4,810 research outputs found
Discrete Nodal Domain Theorems
We give a detailed proof for two discrete analogues of Courant's Nodal Domain
Theorem
Sea Water Aging ofGlass Reinforced Composites:Shear Behaviour andDamage Modelling
International audienceThis paper presents results from a study of the wet aging of four thermoset resins and their [0°/90°] stitched glass fibre reinforced composites. The matrix resins are orthophthalic polyester, isophthalic polyester, vinyl ester and epoxy. Resins and composites were aged for 18 months, under three immersion conditions: 20°C sea water, 50°C sea water and 50°C distilled water. Tensile tests,on resins and at 45° to fibre direction of composites, both before and after aging enable the influence of matrix resin and aging medium on weight changes and matrix dominated property degradation to be evaluated. This has enabled a unique data set to be obtained. A large part of the shear property loss after aging is recovered after drying. An original application of damage mechanics parameters is used to quantify the changes in composite shear behaviour, in order to provide a more complete representation of the inelastic response
The Cytoskeleton Under External Fluid Mechanical Forces: Hemodynamic Forces Acting on the Endothelium
The endothelium, a single layer of cells that lines all blood vessels, is the focus of intense interest in biomechanics because it is the principal recipient of hemodynamic shear stress. In arteries, shear stress has been demonstrated to regulate both acute vasoregulation and chronic adaptive vessel remodeling and is strongly implicated in the localization of atherosclerotic lesions. Thus, endothelial biomechanics and the associated mechanotransduction of shear stress are of great importance in vascular physiology and pathology. Here we discuss the important role of the cytoskeleton in a decentralization model of endothelial mechanotransduction. In particular, recent studies of four-dimensional cytoskeletal motion in living cells under external fluid mechanical forces are summarized together with new data on the spatial distribution of cytoskeletal strain. These quantitative studies strongly support the decentralized distribution of luminally imposed forces throughout the endothelial cell
Imaging Live Cells Under Mechanical Stress
Cellular responses to mechanical stimuli are implicated in the structural and functional adaptation of many tissues. For example, cellular mechanisms mediate bone and skeletal muscle remodeling during mechanical loading, lung function during ventilator-induced injury, hearing loss in the inner ear, and blood flow-mediated cardiovascular pathophysiology. Since much of our own work investigates vascular biomechanics, we will focus in this chapter on the techniques used to study vascular endothelial cells in vitro; however, similar techniques can be used to study other cell types
The potential of Red Supergiants as extra-galactic abundance probes at low spectral resolution
Red Supergiants (RSGs) are among the brightest stars in the local universe,
making them ideal candidates with which to probe the properties of their host
galaxies. However, current quantitative spectroscopic techniques require
spectral resolutions of R>17,000, making observations of RSGs at distances
greater than 1Mpc unfeasible. Here we explore the potential of quantitative
spectroscopic techniques at much lower resolutions, R ~2-3000. We take archival
J-band spectra of a sample of RSGs in the Solar neighbourhood. In this spectral
region the metallic lines of FeI, MgI, SiI and TiI are prominent, while the
molecular absorption features of OH, H_2O, CN and CO are weak. We compare these
data with synthetic spectra produced from the existing grid of model
atmospheres from the MARCS project, with the aim of deriving chemical
abundances. We find that all stars studied can be unambiguously fit by the
models, and model parameters of log g, effective temperatures Teff,
microturbulence and global metal content may be derived. We find that the
abundances derived for the stars are all very close to Solar and have low
dispersion, with an average of [logZ]=0.13+/-0.14. The values of Teff fit by
the models are ~150K cooler than the stars' literature values for earlier
spectral types when using the Levesque et al. temperature scale, though this
temperature discrepancy has very little systematic effect on the derived
abundances as the equivalent widths (EWs) of the metallic lines are roughly
constant across the full temperature range of RSGs. Instead, elemental
abundances are the dominating factor in the EWs of the diagnostic lines. Our
results suggest that chemical abundance measurements of RSGs are possible at
low- to medium-resolution, meaning that this technique is a viable
infrared-based alternative to measuring abundance trends in external galaxies.
[Abridged]Comment: 10 pages, 7 figures. Accepted for publication in MNRAS
Spectral projections and resolvent bounds for partially elliptic quadratic differential operators
We study resolvents and spectral projections for quadratic differential
operators under an assumption of partial ellipticity. We establish
exponential-type resolvent bounds for these operators, including
Kramers-Fokker-Planck operators with quadratic potentials. For the norms of
spectral projections for these operators, we obtain complete asymptotic
expansions in dimension one, and for arbitrary dimension, we obtain exponential
upper bounds and the rate of exponential growth in a generic situation. We
furthermore obtain a complete characterization of those operators with
orthogonal spectral projections onto the ground state.Comment: 60 pages, 3 figures. J. Pseudo-Differ. Oper. Appl., to appear.
