Dielectrophoretically assembled particles: feasibility for optofluidic systems

This work presents the dielectrophoretic manipulation of sub-micron particles suspended in water and the investigation of their optical responses using a microfluidic system. The particles are made of silica and have different diameters of 600, 450, and 250 nm. Experiments show a very interesting feature of the curved microelectrodes, in which the particles are pushed toward or away from the microchannel centerline depending on their levitation heights, which is further analyzed by numerical simulations. In doing so, applying an AC signal of 12 V_p–p and 5 MHz across the microelectrodes along with a flow rate of 1 μl/min within the microchannel leads to the formation of a tunable band of particles along the centerline. Experiments show that the 250 nm particles guide the longitudinal light along the microchannel due to their small scattering. This arrangement is employed to study the feasibility of developing an optofluidic system, which can be potentially used for the formation of particles-core/liquid-cladding optical waveguides.     

On-chip separation of Lactobacillus bacteria from yeasts using dielectrophoresis

Dielectrophoresis, the induced motion of dielectric particles in non-uniform electric fields, enables the separation of suspended bio-particles based on their dimensions or dielectric properties. This work presents a microfluidic system, which utilises a combination of dielectrophoretic (DEP) and hydrodynamic drag forces to separate Lactobacillus bacteria from a background of yeasts. The performance of the system is demonstrated at two operating frequencies of 10?MHz and 100?kHz. At 10?MHz, we are able to trap the yeasts and bacteria at different locations of the microelectrodes as they experience different magnitudes of DEP force. Alternatively, at 100?kHz we are able to trap the bacteria along the microelectrodes, while repelling the yeasts from the microelectrodes and washing them away by the drag force. These separation mechanisms might be applicable to automated lab-on-a-chip systems for the rapid and label-free separation of target bio-particles.     

Recent Progress in Fiber Optofluidic Lasing and Sensing

Fiber optofluidic laser(FOFL)integrates optical fiber microcavity and microfluidic channel and provides many unique advantages for sensing applications.FOFLs no...     

Liquid�Cliquid fluorescent waveguides using microfluidic-drifting-induced hydrodynamic focusing

We demonstrate fluorescent liquid-core/liquid-cladding (L ²) waveguides focused in three-dimensional (3-D) space based on a 3-D hydrodynamic focusing technique. In the proposed system, the core and vertical cladding streams are passed through a curved 90° corner in a microfluidic channel, leading to the formation of a pair of counter rotating vortices known as the Dean vortex. As a result, the core fluid is completely confined within the cladding fluid and does not touch the top and bottom poly(dimethylsiloxane) (PDMS) surfaces of the microfluidic channel. Because the core stream was not in contact with the PDMS channel, whose refractive index contrast and optical smoothness with the core fluid are lower than that between the core and the cladding fluids, the 3-D focused L ² waveguide exhibited a higher captured fraction (η) and lower propagation loss when compared to conventional two-dimensional (2-D) focused L ² waveguides. Because the proposed 3-D focused L ² waveguides can be generated in planar PDMS microfluidic devices, such optofluidic waveguides can be integrated with precise alignment together with other in-plane microfluidic and optical components to achieve micro-total analysis systems (μ-TAS).     

Optofluidic sensor system with Ge PIN photodetector for CMOS-compatible sensing

Vertical optofluidic biosensors based on refractive index sensing promise highest sensitivities at smallest area footprint. Nevertheless, when it comes to large-scale fabrication and application of such sensors, cheap and robust platforms for sample preparation and supply are needed—not to mention the expected ease of use in application. We present an optofluidic sensor system using a cyclic olefin copolymer microfluidic chip as carrier and feeding supply for a complementary metal–oxide–semiconductor compatibly fabricated Ge PIN photodetector. Whereas typically only passive components of a sensor are located within the microfluidic channel, here the active device is directly exposed to the fluid, enabling top-illumination. The capability for detecting different refractive indices was verified by different fluids with subsequent recording of the optical responsivity. All components excel in their capability to be transferred to large-scale fabrication and further integration of microfluidic and sensing systems. The photodetector itself is intended to serve as a platform for further sophisticated collinear sensing approaches.     

