Controlled Multidirectional Particle Transportation by Magnetic Artificial Cilia

Programmable metachronal motion of closely packed magnetic artificial cilia

Despite recent advances in artificial cilia technologies, the application of metachrony, which is the collective wavelike motion by cilia moving out-of-phase, has been severely hampered by difficulties in controlling closely packed artificial cilia at micrometer length scales. Moreover, there has been no direct experimental proof yet that a metachronal wave in combination with fully reciprocal ciliary motion can generate significant microfluidic flow on a micrometer scale as theoretically predicted. In this study, using an in-house developed precise micro-molding technique, we have fabricated closely packed magnetic artificial cilia that can generate well-controlled metachronal waves. We studied the effect of pure metachrony on fluid flow by excluding all symmetry-breaking ciliary features. Experimental and simulation results prove that net fluid transport can be generated by metachronal motion alone, and the effectiveness is strongly dependent on cilia spacing. This technique not only offers a biomimetic experimental platform to better understand the mechanisms underlying metachrony, it also opens new pathways towards advanced industrial applications.

Recent progress on the development of bioinspired surfaces with high aspect ratio microarray structures: From fabrication to applications

Surfaces with high aspect ratio microarray structures can implement sophisticated assignment in typical fields including microfluidics, sensor, biomedicine, et al. via regulating their deformation or the material properties. Inspired by natural materials and systems, for example sea cockroaches, water spiders, cacti, lotus leaves, rice leaves, and cedar leaves, many researchers have focused on microneedle functional surface studies. When the surface with high aspect ratio microarray structures is stimulated by the external fields, such as optical, electric, thermal, magnetic, the high aspect ratio microarray structures can undergo hydrophilic and hydrophobic switching or shape change, which may be gifted the surfaces with the ability to perform complex task, including directional liquid/air transport, targeted drug delivery, microfluidic chip sensing. In this review, the fabrication principles of various surfaces with high aspect ratio microarray structures are classified and summarized. Mechanisms of liquid manipulation on hydrophilic/hydrophobic surfaces with high aspect ratio microarray structures are clarified based on Wenzel model, Cassie model, Laplace pressure theories and so on. Then the intelligent control strategies have been demonstrated. The applications in microfluidic, drug delivery, patch sensors have been discussed. Finally, current challenges and new insights of future prospects for dynamic manipulation of liquid/air based on biomimetic surface with high aspect ratio microarray structures are also addressed.

Locally Addressable Energy Efficient Actuation of Magnetic Soft Actuator Array Systems

Advances in magnetoresponsive composites and (electro-)magnetic actuators have led to development of magnetic soft machines (MSMs) as building blocks for small-scale robotic devices. Near-field MSMs offer energy efficiency and compactness by bringing the field source and effectors in close proximity. Current challenges of near-field MSM are limited programmability of effector motion, dimensionality, ability to perform collaborative tasks, and structural flexibility. Herein, a new class of near-field MSMs is demonstrated that combines microscale thickness flexible planar coils with magnetoresponsive polymer effectors. Ultrathin manufacturing and magnetic programming of effectors is used to tailor their response to the nonhomogeneous near-field distribution on the coil surface. The MSMs are demonstrated to lift, tilt, pull, or grasp in close proximity to each other. These ultrathin (80 µm) and lightweight (100 gm-2 ) MSMs can operate at high frequency (25 Hz) and low energy consumption (0.5 W), required for the use of MSMs in portable electronics.

Microfluidic systems for particle capture and release: A review

Microfluidic technology has emerged as a promising tool in various applications, including biosensing, disease diagnosis, and environmental monitoring. One of the notable features of microfluidic devices is their ability to selectively capture and release specific cells, biomolecules, bacteria, and particles. Compared to traditional bulk analysis instruments, microfluidic capture-and-release platforms offer several advantages, such as contactless operation, label-free detection, high accuracy, good sensitivity, and minimal reagent requirements. However, despite significant efforts dedicated to developing innovative capture mechanisms in the past, the release and recovery efficiency of trapped particles have often been overlooked. Many previous studies have focused primarily on particle capture techniques and their efficiency, disregarding the crucial role of successful particle release for subsequent analysis. In reality, the ability to effectively release trapped particles is particularly essential to ensure ongoing, high-throughput analysis. To address this gap, this review aims to highlight the importance of both capture and release mechanisms in microfluidic systems and assess their effectiveness. The methods are classified into two categories: those based on physical principles and those using biochemical approaches. Furthermore, the review offers a comprehensive summary of recent applications of microfluidic platforms specifically designed for particle capture and release. It outlines the designs and performance of these devices, highlighting their advantages and limitations in various target applications and purposes. Finally, the review concludes with discussions on the current challenges faced in the field and presents potential future directions.

