Markus Zahn

Transcranial direct current stimulation: A computer-based human model study

OBJECTIVES Interest in transcranial direct current stimulation (tDCS) in clinical practice has been growing, however, the knowledge about its efficacy and mechanisms of action remains limited. This paper presents a realistic magnetic resonance imaging (MRI)-derived finite element model of currents applied to the human brain during tDCS. EXPERIMENTAL DESIGN Current density distributions were analyzed in a healthy human head model with varied electrode montages. For each configuration, we calculated the cortical current density distributions. Analogous studies were completed for three pathological models of cortical infarcts. PRINCIPAL OBSERVATIONS The current density magnitude maxima injected in the cortex by 1 mA tDCS ranged from 0.77 to 2.00 mA/cm(2). The pathological models revealed that cortical strokes, relative to the non-pathological solutions, can elevate current density maxima and alter their location. CONCLUSIONS These results may guide optimized tDCS for application in normal subjects and patients with focal brain lesions.

Interdigital sensors and transducers

This review paper focuses on interdigital electrodes-a geometric structure encountered in a wide variety of sensor and transducer designs. Physical and chemical principles behind the operation of these devices vary so much across different fields of science and technology that the common features present in all devices are often overlooked. This paper attempts to bring under one umbrella capacitive, inductive, dielectric, piezoacoustic, chemical, biological, and microelectromechanical interdigital sensors and transducers. The paper also provides historical perspective, discusses fabrication techniques, modeling of sensor parameters, application examples, and directions of future research.

Moisture equilibrium in transformer paper-oil systems

This paper provides an overview of the classic moisture equilibrium curves and their history and provides useful information on the relationships among them and their validity.

Propagation and structure of streamers in liquid dielectrics

Our purpose is to present a critical review of the current understanding of streamer propagation in dielectric liquids in order to help define the direction of future research. We show that the molecular structure has a significant effect on streamer propagation. The main parameter affecting propagation is the electronic affinity of the liquid molecules.

Three-dimensional head model Simulation of transcranial magnetic stimulation

This paper presents a finite element method used to evaluate the induced current density in a realistic model of the human head exposed to a time varying magnetic field. The tissue electric properties were varied to ascertain their influence on the induced currents. Current density magnitude and vector plots were generated throughout the tissue layers to determine the effects of tissue boundaries on the field. The current density magnitude correlated to the conductivity of the tissue in all the cases tested except where the tissue permittivity was raised to a level to allow for displacement currents. In this case, the permittivity of the tissue was the dominant factor. Current density components normal to the tissue interface were shown to exist in all solutions within the cortex contrary to the predictions of present models that rely on symmetrical geometries. Additionally, modifications in the cortical geometry were shown to perturb the field so that the site of activation could be altered in diseased patient populations. Finally, by varying the tissue permittivity values and the source frequency, we tested the effects of alpha dispersion theories on transcranial magnetic stimulation.

Magnetic Fluid and Nanoparticle Applications to Nanotechnology

Magnetic field based micro/nanoelectromechanical systems (MEMS/NEMS) devices are proposed that use 10 nm diameter magnetic particles, with and without a carrier fluid, for a new class of nanoduct flows, nanomotors, nanogenerators, nanopumps, nanoactuators, and other similar nanoscale devices. A few examples of macroscopic ferrohydrodynamic instabilities that result in patterns, lines, and structures are shown that can be scaled down to sub-micron dimensions.

Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids

Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the char...

Transcranial magnetic stimulation and stroke: A computer-based human model study

This paper explores how transcranial magnetic stimulation (TMS) induced currents in the brain are perturbed by electrical and anatomical changes following a stroke in its chronic stage. Multiple MRI derived finite element head models were constructed and evaluated to address the effects that strokes can have on the induced stimulating TMS currents by comparing stroke models of various sizes and geometries to a healthy head model under a number of stimulation conditions. The TMS induced currents were significantly altered for stimulation proximal to the lesion site in all of the models analyzed. The current density distributions were modified in magnitude, location, and orientation such that the population of neural elements that are stimulated will be correspondingly altered. The current perturbations were minimized for conditions tested where the coil was far removed from the lesion site, including models of stimulation contralateral to the lesioned hemisphere. The present limitations of TMS to the peri-lesional cortex are explored, ultimately concluding that conventional clinical standards for stimulation are unreliable and potentially dangerous predictors of the site and degree of stimulation when TMS is applied proximal to infarction site.

