Sur le volume élémentaire représentatif à considérer pour la modélisation de l'os compact

L'os cortical est essentiellement biphasique (pores+matrice). Pour le calcul de propriétés effectives, il faut choisir un volume représentatif (VER). Ce choix est délicat car 1) le tissu est hétérogène à plusieurs échelles (i.e. zones plus ou moins poreuses en moyenne) ; 2) la couche corticale peut être très fine. Nous montrerons que le choix d’un VER de taille millimétrique permet de calculer de manière satisfaisante des propriétés effectives, dans un sens que l'on précisera

Compréhension et modélisation du régime mixte : synthèse des avancées et perspectives d'applications industrielles : Frottement et lubrification en mise en forme = Understanding and modelling the mixed lubrication regime: summary of achievements and perspectives of industrial applications

Free access articleInternational audienceL'ensemble des travaux décrits précédemment, et résumés ici, a d'ores et déjà fait l'objet de mises en application diverses, qui sont décrites dans cette courte conclusion: explication d'observations sur sites de production, modèles de connaissance, nouvelles idées de formulation de lubrifiants, qui ne demandent qu'à être testées. Bien sûr, l'ensemble des besoins de connaissances et de nouveaux outils n'a pas été satisfait, et quelques pistes de travaux futurs sont dégagées = The works described previously, and summarized here, have already been applied under various forms, to be described hereafter: explanation of industrial observations, knowledge models, new ideas for lubricant formulation, which now have to be tested. Of course, not all the needs of industry have been fulfilled, so that perspectives for further research are highlighted

Nonlinear ultrasound monitoring of fatigue microdamage accumulation in cortical bone

Accumulation of bone micro-damage is suspected to lead to severe impairment of mechanical properties with an increase in skeletal fragility and fracture risk. The objective of the study was to evaluate the potential of Nonlinear Resonant Ultrasound Spectroscopy (NRUS) for measuring micro-damage accumulation in cortical bone using four-point bending cycling fatigue. Sixteen human cortical bone specimens were machined as parallelepiped beams. Damage progression was controlled by measuring the linear elastic beam theory modulus (E LEBT ), known to reflect microdamage accumulation. Before and between each damage step, the nonlinear ultrasonic elastic coefficient was measured by NRUS. At the end of each cycling fatigue, a subset of bone samples was measured by μCT at the European Synchrotron Radiation Facility. Results showing a progressive increase of nonlinear ultrasonic elastic coefficient along fatigue cycling suggest that NRUS measurements are sensitive to micro-damage accumulation. The results mentioned above were validated using synchrotron radiation μCT. The variation of elastic nonlinearity was found to be significantly correlated to the variation of number density of small microcracks which almost doubled in damaged regionsThis research was supported by the Agence Nationale pour la Recherche (ANR), France (Grant BONUS_07BLAN0197

Elasticité et porosité de l'os cortical humain : modèles et expériences

A l'échelle millimétrique, l'os cortical est vu comme une matrice minéralisée traversée de pores (canaux de Havers). Nous avons mesuré la porosité et l'élasticité de 21 échantillons (10 donneurs) et nous montrons que la rigidité de la matrice a une influence très faible sur l'élasticité apparente. Ces données permettent pour la première fois une analyse critique des modèles de changement d'échelle (homogénéisation asymptotique, Mori-Tanaka, bornes, etc.). Nous établissons que la densité apparente à l'échelle millimétrique peut être reliée au tenseur d'élasticité anisotrope grâce aux modèles

First report of the epidemiological surveillance network for equine infectious and contagious abortions

Towards the end of 2008, the RESPE epidemiological surveillance network created a sub-network to monitor equine infectious diseases responsible for abortions and neonatal losses in France, with medical, economic and/or zoonotic relevance. These diseases include herpes viral diseases [EHV1, EHV4], equine viral arteritis (EVA) and leptospirosis, a zoonosis considered as an emerging disease in horses. The herpes virus EHV-1 remains a major cause of infectious abortion in mares (5% of all abortions). Cases remain sporadic, and although they occur in groups which can be quite large, vaccination may help limit the spread of infection. On the other hand, abortions due to HVE-4 are rare. Leptospira are suspected in 7% and probably involved in 2% of cases. No abortion due to equine viral arteritis has been reported over the past two years. To meet the set objectives with greater efficiency, the number of case reports must be increased by broadening the geographical and zootechnical limits, and by monitoring systematically all positive and/or suspected foci through contact with the person reporting the cases and/or the breeder. The accumulation of such data is the only way we will improve our understanding of the circulation of pathogens in farms, and thus be able to implement precautionary measures and evaluate the impact of vaccination.Le réseau d'épidémiosurveillance en pathologie équine (RESPE) a mis en place, à la fin de 2008, un sous-réseau consacré à la surveillance des causes d'avortement et de perte néonatale ayant une importance sanitaire, économique et/ou zoonosique. C'est à dire les herpès viroses [HVE1, HVE4], l'artérite virale [VAE] et la leptospirose, zoonose considérée comme maladie émergente chez le Cheval. Durant ces deux premières années, les objectifs ont été globalement atteints. La surveillance des trois catégories étiologiques d'avortement a permis la diffusion d'alertes lors de cas positifs. L'avortement herpétique (HVE1) reste la première cause virale d'avortement infectieux de la Jument (5% de l'ensemble des avortements). Ces cas d'avortements herpétiques sont restés sporadiques ; bien qu'étant observés dans des effectifs parfois importants, la vaccination limite la diffusion de l'infection. Le virus HVE 4 est abortif, mais l'avortement dû à celui-ci est rare. Les leptospires sont suspectés dans 7 % et probablement impliqués dans 2% des cas. Aucun avortement dû au virus de l'artérite n'a été constaté au cours de ces deux années. Cependant, afin de mieux répondre aux objectifs fixés, il est nécessaire d'augmenter le nombre de déclarations. Il est indispensable de mettre en place un suivi systématique des foyers positifs et/ou suspects afin d'augmenter les informations épidémiologiques. Seule l'accumulation de ces données permettra de comprendre la circulation des agents pathogènes dans les élevages et autorisera la mise en place de mesures préventives, ainsi que l'évaluation de la vaccination

