Sheathless hydrophoretic particle focusing in a microchannel with exponentially increasing obstacle arrays

We present a novel microfluidic device with exponentially increasing obstacle arrays to enable sheathless particle focusing. The anisotropic fluidic resistance of slant obstacles generates transverse flows, along which particles are focused to one sidewall. In the successive channel with exponentially increasing widths, bent obstacles extended from the slant obstacles increase the focusing efficiency of the particles. With the device, we achieved the focusing efficiency of 76%, 94%, and 98% for 6, 10, and 15 microm beads, respectively. The focusing efficiency of the particles can be further improved in the devices with more extension steps. In addition, using the microfluidic devices with the symmetric structure of the slant and bent obstacles, we achieved complete focusing of biological cells to the centerline of a channel within 1.7% coefficient of variation. The results demonstrated the sheathless hydrophoretic focusing of microparticles and cells with the advantages of a sheathless method, passive operation, single channel, and flow rate independence.     

Optically Coated Mirror‐Embedded Microchannel to Measure Hydrophoretic Particle Ordering in Three Dimensions

Three‐dimensional (3D) measurement of the behavior of microfluidic particles is vital for improving their operational efficiency and characterization. In particular, it is important to measure particle motions in 3D for exact characterization of hydrophoresis, which utilizes 3D convective flows for size separation. Herein, the 3D measurement of hydrophoretic particle ordering for the exact characterization of hydrophoresis by using an optically coated mirror‐embedded microchannel is reported. The mirror, ideally at 45°, reflects the side view of the channel and enables 3D positional information to be obtained easily from two different orthogonal‐axis images. With this method, it is shown that hydrophoresis is governed by convective vortices and steric hindrance. It is also observed that hydrophoresis enables 3D particle focusing without sheath flows and accurate flow‐rate control. The mechanism of hydrophoresis is finally verified by conducting a computational simulation and comparing the simulation results with the experimental measurements. The hydrophoretic method can be straightforwardly integrated as a 3D particle‐focusing component in integrated microfluidic systems. The mirror‐embedded channel can also be readily fabricated in a single cast of polydimethylsiloxane, thus offering low‐cost, easy implementation of 3D particle measurement.     

Variable optical attenuator with a polymer-stabilized dual-frequency liquid crystal

A transmission-type variable optical attenuator (VOA) based on a polymer-stabilized dual-frequency liquid crystal (PSDFLC) is demonstrated at gamma = 1.55 microm. The VOA is highly transparent in the voltage-off state but scatters light in the voltage-on state. By using a birefringent beam displacer incorporated with half-wave plates, we can obtain a VOA that is polarization independent and that exhibits a 31 dB dynamic range. The polymer networks and dual-frequency effect together reduce the response time (rise + decay) of a 16 microm PSDFLC cell to 30 ms at room temperature and at a voltage of 24 Vrms.     

Gravity-driven microfluidic particle sorting device with hydrodynamic separation amplification

This paper describes a simple microfluidic sorting system that can perform size profiling and continuous mass-dependent separation of particles through combined use of gravity (1 g) and hydrodynamic flows capable of rapidly amplifying sedimentation-based separation between particles. Operation of the device relies on two microfluidic transport processes: (i) initial hydrodynamic focusing of particles in a microchannel oriented parallel to gravity and (ii) subsequent sample separation where positional difference between particles with different mass generated by sedimentation is further amplified by hydrodynamic flows whose streamlines gradually widen out due to the geometry of a widening microchannel oriented perpendicular to gravity. The microfluidic sorting device was fabricated in poly(dimethylsiloxane), and hydrodynamic flows in microchannels were driven by gravity without using external pumps. We conducted theoretical and experimental studies on fluid dynamic characteristics of laminar flows in widening microchannels and hydrodynamic amplification of particle separation. Direct trajectory monitoring, collection, and post-analysis of separated particles were performed using polystyrene microbeads with different sizes to demonstrate rapid (<1 min) and high-purity (>99.9%) separation. Finally, we demonstrated biomedical applications of our system by isolating small-sized (diameter <6 microm) perfluorocarbon liquid droplets from polydisperse droplet emulsions, which is crucial in preparing contrast agents for safe, reliable ultrasound medical imaging, tracers for magnetic resonance imaging, or transpulmonary droplets used in ultrasound-based occlusion therapy for cancer treatment. Our method enables straightforward, rapid, real-time size monitoring and continuous separation of particles in simple stand-alone microfabricated devices without the need for bulky and complex external power sources. We believe that this system will provide a useful tool to separate colloids and particles for various analytical and preparative applications and may hold potential for separation of cells or development of diagnostic tools requiring point-of-care sample preparation or testing.     

