbles of three lengths are trapped in microtubes that are embedded and three-dimensionally aligned inside the drone body using two-photon polymerization. These bubbles can generate on-demand 3-D propulsion through microstreaming when they are selectively excited at their individual resonance frequencies that depend on the bubble sizes. In order to equip the drone with highly stable maneuverability, a non-uniform mass distribution of the drone body is carefully designed to spontaneously restore the drone to the upright position from disturbances. A mathematical model of the restoration mechanism is developed to predict the restoration behavior showing a good agreement with the experimental data. The present swimming microdrone potentially lends itself to a robust 3-D maneuverable microscale mobile cargo navigating in vitro and in vivo for biomedical applications.

Microfluidic devices powered by integrated elasto-magnetic pumps

We show how an asymmetric elasto-magnetic system provides a novel integrated pumping solution for lab-on-a-chip and point of care devices. This monolithic pumping solution, inspired by Purcell's 3-link swimmer, is integrated within a simple microfluidic device, bypassing the requirement of external connections. We experimentally prove that this system can provide tuneable fluid flow with a flow rate of up to 600 μL h-1. This fluid flow is achieved by actuating the pump using a weak, uniform, uniaxial, oscillating magnetic field, with field amplitudes in the range of 3-6 mT. Crucially, the fluid flow can be reversed by adjusting the driving frequency. We experimentally prove that this device can successfully operate on fluids with a range of viscosities, where pumping at higher viscosity correlates with a decreasing optimal driving frequency. The fluid flow produced by this device is understood here by examining the non-reciprocal motion of the elasto-magnetic component. This device has the capability to replace external pumping systems with a simple, integrated, lab-on-a-chip component.

Single-Cell Elasticity Measurement with an Optically Actuated Microrobot

A cell elasticity measurement method is introduced that uses polymer microtools actuated by holographic optical tweezers. The microtools were prepared with two-photon polymerization. Their shape enables the approach of the cells in any lateral direction. In the presented case, endothelial cells grown on vertical polymer walls were probed by the tools in a lateral direction. The use of specially shaped microtools prevents the target cells from photodamage that may arise during optical trapping. The position of the tools was recorded simply with video microscopy and analyzed with image processing methods. We critically compare the resulting Young’s modulus values to those in the literature obtained by other methods. The application of optical tweezers extends the force range available for cell indentations measurements down to the fN regime. Our approach demonstrates a feasible alternative to the usual vertical indentation experiments.

Propulsion and energetics of a minimal magnetic microswimmer

In this manuscript we describe the realization of a minimal hybrid microswimmer, composed of a ferromagnetic nanorod and a paramagnetic microsphere. The unbounded pair is propelled in water upon application of a swinging magnetic field that induces a periodic relative movement of the two composing elements, where the nanorod rotates and slides on the surface of the paramagnetic sphere. When taken together, the processes of rotation and sliding describe a finite area in the parameter space, which increases with the frequency of the applied field. We develop a theoretical approach and combine it with numerical simulations, which allow us to understand the dynamics of the propeller and explain the experimental observations. Furthermore, we demonstrate a reversal of the microswimmer velocity by varying the length of the nanorod, as predicted by the model. Finally, we determine theoretically and in experiments the Lighthill's energetic efficiency of this minimal magnetic microswimmer.

Magnetically Powered Biodegradable Microswimmers

The propulsive efficiency and biodegradability of wireless microrobots play a significant role in facilitating promising biomedical applications. Mimicking biological matters is a promising way to improve the performance of microrobots. Among diverse locomotion strategies, undulatory propulsion shows remarkable efficiency and agility. This work proposes a novel magnetically powered and hydrogel-based biodegradable microswimmer. The microswimmer is fabricated integrally by 3D laser lithography based on two-photon polymerization from a biodegradable material and has a total length of 200 μm and a diameter of 8 μm. The designed microswimmer incorporates a novel design utilizing four rigid segments, each of which is connected to the succeeding segment by spring to achieve undulation, improving structural integrity as well as simplifying the fabrication process. Under an external oscillating magnetic field, the microswimmer with multiple rigid segments connected by flexible spring can achieve undulatory locomotion and move forward along with the directions guided by the external magnetic field in the low Reynolds number (Re) regime. In addition, experiments demonstrated that the microswimmer can be degraded successfully, which allows it to be safely applied in real-time in vivo environments. This design has great potential in future in vivo applications such as precision medicine, drug delivery, and diagnosis.

