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Understanding the niche interactions between blood and bone through the in vitro co-culture of osteo-competent cells and endothelial cells is a key factor in unraveling therapeutic potentials in bone regeneration. This can be additionally supported by employing numerical simulation techniques to assess local physical factors, such as oxygen concentration, and mechanical stimuli, such as shear stress, that can mediate cellular communication. In this study, we developed a Mesenchymal Stem Cell line (MSC) and a Human Umbilical Vein Endothelial Cell line (HUVEC), which were co-cultured under flow conditions in a three-dimensional, porous, natural pullulan/dextran scaffold that was supplemented with hydroxyapatite crystals that allowed for the spontaneous formation of spheroids. After 2 weeks, their viability was higher under the dynamic conditions (>94%) than the static conditions (<75%), with dead cells central in the spheroids. Mineralization and collagen IV production increased under the dynamic conditions, correlating with osteogenesis and vasculogenesis. The endothelial cells clustered at the spheroidal core by day 7. Proliferation doubled in the dynamic conditions, especially at the scaffold peripheries. Lattice Boltzmann simulations showed negligible wall shear stress in the hydrogel pores but highlighted highly oxygenated zones coinciding with cell proliferation. A strong oxygen gradient likely influenced endothelial migration and cell distribution. Hypoxia was minimal, explaining high viability and spheroid maturation in the dynamic conditions. [ABSTRACT FROM AUTHOR]

The traditional 3D culture systems in vitro lack the biological and mechanical spatiotemporal stimuli characteristic to native tissue development. In our study, we combined porous polysaccharide-based hydrogel scaffolds with a bioreactor-type perfusion device that generates favorable mechanical stresses while enhancing nutrient transfers. MC3T3E1 mouse osteoblasts were seeded in the scaffolds and cultivated for 3 weeks under dynamic conditions at a perfusion rate of 10 mL min−1. The spatial distribution of the cells labeled with superparamagnetic iron oxide nanoparticles was visualized by MRI. Confocal microscopy was used to assess cell numbers, their distribution inside the scaffolds, cell viability, and proliferation. The oxygen diffusion coefficient in the hydrogel was measured experimentally. Numerical simulations of the flow and oxygen transport within the bioreactor were performed using a lattice Boltzmann method with a two-relaxation time scheme. Last, the influence of cell density and spheroid size on cell oxygenation was investigated. The cells spontaneously organized into spheroids with a diameter of 30–100 μm. Cell viability remained unchanged under dynamic conditions but decreased under static culture. The cell proliferation (Ki67 expression) in spheroids was not observed. The flow simulation showed that the local fluid velocity reached 27 mm s−1 at the height where the cross-sectional area of the flow was the smallest. The shear stress exerted by the fluid on the scaffolds may locally rise to 100 mPa, compared with the average value of 25 mPa. The oxygen diffusion coefficient in the hydrogel was 1.6 × 10 − 9 m2 s−1. The simulation of oxygen transport and consumption confirmed that the cells in spheroids did not suffer from hypoxia when the bioreactor was perfused at 10 mL min−1, and suggested the existence of optimal spheroid size and spacing for appropriate oxygenation. Collectively, these findings enabled us to define the optimal conditions inside the bioreactor for an efficient in vitro cell organization and survival in spheroids, which are paramount to future applications with organoids. [ABSTRACT FROM AUTHOR]

Perforations contribute to liquid redistribution in corrugated sheet packings. We focus on a simplified but relevant experimental configuration where a vertical perforated flat sheet is supplied with liquid on its front side. We examine how the perforations irrigate the back of the plate. We successively consider a single perforation, a spanwise row of perforations, and a staggered array of perforations. We quantify the liquid transfer through a single row of perforations and find that the transferred flow rate per unit perforation diameter varies linearly with the supply flow rate per unit width. We also analyze the spreading of the rivulets leaking from the perforations, their merging into a continuous wavy film, and the leveling of this film as it falls down the plate. The spreading and the merging exhibit a power‐law behavior in agreement with theoretical models. The leveling exhibits exponential decay behavior. [ABSTRACT FROM AUTHOR]

