Finite element simulations of hydrodynamic trapping in microfluidic particle-trap array systems

Computational fluid dynamic (CFD) simulation is a powerful tool in the design and implementation of microfluidic systems, especially for systems that involve hydrodynamic behavior of objects such as functionalized microspheres, biological cells, or biopolymers in complex structures. In this work, we investigate hydrodynamic trapping of microspheres in a novel microfluidic particle-trap array device by finite element simulations. The accuracy of the time-dependent simulation of a microsphere''s motion towards the traps is validated by our experimental results. Based on the simulation, we study the fluid velocity field, pressure field, and force and stress on the microsphere in the device. We further explore the trap array''s geometric parameters and critical fluid velocity, which affect the microsphere''s hydrodynamic trapping. The information is valuable for designing microfluidic devices and guiding experimental operation. Besides, we provide guidelines on the simulation set-up and release an openly available implementation of our simulation in one of the popular FEM softwares, COMSOL Multiphysics. Researchers may tailor the model to simulate similar microfluidic systems that may accommodate a variety of structured particles. Therefore, the simulation will be of particular interest to biomedical research involving cell or bead transport and migration, blood flow within microvessels, and drug delivery.     

A microfluidic device enabling high-efficiency single cell trapping

Single cell trapping increasingly serves as a key manipulation technique in single cell analysis for many cutting-edge cell studies. Due to their inherent advantages, microfluidic devices have been widely used to enable single cell immobilization. To further improve the single cell trapping efficiency, this paper reports on a passive hydrodynamic microfluidic device based on the “least flow resistance path” principle with geometry optimized in line with corresponding cell types. Different from serpentine structure, the core trapping structure of the micro-device consists of a series of concatenated T and inverse T junction pairs which function as bypassing channels and trapping constrictions. This new device enhances the single cell trapping efficiency from three aspects: (1) there is no need to deploy very long or complicated channels to adjust flow resistance, thus saving space for each trapping unit; (2) the trapping works in a “deterministic” manner, thus saving a great deal of cell samples; and (3) the compact configuration allows shorter flowing path of cells in multiple channels, thus increasing the speed and throughput of cell trapping. The mathematical model of the design was proposed and optimization of associated key geometric parameters was conducted based on computational fluid dynamics (CFD) simulation. As a proof demonstration, two types of PDMS microfluidic devices were fabricated to trap HeLa and HEK-293T cells with relatively significant differences in cell sizes. Experimental results showed 100% cell trapping and 90% single cell trapping over 4 × 100 trap sites for these two cell types, respectively. The space saving is estimated to be 2-fold and the cell trapping speed enhancement to be 3-fold compared to previously reported devices. This device can be used for trapping various types of cells and expanded to trap cells in the order of tens of thousands on 1-cm² scale area, as a promising tool to pattern large-scale single cells on specific substrates and facilitate on-chip cellular assay at the single cell level.     

Geometrical effects in microfluidic-based microarrays for rapid, efficient single-cell capture of mammalian stem cells and plant cells

In this paper, a detailed numerical and experimental investigation into the optimisation of hydrodynamic micro-trapping arrays for high-throughput capture of single polystyrene (PS) microparticles and three different types of live cells at trapping times of 30 min or less is described. Four different trap geometries (triangular, square, conical, and elliptical) were investigated within three different device generations, in which device architecture, channel geometry, inter-trap spacing, trap size, and trap density were varied. Numerical simulation confirmed that (1) the calculated device dimensions permitted partitioned flow between the main channel and the trap channel, and further, preferential flow through the trap channel in the absence of any obstruction; (2) different trap shapes, all having the same dimensional parameters in terms of depth, trapping channel lengths and widths, main channel lengths and widths, produce contrasting streamline plots and that the interaction of the fluid with the different geometries can produce areas of stagnated flow or distorted field lines; and (3) that once trapped, any motion of the trapped particle or cell or a shift in its configuration within the trap can result in significant increases in pressures on the cell surface and variations in the shear stress distribution across the cell’s surface. Numerical outcomes were then validated experimentally in terms of the impact of these variations in device design elements on the percent occupancy of the trapping array (with one or more particles or cells) within these targeted short timeframes. Limitations on obtaining high trap occupancies in the devices were shown to be primarily a result of particle aggregation, channel clogging and the trap aperture size. These limitations could be overcome somewhat by optimisation of these device design elements and other operational variables, such as the average carrier fluid velocity. For example, for the 20 μm polystyrene microparticles, the number of filled traps increased from 32% to 42% during 5–10 min experiments in devices with smaller apertures. Similarly, a 40%–60% reduction in trapping channel size resulted in an increase in the amount of filled traps, from 0% to almost 90% in 10 min, for the human bone marrow derived mesenchymal stem cells, and 15%–85% in 15 min for the human embryonic stem cells. Last, a reduction of the average carrier fluid velocity by 50% resulted in an increase from 80% to 92% occupancy of single algae cells in traps. Interestingly, changes in the physical properties of the species being trapped also had a substantial impact, as regardless of the trap shape, higher percent occupancies were observed with cells compared to single PS microparticles in the same device, even though they are of approximately the same size. This investigation showed that in microfluidic single cell capture arrays, the trap shape that maximizes cell viability is not necessarily the most efficient for high-speed single cell capture. However, high-speed trapping configurations for delicate mammalian cells are possible but must be optimised for each cell type and designed principally in accordance with the trap size to cell size ratio.     

