Effective permeability of a gap junction with age-structured switching

We analyze the diffusion equation in a bounded interval with a stochastically gated interior barrier at the center of the domain. This represents a stochastically gated gap junction linking a pair of identical cells. Previous work has modeled the switching of the gate as a two-state Markov process and used the theory of diffusion in randomly switching environments to derive an expression for the effective permeability of the gap junction. In this paper we extend the analysis of gap junction permeability to the case of a gate with age-structured switching. The latter could reflect the existence of a set of hidden internal states such that the statistics of the non-Markovian two-state model matches the statistics of a higher-dimensional Markov process. Using a combination of the method of characteristics and transform methods, we solve the partial differential equations for the expectations of the stochastic concentration, conditioned on the state of the gate and after integrating out the residence time of the age-structured process. This allows us to determine the jump discontinuity of the concentration at the gap junction and thus the effective permeability. We then use stochastic analysis to show that the solution to the stochastic PDE is a certain statistic of a single Brownian particle diffusing in a stochastically fluctuating environment. In addition to providing a simple probabilistic interpretation of the stochastic PDE, this representation enables an efficient numerical approximation of the solution of the PDE by Monte Carlo simulations of a single diffusing particle. The latter is used to establish that our analytical results match those obtained from Monte Carlo simulations for a variety of age-structured distributions

One-Dimensional Tunable Josephson Metamaterials - Eindimensionale stimmbare Josephson Metamaterialien

This thesis presents a novel approach to the experimental realization of tunable, superconducting metamaterials. Therefore, conventional resonant meta-atoms are replaced by meta-atoms that contain Josephson junctions, which renders their resonance frequency tunable by an external magnetic field. This tunability is theoretically and experimentally investigated in one-dimensional magnetic and electric metamaterials. For the magnetic metamaterial, the effective, magnetic permeability is determined

Low carbon housing: lessons from Elm Tree Mews

This report sets out the findings from a low carbon housing trial at Elm Tree Mews, York, and discusses the technical and policy issues that arise from it. The Government has set an ambitious target for all new housing to be zero carbon by 2016. With the application of good insulation, improved efficiencies and renewable energy, this is theoretically possible. However, there is growing concern that, in practice, even existing carbon standards are not being achieved and that this performance gap has the potential to undermine zero carbon housing policy. The report seeks to address these concerns through the detailed evaluation of a low carbon development at Elm Tree Mews. The report: * evaluates the energy/carbon performance of the dwellings prior to occupation and in use; * analyses the procurement, design and construction processes that give rise to the performance achieved; * explores the resident experience; * draws out lessons for the development of zero carbon housing and the implications for government policy; and * proposes a programme for change, designed to close the performance gap

Development of a 3D In Vitro Model of the Blood-Brain Barrier in Layered Microfluidic Devices.

The endothelial cells lining the capillaries that supply the brain with oxygen and nutrients present a highly regulated transport barrier known as the blood-brain barrier (BBB). These endothelial cells are characterized by thick cell membranes, low number of endocytic vesicles, absence of fenestrae, and highly organized tight junctions that restrict molecular diffusion across the paracellular space. The integrity and function of the BBB is finely regulated by several environmental conditions including endothelial cell-to-cell contact, communication with other neural cells such as astrocytes and pericytes, and the local concentration of secreted chemical factors. Several groups have cultured primary and immortalized brain capillary endothelial cells to develop an in vitro model that mimics the BBB for the purpose of screening transport properties of new drug molecules designed for treatment of central nervous system (CNS) disorders. However, these in vitro models generally failed to mimic the restrictive transport properties of the BBB due to the formation of “loose” tight junctions, lower expression of specific carriers, or limited cell viability. We developed a 3D in vitro model of the BBB by culturing brain endothelial cells with pericytes and astrocytes in layered microfluidic channels. We hypothesized that the proposed model would improve endothelial cell polarization and enhance the formation of tight junctions, provide better endothelial cell-to-cell contact that is important for barrier development, and prevent the dilution of secreted neurotrophic factors, and these conditions collectively led to the development of an in vitro model that can truly mimic the BBB.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111555/1/jackwang_1.pd

Isolated and Bidirectional DC-DC Converter for Electric Vehicles

O estado da arte iniciou com a análise na literatura de topologias de conversores DC-DC. Técnicas de modulação são estudadas com vista a melhorar a eficiência de conversão, realçando as vantagens e limitações inerentes das mesmas. Após a análise da literatura, o foco projeto passou a ser a topologias de dupla ponte com dispositivos ativos e com isolamento galvânico intermédio entre as duas pontes (conhecido em inglês por dual active bridge). Algumas técnicas de modulação que permitem o funcionamento do conversor são analisadas no documento e suportadas com resultados obtidos em ambiente de simulação. O dimensionamento do transformador de potência é realizado assim como a descrição dos passos. É relizado uma análise de mercado de dispositivos de comutação com a tecnologia "Silicon Carbide" e são apresentados estimativas de perdas e eficiência de operação na utilização de transistores com a techonoloa SiC no conversor analisado. Os resultados são obtidos com recurso a simulações computacionais que através de modelos de aproximação permitem aproximar o conversor a uma situação mais proxima da real. Em termos de implementação, é esperado a implementação um circuito de comando para dois MOSFETS com tecnologia SiC com a configuração em meia ponte ligada a uma carga

Physics of thin-film ferroelectric oxides

This review covers the important advances in recent years in the physics of thin film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin film form. We introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this we cover the enormous progress that has been made in the first principles computational approach to understanding ferroelectrics. We then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, we look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in non conventional nanoscale geometries.Comment: This is an invited review for Reviews of Modern Physics. We welcome feedback and will endeavour to incorporate comments received promptly into the final versio

Observation of metastable states in a superconducting Josephson circuit using an Andreev interferometer

We report on measurements of macroscopic quantum states in superconducting Josephson circuits using a highly sensitive hybrid quantum interferometer as the readout probe. The investigated Josephson circuit is one of the leading candidates as solid-state qubits (persistent current qubit), which are known to exhibit macroscopic quantum states with atomic-like properties. The readout device is a modified Andreev interferometer with semi-metallic normal segment in a “folded" geometry, and is designed to reduce the back action during measurement, as well as minimising the electromagnetic coupling between the circuit and the environment. A pulsed lock-in technique has been developed to perform continuous readout of the superconductor phase difference using pulse lengths down to 10 ns. The technique enables to control the energy of the probing quasiparticles in the normal segment of the interferometer, which in turn allows to control of the supercurrent owing in the SNS junction and prevents electron heating of the normal segment. An experimental set-up was designed and installed in a dilution fridge consisting of shielded wiring, magnetic screens, RF tight sample holder and printed circuit board to allow the injection of high frequency excitation signals, while minimising the environment effect on the qubit through careful electrical filtering. The effect of strong RF irradiation on Andreev interferometers allowed us to estimate the response time of the readout device to be less than 40 ps. The measurements show that two macroscopically distinct metastable states exist when the device is biased at the qubit degeneracy point, between which the system makes transitions that can be continuously monitored. Real time kinetics of the system has been investigated at different magnetic fluxes, pulse parameters, temperature and RF radiation. Based on statistical analysis of the transitions, we argue that the metastability is connected with macroscopic quantum tunnelling effects rather than thermal excitation. The experimental data support the hypothesis of a large low frequency noise causing low transition rates