Local flow sensing on helical microrobots for semi-automatic motion adaptation

Helical microrobots with dimensions below 100 µm could serve many applications for manipulation and sensing in small, closed environments such as blood vessels or inside microfluidic chips. However, environmental conditions such as surface stiction from the channel wall or local flow can quickly result in the loss of control of the microrobot, especially for untrained users. Therefore, to automatically adapt to changing conditions, we propose an algorithm that switches between a surface-based motion of the microrobot and a 3D swimming motion depending on the local flow value. Indeed swimming is better for avoiding obstacles and difficult surface stiction areas but it is more sensitive to the flow than surface motion such as rolling or spintop motion. First, we prove the flow sensing ability of helical microrobots based on the difference between the tracked and theoretical speed. For this, a 50 µm long and 5 µm diameter helical microrobot measures the flow profile shape in two different microchannels. These measurements are then compared with simulation results. Then, we demonstrate both swimming and surface-based motion using closed-loop control. Finally, we test our algorithm by following a 2D path using closed-loop control, and adapting the type of motion depending on the flow speed measured by the microrobot. Such results could enable simple high-level control that could expand the development of microrobots toward applications in complex microfluidic environments.

Magnetically Driven Undulatory Microswimmers Integrating Multiple Rigid Segments

Mimicking biological locomotion strategies offers important possibilities and motivations for robot design and control methods. Among bioinspired microrobots, flexible microrobots exhibit remarkable efficiency and agility. These microrobots traditionally rely on soft material components to achieve undulatory propulsion, which may encounter challenges in design and manufacture including the complex fabrication processes and the interfacing of rigid and soft components. Herein, a bioinspired magnetically driven microswimmer that mimics the undulatory propulsive mechanism is proposed. The designed microswimmer consists of four rigid segments, and each segment is connected to the succeeding segment by joints. The microswimmer is fabricated integrally by 3D laser lithography without further assembly, thereby simplifying microrobot fabrication while enhancing structural integrity. Experimental results show that the microswimmer can successfully swim forward along guided directions via undulatory locomotion in the low Reynolds number (Re) regime. This work demonstrates for the first time that the flexible characteristic of microswimmers can be emulated by 3D structures with multiple rigid segments, which broadens possibilities in microrobot design. The proposed magnetically driven microswimmer can potentially be used in biomedical applications, such as medical diagnosis and treatment in precision medicine.

Programmable Locomotion Mechanisms of Nanowires with Semihard Magnetic Properties Near a Surface Boundary

We report on the simplest magnetic nanowire-based surface walker that is able to change its propulsion mechanism near a surface boundary as a function of the applied rotating magnetic field frequency. The nanowires are made of CoPt alloy with semihard magnetic properties synthesized by means of template-assisted galvanostatic electrodeposition. The semihard magnetic behavior of the nanowires allows for programming their alignment with an applied magnetic field as they can retain their magnetization direction after premagnetizing them. By engineering the macroscopic magnetization, the nanowires' speed and locomotion mechanism are set to tumbling, precession, or rolling depending on the frequency of an applied rotating magnetic field. Also, we present a mathematical analysis that predicts the translational speed of the nanowire near the surface, showing a very good agreement with experimental results. Interestingly, the maximal speed is obtained at an optimal frequency (∼10 Hz), which is far below the theoretical step-out frequency (∼345 Hz). Finally, vortices are found by tracking polystyrene microbeads, trapped around the CoPt nanowire, when they are propelled by precession and rolling motion.

Automatic Manipulation of Magnetically Actuated Helical Microswimmers in Static Environments

Electromagnetically actuated microswimmers have been widely used in various biomedical applications due to their minor invasive traits and their easy access to confined environments. In order to guide the microswimmers autonomously towards a target, an obstacle-free path must be computed using path planning algorithms, meanwhile a motion controller must be formulated. However, automatic manipulations of magnetically actuated microswimmers are underdeveloped and still are challenging topics. In this paper, we develop an automatic manipulation system for magnetically actuated helical microswimmers in static environments, which mainly consists of a mapper, a path planner, and a motion controller. First, the mapper processes the captured image by morphological transformations and then labels the free space and the obstacle space. Second, the path planner explores the obstacle-free space to find a feasible path from the start to the goal by a global planning algorithm. Last, the motion controller guides the helical microswimmers along the desired path by a closed-loop algorithm. Experiments are conducted to verify the effectiveness of the proposed automatic manipulation. Furthermore, our proposed approach presents the first step towards applications of microswimmers for targeted medical treatments, such as micromanipulation, targeted therapy, and targeted drug delivery.

Biohybrid and Bioinspired Magnetic Microswimmers

Many motile microorganisms swim and navigate in chemically and mechanically complex environments. These organisms can be functionalized and directly used for applications (biohybrid approach), but also inspire designs for fully synthetic microbots. The most promising designs of biohybrids and bioinspired microswimmers include one or several magnetic components, which lead to sustainable propulsion mechanisms and external controllability. This Review addresses such magnetic microswimmers, which are often studied in view of certain applications, mostly in the biomedical area, but also in the environmental field. First, propulsion systems at the microscale are reviewed and the magnetism of microswimmers is introduced. The review of the magnetic biohybrids and bioinspired microswimmers is structured gradually from mostly biological systems toward purely synthetic approaches. Finally, currently less explored parts of this field ranging from in situ imaging to swarm control are discussed.

