418 research outputs found
Frequency modulated self-oscillation and phase inertia in a synchronized nanowire mechanical resonator
Synchronization has been reported for a wide range of self-oscillating
systems. However, even though it has been predicted theoretically for several
decades, the experimental realization of phase self-oscillation, sometimes
called phase trapping, in the high driving regime has been studied only
recently. We explored in detail the phase dynamics in a synchronized field
emission SiC nanoelectromechanical system with intrinsic feedback. A richer
variety of phase behavior has been unambiguously identified, implying phase
modulation and inertia. This synchronization regime is expected to have
implications for the comprehension of the dynamics of interacting
self-oscillating networks and for the generation of frequency modulated signals
at the nanoscal
Role of fluctuations and nonlinearities on field emission nanomechanical self-oscillators
A theoretical and experimental description of the threshold, amplitude, and
stability of a self-oscillating nanowire in a field emission configuration is
presented. Two thresholds for the onset of self-oscillation are identified, one
induced by fluctuations of the electromagnetic environment and a second
revealed by these fluctuations by measuring the probability density function of
the current. The ac and dc components of the current and the phase stability
are quantified. An ac to dc ratio above 100% and an Allan deviation of 1.3x10-5
at room temperature can be attained. Finally, it is shown that a simple
nonlinear model cannot describe the equilibrium effective potential in the
self-oscillating regime due to the high amplitude of oscillations
Fast determination of coarse grained cell anisotropy and size in epithelial tissue images using Fourier transform
Mechanical strain and stress play a major role in biological processes such
as wound healing or morphogenesis. To assess this role quantitatively, fixed or
live images of tissues are acquired at a cellular precision in large fields of
views. To exploit these data, large numbers of cells have to be analyzed to
extract cell shape anisotropy and cell size. Most frequently, this is performed
through detailed individual cell contour determination, using so-called
segmentation computer programs, complemented if necessary by manual detection
and error corrections. However, a coarse grained and faster technique can be
recommended in at least three situations. First, when detailed information on
individual cell contours is not required, for instance in studies which require
only coarse-grained average information on cell anisotropy. Second, as an
exploratory step to determine whether full segmentation can be potentially
useful. Third, when segmentation is too difficult, for instance due to poor
image quality or too large a cell number. We developed a user-friendly, Fourier
transform-based image analysis pipeline. It is fast (typically cells per
minute with a current laptop computer) and suitable for time, space or ensemble
averages. We validate it on one set of artificial images and on two sets of
fully segmented images, one from a Drosophila pupa and the other from a chicken
embryo; the pipeline results are robust. Perspectives include \textit{in vitro}
tissues, non-biological cellular patterns such as foams, and stacks.Comment: 13 pages; 9 figure
A migrating epithelial monolayer flows like a Maxwell viscoelastic liquid
We perform a bidimensional Stokes experiment in an active cellular material:
an autonomously migrating monolayer of Madin-Darby Canine Kidney (MDCK)
epithelial cells flows around a circular obstacle within a long and narrow
channel, involving an interplay between cell shape changes and neighbour
rearrangements. Based on image analysis of tissue flow and coarse-grained cell
anisotropy, we determine the tissue strain rate, cell deformation and
rearrangement rate fields, which are spatially heterogeneous. We find that the
cell deformation and rearrangement rate fields correlate strongly, which is
compatible with a Maxwell viscoelastic liquid behaviour (and not with a
Kelvin-Voigt viscoelastic solid behaviour). The value of the associated
relaxation time is measured as ~min, is observed to be
independent of obstacle size and division rate, and is increased by inhibiting
myosin activity. In this experiment, the monolayer behaves as a flowing
material with a Weissenberg number close to one which shows that both elastic
and viscous effects can have comparable contributions in the process of
collective cell migration.Comment: 17 pages, 15 figure
Self-oscillations in field emission nanowire mechanical resonators: a nanometric DC-AC conversion
We report the observation of self-oscillations in a bottom-up
nanoelectromechanical system (NEMS) during field emission driven by a constant
applied voltage. An electromechanical model is explored that explains the
phenomenon and that can be directly used to develop integrated devices. In this
first study we have already achieved ~50% DC/AC (direct to alternative current)
conversion. Electrical self-oscillations in NEMS open up a new path for the
development of high speed, autonomous nanoresonators, and signal generators and
show that field emission (FE) is a powerful tool for building new
nano-components
Simple modeling of self-oscillation in Nano-electro-mechanical systems
We present here a simple analytical model for self-oscillations in
nano-electro-mechanical systems. We show that a field emission self-oscillator
can be described by a lumped electrical circuit and that this approach is
generalizable to other electromechanical oscillator devices. The analytical
model is supported by dynamical simulations where the electrostatic parameters
are obtained by finite element computations.Comment: accepted in AP
Sensing and cooling of a nanomechanical resonator with an electron beam stimulated internal feedback and a capacitive force
A model for the cooling properties of a nanocantilever by a free electron
beam is presented for a capacitive interaction. The optimal parameters for
position sensing and cooling applications are estimated from previous
experimental conditions. In particular , we demonstrate that a purely
capacitive force and an electron beam stimulated internal feedback can lower
the temperature of a nanocantilever by several orders of magnitude in striking
contrast with the conventional electrostatic damping regime. We propose a step
by step protocol to extract the interdependent parameters of the experiments.