Optofluidics technology based on colloids and their assemblies

Optofluidic technology is believed to provide a breakthrough for the currently underlying problems in microfluidics and photonics/optics by complementary integration of fluidics and photonics. The key aspect of the optofluidics technology is based on the use of fluidics for tuning the optical properties and addressing various functional materials inside of microfluidic channels which have build-in photonic structures. Through the optofluidic integrations, fluidics enhances the controllability and tunability of optical systems. In particular, colloidal dispersion gives novel properties such as photonic band-gaps and enhanced Raman spectrum that conventional optofluidic devices cannot exhibit. In this paper, the state of the art of the colloidal dispersions is reviewed especially for optofluidic applications. From isolated singlet colloidal particles to colloidal clusters, their self-organized assemblies lead to optical manipulation of the photonic/optical properties and responses. Finally, we will discuss the prospects of the integrated optofluidics technology based on colloidal systems.     

A microfluidic system combining acoustic and dielectrophoretic particle preconcentration and focusing

Microfabricated systems have recently become useful for routing particles to precise locations in microfluidic channels. In this paper we discuss the modeling, fabrication and characterization of such a platform that combines acoustic forces and ac dielectrophoresis (DEP). This system integrates a bulk lead zirconate titanate (PZT) slab with substrate patterned microelectrodes for DEP manipulation of particles. Moreover, a one-dimensional transmission line model is presented to understand the coupling of the acoustic and dielectrophoretic transducers with the microdevice. While the acoustic model does not predict the lateral coupling in the system, it does provide some insight into axial (thickness-mode) frequencies of operation. Experiments are also conducted in which particles were routed into a large (0.75 mm wide) microchannel and preconcentrated and focused into coarse bundles by coupling an acoustic wave into the channel. Subsequently, particles are further focused into single file particle streams using interdigitated DEP electrodes. This system can be used for high throughput assays for which it is necessary to isolate and investigate small bundles of particles and single particles.     

Miniaturized flow cytometer with 3D hydrodynamic particle focusing and integrated optical elements applying silicon photodiodes

In this study, the design, realization and measurement results of a novel optofluidic system capable of performing absorbance-based flow cytometric analysis is presented. This miniaturized laboratory platform, fabricated using SU-8 on a silicon substrate, comprises integrated polymer-based waveguides for light guiding and a biconcave cylindrical lens for incident light focusing. The optical structures are detached from the microfluidic sample channel resulting in a significant increase in optical sensitivity. This allows the application of standard solid-state laser and standard silicon-based photodiodes operated by lock-in-amplification resulting in a highly practical and effective detection system. The easy-to-fabricate single-layer microfluidic structure enables independently adjustable 3D hydrodynamic sample focusing to an arbitrary position in the channel. To confirm the fluid dynamics and raytracing simulations and to characterize the system, different sets of microparticles and T-lymphocyte cells (Jurkat cell line) for vital staining were investigated by detecting the extinction (axial light loss) signal. The analytical classification via signal peak height/width demonstrates the high sensitivity and sample discrimination capability of this compact low-cost/low-power microflow cytometer.     

Two optofluidic devices for the refractive index measurement of small volume of fluids

Two simple optofluidic devices based on a microprism and a refraction channel, respectively, are proposed for measuring the refractive index of fluids. The microprism chip consists of an optical waveguide channel and a single triangular chamber filled with the test fluid, while the refraction channel chip consists of a single turning channel which functions as a liquid-core/solid-cladding optical waveguide. In both chips, the refractive index of the test fluid is calculated from a CCD image of the refracted ray in accordance with simple geometrical optics principles. The experimental results show that the refraction channel chip provides a more accurate measurement performance than the microprism chip; particularly for colloidal samples. However, the refraction channel chip is suitable only for the measurement of fluids with a refractive index greater than that of the chip substrate. Overall, the results presented in this study show that both devices provide a simple, low cost and effective means of determining the refractive index of a wide range of common test fluids with nano-liter volume.     

