Christiane Thielemann

Triangular neuronal networks on microelectrode arrays: an approach to improve the properties of low-density networks for extracellular recording

Multi-unit recording from neuronal networks cultured on microelectrode arrays (MEAs) is a widely used approach to achieve basic understanding of network properties, as well as the realization of cell-based biosensors. However, network formation is random under primary culture conditions, and the cellular arrangement often performs an insufficient fit to the electrode positions. This results in the successful recording of only a small fraction of cells. One possible approach to overcome this limitation is to raise the number of cells on the MEA, thereby accepting an increased complexity of the network. In this study, we followed an alternative strategy to increase the portion of neurons located at the electrodes by designing a network in confined geometries. Guided settlement and outgrowth of neurons is accomplished by taking control over the adhesive properties of the MEA surface. Using microcontact printing a triangular two-dimensional pattern of the adhesion promoter poly-D-lysine was applied to the MEA offering a meshwork that at the same time provides adhesion points for cell bodies matching the electrode positions and gives frequent branching points for dendrites and axons. Low density neocortical networks cultivated under this condition displayed similar properties to random networks with respect to the cellular morphology but had a threefold higher electrode coverage. Electrical activity was dominated by periodic burst firing that could pharmacologically be modulated. Geometry of the network and electrical properties of the patterned cultures were reproducible and displayed long-term stability making the combination of surface structuring and multi-site recording a promising tool for biosensor applications.

Measurements of the mechanical behaviour of micromachined silicon and silicon-nitride membranes for microphones, pressure sensors and gas flow meters

Abstract This paper mainly presents the experimental determination of the small deflection behaviour of boron-implanted silicon-nitride and highly boron-doped silicon diaphragms for micromachined silicon subminiature microphones. The additional implantation of boron into silicon-nitride diaphragms reduces the intrinsic stress in the deposited amorphous films. The minimum detectable deflection, using a Mach-Zehnder interferometer, is about 0.02 nm for dynamic measurements (A-weighted filtering). The largest measurable deflection (where nonlinearities of the interferometer are negligible) is strongly influenced by the wavelength of the laser and is about 10 nm. Thus, applying this method to pressure sensors and gas flow meters, the pressure range is restricted. In order to achieve a high sensitivity of the measuring apparatus and a low detectable deflection amplitude a feedback configuration stabilizes the interferometer in the most sensitive operation points.

Silicon-based inorganic electrets for application in micromachined devices

Layers of SiO/sub 2/ and Si/sub 3/N/sub 4/ have good mechanical properties for application in micro-machined electret capacitor microphones. These materials are investigated as single or double layers in terms of chargeability and long-term charge stability. The main emphasis is put on the miniaturization of electret layers. The lateral dimensions of the electrets are reduced to 2 mm and the charge decay characteristics under different environmental conditions are described. According to the experimental data, multilayer samples of silicon dioxide and nitride possess good chargeability and higher charge stability compared to the well investigated single layers. Typically, the double layers loose /spl sim/10% of their surface potential when annealed for 200 min at 300/spl deg/C and show a peak of the thermally stimulated current at 430/spl deg/C. It can also be shown that the miniaturization of samples does not necessarily cause a faster charge decay.

A novel automated spike sorting algorithm with adaptable feature extraction

To study the electrophysiological properties of neuronal networks, in vitro studies based on microelectrode arrays have become a viable tool for analysis. Although in constant progress, a challenging task still remains in this area: the development of an efficient spike sorting algorithm that allows an accurate signal analysis at the single-cell level. Most sorting algorithms currently available only extract a specific feature type, such as the principal components or Wavelet coefficients of the measured spike signals in order to separate different spike shapes generated by different neurons. However, due to the great variety in the obtained spike shapes, the derivation of an optimal feature set is still a very complex issue that current algorithms struggle with. To address this problem, we propose a novel algorithm that (i) extracts a variety of geometric, Wavelet and principal component-based features and (ii) automatically derives a feature subset, most suitable for sorting an individual set of spike signals. Thus, there is a new approach that evaluates the probability distribution of the obtained spike features and consequently determines the candidates most suitable for the actual spike sorting. These candidates can be formed into an individually adjusted set of spike features, allowing a separation of the various shapes present in the obtained neuronal signal by a subsequent expectation maximisation clustering algorithm. Test results with simulated data files and data obtained from chick embryonic neurons cultured on microelectrode arrays showed an excellent classification result, indicating the superior performance of the described algorithm approach.