Microarray of non-connected gold pads used as high density electric traps for parallelized pairing and fusion of cells

Cell fusion consists of inducing the formation of a hybridoma cell containing the genetic properties of the progenitor cells. Such an operation is usually performed chemically or electrically. The latter method, named electrofusion, is considered as having a strong potential, due to its efficiency and non-toxicity, but deserves further investigations prior to being applicable for key applications like antibody production and cancer immunotherapy. Indeed, to envision such applications, a high amount of hybrid cells is needed. In this context, we present in this paper a device for massive cell pairing and electrofusion, using a microarray of non-connected conductive pads. The electrofusion chamber––or channel––exposes cells to an inhomogeneous electric field, caused by the pads array, enabling the trapping and pairing of cells with dielectrophoresis (DEP) forces prior to electrofusion. Compared to a mechanical trapping, such electric trapping is fully reversible (on/off handling). The DEP force is contactless and thus eases the release of the produced hybridoma. Moreover, the absence of wire connections on the pads permits the high density trapping and electrofusion of cells. In this paper, the electric field mapping, the effect of metallic pads thickness, and the transmembrane potential of cells are studied based on a numerical model to optimize the device. Electric calculations and experiments were conducted to evaluate the trapping force. The structure was finally validated for cell pairing and electrofusion of arrays of cells. We believe that our approach of fully electric trapping with a simple structure is a promising method for massive production of electrofused hybridoma.     

Photoresponsive microvalve for remote actuation and flow control in microfluidic devices

Microvalves with different actuation methods offer great integrability and flexibility in operation of lab-on-chip devices. In this work, we demonstrate a hydrogel-based and optically controlled modular microvalve that can be easily integrated within a microfluidic device and actuated by an off-chip laser source. The microvalve is based on in-channel trapping of microgel particles, which are composed of poly(N-isopropylacrylamide) and polypyrrole nanoparticles. Upon irradiation by a near-infrared (NIR) laser, the microgel undergoes volumetric change and enables precisely localized fluid on/off switching. The response rate and the “open” duration of the microvalve can be simply controlled by adjusting the laser power and exposure time. We showed that the trapped microgel can be triggered to shrink sufficiently to open a channel within as low as ∼1–2 s; while the microgel swells to re-seal the channel within ∼6–8 s. This is so far one of the fastest optically controlled and hydrogel-based microvalves, thus permitting speedy fluidic switching applications. In this study, we successfully employed this technique to control fluidic interface between laminar flow streams within a Y-junction device. The optically triggered microvalve permits flexible and remote fluidic handling, and enables pulsatile in situ chemical treatment to cell culture in an automatic and programmed manner, which is exemplified by studies of chemotherapeutic drug induced cell apoptosis under different drug treatment strategies. We find that cisplatin induced apoptosis is significantly higher in cancer cells treated with a pulsed dose, as compared to continuous flow with a sustained dose. It is expected that our NIR-controlled valving strategy will provide a simple, versatile, and powerful alternative for liquid handling in microfluidic devices.     

