Bidirectional actuation of ferrofluid using micropatterned planar coils assisted by bias magnetic fields

This paper reports a ferrofluid control method that enables both attraction and repelling of ferrofluid on micropatterned planar coils coupled with permanent magnets. A combinational use of a controlled magnetic field and a bias field is shown to provide lateral forces that attract/repel the ferrofluid to/from the coil depending on the direction of the current passed through the coil. Active mirror devices whose mirrors are switched by ferrofluids are developed as a proof-of-concept of the actuation method toward the application to imaging devices and optical switches. The planar devices lithographically fabricated to have arrays of mirror-coil cells are used to demonstrate activation/deactivation of individual cells enabled by the bidirectional radial motion of the ferrofluid layer with ∼100 μm thickness. The static and dynamic behaviors of the ferrofluid in the devices are characterized through an image processing approach. Multiple mirror cells are selectively and simultaneously operated to show enhanced ferrofluid control uniquely available with the two modes of the actuation as well as to demonstrate pattern generation with the arrays.     

Novel electromagnetic actuation (EMA) method for 3-dimensional locomotion of intravascular microrobot

A new electromagnetic actuation (EMA) method is proposed for 3-dimensional locomotion of a microrobot. Generally, the EMA system uses Helmholtz coils and Maxwell coils. The Helmholtz coil pair generates a uniform magnetic flux density and the Maxwell coil pair generates a uniform gradient magnetic flux. The microrobot can be aligned to the desired direction by the Helmholtz coils and then, be propelled in the aligned direction by the Maxwell coils. However, many previous EMA systems have been restricted to 2-dimensional planar actuation. The EMA system proposed in this paper consists of a pair of stationary Helmholtz–Maxwell coils and a pair of rotational Helmholtz–Maxwell coils. This new EMA system can manipulate a microrobot in 3-dimensional space. For accurate actuation of a microrobot, the gravitational force, which influences the motion of microrobot, has to be analyzed and compensated. Through various experiments, the performance of the proposed EMA system was evaluated. Finally, a microrobot was test-driven in a blood vessel phantom, and the result of the test drive verified the feasibility of 3-dimensional motion of a microrobot by the new EMA system.     

Numerical and experimental investigations of the formation process of ferrofluid droplets

This paper reports both experimental and numerical investigations of the formation process of ferrofluid droplets in a flow focusing configuration with and without an applied magnetic field. In the experiment, the homogenous magnetic field was generated using an electromagnet. The magnetic field in the flow direction affects the formation process and changes the size of the droplets. The change in the droplet size depends on the magnetic field strength and the flow rates. A numerical model was used to investigate the force balance during the droplet breakup process. A linearly magnetizable fluid was assumed. Particle level set method was employed to capture the interface movement between the continuous fluid and the dispersed fluid. Results of the droplet formation process and the flow field are discussed for both cases with and without the magnetic field. Finally, experimental and numerical results are compared.     

Magnetic concentration of particles and cells in ferrofluid flow through a straight microchannel using attracting magnets

Concentrating particles to a detectable level is often necessary in many applications. Although magnetic force has long been used to enrich magnetic (or magnetically tagged) particles in suspensions, magnetic concentration of diamagnetic particles is relatively new and little reported. We demonstrate in this work a simple magnetic technique to concentrate polystyrene particles and live yeast cells in ferrofluid flow through a straight rectangular microchannel using negative magnetophoresis. The magnetic field gradient is created by two attracting permanent magnets that are placed on the top and bottom of the planar microfluidic device and held in position by their natural attractive force. The magnet–magnet distance is mainly controlled by the thickness of the device substrate and can be made small, allowing for the use of a dilute ferrofluid in the developed magnetic concentration technique. This advantage not only enables a magnetic/fluorescent label-free handling of diamagnetic particles, but also renders such handling biocompatible.     

胶囊内窥镜机器人的外磁场驱动方法

研究了利用外部磁场驱动胶囊内窥镜机器人的方法. 我们将梯度线圈和匀场线圈进行组合，通过调整线圈加载电流以及线圈绕可平移病床的旋转来合成空间上梯度均匀、低场强区域可动态调整的驱动磁场，作用于机器人内置的永磁体,从而获得期望的驱动力和辅助转矩.仿真表明组合线圈构成的磁场环境对于驱动相当有利. 我们提出的驱动方法具有良好的可控性和安全性，为进一步研究梯度磁场驱动适于体内各种腔管的微机器人奠定了基础.     

