150 research outputs found
NDT of Specimen of Complex Geometry Using Ultrasonic Adaptive Techniques - The F.A.U.S.T. System
Phased array techniques, providing an electronic control of the beam, are widely used in ultrasonic imaging. Such techniques, making use of array transducers with delayed transmission pulse on each element, allow to steer and focus the beam, enabling various testing configurations and imaging procedures : sector scanning and tomography, tracking echoes, depth focusing. In nuclear industry, various configurations of geometry and materials are encountered, which require many different testing configurations. The CEA (French Atomic Energy Commission) has developed an adaptive system based on phased array techniques dynamically controlled by a multi-channel acquisition system: theF.A.U.S.T. (Focusing Adaptive UltraSonic Tomography) system. This system aims at improving the performances of nondestructive testing, particularly for what concerns the adaptability to different control configurations and defect characterization. Previous works have described this system, its performances for beam forming and also its specific abilities for defect characterization using beam steering or spatial amplitude distribution at reception [1, 2]
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Ca2+ waves coordinate purinergic receptor–evoked integrin activation and polarization
The integrin GPIIb/IIIa is highly abundant on the surface of platelets and can be activated by intracellular Ca2+ signaling in an “inside-out” manner to bind to the adhesive ligand fibrinogen. Bye et al. imaged intracellular Ca2+ signaling and fibrinogen binding events in primary rat megakaryocytes activated through the ADP-stimulated receptors P2Y1 and P2Y12. The authors found that signaling by both receptors was required for full integrin activation, which depended on P2Y1-stimulated Ca2+ signaling and P2Y12-stimulated activation of the kinase PI3K. In addition, fibrinogen binding became polarized in these cells in a manner dependent on the direction of ADP-stimulated Ca2+ waves
Yield stress and elastic modulus of suspensions of noncolloidal particles in yield stress fluids
We study experimentally the behavior of isotropic suspensions of noncolloidal
particles in yield stress fluids. This problem has been poorly studied in the
literature, and only on specific materials. In this paper, we manage to develop
procedures and materials that allow us to focus on the purely mechanical
contribution of the particles to the yield stress fluid behavior, independently
of the physicochemical properties of the materials. This allows us to relate
the macroscopic properties of these suspensions to the mechanical properties of
the yield stress fluid and the particle volume fraction, and to provide results
applicable to any noncolloidal particle in any yield stress fluid. We find that
the elastic modulus-concentration relationship follows a Krieger-Dougherty law,
and we show that the yield stress-concentration relationship is related to the
elastic modulus-concentration relationship through a very simple law, in
agreement with results from a micromechanical analysis
Broadband Fields Radiated in a Solid by Water-Coupled Transducers: A Comparison of Approximate Models, Numerical Approaches and Experiments
In number of configurations, ultrasonic tests in the French nuclear industry are made using water-coupled focused transducers. To study the influence of the various parameters involved in transducer/piece configurations, model-based predictions of the field radiated by transducers are very useful. A model (called Champ-Sons) has been developed at the French Atomic Energy Commission (CEA) to calculate the field radiated by focused or unfocused transducer through liquid/solid interface at normal or oblique incidence [1]. It can deal with radiating surface of complex (3-D) shape (spherical focusing, Fermat’s surfaces, multiple-elements [2] etc.). The calculation is done directly in the time domain for broadband sources and in the frequency domain for narrowband sources. In its present form Champ-Sons deals with either plane or cylindrical interfaces between a fluid and an isotropic solid. It is implemented in a user-friendly software developed at the CEA called CIVA [3] for NDT data processing (eddy-current, ultrasonics, neutrongraphy, radiography). Since non-canonical configurations are considered and pure numerical schemes are too computer intensive, the model treats the refraction at the fluid/solid interface in an approximate way. It has been validated experimentally [1]
Application of Ultrasonic Beam Modeling to Phased Array Testing of Complex Geometry Components
