Magnetic particle dosing and size separation in a microfluidic channel

Separation of functional magnetic particles or magnetically labeled entities is a key feature for bioanalytical or biomedical applications and therefore also an important component of lab-on-a-chip devices for biological applications. We present a novel integrated microfluidic magnetic bead manipulation device, comprising dosing of magnetic particles, controlled release and subsequent magnetophoretic size separation with high resolution. The system is designed to meet the requirements of specific bioassays, in particular of on-chip agglutination assays for the detection of rare analytes by particle coupling as doublets. Integrated soft-magnetic microtips with different shapes provide the magnetic driving force of the bead manipulation protocol. The magnetic tips that serve as field concentrators of an external electromagnetic field, are positioned in close contact to a microfluidic channel in order to generate high magnetic actuation forces. Mixtures of 1.0 μm and 2.8 μm superparamagnetic beads have been used to characterize the system. Magnetophoretic size separation with high resolution was performed in static conditions and in continuous flow mode. In particular, we could demonstrate the separation of 1.0 μm single beads and doublets in a sample flow.     

集成惯性聚焦结构的介电泳微流控芯片的粒子连续分离

设计并制造了一种带有惯性聚焦结构的介电泳微流控芯片,以实现不同介电性质的粒子连续分离.采用MEMS工艺制作了介电泳微流控芯片:通道入口侧壁设置一对梯形结构使经过的粒子受惯性升力的作用聚焦到通道两侧;通道底部光刻一组夹角为90°的倾斜叉指电极产生非均匀电场,利用介电泳力和流体曳力的合力使通道两侧不同的粒子发生角度不同的偏转进入不同通道,从而实现分离.将酵母菌细胞和聚苯乙烯小球作为实验样本,分析了流速和交流电压对分离的影响,确定了二者分离的最优条件并进行分离.实验结果表明,将电导率为20μS/cm的样本溶液以5μL/min的流速注入到通道中,施加6 Vp-p、10 kHz的正弦信号,酵母菌细胞沿电极运动至夹角处后沿通道中心排出,聚苯乙烯小球沿通道两侧排出,成功实现分离,平均分离效率达92.8％、平均分离纯度达90.7％.     

Multiplex Inertio-Magnetic Fractionation (MIMF) of magnetic and non-magnetic microparticles in a microfluidic device

Separation of multiple microparticles at high throughput is highly required in different applications such as diagnostics and immunomagnetic detection. We present a microfluidic device for multiplex (i.e., duplex to fourplex) fractionation of magnetic and non-magnetic microparticles using a novel hybrid technique based on interactions between flow-induced inertial forces and countering magnetic forces in a simple expansion microchannel with a side permanent magnet. Separation of more than two types of particles solely by inertia or magnetic forces in a straight microchannel is challenging due to the inherent limitations of each technique. By combining inertial and magnetic forces in a straight microchannel and addition of a downstream expansion hydrodynamic separator, we overcame these limitations and achieved duplex to fourplex fractionation of magnetic and non-magnetic microparticles with high throughput and efficiency. Particle fractionation performance in our device was first optimized with respect to parameters such as flow rate and aspect ratio of the channel to attain coexistence of inertial and magnetic focusing of particles. Using this scheme, we achieved duplex fractionation of particles at high throughput of 10⁹ particles per hour. Further, we conducted experiments with three magnetic particles (5, 11 and 35 µm) to establish their size-dependent ordering in the device under combined effects of magnetic and inertial forces. We then used the findings for fourplex fractionation of 5, 11 and 35 µm magnetic particles from non-magnetic particles of various sizes (10–19 µm). This Multiplex Inertio-Magnetic Fractionation (MIMF) technique offers a simple tool to handle complex and heterogeneous samples and can be used for affinity-based immunomagnetic separation of multiple biological substances in fluidic specimens in the future.     

