Harald van Lintel

Effect of the surface charge on ion transport through nanoslits

A description of ion transport through geometrically defined nanoslits is presented. It is characterized by the effective surface charge density and was obtained by impedance spectroscopy measurements of electrolytes with different physico-chemical properties. The fluid channels were fabricated in a Pyrex-Pyrex field assisted bonding process with an intermediate layer of amorphous silicon. The height of the nanoslits was defined by the 50 nm thickness of the amorphous silicon layer. Two microfluidic channels, containing electrodes for the characterization of the nanoslits, maintained fresh liquid on both sides of the nanoapertures. By changing the KCl concentration of the electrolyte, a conductance plateau (in log-log scale) was observed due to the dominance of the effective surface charge density, resulting in an excess of mobile counterions in the nanoslits at low salt concentrations. The effective surface charge density of the Pyrex nanoslits could be modified by changing the pH of the solution. It was verified that at higher pH values the nanoslit conductance increased. Field-effect experiments allowed changing the effective surface charge density as well. The polarity of the external voltage could be chosen such that the effective surface charge density was increased or decreased, resulting in a higher or lower nanoslit conductance. This regulation of ionic flow can be exploited for the fabrication of nanofluidic devices

Development of a microfluidics biosensor for agarose-bead immobilized Escherichia coli bioreporter cells for arsenite detection in aqueous samples

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 μg As per L; elsewhere, 50 μg 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 × 500 μm(2) cages and exposed to aqueous samples containing arsenic. Cell-beads frozen at -20 °C in the microfluidic chip retained inducibility for up to a month and arsenic samples with 10 or 50 μg L(-1) could be reproducibly discriminated from the blank. In the 0-50 μg 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 μg L(-1) was 75-120 minutes, and 120-180 minutes for a concentration of 10 μg L(-1).

Polyimide/SU-8 catheter-tip MEMS gauge pressure sensor

This paper describes the development of a polyimide/SU-8 catheter-tip MEMS gauge pressure sensor. Finite element analysis was used to investigate critical parameters, impacting on the device design and sensing characteristics. The sensing element of the device was fabricated by polyimide-based micromachining on a flexible membrane, using embedded thin-film metallic wires as piezoresistive elements. A chamber containing this flexible membrane was sealed using an adapted SU-8 bonding technique. The device was evaluated experimentally and its overall performance compared with a commercial silicon-based pressure sensor. Furthermore, the device use was demonstrated by measuring blood pressure and heart rate in vivo.

Compact portable biosensor for arsenic detection in aqueous samples with Escherichia coli bioreporter cells

We present a compact portable biosensor to measure arsenic As(III) concentrations in water using Escherichia coli bioreporter cells. Escherichia coli expresses green fluorescent protein in a linearly dependent manner as a function of the arsenic concentration (between 0 and 100 μg/L). The device accommodates a small polydimethylsiloxane microfluidic chip that holds the agarose-encapsulated bacteria, and a complete optical illumination/collection/detection system for automated quantitative fluorescence measurements. The device is capable of sampling water autonomously, controlling the whole measurement, storing and transmitting data over GSM networks. We demonstrate highly reproducible measurements of arsenic in drinking water at 10 and 50 μg/L within 100 and 80 min, respectively.

Study of micro-glow discharges as ion sources for ion mobility spectrometry

We develop a microionizer as the ion source in a miniaturized spectrometerlike gas analyzer, operating at atmospheric pressure. Several options for the ionizing principle were considered. The application of dc glow discharge in a microsystem was studied in detail and first devices were fabricated and tested. We obtained stable dc plasmas in micromachined electrode gaps from 1 to 50 μm width, at pressures up to 105 Pa in various gases. With 1 and 3 μm gaps, stable glow was achieved at atmospheric pressure in Ar and N2, respectively. In a macrosystem, we extracted ions from a dc glow discharge and controlled the ion flow with a grid.