For several years, the French Atomic Energy Commission (CEA) has developed phased array techniques to improve defect characterization and adaptability to various inspection configurations [1]. Such techniques allow to steer and focus the ultrasonic beam radiated by a transducer split into a set of individually addressed elements, using amplitude and delay laws. For most conventional systems, those delay laws are extracted from geometric ultrasonic paths between each element of the array and a geometric focusing applied to perform beam-forming abilities [2] for simple geometry components (e.g. beam- steering over a plane specimen), whereas experimental delays can be supplied to the array at transmission and reception to optimally adapt the ultrasonic beam to the detected defect, in a so-called self-focusing process [3,4]. This method, relevant for complex material or geometry leading to phase distortion or complex paths that cannot be predicted by simple geometrical calculations, obviously requires the existence of a reflector and the ultrasonic beam radiated by the experimental delay law cannot be known. Therefore this technique is used to improve defect detection (optimal sensibility) rather than defect characterization. To assess complex geometry components inspection with an adaptive system, the CEA has developed new modeling devoted to predict the ultrasonic field radiated by arbitrary transducers through complex geometry and material specimen [5]. A model allows to compute optimized delay laws to preserve the characteristics of the beam through the complex surface, as well as the actual radiated field using those delays. This paper presents two applications of this model : the inspection of a misaligned specimen, and the inspection of an irregular surface
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Evidence for shear-mediated Ca2+ entry through mechanosensitive cation channels in human platelets and a megakaryocytic cell line
The role of mechanosensitive (MS) Ca2+-permeable ion channels in platelets is unclear, despite the importance of shear stress in platelet function. We sought to investigate the expression and functional relevance of MS channels in human platelets. The effect of shear stress on Ca2+ entry in human platelets and Meg-01 megakaryocytic cells loaded with Fluo-3 was examined by confocal microscopy. Cells were attached to microscope slides within flow chambers that allowed application of physiological and pathological shear stress. Arterial shear (1002.6s-1) induced a sustained increase in intracellular calcium ([Ca2+]i) in Meg-01 cells and enhanced the frequency of repetitive Ca2+ transients by 80% in platelets. These Ca2+ increases were abrogated by the MS channel inhibitor GsMTx-4 or by chelation of extracellular Ca2+. Thrombus formation was studied on collagen-coated surfaces using 3,3'-dihexyloxacarbocyanine iodide (DiOC6)-stained platelets. In addition, [Ca2+]i and functional responses of washed platelet suspensions were studied with Fura-2 and light transmission aggregometry, respectively. Thrombus size was reduced 50% by GsMTx-4 independently of P2X1 receptors. In contrast, GsMTx-4 had no effect on collagen-induced aggregation and on Ca2+ influx via TRPC6 or Orai1 channels, and caused only a minor inhibition of P2X1-dependent Ca2+ entry. The Piezo1 agonist, Yoda1, potentiated shear-dependent platelet Ca2+ transients by 170%. Piezo1 mRNA transcripts and protein were detected in both platelets and Meg-01 cells using qRT-PCR and Western blotting. We conclude that platelets and Meg-01 cells express the MS cation channel Piezo1, which may contribute to Ca2+ entry and thrombus formation under arterial shear stress
Broadband Fields Radiated in a Solid by Water-Coupled Transducers: A Comparison of Approximate Models, Numerical Approaches and Experiments
In number of configurations, ultrasonic tests in the French nuclear industry are made using water-coupled focused transducers. To study the influence of the various parameters involved in transducer / piece configurations, model-based predictions of the field radiated by transducers are very useful. A model (called Champ-Sons) has been developed at th
A Characterization of Scale Invariant Responses in Enzymatic Networks
An ubiquitous property of biological sensory systems is adaptation: a step
increase in stimulus triggers an initial change in a biochemical or
physiological response, followed by a more gradual relaxation toward a basal,
pre-stimulus level. Adaptation helps maintain essential variables within
acceptable bounds and allows organisms to readjust themselves to an optimum and
non-saturating sensitivity range when faced with a prolonged change in their
environment. Recently, it was shown theoretically and experimentally that many
adapting systems, both at the organism and single-cell level, enjoy a
remarkable additional feature: scale invariance, meaning that the initial,
transient behavior remains (approximately) the same even when the background
signal level is scaled. In this work, we set out to investigate under what
conditions a broadly used model of biochemical enzymatic networks will exhibit
scale-invariant behavior. An exhaustive computational study led us to discover
a new property of surprising simplicity and generality, uniform linearizations
with fast output (ULFO), whose validity we show is both necessary and
sufficient for scale invariance of enzymatic networks. Based on this study, we
go on to develop a mathematical explanation of how ULFO results in scale
invariance. Our work provides a surprisingly consistent, simple, and general
framework for understanding this phenomenon, and results in concrete
experimental predictions
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