Analytical model of microfluidic transport of non-magnetic particles in ferrofluids under the influence of a permanent magnet

This study describes an analytical model and experimental verifications of transport of non-magnetic spherical microparticles in ferrofluids in a microfluidic system that consists of a microchannel and a permanent magnet. The permanent magnet produces a spatially non-uniform magnetic field that gives rise to a magnetic buoyancy force on particles within ferrofluid-filled microchannel. We obtained trajectories of particles in the microchannel by (1) calculating magnetic buoyancy force through the use of an analytical expression of magnetic field distributions and a nonlinear magnetization model of ferrofluids, (2) deriving governing equations of motion for particles through the use of analytical expressions of dominant magnetic buoyancy and hydrodynamic viscous drag forces, (3) solving equations of motion for particles in laminar flow conditions. We studied effects of particle size and flow rate in the microchannel on the trajectories of particles. The analysis indicated that particles were increasingly deflected in the direction that was perpendicular to the flow when size of particles increased, or when flow rate in the microchannel decreased. We also studied ??wall effect?? on the trajectories of particles in the microchannel when surfaces of particles were in contact with channel wall. Experimentally obtained trajectories of particles were used to confirm the validity of our analytical results. We believe this study forms the theoretical foundation for size-based particle (both synthetic and biological) separation in ferrofluids in a microfluidic device. The simplicity and versatility of our analytical model make it useful for quick optimizations of future separation devices as the model takes into account important design parameters including particle size, property of ferrofluids, magnetic field distribution, dimension of microchannel, and fluid flow rate.     

Microfluidic separation of magnetic particles with soft magnetic microstructures

This paper demonstrates simple and cost-effective microfluidic devices for enhanced separation of magnetic particles by using soft magnetic microstructures. By injecting a mixture of iron powder and polydimethylsiloxane (PDMS) into a prefabricated channel, an iron–PDMS microstructure was fabricated next to a microfluidic channel. Placed between two external permanent magnets, the magnetized iron–PDMS microstructure induces localized and strong forces on the magnetic particles in the direction perpendicular to the fluid flow. Due to the small distance between the microstructure and the fluid channel, the localized large magnetic field gradients result a vertical force on the magnetic particles, leading to enhanced separation of the particles. Numerical simulations were developed to compute the particle trajectories and agreed well with experimental data. Systematic experiments and numerical simulation were conducted to study the effect of relevant factors on the transport of superparamagnetic particles, including the shape of iron–PDMS microstructure, mass ratio of iron–PDMS composite, width of the microfluidic channel, and average flow velocity.     

Microfluidic devices in superconducting magnets: on-chip free-flow diamagnetophoresis of polymer particles and bubbles

Superconducting magnets enable the study of high magnetic fields on materials and objects, for example in material synthesis, self-assembly or levitation experiments. The setups employed often lack in precise spatial control of the object of interest within the bore of the magnet. Microfluidic technology enables accurate manipulation of fluidic surroundings and we have investigated the integration of microfluidic devices into superconducting magnets to enable controlled studies of objects in high magnetic fields. Polymeric microparticles similar in size to biological cells were manipulated via diamagnetic repulsion. The particles were suspended in an aqueous paramagnetic medium of manganese (II) chloride and pumped into a microfluidic chip, where they were repelled in continuous flow by the high magnetic field. The extent of deflection was studied as a function of increasing (1) particle size, (2) paramagnetic salt concentration, and (3) magnetic field strength. Optimizing these parameters allowed for the spatial separation of two particle populations via on-chip free-flow diamagnetophoresis. Finally, preliminary findings on the repulsion of air bubbles are shown.     

Microfluidic device for continuous magnetophoretic separation of white blood cells

This paper presents a microfluidic device for magnetophoretic separation of red blood cells from blood under continuous flow. The separation method consists of continuous flow of a blood sample (diluted in PBS) through a microfluidic channel which presents on the bottom “dots” of ferromagnetic layer. By applying a magnetic field perpendicular on the flowing direction, the ferromagnetic “dots” generate a gradient of magnetic field which amplifies the magnetic force. As a result, the red blood cells are captured on the bottom of the microfluidic channel while the rest of the blood is collected at the outlet. Experimental results show that an average of 95% of red blood cells is trapped in the device.     