3D focusing of nanoparticles in microfluidic channels

Dynamic focusing of particles can be used to centre particles in a fluid stream, ensuring the passage of the particles through a specified detection volume. This paper describes a method for focusing nanoparticles using dielectrophoresis. The method differs from other focusing methods in that it manipulates the particles and not the fluid. Experimental focusing is demonstrated for a range of different particle types, and discussed in terms of the operational limits of the device. Dynamic numerical simulations of the particle motion in the device are presented and compared with the experimental results. The potential of the device for nanoparticle control and manipulation in microfluidic chips is discussed.     

Optical characteristics of a refractive optical attenuator with respect to the wedge angles of a silicon optical leaker

We design, fabricate, and characterize the micromachined refractive variable optical attenuator (VOA) with a wedge-shaped silicon optical leaker (SOL). The vertical structures of the VOA device can be simply fabricated by deep reactive ion etching with no sidewall metallization, and the 8 degrees angled fibers are employed for a high return loss even in air-ambient conditions. The SOL successively transmits and refracts part of the incident light far outside the acceptance angle of the output fiber, showing an effective optical attenuation. The fabricated VOA gives high optical performances, such as a response time of 6 ms, a return loss of 39 dB, an insertion loss of 0.6 dB, an attenuation range of 43 dB, and a polarization-dependent loss (PDL) of a 10% attenuation level, including a wavelength-dependent loss. The optical characteristics of the VOA are also theoretically investigated with respect to the wedge angles of the SOL. The experimental characteristics are in good agreement with the theoretical values calculated, considering light scattered from the endface of an optical fiber and sidewall of the SOL. The PDL estimation was confirmed especially to sufficiently explain the fundamental characteristic of the PDL for the refractive VOA.     

A new micromechanism for transformation of small displacements to large rotations for a VOA

New movement translation micromechanism (MTM) is proposed to transfer and amplify small in-plane displacement or movement into large out-of-plane vertical displacement or rotation. Based on this MTM, we may just apply 3-V dc load to generate 3.1-/spl mu/m in-plane movement, then 26.4/spl deg/ rotation angle of pop-up micromirror can be subsequently derived. An axial aligned fiber-to-fiber variable optical attenuator (VOA) device using the MTM, a U-shaped electrothermal actuator array, and the pop-up micromirror to reflect the attenuated light toward out-of-plane direction is designed and characterized. The proposed new VOA device achieves 37-dB attenuation range under 3-V dc load, while return loss, polarization-dependent loss, and wavelength-dependent loss at attenuation of 3 dB are measured as -45B, 0.05, and 0.28 dB. This new concept of steering a portion of input light toward out-of-plane direction is proven to be feasible for VOA applications.     

On-chip high-speed sorting of micron-sized particles for high-throughput analysis

A new design of particle sorting chip is presented. The device employs a dielectrophoretic gate that deflects particles into one of two microfluidic channels at high speed. The device operates by focussing particles into the central streamline of the main flow channel using dielectrophoretic focussing. At the sorting junction (T- or Y-junction) two sets of electrodes produce a small dielectrophoretic force that pushes the particle into one or other of the outlet channels, where they are carried under the pressure-driven fluid flow to the outlet. For a 40 microm wide and high channel, it is shown that 6 microm diameter particles can be deflected at a rate of 300/s. The principle of a fully automated sorting device is demonstrated by separating fluorescent from non-fluorescent latex beads.     

