Ute Schreiber

Choosing electrodes for deep brain stimulation experiments--electrochemical considerations.

Deep brain stimulation (DBS) is a therapy of movement disorders including Parkinsons disease (PD). Commercially available electrodes for animal models of Parkinsons disease vary in geometry and material. We characterized such electrodes and found a drift in their properties within minutes and up to about 60 h after immersion in cell culture medium, both with and without a stimulation signal. Electrode properties could largely be restored by proteolytic treatment for platinum/iridium electrodes but not for stainless steel ones. Short-term drift and irreversible aging could be followed by impedance measurements. Aging was accompanied by metal corrosion and erosion of the plastic insulation. For both materials, the degradation rates depended on the current density at the electrode surfaces. Fourier analysis of the DBS pulse (60 micros, repetition rate 130 Hz) revealed harmonic frequencies spanning a band of more than three decades, with significant harmonics up to the MHz range. The band is located in a window imposed by electrode processes and capacitive cell membrane bridging at the low and high frequency ends, respectively. Even though electrode processes are reduced at higher frequencies they only vanish above 1 MHz and cannot be avoided. Therefore, the use of inert electrode materials is of special importance. The neurotoxicity of iron makes avoiding stainless steel electrodes imperative. Future developments need to avoid the use of corrosive materials and current density hot spots at the electrode surface, and to reduce low frequency components in the DBS pulses in order to diminish electrode processes.

Algebraic multigrid for complex symmetric matrices and applications

This paper is concerned with the numerical study of an algebraic multigrid preconditioner for complex symmetric system matrices. We use several different Krylov subspace methods as an outer iteration, namely the QMR method proposed by Freund and Nachtigal, the BiCGCR method of Clemens, and the CSYM method of Bunse-Gerstner and Stoever. In addition, we compare the results with the standard Jacobi preconditioner for complex symmetric problems. We test our approach on the numerical simulation of high-voltage insulators.

Modeling and Simulation of Electro-Quasistatic Fields

We consider time-dependent electromagnetic fields with negligible propagation effect. These fields are called quasistatic. While the case with negligible displacement current (mainly inductive effects; magneto-quasistatics) is usually treated in classical theory, the case of fields free of eddy currents (mainly capacitive; electro-quasistatics) is still not too common Yet, the electro-quasistatic model is applicable in many different constellations, especially for microelectronic devices, too.

Electro-quasistatic calculation of electric field strength on high-voltage insulators with an algebraic multigrid algorithm

This paper is concerned with the numerical study of a new algebraic multigrid preconditioner for complex symmetric system matrices. The authors use several different Krylov subspace methods as an outer iteration, namely the quasi-minimal residual method of Freund and Nachtigal, the bi-orthogonal conjugate gradient conjugate residual method of Clemens, and the complex symmetric matrix structure method of Bunse-Gerstner and Stoever. The authors compare the results with the standard Jacobi-preconditioner and test their approach on the numerical simulation of high-voltage insulators.

The Electro—Quasistatic Model in Different Applications

An electromagnetic field can be considered as slowly varying if the wavelength is large compared to the problem region. In the electro-quasistatic case it then may be assumed that the time-derivative of the magnetic flux is negligible, whereas the displacement currents have to be taken into account. Under these assumptions Maxwell’s equations for time harmonic fields reduce to a complex Poisson’s equation and discretization yields a complex symmetric system of equations. Krylov-subspace methods with an algebraic multigrid (AMG) preconditioner are used for fast solution. The electro-quasistatic model is applicable in many different constellations. This paper deals with applications from three different fields: high-voltage engineering, neural sensor-actor systems and the influence of slowly varying fields on human tissue.

Simulation of Electric Field Strength and Force Density on Contaminated H-V Insulators

This paper reports on the simulation of the behavior of single water droplets on a horizontally arranged polymeric surface under the influence of an applied electric a.c. field. The goal of this investigation are predictions for the aging performance of contaminated insulators in high-voltage power problems.

Coupled Calculation of Electromagnetic Fields and Mechanical Deformation

Interest in multi-disciplinary simulations as a means of solving coupled electromagnetic-mechanical problems is increasing. The development of adapted simulation codes is an answer but these codes are often very specialized and are not always applicable to physically similar problems. For this reason it is advisable to create coupling software for the existing codes. The MpCCI-library is such a coupling software. In this paper we will present the results for a coupled simulation of electromagnetic fields in an accelerating cavity and its structural deformation by Lorentz force via the MpCCI-library.