Electrostatically actuated metal-droplet microswitches integrated on CMOS chip

The dominance of surface tension over inertia in microscale and favorable scale effect for electrostatic actuation allow electrostatically driven metal-droplet systems practical. Because of such potential advantages as low contact resistance, naturally bistable operation, and high switch density, the liquid-metal droplet switch is an excellent candidate for reconfigurable circuit interconnections. Following earlier droplet microswitch examples and related studies of metal-droplet behavior, we report the first functioning droplet switch directly integrated on top of a functional CMOS circuit. While the surface tension dominance makes the droplet switches practical as a mechanical system and also brings bistability, it also requires a high electric field to move the droplet. We implement the concept of physical surface modification to lower the driving voltage to a value that a commercial CMOS process can provide. Unlike previous droplet switches, the reported device is planar-processed to allow the integration with the underlying CMOS circuits. The integrated switch is made functional by such provisions as self-limiting actuation and by optimizing the electrostatic force in the planar configuration and avoiding liquid-metal "flooding" into surface patterns. A fabrication process for low driving voltage and high compatibility is developed to integrate the droplet switch on the custom-developed CMOS chip. A packaging method adapted from well-established microelectronic packaging isolates the active switch space from the surrounding environment. Low driving voltage (as low as 15 V) and millisecond switching speed are achieved by the current on-chip device. While the current device uses /spl sim/150 /spl mu/m droplets for demonstration, additional theoretical and experimental results indicate that further miniaturization would lead to smaller devices and lower operation voltage.     

Enhanced locomotive and drilling microrobot using precessional and gradient magnetic field

We propose a new electromagnetic actuation (EMA) system for an intravascular microrobot with steering, locomotion and drilling functions. The EMA system consists of 3 pairs of Helmholtz coil and 1 pair of Maxwell coil. Generally, Helmholtz coils can align a microrobot in a desired direction by generating a uniform magnetic flux. If the uniform magnetic field generated by Helmholtz coils can be rotated, a microrobot with Helmholtz coils can also be rotated. On the other hand, a Maxwell coil, which generates a constant gradient magnetic flux, can supply the propulsion force for the microrobot. A microrobot actuated by the proposed EMA system has a spiral shaped body containing two magnets with different magnetization directions. With the proposed EMA system, the microrobot can move to the target region and perform drilling there by the precessional magnetic field of the Helmholtz coil pairs. The propulsion force for the microrobot is produced by the gradient magnetic field generated by the Maxwell coil pair. The moving velocity and the drilling performance of the microrobot can be increased by the propulsion force of the Maxwell coil pair. Through various tests, the feasibility and enhancement of the microrobot actuated by the proposed EMA system were verified.     

基于平面线圈的基波脉振磁场构造方法研究

为了减小电磁式时栅位移传感器的原始误差,提出了一种基于平面线圈的基波脉振磁场构造方法.通过研究现有电磁式时栅磁场构造方法和平面线圈磁场分布特性,利用各次谐波畸变率THD(Total Harmonic Distortion)最小的优化算法,得到平面线圈最优化布置参数,并且在数值分析和有限元分析软件中得到了验证.根据这些参数设置,设计了基于多匝方形平面线圈的新型电磁式时栅位移传感器.在150 mm量程内,新型时栅原始误差为-32μm~23μm,较现有电磁式时栅位移传感器减小了42.3%.     

Design multiple-layer gradient coils using least-squares finite element method

The design of gradient coils for magnetic resonance imaging is an optimization task in which a specified distribution of the magnetic field inside a region of interest is generated by choosing an optimal distribution of a current density geometrically restricted to specified non-intersecting design surfaces, thereby defining the preferred coil conductor shapes. Instead of boundary integral type methods, which are widely used to design coils, this paper proposes an optimization method for designing multiple layer gradient coils based on a finite element discretization. The topology of the gradient coil is expressed by a scalar stream function. The distribution of the magnetic field inside the computational domain is calculated using the least-squares finite element method. The first-order sensitivity of the objective function is calculated using an adjoint equation method. The numerical operations needed, in order to obtain an effective optimization procedure, are discussed in detail. In order to illustrate the benefit of the proposed optimization method, example gradient coils located on multiple surfaces are computed and characterised.     