Recent Advances in Magnetic Polymer Composites for BioMEMS: A Review

The objective of this review is to investigate the potential of functionalized magnetic polymer composites for use in electromagnetic micro-electro-mechanical systems (MEMS) for biomedical applications. The properties that make magnetic polymer composites particularly interesting for application in the biomedical field are their biocompatibility, their adjustable mechanical, chemical, and magnetic properties, as well as their manufacturing versatility, e.g., by 3D printing or by integration in cleanroom microfabrication processes, which makes them accessible for large-scale production to reach the general public. The review first examines recent advancements in magnetic polymer composites that possess unique features such as self-healing capabilities, shape-memory, and biodegradability. This analysis includes an exploration of the materials and fabrication processes involved in the production of these composites, as well as their potential applications. Subsequently, the review focuses on electromagnetic MEMS for biomedical applications (bioMEMS), including microactuators, micropumps, miniaturized drug delivery systems, microvalves, micromixers, and sensors. The analysis encompasses an examination of the materials and manufacturing processes involved and the specific fields of application for each of these biomedical MEMS devices. Finally, the review discusses missed opportunities and possible synergies in the development of next-generation composite materials and bioMEMS sensors and actuators based on magnetic polymer composites.

3D-printed micrometer-scale wireless magnetic cilia with metachronal programmability

Biological cilia play essential roles in self-propulsion, food capture, and cell transportation by performing coordinated metachronal motions. Experimental studies to emulate the biological cilia metachronal coordination are challenging at the micrometer length scale because of current limitations in fabrication methods and materials. We report on the creation of wirelessly actuated magnetic artificial cilia with biocompatibility and metachronal programmability at the micrometer length scale. Each cilium is fabricated by direct laser printing a silk fibroin hydrogel beam affixed to a hard magnetic FePt Janus microparticle. The 3D-printed cilia show stable actuation performance, high temperature resistance, and high mechanical endurance. Programmable metachronal coordination can be achieved by programming the orientation of the identically magnetized FePt Janus microparticles, which enables the generation of versatile microfluidic patterns. Our platform offers an unprecedented solution to create bioinspired microcilia for programmable microfluidic systems, biomedical engineering, and biocompatible implants.

Ordered Magnetic Cilia Array Induced by the Micro-cavity Effect for the In Situ Adjustable Pressure Sensor

Cilia are fundamental functional structures in natural biology. As the primary option of artificial cilia, magnetic cilia have been drawing extensive attention due to their excellent biocompatibility, sensitive response, and contactless actuation. However, most of the ordered magnetic cilia are fabricated by molds, suffering from high cost and low efficiency. In this paper, an ultrafast fabrication method of ordered cilia array using the micro-cavity inducing effect was proposed. With the impact of static and dynamic magnetic fields, the fine cilia were first formed in out-cavity area and then converged above cavities forming complete cilia structures. The mechanism of the micro-cavity inducing effect was further revealed. Finally, the ordered cilia array was used to develop the pressure sensor with variable stiffness, making the in situ adjustment of the sensor performance possible. The ordered cilia array was applied as a micro-mixer and largely improved the mixing efficiency for different mediums. The ordered cilia array also successfully served as the info carrier for rapid sub-encryption. This method allows the fast and controlled forming of ordered cilia arrays within 30 s, and the cilia structure can be adjusted in a large range of aspect ratios (1-9), providing an approach to large-scale producing the magnetic cilia for different applications.

Metachronal patterns by magnetically-programmable artificial cilia surfaces for low Reynolds number fluid transport and mixing

Motile cilia can produce net fluid flows at low Reynolds number because of their asymmetric motion and metachrony of collective beating. Mimicking this with artificial cilia can find application in microfluidic devices for fluid transport and mixing. Here, we study the metachronal beating of nonidentical, magnetically-programmed artificial cilia whose individual non-reciprocal motion and collective metachronal beating pattern can be independently controlled. We use a finite element method that accounts for magnetic forces, cilia deformation and fluid flow in a fully coupled manner. Mimicking biological cilia, we study magnetic cilia subject to a full range of metachronal driving patterns, including antiplectic, symplectic, laeoplectic and diaplectic waves. We analyse the induced primary flow, secondary flow and mixing rate as a function of the phase lag between cilia and explore the underlying physical mechanism. Our results show that shielding effects between neighboring cilia lead to a primary flow that is larger for antiplectic than for symplectic metachronal waves. The secondary flow can be fully explained by the propagation direction of the metachronal wave. Finally, we show that the mixing rate can be strongly enhanced by laeoplectic and diaplectic metachrony resulting in large velocity gradients and vortex-like flow patterns.

