Electric manipulation of bioparticles and macromolecules on microfabricated electrodes

Plasmofluidic-Based Near-Field Optical Trapping of Dielectric Nano-Objects Using Gold Nanoislands Sensor Chips

Near-field optical manipulation has been widely used for guiding and trapping nanoscale objects close to an optical-active interface. This near-field manipulation opens opportunities for next-generation biosensing with the capability of large-area trapping and in situ detection. In this article, we used the finite element method (FEM) to analyze the motion mechanism of nano-objects (50-500 nm) in the near-field optics, especially localized surface plasmon resonance (LSPR). The size-dependent optical forces and hydrodynamic forces of subwavelength nanoparticles (<500 nm) in different hydrodynamic velocity fields were calculated. When the strength of the local electric field was increased, LSPR with two-dimensional gold nanoislands (AuNIs) showed improved capability for manipulating nano-objects near the vicinity of the AuNI interface. Through the experiments of in situ interferometric testing 50-500 nm nano-objects with constant number concentration or volume fraction, it was confirmed that the local plasmonic near-field was able to trap the dielectric polystyrene beads smaller than 200 nm. The plasmofluidic system was further verified by testing biological nanovesicles such as exosomes (40-200 nm) and high- and low-density lipoproteins (10-200 nm). This concept of direct dielectric nano-objects manipulation enables large-scale parallel trapping and dynamic sensing of biological nanovesicles without the need of molecular binding tethers or labeling.

Two-dimensional computational method for generating planar electrode patterns with enhanced volumetric electric fields and its application to continuous dielectrophoretic bacterial capture

An array of microfabricated interdigitated electrodes (IDEs) is one of the most commonly used forms of electrode geometry for dielectrophoretic manipulation of biological particles in microfluidic biochips owing to simplicity of fabrication and ease of analysis. However, the dielectrophoretic force dramatically reduces as the distance from the electrode surface increases; therefore, the effective region is usually close to the electrode surface for a given electric potential difference. Here, we present a novel two-dimensional computational method for generating planar electrode patterns with enhanced volumetric electric fields, which we call the "microelectrode discretization (MED)" method. It involves discretization and reconstruction of planar electrodes followed by selection of the electrode pattern that maximizes a novel objective function, factor S, which is determined by the electric potentials on the electrode surface alone. In this study, IDEs were used as test planar electrodes. Two arrays of IDEs and respective MED-optimized electrodes were implemented in microfluidic devices for the selective capture of Escherichia coli against 1 μm-diameter polystyrene beads, and we experimentally observed that 1.4 to 35.8 times more bacteria were captured using the MED-optimized electrodes than the IDEs (p < 0.0016), with a bacterial purity against the beads of more than 99.8%. This simple design method offered simplicity of fabrication, highly enhanced electric field, and uniformity of particle capture, and can be used for many dielectrophoresis-based sensors and microfluidic systems.

A low voltage nanopipette dielectrophoretic device for rapid entrapment of nanoparticles and exosomes extracted from plasma of healthy donors

An insulator-based dielectrophoresis (iDEP) is a label-free method that has been extensively utilized for manipulation of nanoparticles, cells, and biomolecules. Here, we present a new iDEP approach that can rapidly trap nanoparticles at the close proximity of a glass nanopipette’s tip by applying 10 V/cm direct current (DC) across the pipette’s length. The trapping mechanism was systemically studied using both numerical modeling and experimental observations. The results showed that the particle trapping was determined to be controlled by three dominant electrokinetic forces including dielectrophoretic, electrophoretic and electroosmotic force. Furthermore, the effect of the ionic strength, the pipette’s geometry, and the applied electric field on the entrapment efficiency was investigated. To show the application of our device in biomedical sciences, we demonstrated the successful entrapment of fluorescently tagged liposomes and unlabeled plasma-driven exosomes from the PBS solution. Also, to illustrate the selective entrapment capability of our device, 100 nm liposomes were extracted from the PBS solution containing 500 nm polystyrene particles at the tip of the pipette as the voltage polarity was reversed.

Point-of-Care Systems for Rapid DNA Quantification in Oncology

The development of new powerful applications and the improvement in fabrication techniques are promising an explosive growth in lab-on-chip use in the upcoming future. As the demand reaches significant levels, the semiconductor industry may enter in the field, bringing its capability to produce complex devices in large volumes, high quality and low cost. The lab-on-chip concept, when applied to medicine, leads to the point-of-care concept, where simple, compact and cheap instruments allow diagnostic assays to be performed quickly by untrained personnel directly at the patient's side. In this paper, some practical and economical considerations are made to support the advantages of point-of-care testing. A series of promising technologies developed by STMicroelectronics on lab-on-chips is also presented, mature enough to enter in the common medical practice. The possible use of these techniques for cancer research, diagnosis and treatment are illustrated together with the benefits offered by their implementation in point-of-care testing.