Continuous-flow in-droplet magnetic particle separation in a droplet-based microfluidic platform

This paper presents a continuous-flow in-droplet magnetic particle separation in a droplet-based microfluidic device for magnetic bead-based bioassays. Two functions, electrocoalescence and magnetic particle manipulation, are performed in this device. A pair of charging metallic needles is inserted into two aqueous channels of the device. By electrostatic force, two different solutions can be merged to be mixed at a junction of droplet generation. The manipulation of magnetic particles is achieved using an externally applied magnetic field. The magnetic particles are separated by the magnetic field to one side of the droplet and extracted by splitting the droplet into two daughter droplets: one contains the majority of the magnetic particles and the other is almost devoid of magnetic particles. The applicability of the continuous-flow in-droplet magnetic particle separation is demonstrated by performing a proof-of-concept immunoassay between streptavidin-coated magnetic beads and biotin labelled with fluorescence. This approach will be useful for various biological and chemical analyses and compartmentalization of small samples.     

不规则直流线圈磁场的计算方法研究

将不规则直流线圈等效为若干段载流直导线的组合,通过坐标变换和磁场变换计算出单根载流直导线的磁场,进而叠加求和得到整个不规则直流线圈的磁场.计算示例表明,该方法计算不规则直流线圈的磁场简单易行,可用于实际工程中线圈磁场的计算.     

Ferrofluid-molding method for polymeric microlens arrays fabrication

This study proposes a method named as ferrofluid-molding method for polymer microlens array fabrication. In this method, the master of the mother mold for microlens molding is an array of ferrofluid droplets. We generated droplet arrays by inducing the droplet’s magnetic hydrodynamic instability under different magnetic fields, and used the field-dependent droplet dimensions to fabricate numerous mold cavities. By this we could fabricate arrays of microlens with different bottom area, height, radius of curvature, and focal length. From our analysis, all the fabricated microlens arrays possessed good uniformity, and the largest numerical aperture of our microlens array was found as 0.54. In addition, we also designed a light uniformity experiment to demonstrate a potential application of our microlens arrays.     

Electric field driven addressing of ATPS droplets in microfluidic chips

The possibility of controlled droplet motion (droplet addressing) mediated by DC electric field in aqueous two-phase systems (ATPS) is here reported for the first time. Three ATPS of polyethylene glycol (PEG)/salt type, namely PEG/phosphate, PEG/sulphate, and PEG/carbonate, were selected for this study. We observed fast motion of salty droplets dispersed in PEG continuous phase induced by electric field of relative low strength. Hence, three fluidic systems with separated electrode chambers for the evaluation of electrophoretic mobilities and for addressing experiments were fabricated. Electrophoretic mobilities of salty droplets always exceeded the value of \(1\times 10^{-7}\, \hbox {m}^2\hbox {V}^{-1}\hbox {s}^{-1}\), which is about by one magnitude higher value than those typically measured in water–oil droplet systems. The electrophoretic mobilities in systems with free surface are the same or even smaller than in closed microfluidic structures, which is accounted mainly to the fact that a significant part of salty droplets is exposed to air and does not contribute to droplet forcing. Series of addressing and merging experiments in a microfluidic chip shows that DC electric field can be used as a powerful tool for smart manipulation of droplets in microfluidic systems with PEG/salt ATPS.     

Diamagnetic particle focusing using ferromicrofluidics with a single magnet

Focusing particles into a tight stream is critical to many applications such as microfluidic flow cytometry and particle sorting. Current magnetic field-induced particle focusing techniques rely on the use of a pair of repulsive magnets, which makes the device integration and operation difficult. We develop herein a new approach to focusing nonmagnetic particles in ferrofluid flow through a T-microchannel using a single permanent magnet. Particles are deflected across the suspending ferrofluid by negative magnetophoresis and confined by a water flow to the center plane of the microchannel, leading to a focused particle stream flowing near the bottom channel wall. Such three-dimensional diamagnetic particle focusing is demonstrated in a sufficiently diluted ferrofluid through both the top and side views of the microchannel. As the suspended particles can be visualized in bright field, this magnetic focusing method is expected to find applications to label-free (i.e., no magnetic or fluorescent labeling) cellular focusing in lab-on-a-chip devices.     

Magnetohydrodynamics in narrow fluidic channels in presence of spatially non-uniform magnetic fields: framework for combined magnetohydrodynamic and magnetophoretic particle transport

In this paper, we bring out the implications of a spatially varying magnetic field towards combined magnetohydrodynamic–magnetophoretic transport in narrow fluidic confinements. We first present a generic framework for describing the flow field that is generated under the combined influences of a driving pressure gradient, an axial electric field, and a spatially varying transverse magnetic field. As a demonstrative example, we derive analytical solutions for the flow field, based on a plausible choice of the mathematical form of the nature of spatial variation of the magnetic field. Proceeding further ahead, we also address the magnetophoretic motion of particles, subjected to such spatially varying magnetic fields. We depict the trajectories of representative spherical particles in the flow-field, as a combined consequence of the magnetohydrodynamic and magnetophoretic forcing mechanisms. We also demonstrate that such combined magnetophoretic and magnetohydrodynamic transport can be employed as a novel technique to separate particles based on sizes and electromagnetic properties.     