This work will aid future developments of ultra sensitive force sensors in
electron microscopes
High-level cefotaxime-resistant Proteus mirabilis strain isolated from a Tunisian intensive care unit ward: CTX-M-8 extended-spectrum β-lactamase coproduced with a plasmid mediated AmpC lactamase
The aimed of this study was to determine the implication of the biochemical and the molecular mechanism and to describe the properties of an extended-spectrum β-lactamase (ESBL) CTX-M-8 which was reported for the first time in Africa. A clinical isolate of Proteus mirabilis FS6449 was isolated from a patient hospitalized at an intensive care unit of the Military Hospital in Tunisia in 2009. Antimicrobial susceptibility was determined with the disk diffusion method according to Clinical and Laboratory Standards Institute (CLSI) guidelines and revealed that this strain was resistant to expanded-spectrum β-lactams. Analysis of P. mirabilis FS6449 by double-disk synergy test yielded a positive result suggesting the production of ESBLs. Sonicate of the isolate hydrolysed cefotaxime and benzylpenicillin. Isoelectric focusing exhibited four β-lactamase bands of isoelectric points (pIs) 5.6, 6, 6.5 and over 7.6. Polymerase chain reaction (PCR) and sequencing experiments revealed the presence of four β-lactamase genes encoding TEM-2, CTX-M-8, TEM-24, and an AmpC enzyme. Among them, the genes encoding TEM-24 and an AmpC enzyme were transferred to the recipient by conjugation experiments.Keywords: Resistance, β-lactamase, Proteus mirabilisAfrican Journal of Biotechnology Vol. 12(21), pp. 3278-328
‘Pole Test’ Measurements in Critical Leg Ischaemia
AbstractBackgroundFor the quantification of critical limb ischaemia (CLI) most vascular surgery units use sphygmo-manometric and transcutaneous oxygen pressure (TcPO2) measurements. However, measurements obtained by cuff-manometry can be overestimated especially in diabetic patients because of medial calcification that makes leg arteries less compressible. TcPO2 measurements present a considerable overlap in the values obtained for patients with different degrees of ischaemia and its reproducibility has been questioned. Arterial wall stiffness has less influence on the pole test, based on hydrostatic pressure derived by leg elevation, and this test seems to provide a reliable index of CLI.ObjectiveThe objective of this study was to evaluate the pole pressure test for detection of critical lower limb ischaemia, correlating results with cuff-manometry and transcutaneous oxygen pressure.DesignUniversity hospital-prospective study.Materials and methodsSeventy-four patients (83 legs) with rest pain or gangrene were evaluated by four methods: pole test, cuff-manometry, TcPO2 and arteriography. CLI was present if the following criteria were met: (a) important arteriographic lesions+rest pain with an ankle systolic pressure (ASP) ≤40mmHg and/or a TcPO2 ≤30mmHg, or (b) important arteriographic lesions+tissue loss with an ASP ≤60mmHg and/or a TcPO2 ≤40mmHg. Fifty-seven lower limbs met the criteria for CLI.ResultsMeasurements obtained by cuff-manometry were significantly higher to those obtained by pole test (mean pressure difference: 40mmHg, p<0.001). The difference between the two methods remained statistically significant for both diabetics (50.73, p<0.001) and non-diabetics (31.46, p<0.001). Mean TcPO2 value was 15.51mmHg and there was no important difference between patients with and without diabetes. Overall, there was a correlation between sphygmomanometry and pole test (r=0.481). The correlation persisted for patients without diabetes (r=0.581), but was not evident in patients with diabetes. Correlation between pole test and TcPO2 was observed only for patients with diabetes (r=0.444). There was no correlation between cuff-manometry and TcPO2. The pole test offered an accuracy of 88% for the detection of CLI. The sensitivity of this test was 95% and the specificity 73%
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