Revised according to referee report, including minor changes to Corollary
1.8. The final publication will be available at link.springer.co
Is treatment of depression cost-effective in people with diabetes? A systematic review of the economic evidence
PublishedJournal ArticleResearch Support, Non-U.S. Gov'tReviewOBJECTIVES: Depression is common in diabetes and linked to a wide range of adverse outcomes. UK policy indicates that depression should be treated using conventional psychological treatments in a stepped care framework. This review aimed to identify current economic evidence of psychological treatments for depression among people with diabetes. METHOD: Electronic search strategies (conducted in MEDLINE, EMBASE, PsycINFO, CINAHL, NHS EED) combined clinical and economic search terms to identify full economic evaluations of the relevant interventions. Prespecified screening and inclusion criteria were used. Standardized data extraction and critical appraisal were conducted and the results summarized qualitatively. RESULTS: Excluding duplicates, 1,516 studies for co-morbid depression and diabetes were screened. Four economic evaluations were identified. The studies found that the interventions improved health status, reduced depression and were cost-effective compared with usual care. The studies were all U.S.-based and evaluated collaborative care programs that included psychological therapies. Critical appraisal indicated limitations with the study designs, analysis and results for all studies. CONCLUSIONS: The review highlighted the paucity of evidence in this area. The four studies indicated the potential of interventions to reduce depression and be cost-effective compared with usual care. Two studies reported costs per QALY gained of USD 267 to USD 4,317, whilst two studies reported the intervention dominated usual care, with net savings of USD 440 to USD 612 and net gains in patient free days or QALYs
The progenitor mass of the magnetar SGR1900+14
Magnetars are young neutron stars with extreme magnetic fields (B >
10^{14}-10^{15}G). How these fields relate to the properties of their
progenitor stars is not yet clearly established. However, from the few objects
associated with young clusters it has been possible to estimate the initial
masses of the progenitors, with results indicating that a very massive
progenitor star (M_prog >40Msun) is required to produce a magnetar. Here we
present adaptive-optics assisted Keck/NIRC2 imaging and Keck/NIRSPEC
spectroscopy of the cluster associated with the magnetar SGR 1900+14, and
report that the initial progenitor star mass of the magnetar was a factor of
two lower than this limit, M_prog=17 \pm 2 Msun. Our result presents a strong
challenge to the concept that magnetars can only result from very massive
progenitors. Instead, we favour a mechanism which is dependent on more than
just initial stellar mass for the production of these extreme magnetic fields,
such as the "fossil-field" model or a process involving close binary evolution.Comment: 9 pages, 8 figs. Accepted to Ap
Role of lateral cell–cell border location and extracellular/transmembrane domains in PECAM/CD31 mechanosensation
Phosphorylation of tyrosine residues on platelet–endothelial cell adhesion molecule-1 (PECAM-1), followed by signal trans- 13 duction events, has been described in endothelial cells following exposure to hyperosmotic and fluid shear stress. However, it is 14 unclear whether PECAM-1 functions as a primary mechanosensor in this process. Utilizing a PECAM-1–null EC-like cell line, we 15 examined the importance of cellular localization and the extracellular and transmembrane domains in PECAM-1 phosphorylation 16 responses to mechanical stress. Tyrosine phosphorylation of PECAM-1 was stimulated in response to mechanical stress in null cells 17 transfected either with full length PECAM-1 or with PECAM-1 mutants that do not localize to the lateral cell–cell adhesion site and 18 that do not support homophilic binding between PECAM-1 molecules. Furthermore, null cells transfected with a construct that 19 contains the intact cytoplasmic domain of PECAM-1 fused to the extracellular and transmembrane domains of the interleukin-2 20 receptor also underwent mechanical stress-induced PECAM-1 tyrosine phosphorylation. These findings suggest that mechano- 21 sensitive PECAM-1 may lie downstream of a primary mechanosensor that activates a tyrosine kinase
The convergence of haemodynamics, genomics, and endothelial structure in studies of the focal origin of atherosclerosis
The completion of the Human Genome Project and ongoing sequencing of mouse, rat and other genomes has led to an explosion of genetics-related technologies that are finding their way into all areas of biological research; the field of biorheology is no exception. Here we outline how two disparate modern molecular techniques, microarray analyses of gene expression and real-time spatial imaging of living cell structures, are being utilized in studies of endothelial mechanotransduction associated with controlled shear stress in vitro and haemodynamics in vivo. We emphasize the value of such techniques as components of an integrated understanding of vascular rheology. In mechanotransduction, a systems approach is recommended that encompasses fluid dynamics, cell biomechanics, live cell imaging, and the biochemical, cell biology and molecular biology methods that now encompass genomics. Microarrays are a useful and powerful tool for such integration by identifying simultaneous changes in the expression of many genes associated with interconnecting mechanoresponsive cellular pathways
- …