Search

For over 30 years, carbon fiber microelectrodes have been the gold standard for measurements related to exocytosis and more generally to the processes taking place at the synaptic level. However, this method has a low throughput and molecules can escape detection due to the featureless nature of the planar microelectrodes it uses. Here we present a new electrochemical sensor that addresses these limitations. It is based on insulated protruding volcano-shaped tips of 2 mu m in diameter housing two individually addressable microelectrodes. The sensor enables volume confined and parallelizable recordings of exocytosis from adherent cells. Exocytotic releases from PC12 cells measured by amperometry on our device have quantal size in agreement with commonly admitted values but happen on a much smaller time scale; mostly within half a millisecond. We demonstrate that this faster kinetics must involve a faster vesicle fusion mechanism and is plausibly due to perturbation of the plasma membrane brought by the topography of our sensor. This suggests that exocytosis kinetics may be manipulated by the adequate substrate geometry, which opens up promising new leads of investigation in the study of synaptic processes.

We present an easy, durable method to generate a partially hydrophilic/hydrophobic poly(dimethylsiloxane) (PDMS) microfluidic device. The functionality of this device was assessed in a double flow focusing design to fabricate core-shell structured poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs).

We present a microfluidic device where micro- and nanoparticles can be continuously functionalized in flow. This device relies on an element called "particle exchanger", which allows for particles to be taken from one medium and exposed to some reagent while minimizing mixing of the two liquids. In the exchanger, two liquids are brought in contact and particles are pushed from one to the other by the application of a dielectrophoretic force. We determined the maximum flow velocity at which all the particles are exchanged for a range of particle sizes. We also present a simple theory that accounts for the behaviour of the device when the particle size is scaled. Diffusion mixing in the exchanger is also evaluated. Finally, we demonstrate particle functionalization within the microfluidic device by coupling a fluorescent tag to avidin-modified 880 nm particles. The concept presented in this paper has been developed for synthesis of modified particles but is also applicable to on-chip bead-based chemistry or cellular biology.

Mechanistic investigations on the previously reported reduction of B-alkylcatecholboranes in the presence of methanol led to the disclosure of a new mechanism involving catechol as a reducing agent. More than just revising the mechanism of this reaction, we disclose here the surprising role of catechol, a chain breaking antioxidant, which becomes a source of hydrogen atoms in an efficient radical chain process.

Cellulose whiskers (CWs) were explored as effective nanostructures to manipulate the microstructure of Nafion® membranes for high-performance fuel cell applications. Electrochemical impedance measurements revealed that not only the proton conductivity of the nanocomposite membranes comprising 5 wt% CWs was remarkably increased, but the higher conductivity was also retained at higher temperatures (>100 °C). Moreover, it was found that the inclusion of CWs into a Nafion® matrix provides considerably suppressed methanol permeability. The direct methanol–air single fuel cell test using 5 M methanol solution at 70 °C showed a higher maximum power density of 91 mW cm−2 for the nanocomposite membrane, compared to 28 and 47 mW cm−2 for unmodified Nafion®. Hence, CWs could be considered as effective and elegant nanostructures to regulate the ionic microstructure and alleviate critical drawbacks of Nafion membranes for commercial direct methanol fuel cell applications.

Here we describe preparation and characterization of novel ion exchange polymer nanocomposites for fuel cells applications. Montmorillonite (MMT) nanoclay is modified using amino benzotriazole (BTA) via ion exchange reaction. X-ray diffraction patterns confirmed modification of MMT. This BTA-MMT was then used to produce Nafion based nanocomposite membranes with exfoliated microstructures at different loading weights. Transport property (proton conductivity and methanol permeability) results revealed that incorporation of BTA-modified MMT into Nafion® results in significantly reduced methanol crossover while maintaining proton conductivity. Direct methanol single fuel cell tests confirmed a cell efficiency of 21% for Nafion/BTA-MMT-3 wt% compared to 13.72% for conventional Nafion117 membrane at 5 M methanol feed, and also a higher power density of 144 mW cm−2 in comparison with 39 mW cm−2 for Nafion 117. Owing to these features, prepared Nafion/BTA-MMT membranes can be considered as promising polymeric electrolytes for fuel cell applications.

Here we show that the transport properties and electrochemical performance of polyelectrolyte membranes are improved through the dispersion of chitosan-wrapped carbon nanotubes, for direct methanol fuel cell applications. Methanol permeability is reduced via improving the interfacial interactions and the solubilization of CNTs in the Nafion matrix, as well as inducing the formation of long-range oriented conduction pathways in the vicinity of the decorated one-dimensional nanostructure. The improved membrane selectivity results in a considerably enhanced fuel cell efficiency (16% vs. 11%) and a power generation capacity more than two times higher (110 mW cm−2vs. 47 mW cm−2) in a concentrated methanol solution (5 M), in comparison with the commercial Nafion®117 membrane.

