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Thermal post-impact behaviour of closed-cell cellular structures with fillers
Matej Vesenjak, Andreas Öchsner, Zoran Ren, 2007, original scientific article

Abstract: The study describes the behavior of regular closed-cell cellular structure with gaseous fillers under impact conditions and consequent post-impact thermal conduction due to the compression of filler gas. Two dependent but different analyses types have been carried out for this purpose: (i) a strongly coupled fluid-structure interaction and (ii) a weakly coupled thermal- structural analysis. This paper describes the structural analyses of the closed-cell cellular structure under impact loading. The explicit code LS-DYNA was used to computationally determine the behavior of cellular structure under compressive dynamic loading, where one unit volume element of the cellular structure has been discretised with finite elements considering a simultaneous strongly coupled interaction with the gaseous pore filler. Closed-cell cellular structures with different relative densities and initial pore pressures have been considered. Computational simulations have shown that the gaseous filler influences the mechanical behavior of cellular structure regarding the loading type, relative density and type of the base material. It was determined that the filler's temperature significantly increases due to the compressive impact loading, which might influence the macroscopic behavior of the cellular structure.
Keywords: mechanics, cellular structures, closed cells, gas fillers, impact loading, fluid-structure interaction, dynamic loads, LS-DYNA, ANSYS CFX 10.0, computational simulations
Published: 31.05.2012; Views: 915; Downloads: 18
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H2 separation and use in fuel cells and CO2 separation and reuse as a reactant in the existing methanol process
Anita Kovač Kralj, Peter Glavič, 2007, original scientific article

Abstract: Fuel-cell efficiencies yield substantial reductions in the emissions of climate-change gases and promise an end to exclusive reliance on carbon fuels for energy. Fuel cells, CO2 reuse, process heat integration, and open gas turbine electricity cogeneration can be optimized simultaneously, using a nonlinear programming (NLP) algorithm. The simplified NLP model contains equations of structural and parametric optimization. This NLP model is used tooptimize complex and energy-intensive continuous processes. This procedure does not guarantee a global cost optimum, but it does lead to good, perhaps near-optimum, designs. The plant, which produces methanol, has a surplus of hydrogen (H2) and CO2 flow rates in purge gas. H2 is separated from the purge gas by an existing pressure swing adsorption (PSA) column. Pure H2 can be usedas fuel in fuel cells. CO2 can be separated from the outlet stream (purge gas) by a membrane or absorption system (absorber and regenerator) or an adsorption system and reused as a reactant in a reactor system. Therefore, theproduct yield can be increased and CO2 emissions can be reduced, simultaneously. CO2 emissions can then be reduced at the source. The retrofitted process can be operated within existing parameters. Using a methanol process as a case study, the CO2 emission flow rate can be reduced by4800 t/a. The additional electricity cogeneration in the gas turbine and in fuel cells and additional flow rates of the raw material could generate an additional profit of 2.54 MEUR/a.
Keywords: chemical processing, methanol production, optimization, nonlinear programming, CO2 reuse, fuel cells, heat integration, energy cogeneration
Published: 31.05.2012; Views: 1629; Downloads: 51
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Spatial network representation of complex living tissues
Dean Korošak, Marjan Rupnik, 2008, published scientific conference contribution

Abstract: Networks were widely used to describe organizational and functional principles of living organisms across various scales. The topology of such biological complex networks often turned out to be "scale-free", with the power-law distribution of number of links per node, robust and modular with underlying self-similar structure. However, the topology of cytoarchitecture in living tissues has not yet received wide attention from the network perspective. Here we discuss the spatial complex network model of coupled clusters of beta cells in pancreatic islets. Networks of cells in pancreatic islets were constructed from the 2D section images presenting fluorescently labelled intercellular spaces obtained by two-photon laser scanning microscopy of whole pancreas tissue slices, and cells conductances measured electrophysiologically using whole-cell patch-clamp. We find that the heterogeneity of beta cells in intact living islets induces scale-free topology of the tissue network. Furthermore, we show that the islet-like structures visually similar to 2D section images can be obtained using Voronoi diagrams of random points.
Keywords: pancreatic islets, betta cells, complex networks, cytoarchitecture
Published: 31.05.2012; Views: 1259; Downloads: 52
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Application of group technology in complex cluster type organizational systems
Slobodan Morača, Miodrag Hadžistević, Igor Drstvenšek, Nikola Radaković, 2010, original scientific article

