Yield stress in magnetorheological suspensions near the limit of maximum-packing fraction

International audienceThis work deals with the magnetic field-induced static yield stress of magnetorheological (MR) suspensions with concentration near the limit of maximum-packing fraction. With this aim, homogeneous suspensions of iron microparticles with 50 vol.% concentration were prepared, and their yield stress measured as a function of the applied magnetic field. In view of the failure of existing models to predict, on the basis of realistic hypotheses, the values of the yield stress of highly concentrated MR suspensions, we developed a new model. Our model considers that field application induces body-centered tetragonal (BCT) structures. Upon shearing, these structures deform in such a way that interparticle gaps appear between neighboring particles of the same chain, whereas the approach of particles of parallel chains ensures the mechanical stability of the whole multi-chain structure. Based on this hypothesis, and using finite element method simulations of interparticle magnetic interactions, our model is able to quantitatively predict the yield stress of highly concentrated MR suspensions. Furthermore, estimations show that the main contribution to the field-dependent part of the yield stress comes from the change in the permeability of the structures as interparticle gaps are enlarged by the shear

Stick-slip instabilities in the shear flow of magnetorheological suspensions

International audienceThis work is devoted to the stick-slip instabilities that appear in the shear flow of highly concentrated suspensions of magnetic microparticles. The effect of the applied magnetic field strength was analyzed in details. With this aim, homogeneous suspensions of iron microparticles with concentration near the limit of maximum-packing fraction were prepared, and shear-flow measurements were performed in a controlled-rate mode using a rheometer provided with a rough parallel-plate geometry. For each given value of the shear rate, the time evolution of the shear stress was monitored for at least 20 min. Saw-tooth-like stress oscillations, typical of stick-slip instabilities, were obtained at low enough shear rate values. The measurements were restricted to small enough oscillations, at which the rheometer was still able to maintain the shear rate constant. From the microscopic viewpoint, these stick-slip instabilities principally appear due to the periodic failure and healing of the field-induced particle structures, as inferred from experimental observations. This hypothesis is corroborated by a theoretical model developed on the basis of the balance of the magnetic and hydrodynamic torques over the particle structures, allows us to predict the correct order of magnitude of the main parameters of the stick-slip instabilities, including the amplitude and period of the stress oscillations

N-like rheograms of concentrated suspensions of magnetic particles

International audienceWe investigate the rheograms of concentrated suspensions of magnetic particles obtained under imposed shear rate in parallel plate geometry. We show that under magnetic field application the usual trend of the rheogram, i.e., increasing shear stress for the whole range of shear rates, is altered by the appearance of a region in which the shear stress decreases as the shear rate is increased. The existence of this region gives to the rheograms an N-like shape. The two initial regions (pre-yield regime) of these N-like rheograms present unstable flow, characterized by the oscillation of the shear stress with time for each imposed value of shear rate. We also show that rheograms obtained at different sample thicknesses approximately overlap in the developed flow regime, whereas there is a tendency of the shear stress to increase as the thickness is decreased in the pre-yield regime. This tendency is likely due to the strengthening of preexisting particle structures by compression as the gap thickness is decreased. Finally, we analyze the effect of the applied magnetic field strength, H, and demonstrate that the rheograms 2 scale with H 1.5 to a single master curve, for the range of applied magnetic fields under study