Rapid and effective enrichment of mononuclear cells from blood using acoustophoresis

Effective separation methods for fractionating blood components are needed for numerous diagnostic and research applications. This paper presents the use of acoustophoresis, an ultrasound based microfluidic separation technology, for label-free, gentle and continuous separation of mononuclear cells (MNCs) from diluted whole blood. Red blood cells (RBCs) and MNCs behave similar in an acoustic standing wave field, compromising acoustic separation of MNC from RBC in standard buffer systems. However, by optimizing the buffer conditions and thereby changing the acoustophoretic mobility of the cells, we were able to enrich MNCs relative to RBCs by a factor of 2,800 with MNC recoveries up to 88%. The acoustophoretic microchip can perform cell separation at a processing rate of more than 1 × 10⁵ cells/s, corresponding to 5 µl/min undiluted whole blood equivalent. Thus, acoustophoresis can be easily integrated with further down-stream applications such as flow cytometry, making it a superior alternative to existing MNC isolation techniques.

Electrokinetically driven continuous-flow enrichment of colloidal particles by Joule heating induced temperature gradient focusing in a convergent-divergent microfluidic structure

Enrichment of colloidal particles in continuous flow has not only numerous applications but also poses a great challenge in controlling physical forces that are required for achieving particle enrichment. Here, we for the first time experimentally demonstrate the electrokinetically-driven continuous-flow enrichment of colloidal particles with Joule heating induced temperature gradient focusing (TGF) in a microfluidic convergent-divergent structure. We consider four mechanisms of particle transport, i.e., advection due to electroosmosis, electrophoresis, dielectrophoresis and, and further clarify their roles in the particle enrichment. It is experimentally determined and numerically verified that the particle thermophoresis plays dominant roles in enrichment of all particle sizes considered in this study and the combined effect of electroosmosis-induced advection and electrophoresis is mainly to transport particles to the zone of enrichment. Specifically, the enrichment of particles is achieved with combined DC and AC voltages rather than a sole DC or AC voltage. A numerical model is formulated with consideration of the abovementioned four mechanisms, and the model can rationalize the experimental observations. Particularly, our analysis of numerical and experimental results indicates that thermophoresis which is usually an overlooked mechanism of material transport is crucial for the successful electrokinetic enrichment of particles with Joule heating induced TGF.

Automated Combinatorial Process for Nanofabrication of Structures Using Bioderivatized Nanoparticles

A fully automated electronic microarray control system (Nanochip 400 System) was used to carry out a combinatorial process to determine optimal conditions for fabricating higher order three-dimensional nanoparticle structures. Structures with up to 40 layers of bioderivatized nanoparticles were fabricated on a 400-test site CMOS microarray using the automated Nanochip 400 System. Reconfigurable electric fields produced on the surface of the CMOS microarray device actively transport, concentrate, and promote binding of 40 nm biotin- and streptavidin-derivatized nanoparticles to selected test sites on the microarray surface. The overall fabrication process including nanoparticle reagent delivery to the microarray device, electronic control of the CMOS microarray and the optical/fluorescent detection, and monitoring of nanoparticle layering are entirely controlled by the Nanochip 400 System. The automated nanoparticle layering process takes about 2 minutes per layer, with 10–20 seconds required for the electronic addressing and binding of nanoparticles, and roughly 60 seconds for washing. The addressing and building process is monitored by changes in fluorescence intensity as each nanoparticle layer is deposited. The final multilayered 3D structures are about 2 μm in thickness and 55 μm in diameter. Multilayer nanoparticle structures and control sites on the microarray were verified by SEM analysis.

Size-selective, biocompatible, manufacturable platform for structuring deformable microsystems

Precise, size-selective assembly and sorting are demonstrated in a low-cost system using manufacturable, replicated polymer templates to guide the assembly. Surface interactions between microscale objects and an assembly template are combined with fluid forces to drive site-selective organization of objects onto the template. Although controlling the organization of deformable objects on deformable surfaces offers a key tool for biological applications, the deformability can potentially interfere with the process that drives size selectivity. Theoretical models of the polymer assembly system were created to predict when selectivity will fail in deformable systems and were validated by comparison with experiments. Selective template-driven assembly of polystyrene microspheres on PDMS templates replicated from silicon masters was carried out using templated assembly by selective removal (TASR), demonstrating the effectiveness of selective assembly with low-cost, manufacturable materials and processes. The assembly of polystyrene microcomponents on PDMS shows high assembly yields and effective selectivity, in agreement with models.