Abstract: The aim of this research was to contribute to the development of structural design procedures of complex - cluster type organizational systems. Industrial clusters can help companies to improve their own market positions, effectiveness, productivity and product quality. Organization of the production process in a company is an extremely complex process itself, and when it is transferred to the cluster level, the result is a complex task which is difficult to solve. For that purpose, this paper analyses the conditions and possibilities that would enable those structures to adapt to changes in the surroundings - flexibility and management adequacy of production and organizational structures - by lowering the degree of complexity. For the time being, no simple models which would enable an increase of process effectiveness in complex organizational units like clusters have been developed. One of the possible solutions which would decrease the complexity of flows and increase process effectiveness within an industrial cluster is the application of Group approach.
Keywords: industrial clusters, group technology, planning, work cells, complexity, flexibility
Published: 31.05.2012; Views: 1191; Downloads: 22
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Heat conduction in closed-cell cellular metals
Matej Vesenjak, Zoran Žunič, Andreas Öchsner, Matjaž Hriberšek, Zoran Ren, 2005, original scientific article

Abstract: The purpose of this research was to describe the thermal transport properties in closed-cell cellular metals. Influence of cell size variations with different pore gases has been investigated with transient computational simulations. Heat conduction through the base material and gas in pores (cavities) was considered, while the convection and radiation were neglected in the initial stage of this research. First, parametric analysis for definingthe proper mesh density and time step were carried out. Then, two-dimensional computational models of the cellular structure, consisting of the base material and the pore gas, have been solved using ANSYS CFX software within the framework of finite volume elements. The results have confirmed theexpectations that the majority of heat is being transferred through the metallic base material with almost negligible heat conduction through the gas in pores. The heat conduction in closed-cell cellular metals is therefore extremely depended on the relative density but almost insensitive regarding tothe gas inside the pore, unless the relative density is very low.
Keywords: heat transfer, cellular metal materials, porous materials, closed cells, gas fillers, computational simulations
Published: 01.06.2012; Views: 1354; Downloads: 67
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Response of monocyte-derived dendritic cells to rapidly solidified nickel-titanium ribbons with shape memory properties
Sergej Tomić, Rebeka Rudolf, Mihael Brunčko, Ivan Anžel, V. Savić, Miodrag Čolić, 2012, original scientific article

Abstract: Ni-Ti Shape Memory Alloys (SMAs) have attracted considerable attention as biomaterials for medical devices. However, the biocompatibility of Ni-Ti SMAs is often unsatisfactory due to their poor surface structure. Here we prepared Rapidly Solidified (RS) Ni-Ti SMA ribbons by melt-spinning and their surface was characterised by Augerelectron spectroscopy, X-ray photoelectron spectrometry and scanning electron microscopy. The biocompatibility of the produced ribbons and their immunomodulatory properties were studied on human monocyte-derived dendritic cells (MoDCs). We showed that melt-spinning of Ni-Ti SMAs can form a thin homogenous oxide layer, which improves their corrosion resistance and subsequent toxicity to MoDCs. Ni-Ti RS ribbons stimulated the maturation of MoDCs, as detected by changes in the cells' morphology and increased expression of HLA-DR, CD86, CD40 and CD83 molecules. However, Ni-Ti RS ribbons enhanced the tolerogenic properties of immature MoDCs, which produced higher levels of IL-10 and IL-27, driving the differentiation of IL-10- and TGF-β-producing CD4+T cells. On the other hand, in the presence of lipopolysaccharide, an important pro-inflammatory biomolecule, Ni-Ti RS ribbons enhanced the allostimulatory and Th1 polarising capacity of MoDCs, whereas the production of Th2 and Th17 cytokines was down-regulated. In conclusion, Ni-Ti RS ribbons possess substantial immunomodulatory properties on MoDCs. These findings might be clinically relevant, because implanted Ni-Ti SMA devices can induce both desired and adverse effects on the immune system, depending on the microenvironmental stimuli.
Keywords: nickel-titanium alloy, biocompatibility, cytokines, immunomodulation, monocyte-derived dendritic cells
Published: 01.06.2012; Views: 948; Downloads: 203
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Calcium dependencies of regulated exocytosis in different endocrine cells
Jurij Dolenšek, Maša Skelin, Marjan Rupnik, 2011, review article