Perforations are one of the recognized geometrical features that contribute to liquid redistribution in corrugated sheet packings. Our experimental study focuses on a simplified but relevant configuration: a thin liquid film flowing on either side of a vertical plate with a circular perforation. We focus on the curtain mode when the liquid fills the perforation. Confocal chromatic imaging reveals a capillary ridge upstream of the perforation, an inertial ridge downstream, and a varicose capillary wave standing on the liquid curtain. We show that the wavelength is selected such that the velocity of the wave both satisfies Taylor's dispersion relation and matches the curtain local speed. We examine the effect of perforation size, supply conditions, and liquid properties on the curtain transition. Lastly, we propose a simple model based on a momentum balance that describes the effect of these parameters on the Reynolds number at which curtain forms. [ABSTRACT FROM AUTHOR]

A new theoretical approach is proposed to explain the dose, dose rate and temperature sensitivity of the radiation-induced absorption (RIA) in glasses. In this paper, a βth-order dispersive kinetic model is used to simulate the growth of the density of color centers in irradiated glasses. This model yields an explanation for the power-law dependence on dose and dose rate usually observed for the RIA in optical fibers. It also leads to an Arrhenius-like relationship between the RIA and the glass temperature during irradiation. With a very limited number of adjustable parameters, the model succeeds in explaining, with a good agreement, the RIA growth of two different optical fiber references over wide ranges of dose, dose rate and temperature. [ABSTRACT FROM AUTHOR]

This paper presents a method for identifying wood morphological parameters from gas apparent permeability measurements. The apparent permeability at a given mean pressure is typically determined from the pressure relaxation kinetics when the gas permeates through the wood sample between two compartments which have different initial pressures. Using the proposed set-up, apparent permeability values ranging from 10 to 10 m can be measured with a mean gas pressure varying from 2 bar down to 35 mbar. Morphological and topological parameters are then identified from the variations in apparent permeability as a function of the mean gas pressure using a pore network model. The network consists of elements such as pipes or orifices connected in series or in parallel. The rarefied gas flow is described in each element by an appropriate model, and the unknowns are determined by an inverse method. This approach was first applied to track-etched polycarbonate membranes for validation purposes. The calculated pore radius and pore density values were compared to observations by environmental scanning electron microscopy. Softwood specimens were then investigated. The mean radius of the pores controlling permeability in the longitudinal and tangential directions was determined and found to be in good agreement with literature data. [ABSTRACT FROM AUTHOR]

A novel device was conceived and built to measure the apparent gaseous permeability of porous media over a large range of permeability values (10−10-10−18 m2) and mean pressures (from 1 bar down to ca. 40 mbar). An extensive series of experimental data are presented and analysed for various porous media: (1) Norway spruce ( Picea abies) and European beech ( Fagus silvatica) in the three anatomical directions, and, for comparison, (2) three simpler porous media, i.e. an autoclaved aerated concrete (AAC, light concrete) and two wood-based panels. For all porous media, the intrinsic permeability, the gas slippage factor, and the effective pore size were determined from the variations of the apparent permeability as a function of mean pressure. These results are in good agreement with those of previous studies for spruce and bring new insights for beech and wood-based materials, in general. For all products, the effective pore sizes identified with the new instrument are closely linked to the medium morphology. In particular, it was found that in spite of the huge anisotropic ratios between wood's longitudinal and tangential directions, the identified pore size is similar and corresponds to anatomical features: openings in margo for spruce and scalariform perforation plates for beech. Besides, the pore size identified for beech in the radial direction suggests that radial permeability is most probably controlled by the openings in ray cells (either pits or intercellular voids). [ABSTRACT FROM AUTHOR]