A microfluidic device for rapid concentration of particles in continuous flow by DC dielectrophoresis

This article presents a microfluidic device (so called concentrator) for rapid and efficient concentration of micro/nanoparticles using direct current dielectrophoresis (DC DEP) in continuous fluid flow. The concentrator is composed of a series of microchannels constructed with PDMS-insulating microstructures to focus efficiently the electric field in the flow direction to provide high field strength and gradient. Multiple trapping regions are formed within the concentrator. The location of particle trapping depends on the strength of the electric field applied. Under the experimental conditions, both streaming movement and DEP trapping of particles simultaneously take place within the concentrator at different regions. The former occurs upstream and is responsible for continuous transport of the particles, whereas the latter occurs downstream and rapidly traps the particles delivered from upstream. The observation agrees with the distribution of the simulated electric field and DEP force. The performance of the device is demonstrated by successfully and effectively concentrating fluorescent nanoparticles. At the sufficiently high electric field, the device demonstrates a trapping efficiency of 100%, which means downstream DEP traps and concentrates all (100%) the incoming particles from upstream. The trapping efficiency of the device is further studied by measuring the fluorescence intensity of concentrated particles in the channel. Typically, the fluorescence intensity becomes saturated in Trap 1 by applying the voltage (400 V) for >2 min, demonstrating that rapid concentration of the nanoparticles (10⁷ particles/ml) is achieved in the device. The microfluidic concentrator described can be implemented in applications where rapid concentration of targets is needed such as concentrating cells for sample preparation and concentrating molecular biomarkers for detection.     

Microfluidic device with dual-channel fluorescence acquisition for quantification/identification of cancer cells

This paper presents an optofluidic device for cell discrimination with two independent interrogation regions. Pumping light is coupled to the device, and cell fluorescence is extracted from the two interrogation zones by using optical fibers embedded in the optofluidic chip. To test the reliability of this device, AU-565 cells—expressing EpCAM and HER2 receptors—and RAMOS cells were mixed in a controlled manner, confined inside a hydrodynamic focused flow in the microfluidic chip and detected individually so that they could be discriminated as positive (signal reception from fluorescently labeled antibodies from the AU-565 cells) or negative events (RAMOS cells). A correlation analysis of the two signals reduces the influence of noise on the overall data.     

Optofluidic waveguides: II. Fabrication and structures

We review fabrication methods and common structures for optofluidic waveguides, defined as structures capable of optical confinement and transmission through fluid filled cores. Cited structures include those based on total internal reflection, metallic coatings, and interference based confinement. Configurations include optical fibers and waveguides fabricated on flat substrates (integrated waveguides). Some examples of optofluidic waveguides that are included in this review are Photonic Crystal Fibers (PCFs) and two-dimensional photonic crystal arrays, Bragg fibers and waveguides, and Anti Resonant Reflecting Optical Waveguides (ARROWs). An emphasis is placed on integrated ARROWs fabricated using a thin-film deposition process, which illustrates how optofluidic waveguides can be combined with other microfluidic elements in the creation of lab-on-a-chip devices.     

Preliminary scanning fluorescence detection of a minute particle running along a waveguide implemented microfluidic channel using a light switching mechanism