Abstract: Exocytotic machinery in neuronal and endocrine tissues is sensitive to changesin intracellular Ca2+ concentration. Endocrine cell models, that are most frequently used to study the mechanisms of regulated exocytosis, are pancreatic beta cells, adrenal chromaffin cells and pituitary cells. To reliably study the Ca2+ sensitivity in endocrine cells, accurate and fast determination of Ca2+ dependence in each tested cell is required. With slow photo-release it is possible to induce ramp-like increase in intracellular Ca2+ concentration ([Ca2+]i) that leads to a robust exocytotic activity. Slow increases in the [Ca2+]i revealed exocytotic phases with different Ca2+ sensitivities that have been largely masked in step-like flash photo-release experiments. Strikingly, in the cells of the three described model endocrine tissues (beta, chromaffin and melanotroph cells), distinct Ca2+ sensitivity ćclassesć of secretory vesicles have been observed: a highly Ca2+-sensitive, amedium Ca2+-sensitive and a low Ca2+- sensitive kinetic phase of secretory vesicle exocytosis. We discuss that a physiological modulation of a cellular activity, e.g. by activating cAMP/PKA transduction pathway, can switch the secretory vesicles between Ca2+ sensitivity classes. This significantly alterslate steps in the secretory release of hormones even without utilizationof an additional Ca2+ sensor protein.
Keywords: Calcium sensitivity, Exocytosis, Insulin-secreting cells, Chromaffin cells, Melanotrophs
Published: 05.06.2012; Views: 1016; Downloads: 25
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Cellular diversity promotes intercellular Ca[sup]2+ wave propagation
Marko Gosak, 2009, original scientific article

Abstract: Calcium ions are an important second messenger in living cells. Calcium signals in form of waves serve as a means of intercellular communication and thus represent a vibrant subject for experimental and theoretical investigations. Here we study the role of cellular variability on the occurrence of ▫$Ca^{2+}$▫ wave propagation in a net of diffusively coupled cells. Dynamics of individual cells is simulated by a mathematical model for ▫$Ca^{2+}$▫ oscillations. Structural diversity of cells is introduced via variations of the bifurcation parameters, which signify cell sensitivity for external stimulation. Remarkably, for sufficient values of variability ▫$Ca^{2+}$▫ waves emerge, which are mostly ordered for intermediate variability strength. We analyze the spatial profile via the autocorrelation function, which confirms aresonance-like response due to the cellular variability. Thus, the reported phenomenon is a novel observation of diversity-induced spatial coherence resonance in a tissue-like media.
Keywords: dynamic systems, waves, calcium oscillations, resonance, diversity-induced resonance, cellular variability, coupled cells, intracellular processes
Published: 07.06.2012; Views: 984; Downloads: 11
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Structure of the Malpighian tubule cells and annual changes in the structure and chemical composition of their spherites in the cave cricket Troglophilus neglectus Krauss, 1878 (Rhaphidophoridae, Saltatoria)
Saška Lipovšek Delakorda, Ilse Letofsky-Papst, Tone Novak, Ferdinand Hofer, Maria Anna Pabst, 2009, original scientific article

Abstract: Periodical changes in the structure of spherites in the Malpighian tubule cells of the cave cricket Troglophilus neglectus were studied to elucidate their role during the cricket's life cycle in natural circumstances. Special interest was given to the dormant overwintering period when we hypothesized that the primary role of spherites is to supply minerals for basic vital processes. The investigation was carried out by light and transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy-loss spectroscopy and energy-filtering TEM. Spherites are present only in the middle Malpighian tubule segment, consisting of Type 1 cells, characterized, among other features, by a round, apically placed nucleus and numerous spherites, and a few Type 2 cells with an elongated nucleus in the centre and sparse spherites. At the beginning of dormancy in November juveniles, minerals are accumulated in spherites and then decline until March.In one-year-old May larvae, spherites are commonly rich in minerals, and from July onwards they are progressively exploited in the adults. Spherite destruction starts with apoptosis in senile October individuals. The findings suggest that the mineral supply of spherites in Malpighian tubules is crucial to supporting vital processes throughout the life cycle of T. neglectus.
Keywords: biology, zoology, cells, spherites, insects, Troglophilus neglectus, analysis
Published: 07.06.2012; Views: 1219; Downloads: 62
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