35 research outputs found
Understanding the impact of carrier mobility and mobile ions on perovskite cell performance
The realization of very high efficiency, stable perovskite solar cells fabricated on a large scale at low cost, has the potential to further lower the cost of photovoltaics. This necessitates an understanding of the properties required of the perovskite material, including the carrier mobility. Perovskite cells also feature mobile ionic species, and the impact of these ions on cell performance- A nd in particular, to what extent and under what circumstances they may limit device performance-is not well understood. Here, we employ an advanced numerical model that allows for the presence of mobile ionic species to probe the relationship between carrier mobility, the presence of ionic species as well as different possible recombination mechanisms within the cell. We show that a high electron and hole conductivity throughout the device is key to avoiding transport losses. For devices operating significantly below their radiative limit, achieving a sufficiently high conductivity requires high carrier mobilities of at least 10cm2/V-s. It is shown that the presence of a single mobile ionic species can lead to effective doping of the perovskite bulk, which is detrimental to cell performance by lowering the conductivity of one type of carrier. The results also indicate that increasing cell VOC closer to its radiative limit is also beneficial for reducing transport losses and pushing cell performance closer to its theoretical limit
Dissociation of EphB2 Signaling Pathways Mediating Progenitor Cell Proliferation and Tumor Suppression
SummarySignaling proteins driving the proliferation of stem and progenitor cells are often encoded by proto-oncogenes. EphB receptors represent a rare exception; they promote cell proliferation in the intestinal epithelium and function as tumor suppressors by controlling cell migration and inhibiting invasive growth. We show that cell migration and proliferation are controlled independently by the receptor EphB2. EphB2 regulated cell positioning is kinase-independent and mediated via phosphatidylinositol 3-kinase, whereas EphB2 tyrosine kinase activity regulates cell proliferation through an Abl-cyclin D1 pathway. Cyclin D1 regulation becomes uncoupled from EphB signaling during the progression from adenoma to colon carcinoma in humans, allowing continued proliferation with invasive growth. The dissociation of EphB2 signaling pathways enables the selective inhibition of the mitogenic effect without affecting the tumor suppressor function and identifies a pharmacological strategy to suppress adenoma growth
Period and chemical evolution of SC stars
The SC and CS stars are thermal-pulsing AGB stars with C/O ratio close to
unity. Within this small group, the Mira variable BH Cru recently evolved from
spectral type SC (showing ZrO bands) to CS (showing weak C2). Wavelet analysis
shows that the spectral evolution was accompanied by a dramatic period
increase, from 420 to 540 days, indicating an expanding radius. The pulsation
amplitude also increased. Old photographic plates are used to establish that
the period before 1940 was around 490 days. Chemical models indicate that the
spectral changes were caused by a decrease in stellar temperature, related to
the increasing radius. There is no evidence for a change in C/O ratio. The
evolution in BH Cru is unlikely to be related to an on-going thermal pulse.
Periods of the other SC and CS stars, including nine new periods, are
determined. A second SC star, LX Cyg, also shows evidence for a large increase
in period, and one further star shows a period inconsistent with a previous
determination. Mira periods may be intrinsically unstable for C/O ~ 1; possibly
because of a feedback between the molecular opacities, pulsation amplitude, and
period. LRS spectra of 6 SC stars suggest a feature at wavelength > 15 micron,
which resembles one recently attributed to the iron-sulfide troilite. Chemical
models predict a large abundance of FeS in SC stars, in agreement with the
proposed association.Comment: 14 pages, 20 figures. MNRAS, 2004, accepted for publication. Janet
Mattei, one of the authors, died on 22 March, 2004. This paper is dedicated
to her memor
A Parameter Study of Type II Supernova Light Curves Using 6 M_odot He Cores
Results of numerical calculations of Type II supernova light curves are
presented. The model progenitor stars have 6 cores and various
envelopes, originating from a numerically evolved 20 star. Five
parameters that affect the light curves are examined: the ejected mass, the
progenitor radius, the explosion energy, the Ni mass, and the extent of
Ni mixing. The following affects have been found: 1) the larger the
progenitor radius the brighter the early--time light curve, with little affect
on the late--time light curve, 2) the larger the envelope mass the fainter the
early light curve and the flatter the slope of the late light curve, 3) the
larger the explosion energy the brighter the early light curve and the steeper
the slope of the late light curve, 4) the larger the Ni mass the
brighter the overall light curve after 20 to 50 days, with no affect on the
early light curve, 5) the more extensive the Ni mixing the brighter the
early light curve and the steeper the late light curve. The primary parameters
affecting the light curve shape are the progenitor radius and the ejected mass.