Cell pairing using microwell array electrodes based on dielectrophoresis

We report a simple device with an array of 10,000 (100 × 100) microwells for producing vertical pairs of cells in individual microwells with a rapid manipulation based on positive dielectrophoresis (p-DEP). The areas encircled with micropoles which fabricated from an electrical insulating photosensitive polymer were used as microwells. The width (14 μm) and depth (25 μm) of the individual microwells restricted the size to two vertically aligned cells. The DEP device for the manipulation of cells consisted of a microfluidic channel with an upper indium tin oxide (ITO) electrode and a lower microwell array electrode fabricated on an ITO substrate. Mouse myeloma cells stained in green were trapped within 1 s in the microwells by p-DEP by applying an alternating current voltage between the upper ITO and the lower microwell array electrode. The cells were retained inside the wells even after switching off the voltage and washing with a fluidic flow. Other myeloma cells stained in blue were then trapped in the microwells occupied by the cells stained in green to form the vertical cell pairing in the microwells. Cells stained in different colors were paired within only 1 min and a pairing efficiency of over 50% was achieved.

Direct assembling methodologies for high-throughput bioscreening

Over the last few decades, high-throughput (HT) bioscreening, a technique that allows rapid screening of biochemical compound libraries against biological targets, has been widely used in drug discovery, stem cell research, development of new biomaterials, and genomics research. To achieve these ambitions, scaffold-free (or direct) assembly of biological entities of interest has become critical. Appropriate assembling methodologies are required to build an efficient HT bioscreening platform. The development of contact and non-contact assembling systems as a practical solution has been driven by a variety of essential attributes of the bioscreening system, such as miniaturization, high throughput, and high precision. The present article reviews recent progress on these assembling technologies utilized for the construction of HT bioscreening platforms.

Chip-based size-selective sorting of biological cells using high frequency acoustic excitation

This work presents the size-selective sorting of single biological cells using the assembly process known as templated assembly by selective removal (TASR). We have demonstrated experimentally, for the first time, the selective placement and sorting of single SF9 cells (clonal isolate derived from Spodoptera frugiperda (Fall Armyworm) IPLB-Sf21-AE cells) into patterned hemispherical sites on rigid assembly templates using TASR. Nearly 100% of the assembly sites on the template were filled with matching cells (with assembly density as high as 900 sites per mm(2)) within short time spans of 3 minutes. 3-D reconstruction of cell profiles and volume analysis of cells trapped inside assembly sites demonstrates that only those cells that match the assembly site precisely (within 0.5 μm) in size are assembled on the template. The assembly conditions are also compatible with the extension of TASR to mammalian cells. TASR-based size-selective structuring and sorting of biological systems represents a valuable tool with potential for implementation in biological applications such as cell sorting for medical research or diagnostics, templating for artificial tissue replication, or isolation of single cells for the study of biological or mechanical behavior.

Fabrication of Line and Grid Patterns with Cells Based on Negative Dielectrophoresis

The rapid, direct fabrication of two-dimensional line patterns with biological cells in a culture medium we report here is based on negative dielectrophoresis (n-DEP). It easily creates a versatile cell micropattern without specially pretreating culture slides. When an alternating electric field, typically 1 MHz, was applied to an InterDigitated band Array (IDA) electrode with four subunits, n-DEP force directs cells toward a weaker of electric field strength region. Cells aligned above attracted bands within 1min. Applying AC voltage for 5 min enables cells to adhere to the cell culture slide. When 12 Vpp is applied, 45-65% cells remain in line after the device is washed and disassembled. Resulting adsorbed cell lines were immersed in a medium to culture cells. n-DEP patterning did not significantly damage cells for growth because of the cell number increased by growth. We fabricated cell grid patterns to demonstrate formation of different patterns. After the device was disassembled and excess cells removed, the culture slide was reassembled with the IDA electrode and was rotated 90° to the previous setup. Second cells were patterned in lines the same way, forming grid patterns on the slide. Micropatterns aligned cells at desired locations enabling a biomimetic structure to be generated with biological functions and to detect cellular response to many kinds of drugs for simultaneous high-throughput screening.

