10,044 research outputs found
A two-fluid model for tissue growth within\ud a dynamic flow environment
We study the growth of a tissue construct in a perfusion bioreactor, focussing on its response to the mechanical environment. The bioreactor system is modelled as a two-dimensional channel containing a tissue construct through which a flow of culture medium is driven. We employ a multiphase formulation of the type presented by G. Lemon, J. King, H. Byrne, O. Jensen and K. Shakesheff in their study (Multiphase modelling of tissue growth using the theory of mixtures. J. Math. Biol. 52(2), 2006, 571–594) restricted to two interacting fluid phases, representing a cell population (and attendant extracellular matrix) and a culture medium, and employ the simplifying limit of large interphase viscous drag after S. Franks in her study (Mathematical Modelling of Tumour Growth and Stability. Ph.D. Thesis, University of Nottingham, UK, 2002) and S. Franks and J. King in their study (Interactions between a uniformly proliferating tumour and its surrounding: Uniform material properties. Math. Med. Biol. 20, 2003, 47–89).\ud
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The novel aspects of this study are: (i) the investigation of the effect of an imposed flow on the growth of the tissue construct, and (ii) the inclusion of a mechanotransduction mechanism regulating the response of the cells to the local mechanical environment. Specifically, we consider the response of the cells to their local density and the culture medium pressure. As such, this study forms the first step towards a general multiphase formulation that incorporates the effect of mechanotransduction on the growth and morphology of a tissue construct. The model is analysed using analytic and numerical techniques, the results of which illustrate the potential use of the model to predict the dominant regulatory stimuli in a cell population
A note on heat and mass transfer from a sphere in Stokes\ud flow at low Péclet number
We consider the low Péclet number, Pe ≪ 1, asymptotic solution for steady-state heat and mass transfer from a sphere immersed in Stokes flow with a Robin boundary condition on its surface, representing Newton cooling or a first-order chemical reaction. The application of van Dyke’s rule up to terms of O(Pe3) shows that the O(Pe3 log Pe) terms in the expression for the average Nusselt/Sherwood number are double those previously derived in the literature. Inclusion of the O(Pe3) terms is shown to increase significantly the range of validity of the expansion
Keplerian discs around post-AGB stars: a common phenomenon?
Aims: We aim at showing that the broad-band SED characteristics of our sample
of post-AGB stars are best interpreted, assuming the circumstellar dust is
stored in Keplerian rotating passive discs.
Methods: We present a homogeneous and systematic study of the Spectral Energy
Distributions (SEDs) of a sample of 51 post-AGB objects. The selection criteria
to define the whole sample were tuned to cover the broad-band characteristics
of known binary post-AGB stars. The whole sample includes 20 dusty RV Tauri
stars from the General Catalogue of Variable Stars (GCVS). We supplemented our
own Geneva optical photometry with literature data to cover a broad range of
fluxes from the UV to the far-IR.
Results: All the SEDs display very similar characteristics: a large IR excess
with a dust excess starting near the sublimation temperature, irrespective of
the effective temperature of the central star. Moreover, when available, the
long wavelength fluxes show a black-body slope indicative of the presence of a
component of large mm sized grains.
Conclusions: We argue that in all systems, gravitationally bound dusty discs
are present. The discs must be puffed-up to cover a large opening angle for the
central star and we argue that the discs have some similarity with the passive
discs detected around young stellar objects. We interpret the presence of a
disc to be a signature for binarity of the central object, but this will need
confirmation by long-term monitoring of the radial velocities. We argue that
dusty RV Tauri stars are those binaries which happen to be in the Population II
instability strip.Comment: 29 pages, 5 figures, accepted for publication in A&
The interplay between tissue growth and scaffold degradation in engineered tissue constructs
In vitro tissue engineering is emerging as a potential tool to meet the high demand for replacement tissue, caused by the increased incidence of tissue degeneration and damage. A key challenge in this field is ensuring that the mechanical properties of the engineered tissue are appropriate for the in vivo environment. Achieving this goal will require detailed understanding of the interplay between cell proliferation, extracellular matrix (ECM) deposition and scaffold degradation.\ud
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In this paper, we use a mathematical model (based upon a multiphase continuum framework) to investigate the interplay between tissue growth and scaffold degradation during tissue construct evolution in vitro. Our model accommodates a cell population and culture medium, modelled as viscous fluids, together with a porous scaffold and ECM deposited by the cells, represented as rigid porous materials. We focus on tissue growth within a perfusion bioreactor system, and investigate how the predicted tissue composition is altered under the influence of (i) differential interactions between cells and the supporting scaffold and their associated ECM, (ii) scaffold degradation, and (iii) mechanotransduction-regulated cell proliferation and ECM deposition.\ud
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Numerical simulation of the model equations reveals that scaffold heterogeneity typical of that obtained from μCT scans of tissue engineering scaffolds can lead to significant variation in the flow-induced mechanical stimuli experienced by cells seeded in the scaffold. This leads to strong heterogeneity in the deposition of ECM. Furthermore, preferential adherence of cells to the ECM in favour of the artificial scaffold appears to have no significant influence on the eventual construct composition; adherence of cells to these supporting structures does, however, lead to cell and ECM distributions which mimic and exaggerate the heterogeneity of the underlying scaffold. Such phenomena have important ramifications for the mechanical integrity of engineered tissue constructs and their suitability for implantation in vivo
The color dependent morphology of the post-AGB star HD161796
Context. Many protoplanetary nebulae show strong asymmetries in their
surrounding shell, pointing to asymmetries during the mass loss phase.