Locust-Inspired Direction-Dependent Transport Based on a Magnetic-Responsive Asymmetric-Microplate-Arrayed Surface

Inspired by the highly efficient jumping mechanism of locusts, a magnetic-responsive asymmetric-microplate-arrayed surface is designed. Elastic energy can be stored in the microplate and rapidly released by loading and removing a magnetic field. Similar to the bouncing behavior of the locust, objects deposited on the surface of the microplate-arrayed surface will bounce suddenly. It is found that the continuous transport behavior can be induced in the moving magnetic field and the direction-dependent transport is well achieved by preparing the secondary microstructure. The results show that both the weight and transport velocity of the transported object in the forward transport direction are much greater than those in the reverse transport direction. Furthermore, the anisotropic transport property can be strengthened with the increase of the height of the secondary structure. Such surfaces can transport objects with either soft or hard stiffness, as well as objects with different geometric configurations, and the transport path can be arbitrarily programmed. Based on the transport mechanism, a flexible microconvey belt is further designed, which can transport objects in any controlled direction. Such a simple technique can provide new design ideas for directional microtransport requirements.

Microfluidic Applications of Artificial Cilia: Recent Progress, Demonstration, and Future Perspectives

Artificial cilia-based microfluidics is a promising alternative in lab-on-a-chip applications which provides an efficient way to manipulate fluid flow in a microfluidic environment with high precision. Additionally, it can induce favorable local flows toward practical biomedical applications. The endowment of artificial cilia with their anatomy and capabilities such as mixing, pumping, transporting, and sensing lead to advance next-generation applications including precision medicine, digital nanofluidics, and lab-on-chip systems. This review summarizes the importance and significance of the artificial cilia, delineates the recent progress in artificial cilia-based microfluidics toward microfluidic application, and provides future perspectives. The presented knowledge and insights are envisaged to pave the way for innovative advances for the research communities in miniaturization.

Microscopic artificial cilia - a review

Cilia are microscopic hair-like external cell organelles that are ubiquitously present in nature, also within the human body. They fulfill crucial biological functions: motile cilia provide transportation of fluids and cells, and immotile cilia sense shear stress and concentrations of chemical species. Inspired by nature, scientists have developed artificial cilia mimicking the functions of biological cilia, aiming at application in microfluidic devices like lab-on-chip or organ-on-chip. By actuating the artificial cilia, for example by a magnetic field, an electric field, or pneumatics, microfluidic flow can be generated and particles can be transported. Other functions that have been explored are anti-biofouling and flow sensing. We provide a critical review of the progress in artificial cilia research and development as well as an evaluation of its future potential. We cover all aspects from fabrication approaches, actuation principles, artificial cilia functions - flow generation, particle transport and flow sensing - to applications. In addition to in-depth analyses of the current state of knowledge, we provide classifications of the different approaches and quantitative comparisons of the results obtained. We conclude that artificial cilia research is very much alive, with some concepts close to industrial implementation, and other developments just starting to open novel scientific opportunities.

Biomimetic Liquid Crystal Cilia and Flagella

Cilia and flagella are a vital part of many organisms. Protozoa such as paramecia rely on the collective and coordinated beating of tubular cilia or flagella for their transport, while mammals depend on the ciliated linings of their bronchia and female reproductive tracts for the continuity of breathing and reproduction, respectively. Over the years, man has attempted to mimic these natural cilia using synthetic materials such as elastomers doped with magnetic particles or light responsive liquid crystal networks. In this review, we will focus on the progress that has been made in mimicking natural cilia and flagella using liquid crystal polymers. We will discuss the progress that has been made in mimicking natural cilia and flagella with liquid crystal polymers using techniques such as fibre drawing, additive manufacturing, or replica moulding, where we will put additional focus on the emergence of asymmetrical and out-of-plane motions.