Continuous separation of cells and particles in microfluidic systems

The progress in microfabrication and lab-on-a-chip technologies has been a major area of development for new approaches to bioanalytics and integrated concepts for cell biology. Fundamental advances in the development of elastomer based microfluidics have been driving factors for making microfluidic technology available to a larger scientific community in the past years. In line with this, microfluidic separation of cells and particles is currently developing rapidly where key areas of interest are found in designing lab-on-a-chip systems that offer controlled microenvironments for studies of fundamental cell biology. More recently industrial interests are seen in the development of micro chip based flow cytometry technology both for preclinical research and clinical diagnostics. This critical review outlines the most recent developments in microfluidic technology for cell and particle separation in continuous flow based systems. (130 references).

Fabrication and evaluation of a ratchet type dielectrophoretic device for particle analysis

Dielectrophoresis is an electrokinetic phenomenon that utilizes an asymmetric electric field to separate analytes based on differences in their polarizabilities relative to that of the suspending medium. One dielectrophoretic device architecture that offers interesting possibilities for particle transport without the use of external flow is the ratchet geometry. This paper describes the fabrication and evaluation of a novel dielectrophoretic ratchet device using a series of fine particles as test probes. The asymmetrical electric field required to selectively transport target analytes was produced using electroformed electrodes which offer the possibility of reducing convective heating and which can be used to construct a device in which all particles located within the fluidic channel are exposed to the applied field. Initial tests of this device were conducted using magnetite and polystyrene fine particles to demonstrate selective particle collection and a separation based on differences in the electrical properties of the analytes employed.

Prediction of trapping zones in an insulator-based dielectrophoretic device

A mathematical model is implemented to study the performance of an insulator-based dielectrophoretic device. The geometry of the device was captured in a computational model that solves Laplace equation within an array of cylindrical insulating structures. From the mathematical model it was possible to predict the location and magnitude of the zones of dielectrophoretic trapping of microparticles. Simulation and experimental results of trapping zones are compared for different operating conditions.

Dielectric and dielectrophoretic properties of DNA

The physical properties of DNA are quite important for molecular genetics as well as for its nanotechnological applications. Studying the interactions of alternating current (AC) electric fields with deoxyribonucleic acid (DNA) allows one to draw conclusions about these properties. These interactions are usually investigated in two different ways. In dielectric spectroscopy, a DNA solution is placed in a homogeneous AC field and electronic parameters are measured over several frequency decades in the Hz to GHz range. These electronic data are then interpreted on the basis of physico-chemical models as a result of certain phenomena on the molecular level. In dielectrophoretic studies, a DNA solution is exposed to an inhomogeneous AC field and the spatial response of few or single molecules is monitored by optical or scanning force microscopy. This response can involve translation, elongation and orientation of the molecular strings. In this review, a survey is given of the literature dealing with the dielectric and dielectrophoretic properties of DNA as well as with applications of DNA dielectrophoresis.

Patterning mouse and human embryonic stem cells using micro-contact printing

Local micro-environmental cues consisting of soluble cytokines, extra-cellular matrix (ECM), and cell-cell contacts are determining factors in stem cell fate. These extrinsic cues form a 'niche' that governs a stem cell's decision to either self-renew or differentiate into one or more cell types. Recently, it has been shown that micro-patterning stem cells in two- and three-dimensions can provide direct control over several parameters of the local micro-environment, including colony size, distance between colonies, ECM substrate, and homotypic or heterotypic cell-cell contact. The protocol described here uses micro-contact printing to pattern ECM onto tissue culture substrates. Cells are seeded onto the patterned substrates in serum-free media and are confined to the patterned features. After patterning, stem cell phenotype is analyzed using quantitative immunocytochemistry and immunohistochemistry.

Directed hybridization of DNA derivatized nanoparticles into higher order structures

Electric field directed hybridization was used to produce twenty layer nanostructures composed of DNA derivatized nanoparticles. Using an electronic microarray device, DNA nanoparticles could be directed and concentrated such that rapid and specific hybridization occurs only on the activated sites. Nanoparticle layers were formed within 30 s of activation and twenty layer structures completed in under an hour. Results demonstrate a unique combination of bottom-up and top-down techniques for nanofabrication.