Inorganic electret membrane for a silicon microphone

Abstract A new inorganic electret material is presented for application in micromachined sensors, wherever a static electric field is needed to replace an external voltage supply, e.g., for airborne sound transducers. The double-layer system of thermal silicon dioxide/CVD nitride is an excellent candidate for a silicon-based electret material. An inorganic electret membrane of this material is suggested for application in a silicon microphone. The processing of the electret membrane chip is described and results of stress-compensated electret layers with good charge stability are presented.

Three-Dimensional Carbon Nanotube Electrodes for Extracellular Recording of Cardiac Myocytes

Low impedance at the interface between tissue and conducting electrodes is of utmost importance for the electrical recording or stimulation of heart and brain tissue. A common way to improve the cell–metal interface and thus the signal-to-noise ratio of recordings, as well as the charge transfer for stimulation applications, is to increase the electrochemically active electrode surface area. In this paper, we propose a method to decrease the impedance of microelectrodes by the introduction of carbon nanotubes (CNTs), offering an extremely rough surface. In a multistage process, an array of multiple microelectrodes covered with high quality, tightly bound CNTs was realized. It is shown by impedance spectroscopy and cardiac myocyte recordings that the transducer properties of the carbon nanotube electrodes are superior to conventional gold and titanium nitride electrodes. These findings will be favorable for any kind of implantable heart electrodes and electrophysiology in cardiac myocyte cultures.

Electromagnetic exposure of scaffold-free three-dimensional cell culture systems.

In recent years, a number of in vitro studies have reported on the possible athermal effects of electromagnetic exposure on biological tissue. Typically, this kind of study is performed on monolayers of primary cells or cell lines. However, two-dimensional cell layer systems lack physiological relevance since cells in vivo are organized in a three-dimensional (3D) architecture. In monolayer studies, cell-cell and cell-ECM interactions obviously differ from live tissue and scale-ups of experimental results to in vivo systems should be considered carefully. To overcome this problem, we used a scaffold-free 3D cell culture system, suitable for the exploration of electrophysiological effects due to electromagnetic fields (EMF) at 900 MHz. Dissociated cardiac myocytes were reaggregated into cellular spheres by constant rotation, and non-invasive extracellular recordings of these so-called spheroids were performed with microelectrode arrays (MEA). In this study, 3D cell culture systems were exposed to pulsed EMFs in a stripline setup. We found that inhomogeneities in the EMF due to electrodes and conducting lines of the MEA chip had only a minor influence on the field distribution in the spheroid if the exposure parameters were chosen carefully.

Graphene electrodes for stimulation of neuronal cells

Graphene has the ability to improve the electrical interface between neuronal cells and electrodes used for recording and stimulation purposes. It provides a biocompatible coating for common electrode materials such as gold and improves the electrode properties. Graphene electrodes are also prepared on SiO2 substrate to benefit from its optical properties like transparency. We perform electrochemical and Raman characterization of gold electrodes with graphene coating and compare them with graphene on SiO2 substrate. It was found that the substrate plays an important role in the performance of graphene and show that graphene on SiO2 substrate is a very promising material combination for stimulation electrodes.

A three-dimensional microelectrode array composed of vertically aligned ultra-dense carbon nanotube networks

Electrodes based on carbon nanotubes are a promising approach to manufacture highly sensitive sensors with a low limit of signal detection and a high signal-to-noise ratio. This is achieved by dramatically increasing the electrochemical active surface area without increasing the overall geometrical dimensions. Typically, carbon nanotube electrodes are nearly planar and composed of randomly distributed carbon nanotube networks having a limited surface gain for a specific geometrical surface area. To overcome this limitation, we have introduced vertically aligned carbon nanotube (VACNT) networks as electrodes, which are arranged in a microelectrode pattern of 60 single electrodes. Each microelectrode features a very high aspect ratio of more than 300 and thus a dramatically increased surface area. These microelectrodes composed of VACNT networks display dramatically decreased impedance over the entire frequency range compared to planar microelectrodes caused by the enormous capacity increase. This is experi...

Micromachined capacitive electret microphone

Silicon-based electret materials are presented for the application in micro-machined sensors. Various dielectric layer systems of silicon dioxide and silicon nitride are investigated and compared to the well known single layer silicon dioxide. The superior material system silicon dioxide/silicon nitride is optimized in terms of charge stability, mechanical behavior and processing capabilities. Only the advantageous combination of low stress and good electret properties allows the realization of electret membranes for silicon microphones. Electret membranes were realized by means of an anisotropic etch process. Various measurements of charge stability were performed and evaluated. For the first time the application of an inorganic electret membrane in a micro-machined capacitive silicon microphone is presented and results are shown. An equivalent noise level of 32 dB could be achieved for an electret microphone with a 4 mm2 membrane area.