Shape-programmable artificial cilia for microfluidics

The artificial ciliary motion has been known not to be hydrodynamically optimal, limiting their associated applications in the microscale flow domain. One of the major hurdles of contemporary artificial cilia is its structural rigidity, which restricts their flexibility. To address this issue, this work proposed a shape-programmable artificial cilia design with distinctive polydimethylsiloxane (PDMS) and magnetic segments distributed throughout the structure, which provided precise control for time-spatial modulation of the whole artificial cilia structure under external magnetic actuation. For the fabrication of the proposed multi-segment artificial cilia, a facile microfabrication process with stepwise mold blocking followed by the PDMS and magnetic composite casting was adopted. The hydrodynamic analysis further elucidated that the proposed artificial cilia beating induced significant flow disturbance within the flow field, and the associated application was demonstrated through an efficient mixing operation.

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Fabrication of artificial cilia was conducted through micromilling and casting methods.

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The weighted index was correlated to the bending angles of artificial cilia.

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Hydrodynamic analysis of artificial cilia was performed through the μPIV analysis.

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A significant improvement in mixing performance was achieved in few seconds.

Amphibious Transport of Fluids and Solids by Soft Magnetic Carpets

One of the major challenges in modern robotics is controlling micromanipulation by active and adaptive materials. In the respiratory system, such actuation enables pathogen clearance by means of motile cilia. While various types of artificial cilia have been engineered recently, they often involve complex manufacturing protocols and focus on transporting liquids only. Here, soft magnetic carpets are created via an easy self-assembly route based on the Rosensweig instability. These carpets can transport not only liquids but also solid objects that are larger and heavier than the artificial cilia, using a crowd-surfing effect.This amphibious transportation is locally and reconfigurably tunable by simple micromagnets or advanced programmable magnetic fields with a high degree of spatial resolution. Two surprising cargo reversal effects are identified and modeled due to collective ciliary motion and nontrivial elastohydrodynamics. While the active carpets are generally applicable to integrated control systems for transport, mixing, and sorting, these effects can also be exploited for microfluidic viscosimetry and elastometry.

Highly motile nanoscale magnetic artificial cilia

Cilia are hair-like microscopic structures present abundantly in our body and producing motions at the smallest scales. They perform a wide range of critical functions and are crucial for the normal functioning of our body. Abnormal functioning of cilia results in a number of diseases jointly known as ciliopathies. Artificially mimicking cilia is aimed at understanding their normal/abnormal functionality and at developing cilia-inspired micro-/nanoengineering devices. In this study, we present a magnetic polymer preparation process yielding a material with optimum properties and a cilia fabrication method producing the smallest highly motile artificial cilia with sizes equal to their biological counterparts. This opens avenues for biological studies and for creating submicrometer manipulation and control.

Among the many complex bioactuators functioning at different scales, the organelle cilium represents a fundamental actuating unit in cellular biology. Producing motions at submicrometer scales, dominated by viscous forces, cilia drive a number of crucial bioprocesses in all vertebrate and many invertebrate organisms before and after their birth. Artificially mimicking motile cilia has been a long-standing challenge while inspiring the development of new materials and methods. The use of magnetic materials has been an effective approach for realizing microscopic artificial cilia; however, the physical and magnetic properties of the magnetic material constituents and fabrication processes utilized have almost exclusively only enabled the realization of highly motile artificial cilia with dimensions orders of magnitude larger than their biological counterparts. This has hindered the development and study of model systems and devices with inherent size-dependent aspects, as well as their application at submicrometer scales. In this work, we report a magnetic elastomer preparation process coupled with a tailored molding process for the successful fabrication of artificial cilia with submicrometer dimensions showing unprecedented deflection capabilities, enabling the design of artificial cilia with high motility and at sizes equal to those of their smallest biological counterparts. The reported work crosses the barrier of nanoscale motile cilia fabrication, paving the way for maximum control and manipulation of structures and processes at micro- and nanoscales.

Directional Transportation on Microplate-Arrayed Surfaces Driven via a Magnetic Field

Directional transportation on micro/nanostructure-arrayed surfaces driven by an external field has attracted increasing attention in numerous domains, and this has led to significant progress in this field. In this study, an efficient method for high-speed transportation of solid objects is proposed based on magnetically responsive microplate arrays with a high aspect ratio. The transport speed is approximately an order of magnitude higher than the existing value. In addition, the speed of the transported objects can be controlled appropriately by the speed of the magnet. Besides, objects with varying shapes and sizes can be transported in both air and water. Further investigation of the transport mechanism reveals a rapid release of the elastic strain energy stored in the microplate. Hence, using this energy, the object can bounce forward quickly. The proposed technique and design aid not only in studies on more efficient, intelligent, or even programmed micro/nanotransportation but also in micro/nanomanipulation.