Effects of Dielectrophoresis on Growth, Viability and Immuno-reactivity of Listeria monocytogenes

Dielectrophoresis (DEP) has been regarded as a useful tool for manipulating biological cells prior to the detection of cells. Since DEP uses high AC electrical fields, it is important to examine whether these electrical fields in any way damage cells or affect their characteristics in subsequent analytical procedures. In this study, we investigated the effects of DEP manipulation on the characteristics of Listeria monocytogenes cells, including the immuno-reactivity to several Listeria-specific antibodies, the cell growth profile in liquid medium, and the cell viability on selective agar plates. It was found that a 1-h DEP treatment increased the cell immuno-reactivity to the commercial Listeria species-specific polyclonal antibodies (from KPL) by ~31.8% and to the C11E9 monoclonal antibodies by ~82.9%, whereas no significant changes were observed with either anti-InlB or anti-ActA antibodies. A 1-h DEP treatment did not cause any change in the growth profile of Listeria in the low conductive growth medium (LCGM); however, prolonged treatments (4 h or greater) caused significant delays in cell growth. The results of plating methods showed that a 4-h DEP treatment (5 MHz, 20 Vpp) reduced the viable cell numbers by 56.8–89.7 %. These results indicated that DEP manipulation may or may not affect the final detection signal in immuno-based detection depending on the type of antigen-antibody reaction involved. However, prolonged DEP treatment for manipulating bacterial cells could produce negative effects on the cell detection by growth-based methods. Careful selection of DEP operation conditions could avoid or minimize negative effects on subsequent cell detection performance.

Microfluidic device for cell capture and impedance measurement

This work presents a microfluidic device to capture physically single cells within microstructures inside a channel and to measure the impedance of a single HeLa cell (human cervical epithelioid carcinoma) using impedance spectroscopy. The device includes a glass substrate with electrodes and a PDMS channel with micro pillars. The commercial software CFD-ACE+ is used to study the flow of the microstructures in the channel. According to simulation results, the probability of cell capture by three micro pillars is about 10%. An equivalent circuit model of the device is established and fits closely to the experimental results. The circuit can be modeled electrically as cell impedance in parallel with dielectric capacitance and in series with a pair of electrode resistors. The system is operated at low frequency between 1 and 100 kHz. In this study, experiments show that the HeLa cell is successfully captured by the micro pillars and its impedance is measured by impedance spectroscopy. The magnitude of the HeLa cell impedance declines at all operation voltages with frequency because the HeLa cell is capacitive. Additionally, increasing the operation voltage reduces the magnitude of the HeLa cell because a strong electric field may promote the exchange of ions between the cytoplasm and the isotonic solution. Below an operating voltage of 0.9 V, the system impedance response is characteristic of a parallel circuit at under 30 kHz and of a series circuit at between 30 and 100 kHz. The phase of the HeLa cell impedance is characteristic of a series circuit when the operation voltage exceeds 0.8 V because the cell impedance becomes significant.

Cell loss in integrated microfluidic device

Cell loss during sample transporting from macro-components to micro-components in integrated microfluidic devices can considerably deteriorate cell detection sensitivity. This intrinsic cell loss was studied and effectively minimized through (a) increasing the tubing diameter connecting the sample storage and the micro-device, (b) applying a hydrodynamic focusing approach for sample delivering to reduce cells contacting and adhesion on the walls of micro-channel and chip inlet; (c) optimizing the filter design with a zigzag arrangement of pillars (13 microm in chamber depth and 0.8 microm in gap) to prolong the effective filter length, and iv) the use of diamond shaped pillar instead of normally used rectangular shape to reduce the gap length between any two given pillar (i.e. pressure drop) at the filter region. Cell trapping and immunofluorescent detection of 12 Giardia lamblia and 12 Cryptosporidium parvum cells in 150 microl solution and 50 MCF-7 breast cancer cells in 150 microl solution was completed within 15 min with trapping efficiencies improved from 79+/-11%, 50.8+/-5.5% and 41.3+/-3.6% without hydrodynamic focusing, respectively, to 90.8+/-5.8%, 89.8+/-16.6% and 77.0+/-9.2% with hydrodynamic focusing.

Membraneless microseparation by asymmetry in curvilinear laminar flows

Membraneless microseparation by asymmetric inertial migration is studied in curvilinear laminar flows and evidence of the microseparation is presented. Along a curvilinear laminar flow, transverse particle migration involves competition between three shear-flow effects; the tubular pinch effect, centrifugal force, and Dean's vortex. Equilibrating control of migration allows for particle separation to different outlets. No filter-media or external force is necessary for the microseparation utilizing only shear-flow characteristics. A double-spiral design effectively controls the migration to optimize microseparation. The concentration ratio of 10 microm beads from the two different outlets was 660 times at 92 mm/s of flow velocity. This new technology has great potential for high-throughput and low cost in bio-agent and particulate separation at both macro and micro scales.