Dielectric properties of water and water/ethylene glycol mixtures for use in pulsed power system design

One class of modern pulse power generators use deionized water as an energy storage, switching and transmission dielectric. Water is chosen for its high dielectric constant and relatively high resistivity, which allows reasonably sized and efficient low-impedance high-voltage pulse lines where pulse durations are less than 100 µs. Water/ethylene glycol mixtures are being researched, so that rotating machinery, rather than the usual Marx generator, can be used as the primary energy store. The high resistivity and high dielectric constant of these mixtures at low temperature permit low-loss operation on millisecond time scales. Simple design criteria linking load parameters and charging circuit characteristics to the liquid dielectric are developed which show that the dielectric constant, breakdown strength, and relaxation time are the primary properties of interest to the pulse power engineer. On time scales greater than 100 µs, injection of space charge, with density q and mobility µ, affects the charging and discharging circuit characteristics, introduces the time constant of the time of flight for injected charge to migrate between electrodes, and increases the effective ohmic conductivity σ to σ + qµ. A drift-dominated conduction model is used to describe measured space-charge effects. Kerr electrooptic field mapping measurements show strong space-charge effects with significant distortions in the electric field distribution a few hundred microseconds after high voltage is applied. The injected charge magnitude and sign depends on the electrode material. Thus by appropriate choice of electrode material combinations and voltage polarity, it is possible to have uncharged liquid, unipolar-charged negative or positive, or bipolar-charged liquid. An important case is that of bipolar injection, which has allowed up to a 40 percent higher applied voltage without breakdown than with no charge injection, and thus a doubling of stored energy due to the space-charge shielding which lowers the electric field strengths at both electrodes. Although injected space charge increases the stored electric energy over the capacitive space-charge-free energy, (1/2)CV2, more energy is required from a source during charging and the energy delivered to a resistive load is reduced because of internal dissipation in the capacitor as the charge is conducted to the electrodes. However, it appears that this extra dissipation due to injected charge can be made negligibly small and well worth the price if the space charge allows higher voltage operation for long charging time or repetitively operated machines.

Preparation of a vegetable oil-based nanofluid and investigation of its breakdown and dielectric properties

Investigations during the last decade have shown that conductive nanoparticles can be dispersed in transformer oils to form nanofluids. Well-dispersed nanoparticles are capable of increasing the breakdown voltage of the oil under power frequency and lightning impulses. They also increase the inception voltages for partial discharge [1]. With increasing moisture content, reduction of the breakdown voltage of the nanofluid at power frequency is significantly smaller than that in the corresponding transformer oil [1]. The electrical and thermal properties of four types of nanofluid, prepared by dispersing Al2O3, Fe3O4, SiO2, and SiC nanoparticles in transformer oils, were described in [2]. It has also been reported that the thermal conductivity of such oil was enhanced by 8% when aluminum nitride nanoparticles were dispersed in it at a loading of 0.5% by weight, and its cooling capability was improved by about 20% [3]. An electrodynamic model has been developed describing streamer formation in transformer oil-based nanofluids, which presents generation, recombination, and transport equations for each charge carrier type [4]. Vegetable insulation oils are based on natural ester oils, which are environmentally friendly and fire resistant [4]���[9]. At the moment, little is known about the preparation of nanofluids using natural ester oils and their dielectric, breakdown, and aging properties. Surface modification of nanoparticles is a very effective procedure to avoid nanoparticle agglomeration in insulating nanofluids [10]���[14]. However, the surface modification procedures used for mineral oils cannot be applied to vegetable oils because of their very different molecular structures. We therefore investigated new approaches to the preparation of vegetable oil-based nanofluids. This paper presents some of the results of a study of the breakdown voltages and dielectric properties of a vegetable oil-based nanofluid. The nanofluid was prepared by dispersing Fe3O4 nanoparticles in a vegetable insulation oil obtained from a laboratory at Chongqing University. Oleic acid was used for surface modification of the nanoparticles.