A ferrofluidic magnetic micropump

A microfluidic pump is described that uses magnetic actuation to push fluid through a microchannel. Operation relies on the use of magnetically-actuated plugs of ferrofluid, a suspension of nanosize ferromagnetic particles. The ferrofluid contacts but is immiscible with the pumped fluid. The prototype circular design demonstrates continuous pumping by regenerating a translating ferrofluidic plug at the conclusion of each pumping cycle. The flow rate can be controlled by adjusting device dimensions or the velocity of an external permanent magnet that directs the motion of the ferrofluid. The ferrofluidic plugs also serve as valves; if the magnetic actuator is stopped, pressure can be maintained with no power consumption. Flow can also be reversed by switching the direction of actuation. The maximum flow rate achieved with minimal backpressure was 45.8 μl/min. The maximum pressure head achieved was 135 mm water (1.2 kPa)     

Numerical simulation of two-phase flows in the presence of a magnetic field

A set of non-dimensional model equations, which can simulate incompressible, immiscible two-phase flows in the presence of a magnetic field, has been derived and solved numerically with a finite difference method using the HSMAC algorithm. In this study, dynamics of a falling droplet of liquid metal into a horizontal liquid metal layer and of a rising air bubble in water subject to a magnetic field are presented as examples. The numerical results reveal that the Lorentz force acts to dampen the motion of electric conducting fluid as the Hartmann number increases. On the other hand, even for ordinary non-conducting fluids such as water and air, a substantial body force due to magnetization influences the air bubble behaviour in water under a gradient magnetic field.     

Mechanical and electrical properties of electroplated copper for MR-imaging coils

Mechanical and electrical properties of electroplated copper films were investigated to be used in an active tracking system for the patient-safe localisation of a catheter in intravascular interventions using magnetic resonance imaging. For this system spiral micro coils with high aspect ratio are fabricated on a planar polyimide foil to be wrapped around a catheter tip. Due to this special assembly method the coils and the copper, respectively, have to withstand high mechanical forces. The influence of the electroplating parameters as current density and shape, temperature and sample motion on the mechanical and electrical properties is investigated. These samples were characterised using nanoindentation, tensile tests, laser profiler and four-point prober. Results show that a low constant current density at a temperature of 40°C and sample motion generate flexible and high-Q copper coils.     

Dynamics of droplet transport induced by electrowetting actuation

This study reports on the dynamics of droplets in the capillary regime induced by electrowetting-on-dielectric actuation. The configuration investigated allows for comparing the experimental results with respect to the predictions of Brochard’s theoretical model (Brochard in Langmuir 5:432–438, 1989). Firstly, side-view observations using stroboscopic recording techniques were used to measure and analyse droplet deformations as well as the front and rear apparent contact angles during motion. Secondly, the influence of viscosity on the droplet velocity as a function of the applied voltage was studied. This has revealed that low Reynolds number droplet motion can be described by the simple laminar viscous model of Brochard. Finally, the influence of the dielectric thickness on the droplet dynamics was studied. It is shown that droplet velocity is limited by a saturation effect of the driving electrostatic force and that this phenomenon is very similar to that occurring in static experiments.     

Propulsion of droplets on micro- and sub-micron ratchet surfaces in the Leidenfrost temperature regime

Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. Here, we show that propulsion for the directed motion of water droplets levitating on heated ratchet surfaces in the Leidenfrost (film boiling) regime is significantly enhanced as the ratchet period decreases down to micro- and sub-micrometers. At the temperature range slightly above the threshold temperature of droplet motion, sub-micron ratchets yield water droplet velocities reaching ~40 cm/s, a speed that has never been achieved with any chemical and topological gradient surfaces. This dramatic increase in the droplet velocity is attributed to an enhanced heat transfer through the local contacts between ratchet peaks and bottom of the droplet. A hydrophobic coating on the ratchet surfaces is found to further increase the droplet velocity and decrease the threshold temperature of the droplet motion. The results suggest that miniaturized ratchet surfaces can potentially be used in diverse applications requiring control over fluid transport and heat transfer such as two phase cooling systems for microprocessors and fuel injection for combustion technology and that for those applications the design of ratchet dimensions and surface chemistry are critically important.