Magnetic Polymers for Magnetophoretic Separation in Microfluidic Devices

Magnetophoresis offers many advantages for manipulating magnetic targets in microsystems. The integration of micro-flux concentrators and micro-magnets allows achieving large field gradients and therefore large reachable magnetic forces. However, the associated fabrication techniques are often complex and costly, and besides, they put specific constraints on the geometries. Magnetic composite polymers provide a promising alternative in terms of simplicity and fabrication costs, and they open new perspectives for the microstructuring, design, and integration of magnetic functions. In this review, we propose a state of the art of research works implementing magnetic polymers to trap or sort magnetic micro-beads or magnetically labeled cells in microfluidic devices.

Magnetophoretic Decoupler for Disaggregation and Interparticle Distance Control

The manipulation of superparamagnetic beads has attracted various lab on a chip and magnetic tweezer platforms for separating, sorting, and labeling cells and bioentities, but the irreversible aggregation of beads owing to magnetic interactions has limited its actual functionality. Here, an efficient solution is developed for the disaggregation of magnetic beads and interparticle distance control with a magnetophoretic decoupler using an external rotating magnetic field. A unique magnetic potential energy distribution in the form of an asymmetric magnetic thin film around the gap is created and tuned in a controlled manner, regulated by the size ratio of the bead with a magnetic pattern. Hence, the aggregated beads are detached into single beads and transported in one direction in an array pattern. Furthermore, the simultaneous and accurate spacing control of multiple magnetic bead pairs is performed by adjusting the angle of the rotating magnetic field, which continuously changes the energy well associated with a specific shape of the magnetic patterns. This technique offers an advanced solution for the disaggregation and controlled manipulation of beads, can allow new possibilities for the enhanced functioning of lab on a chip and magnetic tweezers platforms for biological assays, intercellular interactions, and magnetic biochip systems.

Enhancement of microalgae growth using magnetic artificial cilia

Microalgae have shown great potential as a source of biofuels, food, and other bioproducts. More recently, microfluidic devices have been employed in microalgae-related studies. However, at small fluid volumes, the options for controlling flow conditions are more limited and mixing becomes largely reliant on diffusion. In this study, we fabricated magnetic artificial cilia (MAC) and implemented them in millimeter scale culture wells and conducted growth experiments with Scenedesmus subspicatus while actuating the MAC in a rotating magnetic field to create flow and mixing. In addition, surface of MAC was made hydrophilic using plasma treatment and its effect on growth was compared with untreated, hydrophobic MAC. The experiments showed that the growth was enhanced by ten and two times with hydrophobic and hydrophilic MAC, respectively, compared with control groups which contain no MAC. This technique can be used to investigate mixing and flow in small sample volumes, and the enhancement in growth can be beneficial for the throughput of screening studies. Moreover, the methods used for creating and controlling MAC can be easily adopted in labs without microfabrication infrastructures, and they can be mastered by people with little prior experience in microfluidics.

Metachronal actuation of microscopic magnetic artificial cilia generates strong microfluidic pumping

Biological cilia that generate fluid flow or propulsion are often found to exhibit a collective wavelike metachronal motion, i.e. neighboring cilia beat slightly out-of-phase rather than synchronously. Inspired by this observation, this article experimentally demonstrates that microscopic magnetic artificial cilia (μMAC) performing a metachronal motion can generate strong microfluidic flows, though, interestingly, the mechanism is different from that in biological cilia, as is found through a systematic experimental study. The μMAC are actuated by a facile magnetic setup, consisting of an array of rod-shaped magnets. This arrangement imposes a time-dependent non-uniform magnetic field on the μMAC array, resulting in a phase difference between the beatings of adjacent μMAC, while each cilium exhibits a two-dimensional whip-like motion. By performing the metachronal 2D motion, the μMAC are able to generate a strong flow in a microfluidic chip, with velocities of up to 3000 μm s^-1 in water, which, different from biological cilia, is found to be a result of combined metachronal and inertial effects, in addition to the effect of asymmetric beating. The pumping performance of the metachronal μMAC outperforms all previously reported microscopic artificial cilia, and is competitive with that of most of the existing microfluidic pumping methods, while the proposed platform requires no physical connection to peripheral equipment, reduces the usage of reagents by minimizing "dead volumes", avoids undesirable electrical effects, and accommodates a wide range of different fluids. The 2D metachronal motion can also generate a flow with velocities up to 60 μm s^-1 in pure glycerol, where Reynolds number is less than 0.05 and the flow is primarily caused by the metachronal motion of the μMAC. These findings offer a novel solution to not only create on-chip integrated micropumps, but also design swimming and walking microrobots, as well as self-cleaning and antifouling surfaces.