The secondary parameters are the explosion energy, Ni mass and Ni
mixing. I find that while in principle the general shape and absolute magnitude
of a light curve indicate a unique set of parameters, in practice it is
difficult to avoid some ambiguity in the parameters. I find that the
nickel--powered diffusion wave and the recombination of helium produce a
prominent secondary peak in all our calculations. The feature is less prominent
when compositional mixing, both Ni mixing and mixing between the
hydrogen and helium layers, occurs. The model photospheric temperatures and
velocities are presented, for comparison to observation.Comment: 39 pages, 15 figures. Astrophysical Journal (Accepted, Dec. 20, 2004
La impresionante diversidad y estructura del bosque tropical a travĂ©s de una gradiente altitudinal en la selva central del PerĂș
Los bosques pre-montanos y montanos son poco estudiados y su composiciĂłn florĂstica es muy poco conocida, aunque Ășltimamente aquĂ se han descubierto nuevas especies de ĂĄrboles. Describimos la diversidad, composiciĂłn florĂstica y estructura del bosque en 13 parcelas permanentes de 1 ha, evaluadas en el 2018 en el Transecto Yanachaga en el PerĂș (400 a 3170msnm). Registramos un total de 6998 ĂĄrboles, 617 especies, 249 gĂ©neros y 82 familias. Existe unas altas correlaciones entre la altitud, la riqueza y diversidad de especies. La mayor riquezaocurre en la parcela PNY-05 a 470 msnm con 202 especies y la menor con 43 especies en la parcela PNY-01 a 3170 mnsm. La altura promedio del dosel es mayor entre los 400 y 800 msnm, y disminuye progresivamente a medida que se va subiendo, presentando alturas mĂnimas entre 2800 y 3170 msnm. Este mismo comportamiento ocurre con respecto al ĂĄrea basal y volumen de madera. Los individuos muestreados estĂĄn representados por especies de ĂĄrboles (88%), palmeras (4%), helechos arborescentes (6.5%), lianas (1.5%) y hemiepĂfitos leñosos (0. 03%). Las f ormas de vi da varĂ an notablemente en el transecto altitudinal, los ĂĄrboles y palmeras son mĂĄs abundantes y diversos en la parte baja, mientras los helechos arborescentes son abundantes por encima de los 1800 m. Existen diferencias en la diversidad, composiciĂłn y estructura de ĂĄrboles entre parcelas y tambiĂ©n si se compara al llano amazĂłnico. Los bosques del Transecto Yanachaga juegan un papel importante, puesto que conservan una alta diversidad de especies y hĂĄbitats
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Within-host evolution of Staphylococcus aureus during asymptomatic carriage
Background
Staphylococcus aureus is a major cause of healthcare associated mortality, but like many important bacterial pathogens, it is a common constituent of the normal human body flora. Around a third of healthy adults are carriers. Recent evidence suggests that evolution of S. aureus during nasal carriage may be associated with progression to invasive disease. However, a more detailed understanding of within-host evolution under natural conditions is required to appreciate the evolutionary and mechanistic reasons why commensal bacteria such as S. aureus cause disease. Therefore we examined in detail the evolutionary dynamics of normal, asymptomatic carriage. Sequencing a total of 131 genomes across 13 singly colonized hosts using the Illumina platform, we investigated diversity, selection, population dynamics and transmission during the short-term evolution of S. aureus.
Principal Findings
We characterized the processes by which the raw material for evolution is generated: micro-mutation (point mutation and small insertions/deletions), macro-mutation (large insertions/deletions) and the loss or acquisition of mobile elements (plasmids and bacteriophages). Through an analysis of synonymous, non-synonymous and intergenic mutations we discovered a fitness landscape dominated by purifying selection, with rare examples of adaptive change in genes encoding surface-anchored proteins and an enterotoxin. We found evidence for dramatic, hundred-fold fluctuations in the size of the within-host population over time, which we related to the cycle of colonization and clearance. Using a newly-developed population genetics approach to detect recent transmission among hosts, we revealed evidence for recent transmission between some of our subjects, including a husband and wife both carrying populations of methicillin-resistant S. aureus (MRSA).
