Electric-field induced shape transition of nematic tactoids

The occurrence of new textures of liquid crystals is an important factor in tuning their optical and photonics properties. Here, we show, both experimentally and by numerical computation, that under an electric field chitin tactoids (i.e. nematic droplets) can stretch to aspect ratios of more than 15, leading to a transition from a spindle-like to a cigar-like shape. We argue that the large extensions occur because the elastic contribution to the free energy is dominated by the anchoring. We demonstrate that the elongation involves hydrodynamic flow and is reversible, the tactoids return to their original shapes upon removing the field.Comment: 8 pages, 6 figures; accepted for publication in Physical Review

Liquid crystalline properties of type I collagen: Perspectives in tissue morphogenesis

Collagen molecules form the major part of tissues like bone, cornea or tendon where they organize into ordered fibrillar networks. The acid-soluble protein spontaneously assembles in liquid crystalline phases, characterized in polarized light microscopy and X-ray diffraction. Collagen fibrillogenesis obtained in condensed media establishes a link between the fibrillar networks described in vivo and the mesomorphic states obtained in vitro. Cellematrix interactions on these biomimetic materials are currently analysed with perspectives in tissue engineering. In a morphogenetic context, we propose the hypothesis of a liquid crystalline order, between soluble precursor molecules, preceding fibrillogenesis

Chitin–Silica Nanocomposites by Self-Assembly

International audienceA new family of chitin–silica nanocomposites has been synthesized by using a versatile colloid-based combination of self-assembly and sol–gel chemistry. Various textures and morphologies can be obtained by adjusting the evaporationbased processes or by applying external fields. After calcination, textures and birefringence are preserved in the resulting mesoporous silicas

Collagène auto-assemblé en support 3D biomimétique fonctionnalisé pour la différenciation de cellules nerveuses

L'objectif de ce travail était de mettre au point un système de culture tridimensionnel compartimenté pour la différentiation de cellules neurales et la croissance des neurites en 3D. Les matériaux biomimétiques permettent l'élaboration de microenvironnements contrôlés qui peuvent orienter la réponse cellulaire. Ils sont particulièrement intéressants pour les études fondamentales visant à étudier des voies de signalisation impliquées dans des processus physiologiques ou pathologiques. Nous nous sommes intéressés à la maladie d'Alzheimer, où l'on observe des neurites dystrophiques associés aux plaques amyloïdes. Aucune relation n'a été réellement établie entre l'interaction neurites - agrégats, leur dystrophie et la mort neuronale. Dans un premier temps, nous avons décrit et caractérisé la structure et les propriétés de matrices de collagène fibrillaire d'épaisseur calibrée. Ensuite, nous avons mis au point la fonctionnalisation de ces matrices avec des facteurs de croissance neurotrophiques (NGF et BDNF). Deux techniques ont été étudiées : l'imprégnation/libération et le couplage covalent. Ces matrices fonctionnalisées ont été validées comme support pour la différenciation de cellules nerveuses (PC-12 et SH-SY5Y) par des études de la morphologie cellulaire. Enfin, nous avons caractérisé des agrégats amyloïdes (Ab) formés à l'intérieur des matrices de collagène par co-précipitation du peptide Ab avec le collagène et nous avons étudié leur toxicité sur les cellules neurales.The objective of this work was to develop a 3D compartmented cell culture set-up that allow the differentiation of nerve cells and the growth of neurites in the matrix depth. Biomimetic materials enable the formation of controlled microenvironments that orient cell behavior. They are particularly interesting for fundamental studies that aim to study signaling pathways involved in physiologic or pathologic processes. We focused on Alzheimer's disease, in which dystrophic neurites are associated to amyloid plaques. No direct relationship has yet been established between Ab aggregates-neurite interaction, neurite dystrophy and cell death. First, we described and characterized the structure and properties of fibrillar collagen matrices with adapted thickness. Then, we adjusted functionalization of these matrices with neurotrophic growth factors (NGF and BDNF). Two methods were studied: impregnation/release and covalent coupling. Cell morphology studies confirmed that these functionalized matrices were efficient supports for nerve cells differentiation (PC-12 and SH-SY5Y). Finally, we have characterized Ab aggregates that were formed inside collagen matrices by coprecipitation of amyloid peptide and collagen and we studied their toxicity on neural cells.MONTPELLIER-BU Pharmacie (341722105) / SudocSudocFranceF

Surface chemical states of cellulose, chitin and chitosan studied by density functional theory and high-resolution photoelectron spectroscopy

International audienceA combined theoretical and experimental approach has been applied to study the 1s electron-energy surface properties of cellulose, chitin, synthesized chitin nanorods and chitosan using density functional theory and high-resolution photoelectron spectroscopy. This allows to reliably distinguish the contributions of surface hydrocarbon impurities in the photoelectron spectra and to examine in detail the chemical states of the polysaccharide surfaces. Although a stoichiometric structure is suggested for the cellulose surface as more likely, a mechanism for possible degradation of the surface including removal of the OH group bonded to glucose ring is also contemplated. The good agreement between theoretical and experimental results allows suggesting a chitosan-like structure for the surfaces of as-prepared chitin and of chitin nanorods. In addition to the dominant concentration of amino NH2 groups on these surfaces, a small amount of acetyl amine NH2COCH3 groups is also observed on the as-prepared chitin. It is possible that protonated amino NH3+ functional groups instead of acetyl amine are present on the crystalline surface of chitin nanorods. The possible destructive role of X-ray radiation on the studied surfaces is also discussed