An electroactive microwell array for trapping and lysing single-bacterial cells

Interest in single-cell analysis has increased because it allows to understand cell metabolism and characterize disease states, cellular adaptation to environmental changes, cell cycles, etc. Here, the authors propose a device to electrically trap and lyse single-bacterial cells in an array format for high-throughput single-cell analysis. The applied electric field is highly deformed and concentrated toward the inside of the microwell structures patterned on the planar electrode. This configuration effectively generates dielectrophoretic force to attract a single cell per well. The microwell has a comparable size to the target bacterial cell making it possible to trap single cells by physically excluding additional cells. Inducing highly concentrated electric potential on the cell membrane can also effectively lyse the trapped single-bacterial cells. The feasibility of the authors' approach was demonstrated by trapping and lysing Escherichia coli cells at the single-cell level. The present microwell array can be used as a basic tool for individual bacterial cell analysis.     

Hydrodynamic particle focusing design using fluid-particle interaction

For passive sheathless particles focusing in microfluidics, the equilibrium positions of particles are typically controlled by micro channels with a V-shaped obstacle array (VOA). The design of the obstacles is mainly based on the distribution of flow streamlines without considering the existence of particles. We report an experimentally verified particle trajectory simulation using the arbitrary Lagrangian-Eulerian (ALE) fluid-particle interaction method. The particle trajectory which is strongly influenced by the interaction between the particle and channel wall is systematically analyzed. The numerical experiments show that the streamline is a good approximation of particle trajectory only when the particle locates on the center of the channel in depth. As the advantage of fluid-particle interaction method is achieved at a high computational cost and the streamline analysis is complex, a heuristic dimensionless design objective based on the Faxen''s law is proposed to optimize the VOA devices. The optimized performance of particle focusing is verified via the experiments and ALE method.     

Robust fluidic connections to freestanding microfluidic hydrogels

Biomimetic scaffolds approaching physiological scale, whose size and large cellular load far exceed the limits of diffusion, require incorporation of a fluidic means to achieve adequate nutrient/metabolite exchange. This need has driven the extension of microfluidic technologies into the area of biomaterials. While construction of perfusable scaffolds is essentially a problem of microfluidic device fabrication, functional implementation of free-standing, thick-tissue constructs depends upon successful integration of external pumping mechanisms through optimized connective assemblies. However, a critical analysis to identify optimal materials/assembly components for hydrogel substrates has received little focus to date. This investigation addresses this issue directly by evaluating the efficacy of a range of adhesive and mechanical fluidic connection methods to gelatin hydrogel constructs based upon both mechanical property analysis and cell compatibility. Results identify a novel bioadhesive, comprised of two enzymatically modified gelatin compounds, for connecting tubing to hydrogel constructs that is both structurally robust and non-cytotoxic. Furthermore, outcomes from this study provide clear evidence that fluidic interconnect success varies with substrate composition (specifically hydrogel versus polydimethylsiloxane), highlighting not only the importance of selecting the appropriately tailored components for fluidic hydrogel systems but also that of encouraging ongoing, targeted exploration of this issue. The optimization of such interconnect systems will ultimately promote exciting scientific and therapeutic developments provided by microfluidic, cell-laden scaffolds.     

Streamline based design guideline for deterministic microfluidic hydrodynamic single cell traps

A prerequisite for single cell study is the capture and isolation of individual cells. In microfluidic devices, cell capture is often achieved by means of trapping. While many microfluidic trapping techniques exist, hydrodynamic methods are particularly attractive due to their simplicity and scalability. However, current design guidelines for single cell hydrodynamic traps predominantly rely on flow resistance manipulation or qualitative streamline analysis without considering the target particle size. This lack of quantitative design criteria from first principles often leads to non-optimal probabilistic trapping. In this work, we describe an analytical design guideline for deterministic single cell hydrodynamic trapping through the optimization of streamline distributions under laminar flow with cell size as a key parameter. Using this guideline, we demonstrate an example design which can achieve 100% capture efficiency for a given particle size. Finite element modelling was used to determine the design parameters necessary for optimal trapping. The simulation results were subsequently confirmed with on-chip microbead and white blood cell trapping experiments.     

