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Biomedical subjects

L Kramer

Publications and source records attributed to L Kramer.

At least 19 recordsLinked to original sources
PubMed2006

Dogs with patent Dirofilaria immitis infection have higher expression of circulating IL-4, IL-10 and iNOS mRNA than those with occult infection.

Dirofilaria immitis is the agent of canine heartworm disease, in which adult worms reside in the pulmonary arteries, producing first stage larvae (microfilariae) that are released into the bloodstream. The present work describes the cytokine and iNOS mRNA expression in the peripheral blood of naturally infected dogs classified as either microfilariemic or amicrofilariemic. Results show that microfilariemic dogs had higher expression of IL-4 and iNOS mRNA than amicrofilariemic dogs. Furthermore, IL-10 mRNA expression was strongly expressed in dogs with circulating microfilariae, compared to only negligible expression in amicrofilariemic dogs. Finally, mf+ status was associated with a predominance in IgG1 production against worm antigens. These results would suggest that circulating mf may stimulate, like in other filarial infections, an immune bias towards unresponsiveness in D. immitis-infected dogs, consenting long-term adult worm survival.

Animals↗
PubMed2006

Corpus callosum diffusion anisotropy correlates with neuropsychological outcomes in twins disconcordant for traumatic brain injury.

Conventional and diffusion tensor MR imaging studies in twins sustaining severe pediatric traumatic brain injury identified reduction in fractional anisotropy (FA) in all regions of the corpus callosum, particularly the posterior body, rostral body, and genu, relative to healthy cotwins. FA from the rostrum, genu, anterior body, posterior body, and isthmus were correlated with measures of reading speed and comprehension; verbal working memory and math fact retrieval scores were correlated only with the rostral body FA.

Adolescent↗
PubMed2005

Numerical study of optically induced director oscillations in nematic liquid crystals: transition to chaos via homoclinic gluings and the role of backflow.

We investigate the director oscillations of a homeotropically aligned nematic liquid crystal that are generated by cw laser radiation incident at a slightly oblique angle. The full nematodynamic equations are solved numerically, and it is shown that the inclusion of backflow leads to a qualitative change of the theoretical bifurcation scenario at moderate to high intensities. Very good correspondence is achieved with recent observations. The route to chaos via a sequence of homoclinic gluing bifurcations, whose existence was suggested by simple models, but doubted by recent calculations and experiments, is shown to exist in a parameter region unexplored by experiments.

Journal Article↗
PubMed2005

Orientational instabilities in nematic liquid crystals with weak anchoring under combined action of steady flow and external fields.

We study the homogeneous and the spatially periodic instabilities in a nematic liquid crystal layer subjected to steady plane Couette or Poiseuille flow. The initial director orientation is perpendicular to the flow plane. Weak anchoring at the confining plates and the influence of the external electric and/or magnetic field are taken into account. Approximate expressions for the critical shear rate are presented and compared with semianalytical solutions in case of Couette flow and numerical solutions of the full set of nematodynamic equations for Poiseuille flow. In particular the dependence of the type of instability and the threshold on the azimuthal and the polar anchoring strength and external fields is analyzed.

Journal Article↗
PubMed2005

Heartworm disease in dogs: unusual clinical cases.

Two unusual cases of heartworm infection are described, including one of spontaneous rupture of cardiac chambers, most likely due to the progressive increase in pressure within the right ventricle caused by Dirofilaria immitis infection. Another case report describes persistent left cranial vena cava, which interfered with the surgical removal of heartworm via the left jugular vein.

Animals↗
PubMed2005

Is Wolbachia complicating the pathological effects of Dirofilaria immitis infections?

Human and animal parasitic filarial nematodes, which often are the cause of severe disease, harbor intracellular bacteria of the genus Wolbachia (Rickettsiaceae). It is thought that these bacteria play an important role in the pathogenesis and immune response to filarial infection. In order to determine the possible role of Wolbachia in heartworm disease, dogs naturally infected with Dirofilaria immitis were studied for specific antibody response to Wolbachia surface protein (WSP). Antibody subclasses were analyzed to determine immune response polarization. Dogs that died from heartworm disease were necropsied, and various organs were studied by immunohistochemistry to determine whether Wolbachia-derived molecules were present in tissue from infected dogs. Humoral response to the WSP was present in all infected dogs and appeared to be predominantly of the Th1-type. Several organs, including lung, liver, and kidney, contained positive-staining cells for WSP, confirming that the canine host does come into contact with Wolbachia-derived molecules.