Streamline-Based Microfluidic Devices for Erythrocytes and Leukocytes Separation

In this paper, we report two devices for the continuous size-based separation of particles, such as blood cells, which is an important step for on-chip blood preparation. Unlike previously demonstrated passive fluidic devices for particle separation, the local geometry of the bifurcated side channels was used as a design parameter. The design of the devices was based on 2-D fluidic simulation of a T-shaped model. This novel approach was proved to be effective in predicting device performance. The critical particle size for separation was clearly defined in the bifurcated region by simulation under the established theoretical framework. We validated the operation principle of the devices by separating 5- and 10-$muhbox{m}$ polystyrene beads. Human leukocytes were also successfully separated from erythrocytes with 97% efficiency. The separation region of the device had a small footprint for the separation of particles in micrometer range, which makes this device a good candidate to be integrated into a lab-on-a-chip system. The particles were collected in different exit channels after they were separated, which facilitated further sensing and processing. Similar to cross-flow filters, particles were separated perpendicular to the flow direction. The filtering effect was achieved with the collection zones established by the fluidic field. Clogging was minimized by designing the minimal channel width of the devices larger than the largest particle diameter. Solvent exchange could be accomplished for particles. $hfill$[2007-0196]      

Fabrication and integration of microscale permanent magnets for particle separation in microfluidics

Microfluidic magnetophoresis is an effective technique to separate magnetically labeled bioconjugates in lab-on-a-chip applications. However, it is challenging and expensive to fabricate and integrate microscale permanent magnets into microfluidic devices with conventional methods that use thin-film deposition and lithography. Here, we propose and demonstrate a simple and low-cost technique to fabricate microscale permanent magnetic microstructures and integrate them into microfluidic devices. In this method, microstructure channels were fabricated next to a microfluidic channel and were injected with a liquid mixture of neodymium (NdFeB) powders and polydimethylsiloxane (PDMS). After the mixture was cured, the resulted solid NdFeB–PDMS microstructure was permanently magnetized to form microscale magnets. The microscale magnets generate strong magnetic forces capable of separating magnetic particles in microfluidic channels. Systematic experiments and numerical simulations were conducted to study the geometric effects of the microscale magnets. It was found that rectangular microscale magnets generate larger \(({\mathbf {H}}\cdot \nabla ) {\mathbf {H}}\) which is proportional to magnetic force and have a wider range of influence than the semicircle or triangle magnets. For multiple connected rectangular microscale magnet, additional geometric parameters, including separation distance, height and width of the individual elements, further influence the particle separation and were characterized experimentally. With an optimal size combination, complete separation of yeast cells and magnetic microparticles of similar sizes (\(4\;\upmu \hbox {m}\)) was demonstrated with the multi-rectangular magnet microfluidic device.     

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.     

Combined circuit/device modeling and simulation of integrated microfluidic systems

A combined circuit/device model for the analysis of integrated microfluidic systems is presented. The complete model of an integrated microfluidic device incorporates modeling of fluidic transport, chemical reaction, reagent mixing, and separation. The fluidic flow is generated by an applied electrical field or by a combined electrical field and pressure gradient. In the proposed circuit/device model, the fluidic network has been represented by a circuit model and the functional units of the /spl mu/-TAS (micro Total Analysis System) have been represented by appropriate device models. We demonstrate the integration of the circuit and the device models by using an example, where the output from the fluidic transport module serves as the input for the other modules such as mixing, chemical reaction and separation. The combined circuit/device model can be used for analysis and design of entire microfluidic systems with very little computational expense, while maintaining the desired level of accuracy.     

Improved concentration and separation of particles in a 3D dielectrophoretic chip integrating focusing, aligning and trapping

This article presents a dielectrophoresis (DEP)-based microfluidic device with the three-dimensional (3D) microelectrode configuration for concentrating and separating particles in a continuous throughflow. The 3D electrode structure, where microelectrode array are patterned on both the top and bottom surfaces of the microchannel, is composed of three units: focusing, aligning and trapping. As particles flowing through the microfluidic channel, they are firstly focused and aligned by the funnel-shaped and parallel electrode array, respectively, before being captured at the trapping unit due to negative DEP force. For a mixture of two particle populations of different sizes or dielectric properties, with a careful selection of suspending medium and applied field, the population exhibits stronger negative DEP manipulated by the microelectrode array and, therefore, separated from the other population which is easily carried away toward the outlet due to hydrodynamic force. The functionality of the proposed microdevice was verified by concentrating different-sized polystyrene (PS) microparticles and yeast cells dynamically flowing in the microchannel. Moreover, separation based on size and dielectric properties was achieved by sorting PS microparticles, and isolating 5 μm PS particles from yeast cells, respectively. The performance of the proposed micro-concentrator and separator was also studied, including the threshold voltage at which particles begin to be trapped, variation of cell-trapping efficiency with respect to the applied voltage and flow rate, and the efficiency of separation experiments. The proposed microdevice has various advantages, including multi-functionality, improved manipulation efficiency and throughput, easy fabrication and operation, etc., which shows a great potential for biological, chemical and medical applications.     

