Comparison of electrode impedances of Pt, PtIr (10% Ir) and Ir-AIROF electrodes used in electrophysiological experiments

In tissue impedance measurements with the 4-electrode assembly, unexpected difficulties may occur because a combination of electrode impedance and stray capacitance in the array of four electrodes, can lead to serious measuring failures in the low-frequency range. An optimal solution to this problem can be obtained if the electrode impedances are frequency independent. A comparative study of the electrode impedances of Pt and PtIr electrodes and of a new electrode material (Ir-AIROF) is reported. It is shown that the impedance of Ir-AIROF electrodes is relatively low and almost frequency independent. Therefore the use of Ir-AIROF electrodes provides a solution to the problem mentioned above

Classical GR as a topological theory with linear constraints

We investigate a formulation of continuum 4d gravity in terms of a constrained topological (BF) theory, in the spirit of the Plebanski formulation, but involving only linear constraints, of the type used recently in the spin foam approach to quantum gravity. We identify both the continuum version of the linear simplicity constraints used in the quantum discrete context and a linear version of the quadratic volume constraints that are necessary to complete the reduction from the topological theory to gravity. We illustrate and discuss also the discrete counterpart of the same continuum linear constraints. Moreover, we show under which additional conditions the discrete volume constraints follow from the simplicity constraints, thus playing the role of secondary constraints. Our analysis clarifies how the discrete constructions of spin foam models are related to a continuum theory with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010 (ERE2010, Granada, Spain

Classical GR as a topological theory with linear constraints

We investigate a formulation of continuum 4d gravity in terms of a constrained topological (BF) theory, in the spirit of the Plebanski formulation, but involving only linear constraints, of the type used recently in the spin foam approach to quantum gravity. We identify both the continuum version of the linear simplicity constraints used in the quantum discrete context and a linear version of the quadratic volume constraints that are necessary to complete the reduction from the topological theory to gravity. We illustrate and discuss also the discrete counterpart of the same continuum linear constraints. Moreover, we show under which additional conditions the discrete volume constraints follow from the simplicity constraints, thus playing the role of secondary constraints. Our analysis clarifies how the discrete constructions of spin foam models are related to a continuum theory with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010 (ERE2010, Granada, Spain

Classical GR as a topological theory with linear constraints

We investigate a formulation of continuum 4d gravity in terms of a constrained topological (BF) theory, in the spirit of the Plebanski formulation, but involving only linear constraints, of the type used recently in the spin foam approach to quantum gravity. We identify both the continuum version of the linear simplicity constraints used in the quantum discrete context and a linear version of the quadratic volume constraints that are necessary to complete the reduction from the topological theory to gravity. We illustrate and discuss also the discrete counterpart of the same continuum linear constraints. Moreover, we show under which additional conditions the discrete volume constraints follow from the simplicity constraints, thus playing the role of secondary constraints. Our analysis clarifies how the discrete constructions of spin foam models are related to a continuum theory with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010 (ERE2010, Granada, Spain

IRAS 19135+3937: An SRd variable as interacting binary surrounded by a circumbinary disc

Semi-regular (SR) variables are not a homogeneous class and their variability is often explained due to pulsations and/or binarity. This study focuses on IRAS 19135+3937, an SRd variable with an infra-red excess indicative of a dusty disc. A time-series of high-resolution spectra, UBV photometry as well as a very accurate light curve obtained by the Kepler satellite, allowed us to study the object in unprecedented detail. We discovered it to be a binary with a period of 127 days. The primary has a low surface gravity and an atmosphere depleted in refractory elements. This combination of properties unambiguously places IRAS 19135+3937 in the subclass of post-Asymptotic Giant Branch stars with dusty discs. We show that the light variations in this object can not be due to pulsations, but are likely caused by the obscuration of the primary by the circumbinary disc during orbital motion. Furthermore, we argue that the double-peaked Fe emission lines provide evidence for the existence of a gaseous circumbinary Keplerian disc inside the dusty disc. A secondary set of absorption lines has been detected near light minimum, which we attribute to the reflected spectrum of the primary on the disc wall, which segregates due to the different Doppler shift. This corroborates the recent finding that reflection in the optical by this type of discs is very efficient. The system also shows a variable Halpha profile indicating a collimated outflow originating around the companion. IRAS 19135+3937 thus encompasses all the major emergent trends about evolved disc systems, that will eventually help to place these objects in the evolutionary context.Comment: Accepted to MNRA

Kennisarrangementen tussen onderwijs en onderzoek Een verkenning van kennisarrangementen tussen PPO en ASG en het agrarisch onderwijs