Analysis of electrotonic coupling in patterned neuronal networks

Microcontact printing of laminin is known as an efficient approach for guiding neuronal cell migration and neurite outgrowth on artificial surfaces. In the present study, ultrathin (approximately 250 microm) brain stem slices of Sprague-Dawley rats (E15-E18) were cultured on laminin-patterned substrates, such that neuronal cells migrating out of the slices formed grid-shaped neuronal networks along the geometry defined by the pattern. The interconnections between neighbouring pairs of neurons within these artificial networks were assessed electrophysiologically by double patch-clamp recordings and optically by microinjection of fluorescent dyes. Both functional and electrotonic synapses were detected. Based on the recorded data and simulations in PSpice, an electrical model for electrotonically coupled cells was derived. In this model the neuritic pathway is described as a cylindric cable, and gap junctions are represented by an ohmic resistor. Applying this model in the data analysis, the average inner radius of neurites could be determined to be approximately 0.1 microm. In addition, evidence was found for a correlation between the path-width of the applied pattern and the diameter of neurites growing along these paths.     

Development of a rare cell fractionation device: application for cancer detection

Isolating rare cells from biological fluids including whole blood or bone marrow is an interesting biological problem. Characterization of a few metastatic cells from cancer patients for further study is desirable for prognosis/diagnosis. Traditional methods have not proven adequate, due to the compositional complexity of blood, with its large numbers of cell types. To separate individual cells based on their mechanical characteristics, we have developed a series of massively parallel microfabricated sieving device. These devices were constructed with four successively narrower regions of channels numbering /spl sim/1800 per region. As cells traversed the device, they encountered each region and stopped at a gap width that prohibited passage due to their size. Cultured neuroblastoma cells, when mixed with whole blood and applied to the device, were retained in the 10-/spl mu/m-wide by 20-/spl mu/m-deep channels. All other cells migrated to the output. A derivative of the same device was utilized to characterize migration of whole blood. Adult white blood cells were retained at the 2.5-/spl mu/m-wide by 5-/spl mu/m-deep channels, while red blood cells passed through these channels. Devices designed to capture rare cells in peripheral circulation for downstream analysis will provide an important tool for diagnosis and treatment.     

Perfusion in microfluidic cross-flow: separation of white blood cells from whole blood and exchange of medium in a continuous flow

We describe a microfluidic technique for separation of particles and cells and a device that employs this technique to separate white blood cells (WBC) from whole human blood. The separation is performed in cross-flow in an array of microchannels with a deep main channel and large number of orthogonal, shallow side channels. As a suspension of particles advances through the main channel, a perfusion flow through the side channels gradually exchanges the medium of the suspension and washes away particles that are sufficiently small to enter the shallow side channels. The microfluidic device is tested with a suspension of polystyrene beads and is shown to efficaciously exchange the carrier medium while retaining all beads. In tests with whole human blood, the device is shown to reduce the content of red blood cells (RBC) by a factor of approximately 4000 with retention of 98% of WBCs. The ratio between WBCs and RBCs reached at an outlet of the device is 2.4 on average. The device is made of a single cast of poly(dimethylsiloxane) sealed with a cover glass and is simple to fabricate. The proposed technique of separation by perfusion in continuous cross-flow could be used to enrich rare populations of cells based on differences in size, shape, and deformability.     