Fabrication and Characterization of a Magnetic Drilling Actuator for Navigation in a Three-dimensional Phantom Vascular Network

Intravascular microrobots have emerged as a promising tool for vascular diseases. They can be wirelessly and precisely manipulated with a high degree of freedom. Previous studies have evaluated their drilling performance and locomotion, and showed the feasibility of using microrobots for biomedical applications in two-dimensional space. However, it is critical to validate micro-drillers in a three-dimensional (3D) environment because gravity plays an important role in a 3D environment and significantly affects the performance of the micro-drillers in vascular networks. In this work, we fabricated magnetic drilling actuators (MDAs) and characterized their locomotion and drilling performance in vascular network-mimicking fluidic channels. The MDAs were precisely manipulated in the fluidic channel network in both horizontal and vertical planes, selecting and moving through the desired path via the junctions of multiple channels. The MDAs also accurately navigated an artificial thrombosis in an artificial 3D vascular network and successfully drilled through it. The results obtained here confirmed the precise manipulation and drilling performance of the developed MDAs in 3D. We think that the MDAs presented in this paper have great potential as intravascular drillers for precise thrombus treatment.

The Rotation of Microrobot Simplifies 3D Control Inside Microchannels

This paper focuses on the control of rotating helical microrobots inside microchannels. We first use a 50 μm long and 5 μm in diameter helical robot to prove that the proximity of the channel walls create a perpendicular force on the robot. This force makes the robot orbit around the channel center line. We also demonstrate experimentally that this phenomenon simplifies the robot control by guiding it on a channel even if the robot propulsion is not perfectly aligned with the channel direction. We then use numerical simulations, validated by real experimental cases, to show different implications on the microrobot control of this orbiting phenomenon. First, the robot can be centered in 3D inside an in-plane microchannel only by controlling its horizontal direction (yaw angle). This means that a rotating microrobot can be precisely controlled along the center of a microfluidic channel only by using a standard 2D microscopy technology. Second, the robot horizontal (yaw) and vertical (pitch) directions can be controlled to follow a 3D evolving channel only with a 2D feedback. We believe this could lead to simplify imaging systems for the potential in vivo integration of such microrobots.

Computationally Informed Design of a Multi-Axial Actuated Microfluidic Chip Device

This paper describes the computationally informed design and experimental validation of a microfluidic chip device with multi-axial stretching capabilities. The device, based on PDMS soft-lithography, consisted of a thin porous membrane, mounted between two fluidic compartments, and tensioned via a set of vacuum-driven actuators. A finite element analysis solver implementing a set of different nonlinear elastic and hyperelastic material models was used to drive the design and optimization of chip geometry and to investigate the resulting deformation patterns under multi-axial loading. Computational results were cross-validated by experimental testing of prototypal devices featuring the in silico optimized geometry. The proposed methodology represents a suite of computationally handy simulation tools that might find application in the design and in silico mechanical characterization of a wide range of stretchable microfluidic devices.

On-Surface Locomotion of Particle Based Microrobots Using Magnetically Induced Oscillation

The low Reynolds number condition presents a fundamental constraint on designing locomotive mechanisms for microscale robots. We report on the use of an oscillating magnetic field to induce on-surface translational motion of particle based microrobots. The particle based microrobots consist of microparticles, connected in a chain-like manner using magnetic self-assembly, where the non-rigid connections between the particles provide structural flexibility for the microrobots. Following the scallop theorem, the oscillation of flexible bodies can lead to locomotion at low Reynolds numbers, similar to the beating motion of sperm flagella. We characterized the velocity profiles of the microrobots by measuring their velocities at various oscillating frequencies. We also demonstrated the directional steering capabilities of the microrobots. This work will provide insights into the use of oscillation as a viable mode of locomotion for particle based microrobots near a surface.

Fabrication and Manipulation of Ciliary Microrobots with Non-reciprocal Magnetic Actuation

Magnetically actuated ciliary microrobots were designed, fabricated, and manipulated to mimic cilia-based microorganisms such as paramecia. Full three-dimensional (3D) microrobot structures were fabricated using 3D laser lithography to form a polymer base structure. A nickel/titanium bilayer was sputtered onto the cilia part of the microrobot to ensure magnetic actuation and biocompatibility. The microrobots were manipulated by an electromagnetic coil system, which generated a stepping magnetic field to actuate the cilia with non-reciprocal motion. The cilia beating motion produced a net propulsive force, resulting in movement of the microrobot. The magnetic forces on individual cilia were calculated with various input parameters including magnetic field strength, cilium length, applied field angle, actual cilium angle, etc., and the translational velocity was measured experimentally. The position and orientation of the ciliary microrobots were precisely controlled, and targeted particle transportation was demonstrated experimentally.