Wageningen UR besteedt veel aandacht aan samenwerking met het agrarisch onderwijs en ondernemers om de kenniscirculatie vanuit het onderzoek beter vorm en inhoud te geven. In Leren met Toekomst (Potters, 2006) wordt sinds 2005 gewerkt aan het bevorderen van structurele samenwerking tussen onderzoek en groen onderwijs, in eerste instantie is gericht op het ontwikkelen van samenwerking in ondernemersnetwerken. Er is gezamenlijk met het onderwijs een concept ontwikkeld en getest, dit concept of kennisarrangement is vervolgens binnen de sectoren plant en dier benut. Naast deze samenwerking in ondernemersnetwerken biedt Leren met Toekomst de mogelijkheid om andere vormen van samenwerking tussen Onderwijs, Ondernemers en Onderzoek te stimuleren en verder ontwikkelen. In 2007 is een aantal projecten uitgevoerd die zich richtten op de versterking van de samenwerking tussen Onderwijs, Ondernemers en Onderzoek. De centrale vraag van Leren met Toekomst is sinds 2006: Hoe kunnen onderwijs en onderzoek elkaar effectief en structureel versterken in hun inspanningen voor de duurzame Nederlandse landbouwsector en vitaal platteland? Daarvoor werkt het project volgens vier pijlers met elk hun eigen doelstellingen en activiteiten: Ondernemersnetwerken als leeromgeving; Kenniscirculatie tussen onderzoek en onderwijs; Beroepspraktijkvorming op netwerkbedrijven; Bruggen bouwen tussen onderzoek en onderwijs. Centrale vraag bij deze verkennende studie is: Welke kennisarrangementen waar ervaring mee is binnen WUR hebben (naast het beproefde Leren met Toekomst concept) hebben de potentie om bij te dragen aan de centrale doelstelling van Leren met Toekomst, te weten een structurele samenwerking tussen onderzoek en onderwijs? Het gaat in deze verkenning met nadruk niet om een diepte studie, maar om een globale verkenning van mogelijkheden en potenties. Met aandacht voor praktische handvatten voor hen die de relatie tussen onderwijs en onderzoek willen versterken. Doelgroep van de verkennende studie zijn de onderzoekers binnen PPO en ASG3

Hysteresis and bi-stability by an interplay of calcium oscillations and action potential firing

Many cell types exhibit oscillatory activity, such as repetitive action potential firing due to the Hodgkin-Huxley dynamics of ion channels in the cell membrane or reveal intracellular inositol triphosphate (IP$_3$) mediated calcium oscillations (CaOs) by calcium-induced calcium release channels (IP$_3$-receptor) in the membrane of the endoplasmic reticulum (ER). The dynamics of the excitable membrane and that of the IP$_3$-mediated CaOs have been the subject of many studies. However, the interaction between the excitable cell membrane and IP$_3$-mediated CaOs, which are coupled by cytosolic calcium which affects the dynamics of both, has not been studied. This study for the first time applied stability analysis to investigate the dynamic behavior of a model, which includes both an excitable membrane and an intracellular IP$_3$-mediated calcium oscillator. Taking the IP$_3$ concentration as a control parameter, the model exhibits a novel rich spectrum of stable and unstable states with hysteresis. The four stable states of the model correspond in detail to previously reported growth-state dependent states of the membrane potential of normal rat kidney fibroblasts in cell culture. The hysteresis is most pronounced for experimentally observed parameter values of the model, suggesting a functional importance of hysteresis. This study shows that the four growth-dependent cell states may not reflect the behavior of cells that have differentiated into different cell types with different properties, but simply reflect four different states of a single cell type, that is characterized by a single model.Comment: 29 pages, 6 figure

Exploring sequence space in search of functional enzymes using microfluidic droplets

Screening of enzyme mutants in monodisperse picoliter compartments, generated at kilohertz speed in microfluidic devices, is coming of age. After a decade of proof-of-principle experiments, workflows have emerged that combine existing microfluidic modules to assay reaction progress quantitatively and yield improved enzymes. Recent examples of the screening of libraries of randomised proteins and from metagenomic sources suggest that this approach is not only faster and cheaper, but solves problems beyond the feasibility scope of current methodologies. The establishment of new assays in this format – so far covering hydrolases, aldolases, polymerases and dehydrogenases – will enable the exploration of sequence space for new catalysts of natural and non-natural chemical transformations.This work was funded by the Engineering and Physical Sciences Research Council (EPSRC; studentship in the Centre for Doctoral Training “Sensor Technologies and Applications” to P.M., EP/L015889/1) and the Biotechnology and Biological Research Council (BBSRC; BB/K013629/1). FH is an ERC Advanced Investigator