Gas permeation through a polymer network

We study the diffusion of gas molecules through a two-dimensional network of polymers with the help of Monte Carlo simulations. The polymers are modeled as non-interacting random walks on the bonds of a two-dimensional square lattice, while the gas particles occupy the lattice cells. When a particle attempts to jump to a nearest-neighbor empty cell, it has to overcome an energy barrier which is determined by the number of polymer segments on the bond separating the two cells. We investigate the gas current $J$ as a function of the mean segment density $\rho$, the polymer length $\ell$ and the probability $q^{m}$ for hopping across $m$ segments. Whereas $J$ decreases monotonically with $\rho$ for fixed $\ell$, its behavior for fixed $\rho$ and increasing $\ell$ depends strongly on $q$. For small, non-zero $q$, $J$ appears to increase slowly with $\ell$. In contrast, for $q=0$, it is dominated by the underlying percolation problem and can be non-monotonic. We provide heuristic arguments to put these interesting phenomena into context.Comment: Dedicated to Lothar Schaefer on the occasion of his 60th birthday. 11 pages, 3 figure

Simulation studies of permeation through two-dimensional ideal polymer networks

We study the diffusion process through an ideal polymer network, using numerical methods. Polymers are modeled by random walks on the bonds of a two-dimensional square lattice. Molecules occupy the lattice cells and may jump to the nearest-neighbor cells, with probability determined by the occupation of the bond separating the two cells. Subjected to a concentration gradient across the system, a constant average current flows in the steady state. Its behavior appears to be a non-trivial function of polymer length, mass density and temperature, for which we offer qualitative explanations.Comment: 8 pages, 4 figure

A non-canonical RNA silencing pathway promotes mRNA degradation in basal fungi

The increasing knowledge on the functional relevance of endogenous small RNAs (esRNAs) as riboregulators has stimulated the identification and characterization of these molecules in numerous eukaryotes. In the basal fungus Mucor circinelloides, an emerging opportunistic human pathogen, esRNAs that regulate the expression of many protein coding genes have been described. These esRNAs share common machinery for their biogenesis consisting of an RNase III endonuclease Dicer, a single Argonaute protein and two RNA-dependent RNA polymerases. We show in this study that, besides participating in this canonical dicer-dependent RNA interference (RNAi) pathway, the rdrp genes are involved in a novel dicer-independent degradation process of endogenous mRNAs. The analysis of esRNAs accumulated in wild type and silencing mutants demonstrates that this new rdrp-dependent dicer-independent regulatory pathway, which does not produce sRNA molecules of discrete sizes, controls the expression of target genes promoting the specific degradation of mRNAs by a previously unknown RNase. This pathway mainly regulates conserved genes involved in metabolism and cellular processes and signaling, such as those required for heme biosynthesis, and controls responses to specific environmental signals. Searching the Mucor genome for candidate RNases to participate in this pathway, and functional analysis of the corresponding knockout mutants, identified a new protein, R3B2. This RNase III-like protein presents unique domain architecture, it is specifically found in basal fungi and, besides its relevant role in the rdrp-dependent dicer-independent pathway, it is also involved in the canonical dicer-dependent RNAi pathway, highlighting its crucial role in the biogenesis and function of regulatory esRNAs. The involvement of RdRPs in RNA degradation could represent the first evolutionary step towards the development of an RNAi mechanism and constitutes a genetic link between mRNA degradation and post-transcriptional gene silencing

Controls on the global distribution of contourite drifts: Insights from an eddy-resolving ocean model

Contourite drifts are anomalously high sediment accumulations that form due to reworking by bottom currents. Due to the lack of a comprehensive contourite database, the link between vigorous bottom water activity and drift occurrence has yet to be demonstrated on a global scale. Using an eddy-resolving ocean model and a new georeferenced database of 267 contourites, we show that the global distribution of modern contourite drifts strongly depends on the configuration of the world’s most powerful bottom currents, many of which are associated with global meridional overturning circulation. Bathymetric obstacles frequently modify flow direction and intensity, imposing additional finer-scale control on drift occurrence. Mean bottom current speed over contourite-covered areas is only slightly higher (2.2cm/s) than the rest of the global ocean (1.1cm/s), falling below proposed thresholds deemed necessary to re-suspend and redistribute sediments (10–15cm/s). However, currents fluctuate more frequently and intensely over areas with drifts, highlighting the role of intermittent, high-energy bottom current events in sediment erosion, transport, and subsequent drift accumulation. We identify eddies as a major driver of these bottom current fluctuations, and we find that simulated bottom eddy kinetic energy is over three times higher in contourite-covered areas in comparison to the rest o.f the ocean. Our work supports previous hypotheses which suggest that contourite deposition predominantly occurs due to repeated acute events as opposed to continuous reworking under average-intensity background flow conditions. This suggests that the contourite record should be interpreted in terms of a bottom current’s susceptibility to experiencing periodic, high-speed current events. Our results also highlight the potential role of upper ocean dynamics in contourite sedimentation through its direct influence on deep eddy circulation

ASPECTS BIOMÉTRIQUES DE LA CROISSANCE PONDÉRALE DU PORCELET. I. - INFLUENCE DU SEXE, DE L'ANNÉE DE NAISSANCE, DU NUMÉRO ET DE LA TAILLE DE LA PORTÉE

International audienc

Photon Counting Spectral CT component analysis of coronary artery atherosclerotic plaque samples

International audienceObjectives. To evaluate the capabilities of Photon Counting Spectral CT to differentiate components of coronary atherosclerotic plaque based on differences in spectral attenuation and iodine-based contrast agent concentration. Methods. 10 calcified and 13 lipid-rich atheromatous plaques from postmortem human coronary arteries were scanned with a Photon Counting Spectral CT scanner. Individual photons were counted and classified in one of 6 energy bins from 25 to 70 keV.Based on a maximum likelihood approach, maps of Photoelectric Absorption (PA), Compton Scattering (CS) and Iodine Concentration (IC) were reconstructed. Intensity measurements were performed on each map in the vessel wall, the surrounding perivascular fat, the lipid-rich and the calcified plaques. PA and CS values are expressed relative to pure water values. Comparison between these different elements was performed using Kruskal-Wallis tests with pairwise post-hoc Mann-Whitney U tests and Sidak p-values adjustment. Results. Results for vessel wall, surrounding perivascular fat, lipid-rich, and calcified plaques were respectively 1.19+/-0.09, 0.73+/-0.05, 1.08+/-0.14, 17.79+/-6.70 for PA; 0.96+/-0.02, 0.83+/-0.02, 0.91+/-0.03, 2.53+/-0.63 for CS, and 83.3+/-10.1, 37.6+/-8.1, 55.2+/-14.0, 4.9+/-20.0 mmol/l for IC, with significant differences between all tissues for PA, CS and IC (p<0.012). Conclusion. This study demonstrates the capability of energy-sensitive photon counting spectral CT to differentiate between calcifications and iodine infused regions of human coronary artery atherosclerotic plaque samples by analyzing differences in spectral attenuation and iodine-based contrast agent concentration. Advances in Knowledge: Photon Counting, Spectral CT is a promising technique to identify plaque components by analyzing differences in iodine-based contrast agent concentration, photoelectric and Compton attenuation