The deposition of non-Brownian particles from turbulent liquid-flow onto channel walls is numerically analyzed. The approach combines Lagrangian particle tracking with a kinematic model of the near-wall shear layer. For nonbuoyant particles, direct interception is the main deposition mechanism and the deposition velocity scales as the particle diameter (in wall units) to the power of 1.7. When wall/particle hydrodynamic interactions are taken into account, the deposition velocity is significantly reduced and the correction factor scales as the cubic root of the wall roughness to particle diameter ratio. For buoyant particles, sedimentation is usually the predominant deposition mechanism and the hydrodynamic interactions significantly affect the deposition velocity when the drainage characteristic time driven by buoyancy is of the order of the particle residence time close to the wall. Last, a wall-function for the suspended particles is proposed. © 2015 American Institute of Chemical Engineers AIChE J, 62: 891-904, 2016 [ABSTRACT FROM AUTHOR]

A pilot has been designed to study the mechanisms associated with liquid aluminum filtration through ceramic foam filters (CFFs). Before entering the filtering bowl, the liquid metal is seeded with massive artificial inclusions. A theoretical analysis of the filtration of massive inclusions shows that there are two preponderant deposition mechanisms, i.e., direct interception and sedimentation, which explain qualitatively the effect of the operating conditions on the measured filtration efficiency. Furthermore, it is observed that the filtration efficiency measured by two Coulter counters reaches a minimum value for a diameter close to 30 μm. This nonmonotonous trend can be explained by the coexistence of two populations of inclusions, intentionally added massive ones and oxide films. It is expected that oxide films are the most numerous inclusion type for diameters less than 25 μm and have an apparent volume much larger than their absolute volume (contrary to massive inclusions), leading to a higher filtration efficiency. In addition, a metallographic analysis of spent filters shows that the density of the captured massive inclusions decreases exponentially between the filter inlet and the filter outlet. The initial filter coefficient can be derived, which is equal to 25 m−1 for a 30-ppi filter grade (with a specific surface area of 1610 m−1), SiC inclusions 50 μm in diameter, and a filtration velocity of 0.9 cm·s−1. [ABSTRACT FROM AUTHOR]

A two-dimensional mathematical model of the initial stage of depth filtration is developed. This model describes the fluid flow and the particle transport and capture in a representative elementary volume of the filter. A typical simulation can be outlined as follows: as a first stage, the flow field is computed in the whole complex void space of the filter using a lattice-Boltzmann method. As a second stage, the trajectory analysis is derived by applying Newton's second law to suspended particles in the flowing fluid. The capture or the reentrainment of the particles encountering the filter surface is determined from a force balance that takes into account the drag force and the solid friction force. This approach, which provides both microscopic data such as the initial capture probability density and macroscopic data such as the filter coefficient, is validated on a model experiment of deep-bed filtration. [ABSTRACT FROM AUTHOR]

By favoring cell proliferation and differentiation, perfusion bioreactors proved efficient at optimizing cell culture. The aim of this study was to quantify cell proliferation within a perfusion bioreactor and correlate it to the wall shear stress (WSS) distribution by combining 3‐D imaging and computational fluid dynamics simulations.NIH‐3T3 fibroblasts were cultured onto a scaffold model made of impermeable polyacetal spheres or Polydimethylsiloxane cubes. After 1, 2, and 3 weeks of culture, constructs were analyzed by micro‐computed tomography (μCT) and quantification of cell proliferation was assessed. After 3 weeks, the volume of cells was found four times higher in the stacking of spheres than in the stacking of cube.3D‐μCT reconstruction of bioreactors was used as input for the numerical simulations. Using a lattice‐Boltzmann method, we simulated the fluid flow within the bioreactors. We retrieved the WSS distribution (PDF) on the scaffolds surface at the beginning of cultivation and correlated this distribution to the local presence of cells after 3 weeks of cultivation. We found that the WSS distributions strongly differ between spheres and cubes even if the porosity and the specific wetted area of the stackings were very similar. The PDF is narrower and the mean WSS is lower for cubes (11 mPa) than for spheres (20 mPa). For the stacking of spheres, the relative occupancy of the surface sites by cells is maximal when WSS is greater than 20 mPa. For cubes, the relative occupancy is maximal when the WSS is lower than 10 mPa. The discrepancies between spheres and cubes are attributed to the more numerous sites in stacking of spheres that may induce 3‐D (multi‐layered) proliferation. [ABSTRACT FROM AUTHOR]