Electric-field-directed assembly of biomolecular-derivatized nanoparticles into higher-order structures

Multilayered structures composed of biomolecule-derivatized nanoparticles can be fabricated by electric-field-directed self-assembly. A microelectrode-array device facilitates the rapid parallel electrophoretic transport and binding of biotin and streptavidin fluorescent nanoparticles to specific sites on the microarray. Control of the current, voltage, and activation time of each of the 400-microarray electrodes allows a combinatorial approach to optimize nanoparticle binding. Under optimal conditions, nanoparticle layers form within 15 s of microelectrode activation, and the directed assembly of more than 50 alternate layers of nanoparticles is complete within an hour. The final multilayered structures are removed from the support by a relatively simple lift-off process. The electric-field process allows the parallel patterned assembly of multilayer structures using extremely low concentrations of nanoparticles and produces minimal nonspecific binding to unactivated sites. These results are significant for the development of rapid, maskless nanofabrication and hierarchical integration of biomolecular-derivatized nanocomponents into higher-order materials and devices.

Automatic microfluidic platform for cell separation and nucleus collection

This study reports a new biochip capable of cell separation and nucleus collection utilizing dielectrophoresis (DEP) forces in a microfluidic system comprising of micropumps and microvalves, operating in an automatic format. DEP forces operated at a low voltage (15 Vp-p) and at a specific frequency (16 MHz) can be used to separate cells in a continuous flow, which can be subsequently collected. In order to transport the cell samples continuously, a serpentine-shape (S-shape) pneumatic micropump device was constructed onto the chip device to drive the samples flow through the microchannel, which was activated by the pressurized air injection. The mixed cell samples were first injected into an inlet reservoir and driven through the DEP electrodes to separate specific samples. Finally, separated cell samples were collected individually in two outlet reservoirs controlled by microvalves. With the same operation principle, the nucleus of the specific cells can be collected after the cell lysis procedure. The pumping rate of the micropump was measured to be 39.8 microl/min at a pressure of 25 psi and a driving frequency of 28 Hz. For the cell separation process, the initial flow rate was 3 microl/min provided by the micropump. A throughput of 240 cells/min can be obtained by using the developed device. The DEP electrode array, microchannels, micropumps and microvalves are integrated on a microfluidic chip using micro-electro-mechanical-systems (MEMS) technology to perform several crucial procedures including cell transportation, separation and collection. The dimensions of the integrated chip device were measured to be 6x7 cm. By integrating an S-shape pump and pneumatic microvalves, different cells are automatically transported in the microchannel, separated by the DEP forces, and finally sorted to specific chambers. Experimental data show that viable and non-viable cells (human lung cancer cell, A549-luc-C8) can be successfully separated and collected using the developed microfluidic platform. The separation accuracy, depending on the DEP operating mode used, of the viable and non-viable cells are measured to be 84 and 81%, respectively. In addition, after cell lysis, the nucleus can be also collected using a similar scheme. The developed automatic microfluidic platform is useful for extracting nuclear proteins from living cells. The extracted nuclear proteins are ready for nuclear binding assays or the study of nuclear proteins.

Dielectrophoresis microsystem with integrated flow cytometers for on-line monitoring of sorting efficiency

Dielectrophoresis (DEP) and flow cytometry are powerful technologies and widely applied in microfluidic systems for handling and measuring cells and particles. Here, we present a novel microchip with a DEP selective filter integrated with two microchip flow cytometers (FCs) for on-line monitoring of cell sorting processes. On the microchip, the DEP filter is integrated in a microfluidic channel network to sort yeast cells by positive DEP. The two FCs detection windows are set upstream and downstream of the DEP filter. When a cell passes through the detection windows, the light scattered by the cell is measured by integrated polymer optical elements (waveguide, lens, and fiber coupler). By comparing the cell counting rates measured by the two FCs, the collection efficiency of the DEP filter can be determined. The chips were used for quantitative determination of the effect of flow rate, applied voltage, conductivity of the sample, and frequency of the electric field on the sorting efficiency. A theoretical model for the capture efficiency was developed and a reasonable agreement with the experimental results observed. Viable and non-viable yeast cells showed different frequency dependencies and were sorted with high efficiency. At 2 MHz, more than 90% of the viable and less than 10% of the non-viable cells were captured on the DEP filter. The presented approach provides quantitative real-time data for sorting a large number of cells and will allow optimization of the conditions for, e.g., collecting cancer cells on a DEP filter while normal cells pass through the system. Furthermore, the microstructure is simple to fabricate and can easily be integrated with other microstructures for lab-on-a-chip applications.