Animals↗
PubMed2005

Pattern-forming instabilities in nematic liquid crystals under oscillatory Couette flow.

We consider instabilities, either homogeneous or periodic in space, which develop in a nematic liquid crystal layer under rectilinear oscillatory Couette flow for planar surface alignment of the director perpendicular to the flow plane. On the basis of a numerical and analytical linear stability analysis we determine the critical amplitude of the oscillatory flow, the wave number, and the symmetry of the destabilizing mode and present a comprehensive phase diagram of the flow instabilities. In particular it is found that by varying the frequency of the Couette flow the instability changes its temporal symmetry. This transition is shown to be related to the inertia effects of the nematic fluid, which become more important with increasing flow frequency. We also show that an electric field applied perpendicularly to the nematic layer can induce an exchange of instabilities with different spatial and temporal symmetries. The theoretical results are compared with experiments, when available.

Journal Article↗
PubMed2005

Theory of director precession and nonlinear waves in nematic liquid crystals under elliptical shear.

We study theoretically the slow director precession and nonlinear waves observed in homeotropically oriented nematic liquid crystals subjected to circular or elliptical Couette and Poiseuille flow and an electric field. From a linear analysis of the nematodynamic equations it is found that in the presence of the flow the electric bend Fréedericksz transition is transformed into a Hopf-type bifurcation. In the framework of an approximate weakly nonlinear analysis we have calculated the coefficients of the modified complex Ginzburg-Landau equation, which slightly above onset describes nonlinear waves with strong nonlinear dispersion. We also derive the equation describing the precession and waves well above the Fréedericksz transition and for small flow amplitudes. Then the nonlinear waves are of diffusive nature. The results are compared with full numerical simulations and with experimental data.

Journal Article↗
PubMed2005

Light-induced instabilities driven by competing helical patterns in long-pitch cholesterics.

We study theoretically the dynamical reorientation phenomena when a long-pitch cholesteric liquid-crystal film with homeotropic alignment is illuminated by a circularly polarized lightwave. In the present case, the natural cholesteric pitch is of the order of (or larger than) the film thickness. The helical cholesteric structure is thus frustrated by the boundary conditions without illumination. However, above a light intensity threshold reorientation occurs and the bifurcation scenario depends strongly on the natural cholesteric pitch. Recalling that a long-pitch cholesteric is achieved in practice by adding a small amount of chiral agents in a nematic liquid crystal, the observed dynamics can be viewed as the result of the competition between intrinsic and extrinsic unidimensional helical patterns. The intrinsic part consists of the helical deformations induced by the chirality of the dopant, whereas the extrinsic part is related to the chirality induced by the optical field through the non-uniform angular momentum transfer of light to a nematic. The all-optical analog in the case of a pure nematic (without chiral dopant), is also discussed.

Anisotropy↗
PubMed2005

Thermal patterning of a critical polymer blend.

Utilizing the Soret effect, we have employed a moderately focused laser beam (30 microm, 20 mW) to write spatial composition patterns into layers of the critical polymer blend poly(dimethyl siloxane)/poly(ethyl-methyl siloxane) (PDMS/PEMS, M(w)=16.4/22.8 kg/mol) both in the one- and in the two-phase region a few degrees above and below the critical temperature T(c)=37.7 degrees C. Because of the critical divergence of the Soret coefficient, moderate temperature gradients are sufficient to induce composition modulations of large amplitude. In the two-phase regime the spinodal demixing pattern can be locally manipulated in a controlled way. 2D simulations based on a modified Cahn-Hilliard equation are able to reproduce the essential spatial and temporal features observed in the experiments.

Journal Article↗
PubMed2005

Influence of the backflow effect on the orientational dynamics induced by light in nematics.