Questions concerning the origin and the onset of deviations from spherical
symmetry are important for our understanding of the evolution of these objects.
Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims.
We aim at detecting signatures of an aspherical outflow, as well as to derive
the properties of it. Methods. We use the imaging polarimeter ExPo (the extreme
polarimeter), a visitor instrument at the William Herschel Telescope, to
accurately image the dust shell surrounding HD 161796 in various wavelength
filters. Imaging polarimetry allows us to separate the faint, polarized, light
from circumstellar material from the bright, unpolarized, light from the
central star. Results. The shell around HD 161796 is highly aspherical. A clear
signature of an equatorial density enhancement can be seen. This structure is
optically thick at short wavelengths and changes its appearance to optically
thin at longer wavelengths. In the classification of the two different
appearances of planetary nebulae from HST images it changes from being
classified as DUPLEX at short wavelengths to SOLE at longer wavelengths. This
strengthens the interpretation that these two appearances are manifestations of
the same physical structure. Furthermore, we find that the central star is
hotter than often assumed and the relatively high observed reddening is due to
circumstellar rather than interstellar extinction.Comment: Accepted for publication in A&
Detecting Extrasolar Planets with Integral Field Spectroscopy
Observations of extrasolar planets using Integral Field Spectroscopy (IFS),
if coupled with an extreme Adaptive Optics system and analyzed with a
Simultaneous Differential Imaging technique (SDI), are a powerful tool to
detect and characterize extrasolar planets directly; they enhance the signal of
the planet and, at the same time, reduces the impact of stellar light and
consequently important noise sources like speckles. In order to verify the
efficiency of such a technique, we developed a simulation code able to test the
capabilities of this IFS-SDI technique for different kinds of planets and
telescopes, modelling the atmospheric and instrumental noise sources. The first
results obtained by the simulations show that many significant extrasolar
planet detections are indeed possible using the present 8m-class telescopes
within a few hours of exposure time. The procedure adopted to simulate IFS
observations is presented here in detail, explaining in particular how we
obtain estimates of the speckle noise, Adaptive Optics corrections, specific
instrumental features, and how we test the efficiency of the SDI technique to
increase the signal-to-noise ratio of the planet detection. The most important
results achieved by simulations of various objects, from 1 M_J to brown dwarfs
of 30 M_J, for observations with an 8 meter telescope, are then presented and
discussed.Comment: 60 pages, 37 figures, accepted in PASP, 4 Tables adde
7-Li(p,n) Nuclear Data Library for Incident Proton Energies to 150 MeV
We describe evaluation methods that make use of experimental data, and
nuclear model calculations, to develop an ENDF-formatted data library for the
reaction p + Li7 for incident protons with energies up to 150 MeV. The
important 7-Li(p,n_0) and 7-Li(p,n_1) reactions are evaluated from the
experimental data, with their angular distributions represented using Lengendre
polynomial expansions. The decay of the remaining reaction flux is estimated
from GNASH nuclear model calculations. The evaluated ENDF-data are described in
detail, and illustrated in numerous figures. We also illustrate the use of
these data in a representative application by a radiation transport simulation
with the code MCNPX.Comment: 11 pages, 8 figures, LaTeX, submitted to Proc. 2000 ANS/ENS
International Meeting, Nuclear Applications of Accelerator Technology
(AccApp00), November 12-16, Washington, DC, US
The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia
We have computed radiative equilibrium models for the gas in the
circumstellar envelope surrounding the hot, classical Be star Cassiopeia. This calculation is performed using a code that incorporates a
number of improvements over previous treatments of the disk's thermal structure
by \citet{mil98} and \citet{jon04}; most importantly, heating and cooling rates
are computed with atomic models for H, He, CNO, Mg, Si, Ca, & Fe and their
relevant ions. Thus, for the first time, the thermal structure of a Be disk is
computed for a gas with a solar chemical composition as opposed to assuming a
pure hydrogen envelope. We compare the predicted average disk temperature, the
total energy loss in H, and the near-IR excess with observations and
find that all can be accounted for by a disk that is in vertical hydrostatic
equilibrium with a density in the equatorial plane of to
. We also discuss the changes in
the disk's thermal structure that result from the additional heating and
cooling processes available to a gas with a solar chemical composition over
those available to a pure hydrogen plasma.Comment: 11 pages, 8 figures high resolution figures available at
http://inverse.astro.uwo.ca/sig_jon07.htm
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