熔锥型光纤声光可变衰减器

利用耦合模理论对光纤熔锥声光器件进行了数值模拟,得到了全光纤声光衰减器传输谱和可调谐性。分析了带宽与声波长、耦合长度的关系。数值分析结果表明,声波在光纤熔锥中引起的轴向电介质微扰、耦合长度和工作波长都会对器件的传输谱产生影响,选择合适的设计参数可以制作较为理想的声光衰减器。实验上获得了损耗小于0．2dB,带宽大于200nm,动态范围为20dB的单模光纤熔锥可变衰减器,所得结果与理论分析相符合。这种器件可用于光纤通信及光纤传感。     

A ferrofluid guided system for the rapid separation of the non-magnetic particles in a microfluidic device

A microfluidic device was fabricated via UV lithography technique to separate non-magnetic fluoresbrite carboxy microspheres (approximately 4.5 microm) in the pH 7 ferrofluids made of magnetite nanoparticles (approximately 10 nm). A mixture of microspheres and ferrofluid was injected to a lithographically developed Y shape microfluidic device, and then by applying the external magnet fields (0.45 T), the microspheres were clearly separated into different channels because of the magnetic force acting on those non-magnetic particles. During this study, various pumping speeds and particle concentrations associated with the various distances between the magnet and the microfluidic device were investigated for an efficient separation. This study may be useful for the separation of biological particles, which are very sensitive to pH value of the solutions.     

Multinode acoustic focusing for parallel flow cytometry

Flow cytometry can simultaneously measure and analyze multiple properties of single cells or particles with high sensitivity and precision. Yet, conventional flow cytometers have fundamental limitations with regards to analyzing particles larger than about 70 μm, analyzing at flow rates greater than a few hundred microliters per minute, and providing analysis rates greater than 50,000 per second. To overcome these limits, we have developed multinode acoustic focusing flow cells that can position particles (as small as a red blood cell and as large as 107 μm in diameter) into as many as 37 parallel flow streams. We demonstrate the potential of such flow cells for the development of high throughput, parallel flow cytometers by precision focusing of flow cytometry alignment microspheres, red blood cells, and the analysis of a CD4+ cellular immunophenotyping assay. This approach will have significant impact toward the creation of high throughput flow cytometers for rare cell detection applications (e.g., circulating tumor cells), applications requiring large particle analysis, and high volume flow cytometry.     

Continuous sorting of microparticles using dielectrophoresis

Sorting of particles such as cells is a critical process for many biomedical applications, and it is challenging to integrate it into an analytical microdevice. We report an effective and flexible dielectrophoresis (DEP)-based microfluidic device for continuous sorting of multiple particles in a microchannel. The particle sorter is composed of two components-a DEP focusing unit and a Movable DEP Trap (MDT). The trap is formed by an array of microelectrodes at the bottom of the channel and a transparent electrode plate placed at the top. The location of the trap is dependent on the configuration of voltages on the array and therefore is addressable. Flowing particles are first directed and focused into a single particle stream by the focusing unit. The streamed particles are then sorted into different fractions using the movable trap by rapidly switching the applied voltage. The performance of the sorter is demonstrated by successfully sorting microparticles in a continuous flow. The proposed DEP-based microfluidic sorter can be implemented in applications such as sample preparation and cell sorting for subsequent analytical processing, where sorting of particles is needed.     

Size separation of supermicrometer particles in asymmetrical flow field-flow fractionation. Flow conditions for rapid elution

The performance of lift-hyperlayer asymmetrical flow field-flow fractionation using rapid elution conditions was tested through the separation of standard polystyrene latex particles of diameters from 2 to 20 microm. Optimization of flowrates was studied not only in order to obtain efficient and rapid separation, but also to work under conditions of various shape and steepness of the axial flow velocity gradient. Using extreme flow conditions, the five widely spaced particle sizes, 20.5-, 15.0-, 9.7-, 5.0-, and 2.0-microm diameter, could be resolved in 6 min, whereas for the narrower size range of 20.5-5.0 microm, 1 min was enough. The size selectivity in the size range 9.7-2.0 microm was studied as a function of flowrates and particle size and was found to be constant. A particle trapping device made it possible to separate particles of sizes > 10 microm, which has previously proven to be difficult in asymmetrical channels.     