We investigate the dynamical phenomena induced by a circularly polarized plane wave incident perpendicularly on a homeotropically aligned nematic layer. We study theoretically the influence of the velocity field (backflow effect) on the bifurcation scenario. Whereas backflow leads to substantial quantitative changes of secondary bifurcation thresholds, the overall bifurcation scenario remains unchanged. In the regime of uniform precession of the director with large reorientation, an unanticipated spatial oscillation of the flow field across the layer is found. Quantitative comparison with experimental large-aspect ratio systems is now possible.

Journal Article↗
PubMed2004

Phase separation in the presence of spatially periodic forcing.

We have investigated theoretically phase separation in the presence of spatially periodic forcing. From the analytic and numeric study of a suitably generalized Cahn-Hilliard equation in one and two dimensions, we find that the forcing amplitude necessary to generate a periodic kink-type state from small random initial conditions depends weakly on wave number. This amplitude is much larger than the one necessary to stabilize the periodic state, i.e., to prevent late-stage coarsening, once it is established. Surprisingly, the destabilizing mode is of long-wave type, which is in contrast to the well-known most rapidly growing coarsening mode in the unforced system. In the Allen-Cahn equation with nonconserved order parameter the relevant modes are always long wave. It appears feasible to observe these effects by imposing a temperature modulation by optical grating which then couples to concentration modulation via the (Ludwig-)Soret effect.

Journal Article↗
PubMed2004

Pinning force in active media.

Pinning of vortices by defects plays an important role in various physical (superconductivity, superfluidity, etc.) or biological (propagation in cardiac muscle) situations. Which defects act as pinning centers? We propose a way to study this general problem by using an advection field to quantify the attraction between an obstacle and a vortex. A full solution is obtained for the real Ginzburg-Landau equation (RGLE). Two pinning mechanisms are found in excitable media. Our results suggest strong analogies with the RGLE when the heterogeneity is excitable. Unpinning from an unexcitable obstacle is qualitatively harder, resulting in a stronger pinning force. We discuss the implications of our results to control vortices and propose experiments in a chemical active medium and in cardiac tissue.

Heart↗
PubMed2004

Isotropic convection scenarios in an anisotropic fluid.

We study a new variant of electroconvection using a homeotropically aligned nematic liquid crystal. The novelty of this system is a direct transition to roll- or square-type patterns controlled by the frequency of the applied voltage with a rich crossover scenario and strong influence of the zigzag instability even at onset. From the weakly nonlinear theory and simulations of an adapted Swift-Hohenberg model one can understand essential features of the phase diagram. In particular we find a quasiperiodic pattern with square symmetry.

Journal Article↗
PubMed2004

Traveling-stripe forcing generates hexagonal patterns.

We study the response of Turing stripe patterns to a simple spatiotemporal forcing. This forcing has the form of a traveling wave and is spatially resonant with the characteristic Turing wavelength. Experiments conducted with the photosensitive chlorine dioxide-iodine-malonic acid reaction reveal a striking symmetry-breaking phenomenon of the intrinsic striped patterns giving rise to hexagonal lattices for intermediate values of the forcing velocity. The phenomenon is understood in the framework of the corresponding amplitude equations, which unveils a complex scenario of dynamical behaviors.

Journal Article↗
PubMed2004

Measurement of the electric form factor of the neutron at Q2=0.5 and 1.0 GeV2/c2.

The electric form factor of the neutron was determined from measurements of the d-->(e-->,e'n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (15ND3) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G(n)(E)=0.0526+/-0.0033(stat)+/-0.0026(sys) and 0.0454+/-0.0054+/-0.0037 at Q(2)=0.5 and 1.0 (GeV/c)(2), respectively.

Journal Article↗
PubMed2004

Direct transition to electroconvection in a homeotropic nematic liquid crystal.

We present an experimental and theoretical investigation of a variant of electroconvection using an unusual nematic liquid crystal in an isotropic configuration (homeotropic alignment). The significance of the system is a direct transition to the convecting state due to the negative conductivity anisotropy and positive dielectric anisotropy. We observe at onset rolls or squares depending on the frequency and amplitude of the applied ac voltage with a strong signature of the zigzag instability. Good agreement with calculations based on the underlying hydrodynamic theory is found. We also construct an extended Swift-Hohenberg model which allows us to capture complex patterns like squares with a quasiperiodic modulation.

Anisotropy↗