Significance
This investigation begins to paint a picture of the within-host evolution of an important bacterial pathogen during its prevailing natural state, asymptomatic carriage. These results also have wider significance as a benchmark for future systematic studies of evolution during invasive S. aureus disease
The global abundance of tree palms
Aim Palms are an iconic, diverse and often abundant component of tropical ecosystems that provide many ecosystem services. Being monocots, tree palms are evolutionarily, morphologically and physiologically distinct from other trees, and these differences have important consequences for ecosystem services (e.g., carbon sequestration and storage) and in terms of responses to climate change. We quantified global patterns of tree palm relative abundance to help improve understanding of tropical forests and reduce uncertainty about these ecosystems under climate change. Location Tropical and subtropical moist forests. Time period Current. Major taxa studied Palms (Arecaceae). Methods We assembled a pantropical dataset of 2,548 forest plots (covering 1,191 ha) and quantified tree palm (i.e., â„10 cm diameter at breast height) abundance relative to coâoccurring nonâpalm trees. We compared the relative abundance of tree palms across biogeographical realms and tested for associations with palaeoclimate stability, current climate, edaphic conditions and metrics of forest structure. Results On average, the relative abundance of tree palms was more than five times larger between Neotropical locations and other biogeographical realms. Tree palms were absent in most locations outside the Neotropics but present in >80% of Neotropical locations. The relative abundance of tree palms was more strongly associated with local conditions (e.g., higher mean annual precipitation, lower soil fertility, shallower water table and lower plot mean wood density) than metrics of longâterm climate stability. Lifeâform diversity also influenced the patterns; palm assemblages outside the Neotropics comprise many nonâtree (e.g., climbing) palms. Finally, we show that tree palms can influence estimates of aboveâground biomass, but the magnitude and direction of the effect require additional work. Conclusions Tree palms are not only quintessentially tropical, but they are also overwhelmingly Neotropical. Future work to understand the contributions of tree palms to biomass estimates and carbon cycling will be particularly crucial in Neotropical forests
A profile of circulating vascular progenitor cells in human neovascular age-related macular degeneration.
BACKGROUND/OBJECTIVE:A subset of neovascular age-related macular degeneration (nvAMD) subjects appears to be refractory to the effects of anti-VEGF treatment and require frequent intravitreal injections. The vascular phenotype of the choroidal neovascular (CNV) lesions may contribute to the resistance. Animal studies of CNV lesions have shown that cells originating from bone marrow are capable of forming varying cell types in the lesions. This raised the possibility of a similar cell population in human nvAMD subjects. MATERIALS AND METHODS:Blood draws were obtained from subjects with active nvAMD while patients were receiving standard of care anti-VEGF injections. Subjects were classified as refractory or non-refractory to anti-VEGF treatment based on previous number of injections in the preceding 12 months. Peripheral blood mononuclear cells (PBMCs) were isolated and CD34-positive cells purified using magnetic bead sorting. The isolated cells were expanded in StemSpan SFEM media to increase cell numbers. After expansion, the cells were split and plated in either endothelial or mesenchymal promoting conditions. Phenotype analysis was performed via qPCR. RESULTS:There was no significant difference in the number of PBMCs and CD34-positive cells between refractory and non-refractory nvAMD subjects. The growth pattern distribution between endothelial and mesenchymal media conditions were very similar between refractory and non-refractory subjects. qPCR and immunostaining demonstrated positive expression of endothelial markers in endothelial media, and markers such as NG2 and αSMA in mesenchymal media. However, analysis of subsequent samples from AMD subjects demonstrated high variability in both the numbers and differentiation properties of this cell population. CONCLUSIONS:CD34+ cells can be isolated from nvAMD subjects and show both endothelial and pericyte-like characteristics after differentiation in certain media conditions. However, nvAMD subjects show high variability in both numbers of cells and differentiation characteristics in repeat sampling. This variability highlights the importance of taking multiple samples from nvAMD subjects for any clinical trials focused on biomarkers for the disease