M. Koudelka-Hep

Active pixel sensor array for high spatio-temporal resolution electrophysiological recordings from single cell to large scale neuronal networks

This paper presents a chip-based electrophysiological platform enabling the study of micro- and macro-circuitry in in-vitro neuronal preparations. The approach is based on a 64x64 microelectrode array device providing extracellular electrophysiological activity recordings with high spatial (21 microm of electrode separation) and temporal resolution (from 0.13 ms for 4096 microelectrodes down to 8 micros for 64 microelectrodes). Applied to in-vitro neuronal preparations, we show how this approach enables neuronal signals to be acquired for investigating neuronal activity from single cells and microcircuits to large scale neuronal networks. The main elements of the platform are the metallic microelectrode array (MEA) implemented in Complementary Metal Oxide Semiconductor (CMOS) technology similar to a light imager, the in-pixel integrated low-noise amplifiers (11 microVrms) and the high-speed random addressing logic. The chip is combined with a real-time acquisition system providing the capability to record at 7.8 kHz/electrode the whole array and to process the acquired signals.

Large-Scale, High-Resolution Data Acquisition System for Extracellular Recording of Electrophysiological Activity

A platform for high spatial and temporal resolution electrophysiological recordings of in vitro electrogenic cell cultures handling 4096 electrodes at a full frame rate of 8 kHz is presented and validated by means of cardiomyocyte cultures. Based on an active pixel sensor device implementing an array of metallic electrodes, the system provides acquisitions at spatial resolutions of 42 mum on an active area of 2.67 mm times 2.67 mm, and in the zooming mode, temporal resolutions down to 8 mus on 64 randomly selected electrodes. The low-noise performances of the integrated amplifier (11 muVrms) combined with a hardware implementation inspired by image/video processing concepts enable high-resolution acquisitions with real-time preprocessing capabilities adapted to the handling of the large amount of acquired data.

Mercury-plated iridium-based microelectrode arrays for trace metals detection by voltammetry: optimum conditions and reliability

An amperometric microsensor for the detection of trace metals in the low or sub nanomolar range is presented. It is obtained by successively evaporating iridium (2000 A) and Si3N4 (2000 A) on a silicon wafer, followed by a photolithographic pattering procedure. It consists of an array of 10 x 10 iridium microdisc electrodes with a recessed depth of 0.2 μm, separated by 50 or 150 μm. The electrical contacts are isolated by a layer of Agolit or Epoxy resin. Scanning electron microscopy and Atomic force microscopy have been used to control the regularity of the microelectrode array geometry and morphology. For the analysis of trace metals, mercury is deposited on the iridium-based microelectrode array. A given array is able to sustain the operations of Hg deposition/dissolution at least 10 times. The reliability of the mercury-plated iridium-based microelectrode arrays has been tested by a series of systematic Square Wave Anodic Striping Voltammetry (SWASV) analyses in synthetic solutions of lead and cadmium ions in the concentration range 1-10 nM. Repeated measurements over long periods of time on a given mercury layer showed good stability when the Epoxy resin was used and good reproducibility (± 4%) for at least 5 h. A good reproducibility was also found between different arrays. Finally, the mercury-plated iridium-based microelectrode arrays were applied to the lead and cadmium speciation in river water, by direct SWASV measurements, without any separation. A detection limit of 50 pM was established for a preconcentration time of 15 min. The results were compared with other techniques, in particular a similar procedure using a single Hg-plated Ir-based microelectrode.

Localised electrochemical impedance spectroscopy with high lateral resolution by means of alternating current scanning electrochemical microscopy

A new method for measuring local interfacial impedance properties with high lateral resolution was developed by combination of electrochemical impedance spectroscopy (EIS) with scanning electrochemical microscopy (SECM). Alternating current scanning electrochemical microscopy (AC-SECM) allowed to identify and visualise microscopic domains of different conductivity/electrochemical activities on solid/liquid interfaces immersed into an electrolyte. The performance of the method was illustrated by imaging an array of Pt-band microelectrodes in solutions of low conductivity in the absence of any redox mediator.

Integrated array sensor for detecting organic solvents

Abstract A new sensor array device has been designed to detect organic solvents; it comprises an array of six interdigital sensors lying upon a micromachined 0.5 μm thick silicon nitride membrane. There are three separate micromachined cells with two sensors per cell. In each cell, a thin film platinum resistance thermometer/heater is sandwiched in the middle of the silicon nitride layer in order to either monitor or control the temperature of the active layer. The array device has a low power consumption of ≈ mW/sensor at 400 °C and is capable of operating at temperatures in excess of 600 °C, making it suitable for inorganic (e.g., SnO2) as well as organic gas-sensitive materials. The sensor array device has been coated with both polymers and semiconducting oxides and its response to toluene, n-propanol and n-octane has been studied. The low power consumption and good thermal stability of this array device make it useful for application in portable gas monitoring equipment.