Effects of particle–fluid coupling on particle transport and capture in a magnetophoretic microsystem

A numerical analysis is presented of the effects of particle–fluid coupling on the transport and capture of magnetic particles in a microfluidic system under the influence of an applied magnetic field. Particle motion is predicted using a computational fluid dynamic CFD-based Lagrangian–Eulerian approach that takes into account dominant particle forces as well as two-way particle–fluid coupling. Two dimensionless groups are introduced that characterize particle capture, one that scales the magnetic and hydrodynamic forces on the particle and another that scales the distance to the magnetic field source. An analysis is preformed to parameterize capture efficiency with respect to the dimensionless numbers for both one-way and two-way particle–fluid coupling. For one-way coupling, in which the flow field is uncoupled from particle motion, correlations are developed that provide insight into system performance towards optimization. The difference in capture efficiency for one-way versus two-way coupling is analyzed and quantified. The analysis demonstrates that one-way coupling, in the dilute limit, provides a conservative estimate of capture efficiency in that it overpredicts the magnetic force needed to ensure particle capture as compared with a more rigorous fully coupled analysis. In two-way coupling there is a cooperative effect between the magnetic force and a particle-induced fluidic force that enhances capture efficiency. Thus, while one-way coupling is useful for rapid parametric screening of particle capture performance, more accurate predictions require two-way particle–fluid coupling. This is especially true when considering higher capture efficiencies and/or higher particle concentrations.     

Microfluidic counterflow centrifugal elutriation system for sedimentation-based cell separation

We present a centrifugal microfluidic system for precise cell/particle sorting using the concept of counterflow centrifugal elutriation (CCE). A conventional CCE system uses a rotor device incorporating a flow-through separation chamber, in which the balance of centrifugal and counterflow drag forces exerted on particles is gradually shifted by changing the flow rate and/or the rotation speed. In the present system, both the centrifugal and the fluid forces are generated through microdevice rotation in order to significantly simplify the setup of the conventional CCE. In addition, the density gradient of the medium is employed to elute particles/cells of different sedimentation velocities stepwise from the separation chamber instead of changing the rotation speed. We successfully separated polymer particles with diameters of 1.0–5.0 μm using a branched loading channel for focusing particles to the center of the separation chamber. We also demonstrated the sorting of blood cells for biological applications. This system may provide a versatile means for cell/particle sorting in a general biological laboratory and function as a unit operation in various centrifugal microfluidic platforms for biochemical experiments and clinical diagnosis.     

A fluidic interconnection system for polymer-based microfluidic devices

A microfabricated fluidic interconnection system for polymer-based microfluidic nebulizer chips is presented and discussed. The new interconnection mechanism can be used to make fluidic connection between external capillary and the polymer microfluidic chip. The connector mechanism was fabricated using a combination of mechanical milling and laser micromachining. Preliminary leakage tests were performed to demonstrate that the interconnection system is leak-free and pressure tests were performed to evaluate the burst pressure (maximum working pressure). The interconnection system has several advantages over commercially available Nanoport™ interconnection system. The new fluidic interconnection system implemented onto a microfluidic nebulizer chip was successfully tested for desorption electrospray ionization mass spectrometry applications. The performance of the chip using the new connector mechanism was excellent demonstrating the usability of the new connector mechanism.     

Effect of reservoir geometry on vortex trapping of cancer cells

Vortex-aided particle separation is a powerful method to efficiently isolate circulating tumor cells from blood, since it allows high throughput and continuous sample separation, with no need for time-consuming sample preprocessing. With this approach, only the larger particles from a heterogeneous sample will be stably trapped in reservoirs that expand from a straight microfluidic channel, allowing for efficient particle sorting along with simultaneous concentration. A possible limitation is related to the loss of particles from vortex traps due to particle–particle interactions that limit the final cellularity of the enriched solution. It is fundamental to minimize this issue considering that a scant number of target cells are diluted in highly cellular blood. In this work, we present a device for size-based particle separation, which exploits the well-consolidated vortex-aided sorting, but new reservoir layouts are presented and investigated in order to increase the trapping efficiency of the chip. Through simulations and experimental validations, we have been able to optimize the device design to increase the maximum number of particles that can be stably trapped in each reservoir and therefore the total efficiency of the chip.     