A microfluidic platform is developed for the synthesis of monodisperse, 100 nm, chitosan based nanoparticles using nanogelation with ATP. The resulting nanoparticles tuned and enhanced transport and electrochemical properties of Nafion based nanocomposite membranes, which is highly favorable for fuel cell applications.

Radical alkylation of 2-(tert-butyl)-2-methyldioxolan-4-one, a chiral equivalent of glycolic acid, occurs with good to high diastereoselectivity that compares favorably with the corresponding enolate alkylation. The importance of the position of the transition state position, early or late, is highlighted.

We present a novel perfusion-based microfluidic platform for label-free drug toxicity screening which can single out non-lethal morphological changes from cellular death using electrical impedance spectroscopy. Minor cellular changes such as cell-cell contacts and major cell injury were identified via impedance phase angle analysis and follow-up of impedance magnitude at different frequencies. Having exposed HepG2/C3A cells to acetaminophen (AP), we showed that continuous drug perfusion caused a time and concentration-dependent impedance decrease. Moreover, perfusion of repeated doses revealed altered dielectric properties of the cell culture after recovery from AP exposure. This study highlights the possibility to sense cellular changes long before cellular death takes place, pointing out the remarkable sensitivity advantage of this technique over standard endpoint viability tests and its interest for toxicology.

Contamination with arsenic is a recurring problem in both industrialized and developing countries. Drinking water supplies for large populations can have concentrations much higher than the permissible levels (for most European countries and the United States, 10 mu g As per L; elsewhere, 50 mu g As per L). Arsenic analysis requires high-end instruments, which are largely unavailable in developing countries. Bioassays based on genetically engineered bacteria have been proposed as suitable alternatives but such tests would profit from better standardization and direct incorporation into sensing devices. The goal of this work was to develop and test microfluidic devices in which bacterial bioreporters could be embedded, exposed and reporter signals detected, as a further step towards a complete miniaturized bacterial biosensor. The signal element in the biosensor is a nonpathogenic laboratory strain of Escherichia coli, which produces a variant of the green fluorescent protein after contact to arsenite and arsenate. E. coli bioreporter cells were encapsulated in agarose beads and incorporated into a microfluidic device where they were captured in 500 x 500 mu m(2) cages and exposed to aqueous samples containing arsenic. Cell-beads frozen at -20 degrees C in the microfluidic chip retained inducibility for up to a month and arsenic samples with 10 or 50 mu g L-1 could be reproducibly discriminated from the blank. In the 0-50 mu g L-1 range and with an exposure time of 200 minutes, the rate of signal increase was linearly proportional to the arsenic concentration. The time needed to reliably and reproducibly detect a concentration of 50 mu g L-1 was 75-120 minutes, and 120-180 minutes for a concentration of 10 mu g L-1

The poor efficiency of microfluidic single cell trapping is currently restricting the full potential of state-of-the-art single cell analyses. Using fluid dynamics simulations in combination with particle image velocimetry to systematically optimize trap architectures, we present a microfluidic chip with enhanced single cell trapping and on-chip culture performance. Upon optimization of trap geometries, we measured trapping efficiencies of up to 97%. Our device also enables the stable, relatively long-term culture of individual non-adherent mammalian cells in high-throughput without a significant decrease in cell viability. As a first application of this platform we demonstrate the automated separation of the two daughter cells generated upon single cell division. The reliable trapping and re-trapping of mammalian cells should for example provide the fundament for novel types of investigations in stem cell and tumour cell biology, which depend on reliable tracking of genealogical relationships such as in stem cell lineage tracking.

The integration of inductors raises the need for highly magnetic monodomain and insulating materials. With this aim in mind, we investigate the synthesis of FeCo nanoparticles using an organometallic approach. The role of the nature of the ligands, of the organometallic precursors and of the experimental conditions are addressed, leading to the formation of nanoparticles displaying different compositions and structural and magnetic properties. After optimization of the reaction conditions, 20 nm monodisperse nanoparticles, self-organized into millimetre long super-crystals, are obtained. They display a structural atomic disorder, due to the kinetics of decomposition of the precursors leading to a loss in their magnetic properties. Organized arrays of these nanoparticles could also be generated onto silicon wafers with thicknesses from 100 nm to several microns. Such densely packed layers are suitable for inductor nanotechnology

We present here a microfluidic device for the chemical modification of particles. In order to alleviate diffusive mixing issues beads are pushed from a starting buffer to a reagent over a wide channel by an array of shifted electrodes. We also show that the effect of reagent diffusion can be compensated by electrophoretic forces.

