In-situ observation and numerical modeling of contact creep and recovery on oriented semi-crystalline polymer surfaces

Semi-crystalline polymers are commonly used in industrial sectors where surfaces undergo many damages like scratches. In order to avoid as much as possible these surface damages, it is necessary to develop mechanical models able to predict such contact mechanical responses. The aim of this work is to study the effect of orientation, through stretching in the solid state, on the contact mechanics for semi-crystalline polymer surfaces. To that purpose, bulk polymers are uniaxially oriented at various stretching ratio, using hot two-mill rolling process, and are characterized by X-ray analysis. Contact creep and recovery tests were performed on these samples thanks to a home-made experimental device [1]. This apparatus allows an in-situ observation of the contact area during creep step on non-transparent materials. Then, the recovery of the residual imprint is quickly (few seconds after the unloading of the indenter) recorded by non-contact methods. This experiment gives access to the true contact geometry and provides valuable information about the early stage of viscoelastic recovery. Hence, the influence of surface orientation on the contact response was investigated on a model semi-crystalline polymer: HDPE to identify structural parameters that govern viscoelastic/viscoplastic behavior of the surface. The effects of strain levels and creep duration on viscoelastic behavior were also studied. As regards the latter parameter, it was shown that creep duration has no major effect on creep step. Nonetheless, for a same imposed strain, residual depths increase with longer creep duration, inducing, in some case, a permanent deformation of the surface. Regarding the effect of orientation, if the contact creep of the non-oriented surface with a spherical indentor displays a circular contact area, the same experiment performed with oriented semi-crystalline polymers shows an elliptical contact area. Two assumptions can be made to explain this contact shape: (a) the polymeric surface displays anisotropic mechanical properties or (b) the sample is isotropic but the contact takes place on a curved surface due to the rolling stage. In order to better understand the in-situ observations of the contact shape during the contact creep and recovery of the residual imprint, numerical modeling of the surface response was performed using MSC MARC® software. The aim is to reproduce, as closely as possible, the contact creep and recovery experiment. To that purpose an axisymmetric numerical model was created, considering the indentation sphere to be infinitely rigid. This model gives access to the true contact radius during the creep phase. First results seem to indicate that the elliptical shape of the contact area is rather govern by the anisotropic mechanical properties of the semi-crystalline polymer surfaces.   Acknowledgement: This research forms part of the research program of the Dutch Polymer Institute (DPI) project #783. The authors would like to acknowledge the funding support ""MARMA"" from Carnot MICA and HOLO3 (Alsace region) company for developing the instrument.   Reference: [1] : T. Chatel, C. Gauthier, H. Pelletier, V. Le Houérou, D. Favier and R. Schirrer. Journal of Physics D: Applied Physics, 2011, 44, 375-40

Different populations and sources of human mesenchymal stem cells (MSC): A comparison of adult and neonatal tissue-derived MSC

The mesenchymal stroma harbors an important population of cells that possess stem cell-like characteristics including self renewal and differentiation capacities and can be derived from a variety of different sources. These multipotent mesenchymal stem cells (MSC) can be found in nearly all tissues and are mostly located in perivascular niches. MSC have migratory abilities and can secrete protective factors and act as a primary matrix for tissue regeneration during inflammation, tissue injuries and certain cancers

Adaptations métaboliques et hormonales chez le rat anti-obèse Lou/C (influences du régime alimentaire et de l'activité physique)

Le meilleur moyen de lutter contre l'obésité est d'associer une diminution des apports caloriques à une augmentation de la dépense énergétique. Ces conditions sont réunies simultanément chez le rat Lou/C, une souche encore mal caractérisée. L'objectif de ce travail était de préciser certaines caractéristiques du rat Lou/C dans les conditions d'élevage standard et d'étudier ses réponses métaboliques et hormonales lors de variations de la dépense énergétique (exercice aigu) ou lors de modifications des apports alimentaires (régime enrichi en lipides). Les divers résultats obtenus indiquent qu'en conditions basales, le rat Lou/C, comparé au rat Wistar, présente les caractéristiques d'un rat résistant à l'obésité. Le Lou/C présente une glycémie de base réduite, ainsi qu'une accentuation du métabolisme des lipides qui se traduit par une plus faible accumulation de gras corporel. Ces caractéristiques ne sont cependant pas essentiellement dues à la réduction spontanée de la prise calorique de cette souche. Dans des conditions d'exercice, les rats Lou/C sont capables de maintenir leur glycémie stable tout en préservant leurs réserves en glycogène hépatique. Ces résultats suggèrent la mise en jeu d'autres voies métaboliques (néoglucogenèse et/ou utilisation des lipides). Soumis à une diète hyperlipidique, les rats Lou/C présentent, une accumulation de lipides réduite par rapport aux rats Wistar. Cependant, ils présentent une augmentation des AGL plasmatiques, une accumulation des triglycérides hépatiques et une détérioration de la tolérance au glucose indiquant qu'ils ne sont pas mieux protégés contre les effets délétères de cette diète. Les différents résultats obtenus pourront permettre de mieux appréhender les mécanismes impliqués dans la régulation pondérale. De plus, ces données suggèrent que les individus résistant à l'obésité ne sont pas forcément mieux protégés que les individus prompts à devenir obèses, contre les effets néfastes des régimes hyperlipidiques.LYON1-BU.Sciences (692662101) / SudocSudocFranceF

