305 research outputs found
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(Non)Parallel Evolution
Parallel evolution across replicate populations has provided evolutionary biologists with iconic examples of adaptation. When multiple populations colonize seemingly similar habitats, they may evolve similar genes, traits, or functions. Yet, replicated evolution in nature or in the laboratory often yields inconsistent outcomes: Some replicate populations evolve along highly similar trajectories, whereas other replicate populations evolve to different extents or in distinct directions. To understand these heterogeneous outcomes, biologists are increasingly treating parallel evolution not as a binary phenomenon but rather as a quantitative continuum ranging from parallel to nonparallel. By measuring replicate populationsâ positions along this (non)parallel continuum, we can test hypotheses about evolutionary and ecological factors that influence the extent of repeatable evolution. We review evidence regarding the manifestation of (non)parallel evolution in the laboratory, in natural populations, and in applied contexts such as cancer. We enumerate the many genetic, ecological, and evolutionary processes that contribute to variation in the extent of parallel evolution
Mechanically adaptive nanocomposites for neural interfacing
The recording of neural signals with microelectrodes that are implanted into the cortex of the brain is potentially useful for a range of clinical applications. However, the widespread use of such neural interfaces has so far been stifled because existing intracortical electrode systems rarely allow for consistent long-term recording of neural activity. This limitation is usually attributed to scar formation and neuron death near the surface of the implanted electrode. It has been proposed that the mechanical property mismatch between existing electrode materials and the brain tissue is a significant contributor to these events. To alleviate this problem, we utilized the architecture of the sea cucumber dermis as a blueprint to engineer a new class of mechanically adaptive materials as substrates for "smartâ intracortical electrodes. We demonstrated that these originally rigid polymer nanocomposites soften considerably upon exposure to emulated physiological and in vivo conditions. The adaptive nature of these bioinspired materials makes them useful as a basis for electrodes that are sufficiently stiff to be easily implanted and subsequently soften to better match the stiffness of the brain. Initial histological evaluations suggest that mechanically adaptive neural prosthetics can more rapidly stabilize neural cell populations at the device interface than rigid systems, which bodes well for improving the functionality of intracortical device
LCPOM: Precise Reconstruction of Polarized Optical Microscopy Images of Liquid Crystals
When viewed with a cross-polarized optical microscope (POM), liquid crystals
display interference colors and complex patterns that depend on the material's
microscopic orientation. That orientation can be manipulated by application of
external fields, which provides the basis for applications in optical display
and sensing technologies. The color patterns themselves have a high information
content. Traditionally, however, calculations of the optical appearance of
liquid crystals have been performed by assuming that a single-wavelength light
source is employed, and reported in a monochromatic scale. In this work, the
original Jones matrix method is extended to calculate the colored images that
arise when a liquid crystal is exposed to a multi-wavelength source. By
accounting for the material properties, the visible light spectrum and the CIE
color matching functions, we demonstrate that the proposed approach produces
colored POM images that are in quantitative agreement with experimental data.
Results are presented for a variety of systems, including radial, bipolar, and
cholesteric droplets, where results of simulations are compared to experimental
microscopy images. The effects of droplet size, topological defect structure,
and droplet orientation are examined systematically. The technique introduced
here generates images that can be directly compared to experiments, thereby
facilitating machine learning efforts aimed at interpreting LC microscopy
images, and paving the way for the inverse design of materials capable of
producing specific internal microstructures in response to external stimuli.Comment: 12 pages, 5 figures (main text). 6 pages, 6 figures (appendices
Stress-transfer in anisotropic and environmentally adaptive cellulose whisker nanocomposites
Quantitative insights into the stress-transfer mechanisms that determine the mechanical properties of tunicate cellulose whisker/poly(vinyl acetate) nanocomposites were gained by Raman spectroscopy. The extent of stress-transfer is influenced by local orientation (or anisotropy) of the whiskers, which in turn is governed by the processing conditions used to fabricate the nanocomposites. Solution-cast materials display no microscopic anisotropy, while samples that were cast and subsequently compression molded contain both isotropic regions as well as domains of locally oriented whiskers. Polarized optical microscopy showed these regions to have dimensions in the hundreds of ÎŒm. Polarized Raman spectroscopy of the 1095 cmâ»Âč Raman band, associated with CâO ring stretching of the cellulose backbone, was used to quantify the local orientation of the cellulose whiskers. Clear and discernible shifts of this Raman band upon uniaxial deformation of nanocomposite films were further used to determine the level of stress experienced by the cellulose whiskers, ultimately reflecting the levels of stress-transfer predominantly between the poly(vinyl acetate) matrix and the tunicate whiskers, but also between the whiskers within the network. In the isotropic regions, where whiskers form a percolating network, the observed Raman shift rate with respect to strain is smaller than in the regions where the whiskers are uniaxially orientated. The Raman shift is strongly affected by the presence of water, leading to a lack of stress-transfer when the samples are fully hydrated, which is clearly detected by the Raman technique. Heating of the nanocomposites above the glass transition temperature of the poly(vinyl acetate) matrix also reduces the stress experienced by the individual whiskers
Experimental results for nulling the effective thermal expansion coefficient of fused silica fibres under a static stress
We have experimentally demonstrated that the effective thermal expansion coefficient of a fused silica fibre can be nulled by placing the fibre under a particular level of stress. Our technique involves heating the fibre and measuring how the fibre length changes with temperature as the stress on the fibre was systematically varied. This nulling of the effective thermal expansion coefficient should allow for the complete elimination of thermoelastic noise and is essential for allowing second generation gravitational wave detectors to reach their target sensitivity. To our knowledge this is the first time that the cancelation of the thermal expansion coefficient with stress has been experimentally observed
Applying the 'no-one worse off' criterion to design Pareto efficient HIV responses in Sudan and Togo
Dust in Spiral Galaxies: Comparing Emission and Absorption to Constrain Small-Scale and Very Cold Structures
The detailed distribution of dust in the disks of spiral galaxies is
important to understanding the radiative transfer within disks, and to
measuring overall dust masses if significant quantities of dust are either very
opaque or very cold. We address this issue by comparing measures of dust
absorption, using the galaxy-overlap technique in the optical, with measures of
the dust grains' thermal emission from 50-2000 micron using ISOPHOT on board
ISO and SCUBA at the JCMT. We examine three spiral galaxies projected partially
in front of E/S0 galaxies --- AM1316-241, NGC 5545, and NGC 5091 (for NGC 5091
we have only optical and ISO data). Adopting an empirical exponential model for
the dust distribution, we compare column densities and dust masses derived from
the absorption and emission techniques. This comparison is sensitive to the
amount of dust mass in small, opaque structures, which would not contribute
strongly to area-weighted absorption measures, and to very cold dust, which
would contribute to optical absorption but provide only a small fraction of the
sub-mm emission. In AM1316-241, we find global dust masses of 2-5 x 10^7
M_solar, both techniques agreeing at the 50% level. NGC 5545 has about half
this dust mass. The concordance of dust masses is well within the errors
expected from our knowledge of the radial distribution of dust, and argues
against any dominant part of the dust mass being so cold or opaque. The 50-2000
micron data are well fitted by modified Planck functions with an emissivity law
beta=-2, at 21 +/- 2 K. We also present 12 micron ISOCAM observations of these
pairs.Comparison of H-alpha and 12 micron images of NGC 5545 indicate that
ISOCAM images are reliable tracers of star formation.Comment: 16 pages, 4 tables, 8 figures, in press for October Astronomical
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