Biocompatible magnetic core–shell nanocomposites for engineered magnetic tissues

International audienceThe inclusion of magnetic nanoparticles into biopolymer matrixes enables the preparation of magnetic field-responsive engineered tissues. Here we describe a synthetic route to prepare biocompatible core– shell nanostructures consisting of a polymeric core and a magnetic shell, which are used for this purpose. We show that using a core–shell architecture is doubly advantageous. First, gravitational settling for core– shell nanocomposites is slower because of the reduction of the composite average density connected to the light polymer core. Second, the magnetic response of core–shell nanocomposites can be tuned by changing the thickness of the magnetic layer. The incorporation of the composites into biopolymer hydrogels containing cells results in magnetic field-responsive engineered tissues whose mechanical properties can be controlled by external magnetic forces. Indeed, we obtain a significant increase of the viscoelastic moduli of the engineered tissues when exposed to an external magnetic field. Because the composites are functionalized with polyethylene glycol, the prepared bio-artificial tissue-like constructs also display excellent ex vivo cell viability and proliferation. When implanted in vivo, the engineered tissues show good biocompatibility and outstanding interaction with the host tissue. Actually, they only cause a localized transitory inflammatory reaction at the implantation site, without any effect on other organs. Altogether, our results suggest that the inclusion of magnetic core–shell nanocomposites into biomater-ials would enable tissue engineering of artificial substitutes whose mechanical properties could be tuned to match those of the potential target tissue. In a wider perspective, the good biocompatibility and magnetic behavior of the composites could be beneficial for many other applications

One-pot synthesis of oxidation-sensitive supramolecular gels and vesicles

Polypeptide-based nanoparticles offer unique advantages from a nanomedicine perspective such as biocompatibility, biodegradability and stimuli-responsive properties to (patho)physiological conditions. Conventionally, self-assembled polypeptide nanostructures are prepared by first synthesizing their constituent amphiphilic polypeptides followed by post-polymerization self-assembly. Herein, we describe the one-pot synthesis of oxidation-sensitive supramolecular micelles and vesicles. This was achieved by polymerization-induced self-assembly (PISA) of the N-carboxyanhydride (NCA) precursor of methionine using polyethylene oxide as stabilizing and hydrophilic block in dimethyl sulfoxide (DMSO). By adjusting the hydrophobic block length and concentration we obtained a range of morphologies from spherical to worm-like micelles, to vesicles. Remarkably, the secondary structure of polypeptides greatly influenced the final morphology of the assemblies. Surprisingly, worm-like micellar morphologies were obtained for a wide range of methionine block lengths and solid contents, with spherical micelles restricted to very short hydrophobic lengths. Worm-like micelles further assembled into oxidation-sensitive, self-standing gels in the reaction pot. Both vesicles and worm-like micelles obtained using this method demonstrated to degrade under controlled oxidant conditions which would expand their biomedical applications such as in sustained drug release or as cellular scaffolds in tissue engineering

Effect of functionalized PHEMA micro- and nano-particles on the viscoelastic properties of fibrin-agarose biomaterials.

Two types of PHEMA-based particles, exhibiting either carboxyl or tertiary ammine functional groups, were incorporated to fibrin-agarose (FA) hydrogels, and the effect of the addition of these synthetic particles on the viscoelastic and microstructural properties of the biomaterials was evaluated. Experimental results indicated that the incorporation of both types of polymeric particles to FA scaffolds was able to improve the biomechanical properties of the biomaterials under steady state and oscillatory shear stresses, resulting in scaffolds characterized by higher values of the storage, loss, and shear moduli. In addition, the microstructural evaluation of the scaffolds showed that the nanoparticles exhibiting carboxyl functional groups were homogeneously distributed across the fibrous network of the hydrogels. The addition of both types of artificial polymeric particles was able to enhance the viscoelastic properties of the FA hydrogels, allowing the biomaterials to reach levels of mechanical consistency under shear stresses in the same range of some human native soft tissues, which could allow these biomaterials to be used as scaffolds for new tissue engineering applications. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 738-745, 2018

Effect of functionalized PHEMA micro- and nano-particles on the viscoelastic properties of fibrin-agarose biomaterials

Two types of PHEMA-based particles, exhibiting either carboxyl or tertiary ammine functional groups, were incorporated to fibrin-agarose (FA) hydrogels, and the effect of the addition of these synthetic particles on the viscoelastic and microstructural properties of the biomaterials was evaluated. Experimental results indicated that the incorporation of both types of polymeric particles to FA scaffolds was able to improve the biomechanical properties of the biomaterials under steady state and oscillatory shear stresses, resulting in scaffolds characterized by higher values of the storage, loss, and shear moduli. In addition, the microstructural evaluation of the scaffolds showed that the nanoparticles exhibiting carboxyl functional groups were homogeneously distributed across the fibrous network of the hydrogels. The addition of both types of artificial polymeric particles was able to enhance the viscoelastic properties of the FA hydrogels, allowing the biomaterials to reach levels of mechanical consistency under shear stresses in the same range of some human native soft tissues, which could allow these biomaterials to be used as scaffolds for new tissue engineering applications.Peer Reviewe