Bubble-free and pulse-free fluid delivery into microfluidic devices

The bubble-free and pulse-free fluid delivery is critical to reliable operation of microfluidic devices. In this study, we propose a new method for stable bubble-free and pulse-free fluid delivery in a microfluidic device. Gas bubbles are separated from liquid by using the density difference between liquid and gas in a closed cavity. The pulsatile flow caused by a peristaltic pump is stabilized via gas compressibility. To demonstrate the proposed method, a fluidic chamber which is composed of two needles for inlet and outlet, one needle for a pinch valve and a closed cavity is carefully designed. By manipulating the opening or closing of the pinch valve, fluids fill up the fluidic chamber or are delivered into a microfluidic device through the fluidic chamber in a bubble-free and pulse-free manner. The performance of the proposed method in bubble-free and pulse-free fluid delivery is quantitatively evaluated. The proposed method is then applied to monitor the temporal variations of fluidic flows of rat blood circulating within a complex fluidic network including a rat, a pinch valve, a reservoir, a peristaltic pump, and the microfluidic device. In addition, the deformability of red blood cells and platelet aggregation are quantitatively evaluated from the information on the temporal variations of blood flows in the microfluidic device. These experimental demonstrations confirm that the proposed method is a promising tool for stable, bubble-free, and pulse-free supply of fluids, including whole blood, into a microfluidic device. Furthermore, the proposed method will be used to quantify the biophysical properties of blood circulating within an extracorporeal bypass loop of animal models.     

A novel alternating current multiple array electrothermal micropump for lab-on-a-chip applications

The AC electrothermal technique is very promising for biofluid micropumping, due to its ability to pump high conductivity fluids. However, compared to electroosmotic micropumps, a lack of high fluid flow is a disadvantage. In this paper, a novel AC multiple array electrothermal (MAET) micropump, utilizing multiple microelectrode arrays placed on the side-walls of the fluidic channel of the micropump, is introduced. Asymmetric coplanar microelectrodes are placed on all sides of the microfluidic channel, and are actuated in different phases: one, two opposing, two adjacent, three, or all sides at the same time. Micropumps with different combinations of side electrodes and cross sections are numerically investigated in this paper. The effect of the governing parameters with respect to thermal, fluidic, and electrical properties are studied and discussed. To verify the simulations, the AC MAET concept was then fabricated and experimentally tested. The resulted fluid flow achieved by the experiments showed good agreement with the corresponding simulations. The number of side electrode arrays and the actuation patterns were also found to greatly influence the micropump performance. This study shows that the new multiple array electrothermal micropump design can be used in a wide range of applications such as drug delivery and lab-on-a-chip, where high flow rate and high precision micropumping devices for high conductivity fluids are needed.     

Retrieval parameter optimization using genetic algorithms

This paper describes our experiments on automatic parameter optimization for the Japanese monolingual retrieval task. Unlike regression approaches, we optimized parameters completely independently of retrieval models enabling the optimized parameter set to illustrate the characteristics of the target test collections. We adopted genetic algorithms as optimization tools and cross-validated with four test collections, namely the CLIR-J-J collections for NTCIR-3 to NTCIR-6. The most difficult retrieval parameters to optimize are the feedback parameters, because there are no principles for calibrating them. Our approach optimized feedback parameters and basic scoring parameters at the same time. Using test sets and validation sets, we achieved effectiveness levels comparable with very strong baselines, i.e., the best-performing NTCIR official runs.     

Dielectrophoretic capture of low abundance cell population using thick electrodes