A New Fabrication Method for Borosilicate Glass Capillary Tubes with Lateral Inlets and Outlets

Abstract In this paper a new fabrication method for borosilicate glass capillary tubes is presented. As the interest in miniaturized total chemical analysis systems (μ-TAS) is increasing, the need for fluidic paths is growing and thus the study of microchannels, microtubes and microcolumns is an important topic of the microfluidic area. The capillary tubes presented here are fabricated by structuring and bonding three borosilicate glass wafers (7740 Corning Pyrex sR). Microchannels with lateral inlets and outlets have been successfully realized and well-defined size and shape have been obtained. Several capillary tubes with widths from 340 to 940 μm have been realized as well as different section shapes, which can be circular, elliptic or quasi-rectangular. The main fabrication steps and first characterizations are reported.

Microreactor and electrochemical detectors fabricated using Si and EPON SU-8

Abstract An electrochemiluminescence (ECL) detector and a microenzymatic reactor (MER) combining Si and SU-8 technologies are described. Both devices were fabricated using standard processing techniques to produce on-wafer sensor elements, which were composed of a platinum or, alternatively, carbon interdigitated electrode array. The platinum array was resting on top of an Si pn photodiode. After these elements were completed, the entire wafer was modified with SU-8, which was structured to form a series of shaped spacers surrounding each device. For the ECL detector, a simple flow-channel was defined whereas for the microreactor a more complex layout defining two chambers separated by a series of SU-8 columns was employed. The upstream chamber of the microreactor was packed with porous glass beads modified with immobilised enzyme glucose oxidase whereas the downstream chamber contained the electrochemical detector. The performance of the ECL detector was assessed by the detection of codeine using ruthenium (II) tris(2,2′-bipyridyl). A detection limit of 100 μM was obtained and pharmaceutical preparations were successfully assayed. The MER was first evaluated by electrochemical determination of glucose and as a next step, a miniature ECL detector was placed on line downstream the MER to perform glucose measurements by ECL. Glucose was determined with detection limits of 2 and 50 μM by electrochemistry and ECL, respectively. This system was found to have a lifetime of at least 1 month when stored at 4°C.

Microelectrode arrays for electrophysiological monitoring of hippocampal organotypic slice cultures

A three-dimensional platinum (Pt) microelectrode array embedded on a micromachined silicon (Si) substrate (porosity of 13%, via hole diameter of 40 /spl mu/m) has been developed. Electrodes are 35-/spl mu/m wide and 20-/spl mu/m high, spaced 200 /spl mu/m apart and arranged in an elliptic geometry. Integrated within a microperfusion chamber, the devices were used for stimulation and recording experiments of hippocampal slice cultures over a period of several days.

Biocompatibility of silicon-based arrays of electrodes coupled to organotypic hippocampal brain slice cultures

In this study we examined the passive biocompatibility of a three-dimensional microelectrode array (MEA), designed to be coupled to organotypic brain slice cultures for multisite recording of electrophysiological signals. Hippocampal (and corticostriatal) brain slices from 1-week-old (and newborn) rats were grown for 4-8 weeks on the perforated silicon chips with silicon nitride surfaces and 40 microm sized holes and compared with corresponding tissue slices grown on conventional semiporous membranes. In terms of preservation of the basic cellular and connective organization, as visualized by Nissl staining, Timm sulphide silver-staining, microtubule-associated protein 2 (MAP2) and glial fibrillary acidic protein (GFAP) immunostaining, the slice cultures grown on chips did not differ from conventionally grown slice cultures. Neither were there any signs of astrogliosis or neurodegeneration around the upper recording part of the 47-microm-high platinum-tip electrodes. Slice cultures grown on a separate set of chips with platinum instead of silicon nitride surfaces also displayed normal MAP2 and GFAP immunostaining. The width of the GFAP-rich zone (glia limitans) at the bottom surface of the slice cultures was the same ( approximately 20 microm) in cultures grown on chips with silicon nitride and platinum surfaces and on conventional insert membranes. The slice cultures grown on chips maintained a normal, subfield differentiated susceptibility to the glutamate receptor agonist N-methyl-D-aspartate (NMDA) and the neurotoxin trimethyltin (TMT), as demonstrated by the cellular uptake of propidium iodide (PI), which was used as a reproducible and quantifiable marker for neuronal degeneration. We conclude that organotypic brain slice cultures can grow on silicon-based three-dimensional microelectrode arrays and develop normally with display of normal subfield differentiated susceptibilities to known excito- and neurotoxins. From this it is anticipated that the set-up, designed for recording of electrophysiological parameters, can be used for long-term studies of defined neuronal networks and provide valuable information on both normal, neurotoxicological and neuropathological conditions.

An array of Pt-tip microelectrodes for extracellular monitoring of activity of brain slices

A microelectrode array (MEA) consisting of 34 silicon nitride passivated Pt-tip microelectrodes embedded on a perforated silicon substrate (porosity 35%) has been realized. The electrodes are 47 microns high, of which only the top 15 microns are exposed Pt-tips having a curvature of 0.5 micron. The MEA is intended for extracellular recordings of brain slices in vitro. Here we report the fabrication, characterization and initial electrophysiological evaluation of the first generation of Pt-tip MEAs.