Maternal Caloric Restriction Implemented during the Preconceptional and Pregnancy Period Alters Hypothalamic and Hippocampal Endocannabinoid Levels at Birth and Induces Overweight and Increased Adiposity at Adulthood in Male Rat Offspring.

Journal Article;Exposure to inadequate nutritional conditions in critical windows of development has been associated to disturbances on metabolism and behavior in the offspring later in life. The role of the endocannabinoid system, a known regulator of energy expenditure and adaptive behaviors, in the modulation of these processes is unknown. In the present study, we investigated the impact of exposing rat dams to diet restriction (20% less calories than standard diet) during pre-gestational and gestational periods on: (a) neonatal outcomes; (b) endocannabinoid content in hypothalamus, hippocampus and olfactory bulb at birth; (c) metabolism-related parameters; and (d) behavior in adult male offspring. We found that calorie-restricted dams tended to have a reduced litter size, although the offspring showed normal weight at birth. Pups from calorie-restricted dams also exhibited a strong decrease in the levels of anandamide (AEA), 2-arachidonoylglycerol (2-AG), arachidonic acid (AA) and palmitoylethanolamide (PEA) in the hypothalamus at birth. Additionally, pups from diet-restricted dams displayed reduced levels of AEA in the hippocampus without significant differences in the olfactory bulb. Moreover, offspring exhibited increased weight gain, body weight and adiposity in adulthood as well as increased anxiety-related responses. We propose that endocannabinoid signaling is altered by a maternal caloric restriction implemented during the preconceptional and pregnancy periods, which might lead to modifications of the hypothalamic and hippocampal circuits, potentially contributing to the long-term effects found in the adult offspring.This work was supported by the Instituto de Salud Carlos III, Ministerio de Economía y Competitividad (PI13/0226 to FRF, CP12/03109 to JS and PSI-2012-35388 to RGH), Red de Trastornos Adictivos (RD12/0028/0001 to FRF), CIBERobn, Consejería de Economía, Innovación y Ciencia, Junta de Andalucía, UE/ERDF (PI45403, CTS-8221, CTS-433 to FRF), Consejería de Salud, Junta de Andalucía (SAS111224 to JS and FRF), and the German Research Foundation DFG (FOR926, project CP1 to BL). MTR-L has been funded by a FPU predoctoral fellowship of the Spanish Ministerio de Educación, Cultura y Deporte (AP-2009-0225); JS holds ‘‘Miguel Servet’’ research contract from the National System of Health, ISCIII (Grant No. CP12/03109).Ye

Discovering HIV related information by means of association rules and machine learning

Acquired immunodeficiency syndrome (AIDS) is still one of the main health problems worldwide. It is therefore essential to keep making progress in improving the prognosis and quality of life of affected patients. One way to advance along this pathway is to uncover connections between other disorders associated with HIV/AIDS-so that they can be anticipated and possibly mitigated. We propose to achieve this by using Association Rules (ARs). They allow us to represent the dependencies between a number of diseases and other specific diseases. However, classical techniques systematically generate every AR meeting some minimal conditions on data frequency, hence generating a vast amount of uninteresting ARs, which need to be filtered out. The lack of manually annotated ARs has favored unsupervised filtering, even though they produce limited results. In this paper, we propose a semi-supervised system, able to identify relevant ARs among HIV-related diseases with a minimal amount of annotated training data. Our system has been able to extract a good number of relationships between HIV-related diseases that have been previously detected in the literature but are scattered and are often little known. Furthermore, a number of plausible new relationships have shown up which deserve further investigation by qualified medical experts