Direct localised measurement of electrical resistivity profile in rat and embryonic chick retinas using a microprobe

Abstract We report an alternative technique to perform a direct and local measurement of electrical resistivities in a layered retinal tissue. Information on resistivity changes along the depth in a retina is important for modelling retinal stimulation by retinal prostheses. Existing techniques for resistivity-depth profiling have the drawbacks of a complicated experimental setup, a less localised resistivity probing and/or lower stability for measurements. We employed a flexible microprobe to measure local resistivity with bipolar impedance spectroscopy at various depths in isolated rat and chick embryo retinas for the first time. Small electrode spacing permitted high resolution measurements and the probe flexibility contributed to stable resistivity profiling. The resistivity was directly calculated based on the resistive part of the impedance measured with the Peak Resistance Frequency (PRF) methodology. The resistivity-depth profiles for both rat and chick embryo models are in accordance with previous mammalian and avian studies in literature. We demonstrate that the measured resistivity at each depth has its own PRF signature. Resistivity profiles obtained with our setup provide the basis for the construction of an electric model of the retina. This model can be used to predict variations in parameters related to retinal stimulation and especially in the design and optimisation of efficient retinal implants.

Miniaturized bacterial biosensor system for arsenic detection holds great promise for making integrated measurement device

Combining bacterial bioreporters with microfluidics systems holds great promise for in-field detection of chemical or toxicity targets. Recently we showed how Escherichia coli cells engineered to produce a variant of green fluorescent protein after contact to arsenite and arsenate can be encapsulated in agarose beads and incorporated into a microfluidic chip to create a device for in-field detection of arsenic, a contaminant of well known toxicity and carcinogenicity in potable water both in industrialized and developing countries. Cell-beads stored in the microfluidics chip at -20°C retained inducibility up to one month and we were able to reproducibly discriminate concentrations of 10 and 50 μg arsenite per L (the drinking water standards for European countries and the United States, and for the developing countries, respectively) from the blank in less than 200 minutes. We discuss here the reasons for decreasing bioreporter signal development upon increased storage of cell beads but also show how this decrease can be reduced, leading to a faster detection and a longer lifetime of the device.

Photo-Polymer Microchannel Technologies and Applications

A set of new technologies to fabricate microchannels using photosensitive polymer materials is presented. They are based on the use of polymers, hereafter called photopolymers, that are cured with UV light under a mask exposure. The basic material is an epoxy-based photopolymer which can be patterned in thick layers with high aspect ratio. Several process variations for making multilayered channel network are described and discussed: a) a process based on the refill of channels with a non-sensitive polymer, b) a process based on interlevel masking and c) a new process involving the lamination of dry films. With all three processes, multilevel channel networks have been fabricated with minimum channel width and height of about 25 micrometers. The photopolymer channel technology offers a good alternative to other technologies such as glass and silicon micromachining or plastic molding when multilevel crossing microchannels are needed. Moreover, the photoplastic channel technology is a relatively low cost technology that could be applied, in some cases, to disposable fluidic chips.

Remotely powered implantable heart monitoring system for freely moving animals

This paper presents a remotely powered implantable heart monitoring system for freely moving animals. The system measures the blood pressure in the left ventricle of the heart and transmits the data to a database unit. The implanted unit is remotely powered over 25 mm at 8 MHz and has autonomous power control system for changing received power levels due to the moving animal. The blood pressure is measured by a piezoresistive sensor die. The data is transmitted by a OOK modulated transmitter at 868 MHz. The system is realized by using discrete components which are available on the market. The overall size of the implanted unit is 26×13×5.5 mm and the overall power consumption is around 7 mW. Experimental results show the effectiveness of the implantable monitoring system.

Study of Gas Ionization Schemes for Micro Devices

For the development of micro gas sensors, studies of field ionization and break-down in small gaps were performed. A finite element simulation was done for an estimate of the achievable field strength near a nanotube tip in small gaps. The simulation also showed the relatively low importance of the gross electrode geometry when significant protrusions are present. For first measurements two types of test structures were fabricated: planar micro devices with thin film platinum electrodes, and a structure with nanotubes on silicon as electrodes. Both types showed characteristic conditioning behaviour and breakdown at voltages of expected magnitude.