Continuous separation of particles using a microfluidic device equipped with flow rate control valves

We propose herein an improved microfluidic system for continuous and precise particle separation. We have previously proposed a method for particle separation called "pinched flow fractionation." Using the previously reported method, particles can be continuously separated according to differences in their diameters, simply by introducing liquid flows with and without particles into a specific microchannel structure. In this study, we incorporated PDMS membrane microvalves for flow rate control into the microfluidic device to improve the separation accuracy. By adjusting the flow rates distributed to each outlet, target particles could be precisely collected from the desired outlet. We succeeded in separating micron and submicron-size polymer particles. This method can be used widely for continuous and precise separation of various kinds of particles, and can function as an important part of microfluidic systems.

Differential labeling of closely spaced biosensor electrodes via electrochemical lithography

Electrochemical biosensors offer the promise of exceptional scalability and parallelizability. To achieve this promise, however, will require the development of new methods for the differential labeling of closely spaced electrodes with specific biomolecules such as DNA or proteins. Here we report a simple, highly selective method for passivating and differentially labeling closely separated gold electrodes with oligonucleotides or other biomolecules. Analogous to photolithography, where a light-sensitive resist is selectively removed to expose specific surfaces to further modification, we passivate gold electrodes with a self-assembled alkanethiol monolayer that protects them from modification. The monolayer is then electrochemically desorbed at relatively low potentials, allowing for the subsequent labeling of the now exposed array element with a specific sensing biomolecule. The observed passivation is highly efficient: using a C11-OH monolayer as the passivating agent, we do not observe any detectable cross-contamination of adjacent electrodes (95 microm separation) upon labeling with a stem-loop DNA probe. Critically, the conditions employed are sufficiently gentle that depassivation reduces the DNA load on adjacent electrodes by only approximately 1%, allowing for the sequential labeling of multiple, closely spaced electrodes. This technology paves the way for labeling multiple array elements sequentially without observable cross-contamination in a fast and controlled manner.

DNA-based bioanalytical microsystems for handheld device applications

This article reviews and highlights the current development of DNA-based bioanalytical microsystems for point-of-care diagnostics and on-site monitoring of food and water. Recent progresses in the miniaturization of various biological processing steps for the sample preparation, DNA amplification (polymerase chain reaction), and product detection are delineated in detail. Product detection approaches utilizing "portable" detection signals and electrochemistry-based methods are emphasized in this work. The strategies and challenges for the integration of individual processing module on the same chip are discussed.

Detection ofCryptosporidium parvum oocysts using a microfluidic device equipped with the SUS micromesh and FITC-labeled antibody

Development of a microfluidic device equipped with micromesh for detection of Cryptosporidium parvum oocyst was reported. A micromesh consisting of 10 x 10 cavities was microfabricated on the stainless steel plate by laser ablation. Each cavity size, approximately 2.7 microm in diameter, was adopted to capture a single C. parvum oocyst. Under negative pressure operation, suspensions containing microbeads or C. parvum oocysts flowed into the microchannel. Due to strong non-specific adsorption of microbeads onto the PDMS microchannel surface during sample injection, the surface was treated with air plasma, followed by treatment with 1% sodium dodecyl sulfate (SDS) solution. This process reduced the non-specific adsorption of microbeads on the microchannel to 10% or less in comparison to a non-treated microchannel. This microfluidic device equipped with the SUS micromesh was further applied for the capture of C. parvum oocysts. Trapped C. parvum oocysts were visualized by staining with FITC-labeled anti-C. parvum oocyst antibody on a micromesh and counted under fluoroscopic observation. The result obtained by our method was consistent with that obtained by direct immunofluorescence assay coupled with immunomagnetic separation (DFA-IMS) method, indicating that the SUS micromesh is useful for counting of C. parvum oocysts. The newly designed microfluidic device exploits a geometry that allowed for the entrapment of oocysts on the micromesh while providing the rapid introduction of a series of reagents and washes through the microfluidic structure. Our data indicate that this microfluidic device is useful for high-throughput counting of C. parvum oocysts from tap water sample.

Microfluidic diffusive filter for apheresis (leukapheresis)

Apheresis is a procedure used to fractionate whole blood into its individual components. Following fractionation, the desired component is isolated and the remaining blood in many cases is returned to the donor. Leukapheresis is one type of apheresis where leukocytes (white blood cells) are selectively removed. This procedure is commonly used for blood transfusions to remove donor leukocytes from being transferred to the recipient. Apheresis also has several therapeutic applications. In this manuscript we discuss the design, fabrication and testing of a continuous flow diffusive filter, fabricated using simple soft lithographic techniques for depletion of leukocytes. This device employs micro sieves that exploit the size and shape difference between the different cell types to obtain depletion of leukocytes from whole blood. Currently, conventional apheresis methods like centrifugation or fiber mesh filtration are commonly used. A theoretical model was developed to determine the optimal shape of the diffuser to ensure that the volumetric flow through individual sieve elements is equal. This device was designed to serve as a passive device that does not require any external manipulation. Results show that for the given device design, isolation of approximately 50% of the inlet erythrocytes (red blood cells), along with depletion of >97% of the inlet leukocytes is possible at a flow rate of 5 microl min(-1). Simple modifications to the geometry and dimensions of the sieves can be made to obtain isolation of plasma.