Enrichment of rare cell populations such as Circulating Tumor Cells (CTCs) is a critical step before performing analysis. This paper presents a polymeric microfluidic device with integrated thick Carbon-PolyDimethylSiloxane composite (C-PDMS) electrodes designed to carry out dielectrophoretic (DEP) trapping of low abundance biological cells. Such conductive composite material presents advantages over metallic structures. Indeed, as it combines properties of both the matrix and doping particles, C-PDMS allows the easy and fast integration of conductive microstructures using a soft-lithography approach while preserving O₂ plasma bonding properties of PDMS substrate and avoiding a cumbersome alignment procedure. Here, we first performed numerical simulations to demonstrate the advantage of such thick C-PDMS electrodes over a coplanar electrode configuration. It is well established that dielectrophoretic force (F_DEP) decreases quickly as the distance from the electrode surface increases resulting in coplanar configuration to a low trapping efficiency at high flow rate. Here, we showed quantitatively that by using electrodes as thick as a microchannel height, it is possible to extend the DEP force influence in the whole volume of the channel compared to coplanar electrode configuration and maintaining high trapping efficiency while increasing the throughput. This model was then used to numerically optimize a thick C-PDMS electrode configuration in terms of trapping efficiency. Then, optimized microfluidic configurations were fabricated and tested at various flow rates for the trapping of MDA-MB-231 breast cancer cell line. We reached trapping efficiencies of 97% at 20 μl/h and 78.7% at 80 μl/h, for 100 μm thick electrodes. Finally, we applied our device to the separation and localized trapping of CTCs (MDA-MB-231) from a red blood cells sample (concentration ratio of 1:10).     

A nanoporous optofluidic microsystem for highly sensitive and repeatable surface enhanced Raman spectroscopy detection

We report the demonstration of an optofluidic surface enhanced Raman spectroscopy (SERS) device that leverages a nanoporous microfluidic matrix to improve the SERS detection performance by more than two orders of magnitude as compared to a typical open microfluidic channel. Although it is a growing trend to integrate optical biosensors into microfluidic channels, this basic combination has been detrimental to the sensing performance when applied to SERS. Recently, however, synergistic combinations between microfluidic functions and photonics (i.e., optofluidics) have been implemented that improve the detection performance of SERS. Conceptually, the simplest optofluidic SERS techniques reported to date utilize a single nanofluidic channel to trap nanoparticle-analyte conjugates as a method of preconcentration before detection. In this work, we leverage this paradigm while improving upon the simplicity by forming a 3D nanofluidic network with packed nanoporous silica microspheres in a microfluidic channel; this creates a concentration matrix that traps silver nanoclusters and adsorbed analytes into the SERS detection volume. With this approach, we are able to achieve a detection limit of 400 attomoles of Rhodamine 6G after only 2 min of sample loading with high chip-to-chip repeatability. Due to the high number of fluidic paths in the nanoporous channel, this approach is less prone to clogging than single nanofluidic inlets, and the loading time is decreased compared to previous reports. In addition, fabrication of this microsystem is quite simple, as nanoscale fabrication is not necessary. Finally, integrated multimode fiber optic cables eliminate the need for optical alignment, and thus the device is relevant for portable and automated applications in the field, including point-of-sample and point-of-care detection. To illustrate a relevant field-based application, we demonstrate the detection of 12 ppb of the organophosphate malathion in water using the nanofluidic SERS microsystem.     

Increased density and coverage uniformity of viruses on a sensor surface by using U-type, T-type, and W-type microfluidic devices

Microorganisms, molecules, or viruses in the fluidic environment are usually at considerably low Reynolds numbers because of small diameters. The viscous forces of molecules and viruses dominate at considerably low Reynolds numbers. This study developed three microfluidic devices, that is, T type, U type, and W type devices, to control the flow movement, which can increase the adhesion density of viruses on the surface of the sensor. The linker 11-mercaptoundecanoic acid (11-MUA) and Turnip yellow mosaic virus (TYMV) were used in this study and measured by a confocal microscope. Fluorescent intensity and coverage of 11-MUA and TYMV were used to identify the adhesion density quantitatively. Results indicate that 11-MUA layers and TYMV disperse randomly by the dipping method. Attachment tests for T-, U-, and W-type devices demonstrated average fluorescence intensities of 1.56, 2.18, and 2.67, respectively, and average fluorescence coverage of 1.31, 1.87, and 2.55 times those of dipping techniques, respectively. The T-type device produced the lowest fluorescence coverage uniformity (10%-80%), whereas the W-type device produced the highest fluorescence coverage uniformity (80%-90%). Fluorescence intensity correlates positively with flow within a specified flow range; however, the exact relationship between fluorescence intensity and flow requires further study. Attachment tests for TYMV virus samples indicated that the W-type device produced an average fluorescence intensity of 3.59 and average fluorescence coverage of 19.13 times greater than those achieved through dipping techniques. Traditional immersion methods achieved fluorescence coverage of 0%-10%, whereas that of the W-type device reached 70%-90%.     