Electrophoretic cell manipulation and electrochemical gene-function analysis based on a yeast two-hybrid system in a microfluidic device

A novel microfluidic device with an array of analytical chambers was developed in order to perform single-cell-based gene-function analysis. A series of analytical processes was carried out using the device, including electrophoretic manipulation of single cells and electrochemical measurement of gene function. A poly(dimethylsiloxane) microstructure with a microfluidic channel (150 microm in width, 10 microm in height) and an analytical chamber (100 x 20 x 10 microm (3)) were fabricated and aligned on a glass substrate with an array of Au microelectrodes. Two microelectrodes positioned in the analytical chamber were employed as a working electrode for the electrophoretic manipulation of cells and electrochemical measurements. A yeast strain ( Saccharomyces cerevisiae Y190) carrying the beta-galactosidase reporter gene was used to demonstrate that the device could detect the enzyme. Target cells flowing through the main channel were introduced into the chamber by electrophoresis using the ground electrode laid on the main channel. When the cell was treated with 17beta-estradiol, gene expression was triggered to produce beta-galactosidase, catalyzing the hydrolysis of p-aminophenyl-beta- D-galactopyranoside to form p-aminophenol (PAP). The enzymatically generated PAP was detected by cyclic voltammetry and amperometry at the single-cell level in the chamber of the device. Generator-collector mode amperometry was also applied to amplify the current response originating from gene expression in the trapped single cells. After electrochemical measurement, the trapped cells were easily released from the chamber using electrophoretic force.     

Noninvasive acoustic cell trapping in a microfluidic perfusion system for online bioassays

Techniques for manipulating, separating, and trapping particles and cells are highly desired in today's bioanalytical and biomedical field. The microfluidic chip-based acoustic noncontact trapping method earlier developed within the group now provides a flexible platform for performing cell- and particle-based assays in continuous flow microsystems. An acoustic standing wave is generated in etched glass channels (600x61 microm2) by miniature ultrasonic transducers (550x550x200 microm3). Particles or cells passing the transducer will be retained and levitated in the center of the channel without any contact with the channel walls. The maximum trapping force was calculated to be 430+/-135 pN by measuring the drag force exerted on a single particle levitated in the standing wave. The temperature increase in the channel was characterized by fluorescence measurements using rhodamine B, and levels of moderate temperature increase were noted. Neural stem cells were acoustically trapped and shown to be viable after 15 min. Further evidence of the mild cell handling conditions was demonstrated as yeast cells were successfully cultured for 6 h in the acoustic trap while being perfused by the cell medium at a flowrate of 1 microL/min. The acoustic microchip method facilitates trapping of single cells as well as larger cell clusters. The noncontact mode of cell handling is especially important when studies on nonadherent cells are performed, e.g., stem cells, yeast cells, or blood cells, as mechanical stress and surface interaction are minimized. The demonstrated acoustic trapping of cells and particles enables cell- or particle-based bioassays to be performed in a continuous flow format.     

Portable thermal lens spectrometer with focusing system

A portable thermal lens spectrometer with a precise focusing system was developed. Astigmatism of the reflected excitation beam from the microchip was used for depth direction focusing. For width direction focusing, the scattering effect of the transmitted probe beam by a microchannel edge was used. The focusing system was evaluated with a 250 microm wide x 50 microm deep microchannel. Focusing resolutions for depth and width directions were 1 and 10 microm, respectively. The repeatability of the thermal lens signal (40 microM xylenecyanol solution) was proved to be approximately 1% coefficient of variance when using these focusing methods. The limit of detection for a xylenecyanol solution was 30 nM, and the absorbance was 4.7 x 10(-6) AU. The sensitivity was 20-100 times higher than that obtained by spectrophotometry. In consequence, a practical thermal lens spectrometer was realized.     

Polymer-based flexible microlens arrays with hermaphroditic focusing properties

Polymer microlens arrays with hermaphroditic focusing behaviors are demonstrated. Each microlens in an arrays exhibits either converging or diverging focus, depending on the polarization direction of the incident light. A polymer film with patterned microlens arrays is flexible, lightweight, and ultrathin (approximately 50 microm). Details of the lens structure, device fabrication, and lens performance are described.