We present a channel geometry that allows for clean switching between different inlets of a microchip without any contamination of the inlets or the downstream flow. We drive this virtual valve with a pneumatic pressure setup that minimizes disturbance of the downstream flow during the switching procedure by simultaneous variation of the pressures applied to the different inlets. We assess the efficiency of the setup by spectroscopic measurement of downstream dye concentrations, and demonstrate its practical utility by sequentially constructing multiple layers of alginate hydrogel. The method is potentially useful for a whole series of further applications, such as changing perfusion liquids for cell culture and cell analysis, metering, chemical- reaction initiation and multi-sample chromatography, to name a few.

In their article, ‘‘AC frequency characteristics of coplanar impedance sensors as design parameters’’, Hong et al.1 present an analytical model for the calculation of the resistance between two electrodes in a microchannel. However, their measurements fit poorly with their suggested model, and they attribute the discrepancies to electrode thickness and a ‘fringing effect’. In this note we show that the discrepancies are due to the neglected channel height in their model, and that by including two additional transforma ions, the resistance between two

The following paper describes a sacrificial layer method for the manufacturing of microfluidic devices in polyimide and SU-8. The technique uses heat-depolymerizable polycarbonates embedded in polyimide or SU-8 for the generation of microchannels and sealed cavities. The volatile decomposition products originating from thermolysis of the sacrificial material escape out of the embedding material by diffusion through the cover layer. The fabrication process was studied experimentally and theoretically with a focus on the decomposition of the sacrificial materials and their diffusion through the polyimide or SU-8 cover layer. It is demonstrated that the sacrificial material removal process is independent of the actual channel geometry and advances linearly with time unlike conventional sacrificial layer techniques. The fabrication method provides a versatile and fast technique for the manufacturing of microfluidic devices for applications in the field of microTAS and Lab-on-a-Chip.

Combining deflective dielectrophoretic barriers with controlled pressure driven liquid flows in microfluidic devices allows accurate handling of particles such as biological cells in suspensions. Working towards cell-based lab-on-a-chip applications, a platform permitting rapid testing of devices having different dielectrophoretic and fluidic subunits was developed. The performance of such a system is shown in the cases of (A) flooding a small number of immobilised cells with a dye and (B) transient buffer swapping of a large number of cells in flow. The transition times for moving cells from one reagent to the other are below 0.5 s in the case of flow-through cell dipping.

We propose a model to determine the influence of different cell properties, such as size, membrane capacitance and cytoplasm conductivity, on the impedance spectrum as measured in a microfabricated cytometer. A dielectric sphere of equivalent complex permittivity is used as a simplified model to describe a biological cell. The measurement takes place between a pair of facing microelectrodes in a microchannel filled with a saline solution. The model incorporates various cell parameters, such as dielectric properties, size and position in the channel. A 3D finite element model is used to evaluate the magnitude of the electric field in the channel and the resultant changes in charge densities at the measurement electrode boundaries as a cell flows past. The charge density is integrated on the electrode surface to determine the displacement current and the channel impedance for the computed frequency range. The complete impedance model combines the finite element model, the electrode-electrolyte interface impedance and stray impedance, which are measured from a real device. The modeled dielectric complex spectra for various cell parameters are discussed and a measurement strategy for cell discrimination with such a system is proposed. We finally discuss the amount of noise and measurement fluctuations of the sensor.

This paper describes the development of polyimide-based microfluidic devices. A layer transfer and lamination technique is used to fabricate flexible microfluidic channels in various shapes and with a wide range of dimensions. High bond strengths can be achieved by cure cycle adaptation and surface treatment of the polyimide layers prior to bonding. The polyimide microchannels can be combined with metallization layers to fabricate electrodes inside and outside channels. The resulting devices can be used for flexible fluidic and electrical connectors, implantable fluid delivery devices, microelectrodes with embedded fluidic channels, chip-based flow cytometry and for a great variety of other applications in medical, chemical or biological research.

A new cytological tool, based on the microCoulter particle counter (microCPC) principle, aimed at diagnostic applications for cell counting and separation in haematology, oncology or toxicology is described. The device measures the spectral impedance of individual cells or particles and allows screening rates over 100 samples s(-1) on a single-cell basis. This analyzer is intended to drive a sorting actuator producing a subsequent cell separation. Size reduction and integration of functions are essential in achieving precise measurements and high throughput. 3D finite element simulations are presented to compare various electrode geometries and their influence on cell parameters estimation. The device is based on a glass-polyimide microfluidic chip with integrated channels and electrodes microfabricated at the length scale of the particles to be investigated (1-20 microm). A laminar liquid flow carries the suspended particles through the measurement area. Each particle's impedance signal is recorded by a differential pair of microelectrodes using the cell surrounding media as a reference. The micromachined chip and processing electronic circuit allow simultaneous impedance measurements at multiple frequencies, ranging from 100 kHz to 15 MHz. In this paper, we describe the microfabrication and characterisation of an on-chip flow-cytometer as the first building block of a complete cell-sorting device. We then discuss the signal conditioning technique and finally impedance measurements of cells and particles of different sizes and types to demonstrate the differentiation of subpopulations in a mixed sample.