Floating-electrode enhanced constriction dielectrophoresis for biomolecular trapping in physiological media of high conductivity

We present an electrokinetic framework for designing insulator constriction-based dielectrophoresis devices with enhanced ability to trap nanoscale biomolecules in physiological media of high conductivity, through coupling short-range dielectrophoresis forces with long-range electrothermal flow. While a 500-fold constriction enables field focusing sufficient to trap nanoscale biomolecules by dielectrophoresis, the extent of this high-field region is enhanced through coupling the constriction to an electrically floating sensor electrode at the constriction floor. However, the enhanced localized fields due to the constriction and enhanced current within saline media of high conductivity (1 S/m) cause a rise in temperature due to Joule heating, resulting in a hotspot region midway within the channel depth at the constriction center, with temperatures of ∼8°–10°K above the ambient. While the resulting vortices from electrothermal flow are directed away from the hotspot region to oppose dielectrophoretic trapping, they also cause a downward and inward flow towards the electrode edges at the constriction floor. This assists biomolecular trapping at the sensor electrode through enabling long-range fluid sampling as well as through localized stirring by fluid circulation in its vicinity.     

Behavior of a train of droplets in a fluidic network with hydrodynamic traps

The behavior of a droplet train in a microfluidic network with hydrodynamic traps in which the hydrodynamic resistive properties of the network are varied is investigated. The flow resistance of the network and the individual droplets guide the movement of droplets in the network. In general, the flow behavior transitions from the droplets being immobilized in the hydrodynamic traps at low flow rates to breaking up and squeezing of the droplets at higher flow rates. A state diagram characterizing these dynamics is presented. A simple hydrodynamic circuit model that treats droplets as fluidic resistors is discussed, which predicts the experimentally observed flow rates for droplet trapping in the network. This study should enable the rational design of microfuidic devices for passive storage of nanoliter-scale drops.     

Stretching DNA by electric field and flow field in microfluidic devices: An experimental validation to the devices designed with computer simulations

We examined the performance of three microfluidic devices for stretching DNA. The first device is a microchannel with a contraction, and the remaining two are the modifications to the first. The modified designs were made with the help of computer simulations [C. C. Hsieh and T. H. Lin, Biomicrofluidics 5(4), 044106 (2011) and C. C. Hsieh, T. H. Lin, and C. D. Huang, Biomicrofluidics 6, 044105 (2012)] and they were optimized for operating with electric field. In our experiments, we first used DC electric field to stretch DNA. However, the experimental results were not even in qualitative agreement with our simulations. More detailed investigation revealed that DNA molecules adopt a globular conformation in high DC field and therefore become more difficult to stretch. Owing to the similarity between flow field and electric field, we turned to use flow field to stretch DNA with the same devices. The evolution patterns of DNA conformation in flow field were found qualitatively the same as our prediction based on electric field. We analyzed the maximum values, the evolution and the distributions of DNA extension at different Deborah number in each device. We found that the shear and the hydrodynamic interaction have significant influence on the performance of the devices.     

Laser-based patterning for fluidic devices in nitrocellulose

In this report, we demonstrate a simple and low cost method that can be reproducibly used for fabrication of microfluidic devices in nitrocellulose. The fluidic patterns are created via a laser-based direct-write technique that induces polymerisation of a photo-polymer previously impregnated in the nitrocellulose. The resulting structures form hydrophobic barriers that extend through the thickness of the nitrocellulose and define an interconnected hydrophilic fluidic-flow pattern. Our experimental results show that using this method it is possible to achieve microfluidic channels with lateral dimensions of ∼100 μm using hydrophobic barriers that form the channel walls with dimensions of ∼60 μm; both of these values are considerably smaller than those that can be achieved with other current techniques used in the fabrication of nitrocellulose-based fluidic devices. A simple grid patterned nitrocellulose device was then used for the detection of C-reactive protein via a sandwich enzyme-linked immunosorbent assay, which served as a useful proof-of-principle experiment.     

纤维素磁性高分子微球的制备方法研究进展

文章从纤维素的应用价值出发,分别介绍了纤维素微球和的纤维素磁性高分子微球的特点及应用范围,详细综述了纤维素及纤维素磁性微球的制备方法。