Fast Orthogonal Decomposition of Volterra Cubic Kernels using Oblique Unfolding

International audienceDiscrete-time Volterra modeling is a central topic in many application areas and a large class of nonlinear systems can be modeled using high-order Volterra series. The problem with Volterra series is that the number of parameters grows very rapidly with the order of the nonlinearity and the memory in the system. In order to efficiently implement this model, kernel eigen-decomposition can be used in the context of a Parallel-Cascade realization of a Volterra kernel. So, using the multilinear SVD (HOSVD) for decomposing high-order Volterra kernels seems natural. In this paper, we propose to drastically reduce the computational cost of the HOSVD by (1) considering the symmetrized Volterra kernel and (2) exploiting the column-redundancy of the associated mode by using an oblique unfolding of the Volterra kernel. Keeping in mind that the complexity of the full HOSVD for a cubic (I×I×I) unstructured Volterra kernel needs 12I4 flops, our solution allows reducing the complexity to 2I4 flops, which leads to a gain equal to six for a sufficiently large size I

Effets du jeûne intermittent sur l'utilisation des substrats énergétiques au cours de l'exercice chez le rat

International audienc

Dopaminergic metabolism in carotid bodies and high-altitude acclimatization in female rats

We tested the hypothesis that ovarian steroids stimulate breathing through a dopaminergic mechanism in the carotid bodies. In ovariectomized female rats raised at sea level, domperidone, a peripheral D 2-receptor antagonist, increased ventilation in normoxia (minute ventilation = +55%) and acute hypoxia (+32%). This effect disappeared after 10 daily injections of ovarian steroids (progesterone + estradiol). At high altitude (3,600 m, Bolivian Institute for High-Altitude Biology-IBBA, La Paz, Bolivia), neutered females had higher carotid body tyrosine hydroxylase activity (the rate-limiting enzyme for catecholamine synthesis: +129%) and dopamine utilization (+150%), lower minute ventilation (-30%) and hypoxic ventilatory response (-57%), and higher hematocrit (+18%) and Hb concentration (+21%) than intact female rats. Consistent signs of arterial pulmonary hypertension (right ventricular hypertrophy) also appeared in ovariectomized females. None of these parameters was affected by gonadectomy in males. Our results show that ovarian steroids stimulate breathing by lowering a peripheral dopaminergic inhibitory drive. This process may partially explain the deacclimatization of post-menopausal women at high altitude.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Molecular system bioenergetics: regulation of substrate supply in response to heart energy demands

This review re-evaluates regulatory aspects of substrate supply in heart. In aerobic heart, the preferred substrates are always free fatty acids, and workload-induced increase in their oxidation is observed at unchanged global levels of ATP, phosphocreatine and AMP. Here, we evaluate the mechanisms of regulation of substrate supply for mitochondrial respiration in muscle cells, and show that a system approach is useful also for revealing mechanisms of feedback signalling within the network of substrate oxidation and particularly for explaining the role of malonyl-CoA in regulation of fatty acid oxidation in cardiac muscle. This approach shows that a key regulator of fatty acid oxidation is the energy demand. Alterations in malonyl-CoA would not be the reason for, but rather the consequence of, the increased fatty acid oxidation at elevated workloads, when the level of acetyl-CoA decreases due to shifts in the kinetics of the Krebs cycle. This would make malonyl-CoA a feedback regulator that allows acyl-CoA entry into mitochondrial matrix space only when it is needed. Regulation of malonyl-CoA levels by AMPK does not seem to work as a master on–off switch, but rather as a modulator of fatty acid import

Prevention of unloading-induced atrophy by vitamin E supplementation: Links between oxidative stress and soleus muscle proteolysis?

Exposure to reduced activity induces skeletal muscle atrophy. Oxidative stress might contribute to muscle wasting via proteolysis activation. This study aimed to test two hypotheses in rats. First, supplementation of the antioxidant vitamin E, prior and during the phase of unloading, would partly counteract unloading-induced soleus muscle atrophy. Secondly, vitamin E supplementation would decrease the rate of muscle proteolysis by reducing expression of calpains, caspases-3, -9, and -12, and E3 ubiquitin ligases (MuRF1 and MAFbx). Soleus muscle atrophy (-49%) induced by 14 days of hindlimb unloading was reduced to only 32% under vitamin E. Vitamin E partly prevented the decrease in type I and IIa fiber size. Supplementation increased HSP72 content and suppressed the rise in muscle level of thiobarbituric acid-reactive substance caused by unloading but failed to modify the lower ratio of reduced vs oxidized glutathione, the higher uncoupling proteins mRNA, and the antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase) observed after unloading. Vitamin E treatment abolished the large upregulation of caspases-9 and -12 and MuRF1 transcripts in unloaded muscle and greatly decreased the upregulation of mu-calpain, caspase-3, and MAFbx mRNA. In conclusion, the protective effect of vitamin E might be due to modulation of muscle proteolysis-related genes rather than to its antioxidant function