An integrated optical leaky waveguide sensor with electrically induced concentration system for the detection of bacteria

An integrated, sensitive and rapid system was developed for the detection of bacteria. The system combined an optical metal-clad leaky waveguide (MCLW) sensor with an electric field. The electric field was used to concentrate Bacillus subtilis var. niger(BG) bacteria spores onto the immobilized anti-BG antibody on the MCLW sensor surface. This sensor combination has been characterised by detecting the scattering from bacterial spores, which are concentrated at the sensor surface, when they are illuminated at the coupling angle; and by detection of fluorescence from labelled antibodies added after the spores had been captured on the surface. The light scattering and fluorescence detection methods gave a detection limit of BG bacterial spores of 1 x 10(3) spores ml(-1) when the electric field was applied for 3 minutes.

Hydrodynamic filtration for on-chip particle concentration and classification utilizing microfluidics

We propose here a new method for continuous concentration and classification of particles in microfluidic devices, named hydrodynamic filtration. When a particle is flowing in a microchannel, the center position of the particle cannot be present in a certain distance from sidewalls, which is equal to the particle radius. The proposed method utilizes this fact, and is performed using a microchannel having multiple side branch channels. By withdrawing a small amount of liquid repeatedly from the main stream through the side channels, particles are concentrated and aligned onto the sidewalls. Then the concentrated and aligned particles can be collected according to size through other side channels (selection channels) in the downstream of the microchannel. Therefore, continuous introduction of a particle suspension into the microchannel enables both particle concentration and classification at the same time. In this method, the flow profile inside a precisely fabricated microchannel determines the size limit of the filtered substances. So the filtration can be performed even when the channel widths are much larger than the particle size, without the problem of channel clogging. In this study, concentrations of polymer microspheres with diameters of 1-3 microm were increased 20-50-fold, and they were collected independently according to size. In addition, selective enrichment of leukocytes from blood was successfully performed.

An efficient cell separation system using 3D-asymmetric microelectrodes

An efficient 3D-asymmetric microelectrode system for high-throughput was designed and fabricated to enhance sorting sensitivities to the dielectric properties-size, morphology, conductivity, and permittivity-of living cells. The principle of the present system is based on the use of the relative strengths of negative dielectrophoretic and drag forces, as in a conventional 3D-microelectrode system. Whereas the typical 3D-microelectrode system has a constant electric field magnitude due to the constant width of the microelectrodes and a fixed gap between face-to-face microelectrodes, the present 3D-asymmetric microelectrode system has electric fields of continuously varying magnitudes along the transverse direction of a channel owing to the changing widths of the electrodes in the half-circular shaped cross section of the microchannel. Thus, varying dielectric forces are generated, leading to increased sorting sensitivity through differentially induced forces to definitely distinct cell types. Numerical analysis verified the improved sensitivity of the present system for sorting living cells. The feasibility of using the newly fabricated system under experimental conditions was tested by demonstrating that a mixed population of mouse P19 embryonic carcinoma (EC) and red blood cells (RBCs) was effectively sorted to different wells depending on their respective relative physical properties.

Current and developing technologies for monitoring agents of bioterrorism and biowarfare

Recent events have made public health officials acutely aware of the importance of rapidly and accurately detecting acts of bioterrorism. Because bioterrorism is difficult to predict or prevent, reliable platforms to rapidly detect and identify biothreat agents are important to minimize the spread of these agents and to protect the public health. These platforms must not only be sensitive and specific, but must also be able to accurately detect a variety of pathogens, including modified or previously uncharacterized agents, directly from complex sample matrices. Various commercial tests utilizing biochemical, immunological, nucleic acid, and bioluminescence procedures are currently available to identify biological threat agents. Newer tests have also been developed to identify such agents using aptamers, biochips, evanescent wave biosensors, cantilevers, living cells, and other innovative technologies. This review describes these current and developing technologies and considers challenges to rapid, accurate detection of biothreat agents. Although there is no ideal platform, many of these technologies have proved invaluable for the detection and identification of biothreat agents.