Tunable hierarchical porosity from self-assembled chitin-silica nano-composites

International audienceWe studied new mesoporous materials with original properties and obtained from self-assembled chitin-silica nano-composites. Our novel synthesis allows the controlled colloidal assembly of a-chitin nanorods (bundles of elongated chitin monocrystals) and siloxane oligomers. Calcination of nanocomposites results in mesoporous silica materials. Their pore volume fraction fPOR (0-0.52) is strongly correlated to the initial chitin content. Using N2 sorption and TEM data, we identify and characterize primary and secondary textural units related to the imprints of chitin monocrystals (2.5 nm wide) and nanorods (20-30 nm wide) respectively. Primary textural units are preserved over a wide fPOR range (linear relationship between pore volume and specific surface area). The coating of monocrystals by siloxane oligomers leads to a siloxane network of fractal nature as deduced from complementary SAXS data. Beyond a critical value fPOR 0 estimated near 0.2, the coating is partial, and the porosity becomes more open and connected. At larger scales, the arrangements of secondary textural units result in complex textures and long-range ordering, showing similarities with textural features found in natural materials. We discuss the competition between entropy-driven transitions typical of anisotropic particles and kinetic arrest due to colloidal gelation and inorganic condensation. Finally, a schematic model for texture formation is given

Nanoparticles as contrast agents for brain nuclear magnetic resonance imaging in Alzheimer's disease diagnosis

International audienceNuclear Magnetic Resonance Imaging (MRI) of amyloid plaques is a powerful non-invasive approach for the early and accurate diagnosis of Alzheimer's disease (AD) along with clinical observations of behavioral changes and cognitive impairment. The present article aims at giving a critical and comprehensive review of recent advances in the development of nanoparticle-based contrast agents for brain MRI. Nanoparticles considered for the MRI of AD must comply with a highly stringent set of requirements including low toxicity and the ability to cross the blood–brain-barrier. In addition, to reach an optimal signal-to-noise ratio, they must exhibit a specific ability to target amyloid plaques, which can be achieved by grafting antibodies, peptides or small molecules. Finally, we propose to consider new directions for the future of MRI in the context of Alzheimer's disease, in particular by enhancing the performances of contrast agents and by including therapeutic functionalities following a theranostic strategy

Power law rheology and strain-induced yielding in acidic solutions of type I-collagen

International audienceAcidic solutions of collagen are in vitro models of procollagen, the neutrosoluble precursor of collagen secreted by cells to build the extra-cellular matrix. Their viscoelastic properties may influence fibrils nucleation and growth during pH-triggered fibrillogenesis, and could thus be of particular interest for the engineering of artificial tissues as well as for the regulation of the structural properties of the extracellular matrix. Here we report on the rheological properties of acidic solutions of collagen over a wide concentration range, 0.6-300 mg mL(-1). At low concentrations, solutions display usual viscoelastic features consistent with those of dilute and semi-dilute solutions of macromolecules. At higher concentrations, both storage and loss moduli, G' and G '', scale as a weak power-law of the frequency omega(alpha) (alpha = 0.15 - 0.3), similar to what is reported for cross-linked actin networks, living cells and tissues. Creep experiments at high concentration reveal a weak power law regime at short times followed by a steady fluid-like regime at longer times. The transition between these two regimes appears at shorter times as stress amplitude increases. By rescaling creep responses, all time-domain data collapse onto a master curve. This defines a new time-stress superposition principle (TSS) and shows that fluidization occurs above a critical strain and after a lag-time scaling as sigma(-1/a). Power law rheology and strain-induced yielding are suggestive of soft-glassy rheology (SGR): concentrated collagen solutions would be close to a soft-glassy transition. Eventually, we compare the evolution of the rheological properties of acidic solutions from dilute to concentrated regimes to the non monotonous variation of fibrils diameter with collagen concentration in neutral gels

Development of Coprocessed Chitin-Calcium Carbonate as Multifunctional Tablet Excipient for Direct Compression, Part 2: Tableting Properties

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Improved silica–titania catalysts by chitin biotemplating

International audienceSilica–titania materials with improved catalytic performance were elaborated as mesoporous microparticlesby combining sol–gel and spray-drying processes with the self-assembly properties of α-chitin nanorodsacting as biotemplates. Three different synthesis approaches are discussed, leading to materials with variedtextural and chemical characteristics studied by SEM, N2 volumetry, TEM, XPS and DR-UV techniques. Theuse of water or ethanol as initial solvent for chitin nanorod suspensions, as well as the mixing conditions ofthe precursors, has been shown to have a significant impact on the final properties. Materials of specificsurface areas of up to 590 m2 g−1 and porous volumes of up to 0.84 mL g−1, with low surface Si/Ti ratio,could be disclosed. Properties were further investigated by employing the silica–titania materials as heterogeneouscatalysts for the sulfoxidation of bulky model compounds. The location of Ti active sites at thepore surface has been maximized and allows for improved productivity