A portable,hand-powered microfluidic device for sorting of biological particles

Manually hand-powered portable microfluidic devices are cheap alternatives for point-of-care diagnostics. Currently, on-field tests are limited by the use of bulky syringe pumps, pressure controller and equipment. In this work, we present a manually operated microfluidic device incorporated with a groove-based channel. We show that the device is capable to effectively sort particles/cells by manual hand powering. First, the grooved-based channel with differently sized polystyrene particles was characterized using syringe pumps to study their distributions under various flow rate conditions. Afterward, the particle mixtures were sorted manually using hand power to verify the capability of this device. Finally, the manually operated device was used to sort platelets from peripheral blood mononuclear cells (PBMCs). The platelets were collected with a purity of ~ 100%. The purity of PBMCs was enhanced from 0.8 to 10.4% after multiple processes which results in an enrichment ratio of 13.8. During the process of manual hand pumping, the flow fluctuation caused by unstable injection will not influence the sorting performance. Due to its simplicity, this manually operated microfluidic chip is suitable for outfield settings.     

Three-dimensional and analytical modeling of microfluidic particle transport in magnetic fluids

We present an analytical model that can predict the three-dimensional (3D) transport of non-magnetic particles in magnetic fluids inside a microfluidic channel coupled with permanent magnets. The magnets produce a spatially non-uniform magnetic field that gives rise to a magnetic buoyancy force on the particles. Resulting 3D trajectories of the particles are obtained by (1) calculating the 3D magnetic buoyancy force exerted on the particles via an analytical distribution of magnetic fields as well as their gradients, together with a nonlinear magnetization model of the magnetic fluids, (2) deriving the 3D hydrodynamic viscous drag force on the particles with an analytical velocity profile of a low Reynolds number ferrohydrodynamic flow in the channel including “wall effect” and magnetoviscous effect of the magnetic fluids, and (3) constituting and solving the governing equations of motion for the particles using the analytical expressions of magnetic buoyancy force and hydrodynamic viscous drag force. We use such a model to study the particles’ trajectories in the channel and investigate the magnitude of their deflections at different flow rates, with different properties of magnetic fluids and different geometrical parameters of the system.     

Inertial microfluidics for continuous particle filtration and extraction

In this paper, we describe a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration. The design takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces. These dominant inertial forces cause particles to move laterally and occupy equilibrium positions along the longer vertical microchannel walls. Using this principle, we demonstrate extraction of 590 nm particles from a mixture of 1.9 μm and 590 nm particles in a straight microfluidic channel with rectangular cross-section. Based on the theoretical analysis and experimental data, we describe conditions required for predicting the onset of particle equilibration in square and rectangular microchannels. The microfluidic channel design has a simple planar structure and can be easily integrated with on-chip microfluidic components for filtration and extraction of wide range of particle sizes. The ability to continuously and differentially equilibrate particles of different size without external forces in microchannels is expected to have numerous applications in filtration, cytometry, and bioseparations.     

Dielectrophoresis-based microfluidic platform to sort micro-particles in continuous flow

Non-invasive separation of particles with different sizes and sensitivities has been a challenge and interest for point-of-care diagnostics and personalized treatment. Dielectrophoresis is widely known as a powerful technique to sort the particles and (most importantly to) distinguish cells and monitor their state without the need for biochemical tags. In this paper, a dielectrophoresis-based microchannel design is proposed which allows for continuous particle sorting and separation under the applied AC field. It is also practical to implement the platform for monitoring cell behavior irregularities caused by certain diseases toward diagnosis and treatment. In this regard, the device employs dielectrophoretic (DEP) force exerted on the particles by only two electrodes with oblique arrangement in the channel. The electrodes are arranged with a bevel angle to the fluid flow direction but they are not parallel and therefore a gradually decreasing electric field is achieved along the channel’s width. As a result, the dielectrophoretic force, acting on the particles of different sizes, would also gradually decrease along channels width which renders the necessary distinguishing lateral displacements of particles for separation. Therefore, the particles with different sizes can be sorted in a continuous-flow regime and be received at multiple outlet reservoirs with no need to turn the electric field on/off. The presented device is fabricated and evaluated in the experiment to prove its feasibility. Afterward, using numerical simulations, we investigate the optimum design parameters in the presented device to enhance device efficiency for separating particles with different size ranges.