Characterization of a bone mimicking phantom by means of circumferential guided waves dispersion curves

International audiencePrevious studies have evidenced the circumferential propagation of waves guided by the roughly cylindrical cortical shell of the femoral neck. We hypothesize that measuring the phase velocities of such waves could yield estimates of the cortical thickness and material properties, which could improve fracture risk prediction. The objective of this study is to test the ability of the DORT method, processing based on time reversal principle, to measure circumferential guided waves in a bone-mimicking tube. The tube has the typical dimensions of the mid-femoral neck. A focused array specifically designed for this study was used for emission and reception. To serve as a noise-free reference, the experiment was also simulated based on an analytical formulation. The DORT method was applied to experimentally recorded and simulated data to retrieve the guided waves dispersion curves. Five branches of modes were obtained. They were identified by comparison with the theoretical dispersion curves of a semi-immersed plate. Experimental branches can be used in an inverse scheme to obtain estimations of the shell properties. This study shows that measuring circumferential guided waves is feasible in a structure with dimensions and material properties (elasticity, attenuation) close to those of the cortical compartment of the femoral neck

Quantitative ultrasound yields biomarkers of bone mechanical competence

The last two decades have witnessed changes in our ability to assess multi-scale mechanical properties of the human skeleton. Quantitative ultrasound (QUS) has played a critical role in this development, providing gains in bone mechanical status assessment, spanning the scales of bone organization ranging from elementary bone structural units to the mm-scale level. QUS technologies can help in characterizing the anisotropic stiffness ex vivo and are prone to provide answers to some questions that remain open regarding the determinants of bone elastic properties. On the other side, in vivo QUS has potential to assess important bone quality factors such as material properties or structure, and to enhance fracture risk prediction

NRT2.1 phosphorylation prevents root high affinity nitrate uptake activity in Arabidopsis thaliana

In Arabidopsis thaliana, NRT2.1 codes for a main component of the root nitrate high-affinity transport system. Previous studies revealed that post-translational regulation of NRT2.1 plays an important role in the control of root nitrate uptake and that one mechanism could correspond to NRT2.1 C-terminus processing. To further investigate this hypothesis, we produced transgenic plants with truncated forms of NRT2.1. It revealed an essential sequence for NRT2.1 activity, located between the residues 494-513. Using a phospho-proteomic approach, we found that this sequence contains one phosphorylation site, at serine 501, which can inactivate NRT2.1 function when mimicking the constitutive phosphorylation of this residue in transgenic plants. This phenotype could neither be explained by changes in abundance of NRT2.1 and NAR2.1, a partner protein of NRT2.1, nor by a lack of interaction between these two proteins. Finally, the relative level of serine 501 phosphorylation was found to be modulated by nitrate in wildtype plants. Altogether, these observations allowed us to propose a model for a new and essential mechanism for the regulation of NRT2.1 activity

Single spacecraft nulling interferometer for exoplanets: preliminary concept

editorial reviewedOne of the most ambitious goals of modern astronomy is to uncover signs of extraterrestrial biological activity, primarily achieved through spectroscopic analysis of light emitted by exoplanets to identify specific atmospheric molecules. Most exoplanets are indirectly identified through techniques like transit or Doppler shift of the host star's flux. Long-term surveys have yielded statistical insights into the occurrence rates of different planet types based on factors such as radius/mass, orbital period, and the spectral type of the host star. Initial estimates of terrestrial planets within the habitable zone have also emerged. However, the difficulty of detecting light from these exoplanets leaves much unknown about their nature, formation, and evolution. As the number of rocky exoplanets around nearby stars rises, questions about their atmospheric composition, evolutionary trajectory, and habitability increase. Direct measurement of an exoplanet's atmospheric composition through its spectral signature in the infrared can provide answers. Measuring the infrared spectrum of these planets poses significant challenges due to the star/planet contrast and very small angular separation from their host stars. Previous research showed that space-based telescopes are mandatory, and unless large primary mirrors (>30m in diameter) can be sent into space, interferometric techniques become essential. Combining light from distant telescopes with interferometric techniques allows access to information at minimal angular separation, operating within the diffraction limit of individual telescopes. Successful demonstrations of on-ground nulling interferometry open a new era for such space-based missions. They are vital to sidestep and tackle these scientific questions. We recently initiated a new study with the European Space Agency to explore the design parameters and the performances related to an interferometric concept based on a single spacecraft and sparse multiple sub-apertures. Launch constraints are linked to the use of an Ariane 6 launch vehicle. Our parametric study covers a range of 1-4 m for the diameter of the telescope and a 10-60 m baseline. The most promising concept working in the infrared range (3-20μm) will be highlighted. This study is conducted by TUDelft in cooperation with KULeuven, CSL/ULiège, and Amos with the support of the European Space Agency