It has long been thought that an optical sensor, such as a light waveguide implemented total analysis system (TAS), is one of the functional components that will be needed to realize a “ubiquitous human healthcare system” in the near future. We have already proposed the fundamental structure for a light waveguide capable of illuminating a living cell or particle running along a microfluidic channel, as well as of detecting fluorescence even from the extremely weak power of such a minute particle. In order to develop novel functions to detect the internal structure of living cells quickly, an angular scanning method that sequentially changes the direction of illumination of the minute cell or particle may be crucial. In this paper, we investigate fluorescence detection from moving particles by switching the laser power delivery path of plural light waveguides as a preliminary experiment toward this novel method. To construct an experimental system able to incorporate a switching light source mechanism cost effectively, we utilized a conventional TAS chip with plural waveguide pairs arranged in parallel, and a forced vibration mechanism on an optical fiber tip by a piezoelectric actuator. With this system, we performed an experiment to detect extremely weak fluorescence using micro particles with a fluorescent substance attached and an optical TAS chip that incorporated a microfluidic channel and three pairs of laser-power-delivering light waveguide cores. We successfully obtained clear, quasi-triangular-shaped pulses in fluorescent signals from resin particles running across the intersection under three different conditions: (1) a particle with approximately the same velocity as that of a forced-vibrated optical fiber tip of approximately 700 mm/s, (2) a particle with velocity 1 digit smaller than that of an optical fiber tip, and (3) a particle with velocity approximately 1/20 that of an optical fiber tip.     

基于石英和载玻片的微电极制备工艺研究

研究以石英片和载玻片为基底的微电极制备工艺,以较常见的叉指型电极为例,基于光刻工艺中各工艺参数要求,重点研究曝光和显影时间对微电极的影响。制得的2种不同基底的电极,采用对比度、精度等关键参数进行分析,最终确定选用载玻片作为芯片基底并获得其最佳工艺参数;制作带电极的PDMS—玻璃微流控芯片,通过实验,成功观察到酵母菌细胞的正、负介电泳现象。     

Nanomanipulation of single influenza virus using dielectrophoretic concentration and optical tweezers for single virus infection to a specific cell on a microfluidic chip

A major problem when analyzing bionanoparticles such as influenza viruses (approximately 100 nm in size) is the low sample concentrations. We developed a method for manipulating a single virus that employs optical tweezers in conjunction with dielectrophoretic (DEP) concentration of viruses on a microfluidic chip. A polydimethylsiloxane microfluidic chip can be used to stably manipulate a virus. The chip has separate sample and analysis chambers to enable quantitative analysis of the virus functions before and after it has infected a target cell. The DEP force in the sample chamber concentrates the virus and prevents it from adhering to the glass substrate. The concentrated virus is transported to the sample selection section where it is trapped by optical tweezers. The trapped virus is transported to the analysis chamber and it is brought into contact with the target cell to infect it. This paper describes the DEP virus concentration for single virus infection of a specific cell. We concentrated the influenza virus using the DEP force, transported a single virus, and made it contact a specific H292 cell.     

Optofluidic waveguides: II. Fabrication and structures

We review fabrication methods and common structures for optofluidic waveguides, defined as structures capable of optical confinement and transmission through fluid filled cores. Cited structures include those based on total internal reflection, metallic coatings, and interference based confinement. Configurations include optical fibers and waveguides fabricated on flat substrates (integrated waveguides). Some examples of optofluidic waveguides that are included in this review are Photonic Crystal Fibers (PCFs) and two-dimensional photonic crystal arrays, Bragg fibers and waveguides, and Anti Resonant Reflecting Optical Waveguides (ARROWs). An emphasis is placed on integrated ARROWs fabricated using a thin-film deposition process, which illustrates how optofluidic waveguides can be combined with other microfluidic elements in the creation of lab-on-a-chip devices.     

Micro-light distribution system via optofluidic cascading prisms

This article reports a micro-light distribution system realized by altering the reflective indices of two optofluidic cascading prisms. Different micron scale light distribution configurations can be tuned via the imposed flow rates of the microfluidic mixers. The variable optical interface’s reflectivity of the cascading prisms is based on the tuning of refractive indices of micromixed fluids within the cascading prisms. The microscale light distribution is achieved via total internal reflection and partial refraction occurs at the fluid–solid optical interfaces. 1 × 3 light switching denotes one optical inlet while the light can be guided via any one of the three optical outlets. The 1 × 3 light switching capability is demonstrated. The light splitting capability to achieve different proportion of light power distribution is also demonstrated and characterized. The optofluidic cascading prisms are integrated with micromixer, prolonging the working life of the optical compartment considerably as the fluids are only consumed when optical tuning is required. The proposed technique eliminates the disadvantage of optofluidic compartments based on liquid–liquid interfaces as liquid–liquid interface possess weaker mechanical stability than solid–liquid interface. The proposed design also does not require continuous supply of fluids as in optofluidic compartments based on liquid–liquid interfaces. The optofluidic cascading prisms can be cascaded further to form a complex planar light distribution system for seamless light distribution in lab-on-a-chip excitation and sensing applications.     

Illumination and scattered light detection using a light source consisting of sub-micrometer defect arrays formed on a extremely flat light waveguide core

We have previously argued that an optical sensor combined total analysis system (TAS) is one of the indispensable functional components needed to realize a “ubiquitous human healthcare” system. To achieve this goal, we have proposed a fundamental structure for illuminating a minute cell or particle running along a microfluidic channel using a flat waveguide construction. It is desirable that the TAS light source should be arranged as close to the specimen flow as possible in order to acquire the necessary optical properties; hence, artificial defects formed on the surface of a flat light waveguide are considered to be a promising candidate for realizing the arbitrary-shaped light source for a highly functional optical TAS structure. Based on this idea, we fabricated a structure, constructing a flat and square light source consisting of rectangular solids, sub-micrometer in size, with a 1-μm thick and a 12-μm wide light waveguide core. We successfully trial-manufactured an optical TAS chip with a fluidic channel containing a 14 × 10-μm cross section, and an extremely flat light waveguide core. We repeatedly confirmed that the defect array could function as an approximately square light source when a 650-nm wavelength laser power was carefully introduced. Furthermore, we developed a hybrid numerical calculation method base on the finite-difference, time-domain method together with the beam propagation method. Utilizing this hybrid method, we evaluated the optical response when a particle runs across the light source while changing the aperture length of a shading mask to obtain signals with both higher intensity and shorter full width at half maximum. The numerical results were compared with experimental results obtained using an image acquisition system, and demonstrated good qualitative accord.     

Optofluidics: a novel generation of reconfigurable and adaptive compact architectures

The integration of microfluidics and microphotonics brings the ability to tune and reconfigure ultra-compact optical devices. This flexibility is essentially provided by three characteristics of fluids that are scalable at the micron-scale: fluid mobility, large ranges of index modulation, and abrupt interfaces that can be easily reshaped. Several examples of optofluidic devices are presented here to illustrate the achievement of flexible devices on (semi) planar and compact platforms. First, we report an integrated geometry for a compact and tunable interferometer that exploits a sharp and mobile air/water interface. We then describe a class of optically controlled devices that rely on the actuation of optically trapped micron-sized objects within a fluid environment. The last architecture results from the infiltration of photonic crystal devices with fluids. This produces tunable and reconfigurable photonic devices, like optical switches. Higher degrees of functionality could be achieved with sophisticated optofluidic platforms that associate complex microfluidic delivery and mixing schemes with microphotonic devices. Moreover, optofluidics offers new opportunities for realizing highly responsive and compact sensors.     

Comparison of spherical and non-spherical particles in microchannels under dielectrophoretic force

This paper focuses on the computational and experimental study of dielectrophoretic (DEP) force based manipulation of spherical and non-spherical particles by taking into consideration of both electrokinetic effects and particle hydrodynamics. The model is first validated with conventional dipole moment theory. The movements of a spherical polystyrene particle and a rod-shape particle under a non-uniform electric field created by a pair of non-symmetrical electrodes in a microfluidic channel are studied, and a good agreement between the simulation and experimental results is obtained. Both experimental and simulation results reveal that the rod-shape particle experiences larger DEP force and moves faster than spherical particle with a similar mass. It was also interestingly found that the shape-dependent DEP force distribution on the microscale rod particle results in its unique behavior, which cannot be captured by traditional DEP theory.