Hacia la mímesis de la seda de araña a partir de poliuretanos con segmentos cortos de unidades rígidas y semiflexibles

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Abstract

La capacidad de absorber energía mecánica por un material viene determinada por el concepto de tenacidad. Materiales con alta tenacidad, tales como la seda de araña, pueden ser empleados en numerosas aplicaciones que van desde la ingeniería civil a la biomedicina, además de seguridad vial o defensa. Los poliuretanos elastoméricos son materiales de gran versatilidad en cuanto a composición y propiedades, resultando atractivos para tratar de mimetizar la seda de araña de manera sintética, debido a ciertas similitudes en cuanto a propiedades y morfología. En este trabajo se presentan los resultados de la síntesis y caracterización de una nueva familia de poliuretanos segmentados, formados por segmentos cortos, tanto de cadenas semiflexibles como de cadenas rígidas o cristalinas. El uso de segmentos cortos semiflexibles, es la innovación principal de este trabajo, ya que usualmente los poliuretanos elastoméricos se preparan con dioles de masas moleculares entre 2000-4000 g mol-1. La introducción de segmentos semiflexibles de baja masa molecular pretende dotar a estos materiales de una mayor capacidad de asociación intermolecular por puente de hidrógeno, manteniendo las propiedades elastoméricas. Los materiales han sido caracterizados mediante espectroscopia infrarroja de transformada de Fourier (FTIR), microscopía de fuerzas atómicas (AFM), calorimetría diferencial de barrido (DSC) y ensayos mecánicos de tracción.

Translated abstract

The capacity to absorb mechanical energy is determined through the concept of toughness. Materials with high toughness such as spider silk, could be used in numerous fields like civil engineering, biomedicine, vial-security or defense. Elastomeric polyurethanes are attractive to try to mimic spider silk through a synthetic pathway due to the materials similitudes in terms of properties and morphology. This work presents results on the synthesis and characterization of a new family of segmented polyurethanes formed by short units of semiflexible and stiff or crystalline segments. The use of semiflexible short units is the principal innovation of this work, since usually the semiflexible segments are composed of diols of molecular weights in the range 2000-4000 g mol-1. The introduction of short length semiflexible segments aims to endow the material a higher capacity of interchain hydrogen bonding at the same time it retains the elastomeric behavior. The materials were characterized by Fourier transformed infrared spectroscopy (FTIR), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and tensile testing.

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Rapid planetesimal formation in turbulent circumstellar discs

Anders Johansen, Jeffrey S. Oishi, Mordecai-Mark Mac Low … (2007)

The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.

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Conformational Evolution of Elongated Polymer Solutions Tailors the Polarization of Light-Emission from Organic Nanofibers

Andrea Camposeo, Israel Greenfeld, Francesco Tantussi … (2014)

Polymer fibers are currently exploited in tremendously important technologies. Their innovative properties are mainly determined by the behavior of the polymer macromolecules under the elongation induced by external mechanical or electrostatic forces, characterizing the fiber drawing process. Although enhanced physical properties were observed in polymer fibers produced under strong stretching conditions, studies of the process-induced nanoscale organization of the polymer molecules are not available, and most of fiber properties are still obtained on an empirical basis. Here we reveal the orientational properties of semiflexible polymers in electrospun nanofibers, which allow the polarization properties of active fibers to be finely controlled. Modeling and simulations of the conformational evolution of the polymer chains during electrostatic elongation of semidilute solutions demonstrate that the molecules stretch almost fully within less than 1 mm from jet start, increasing polymer axial orientation at the jet center. The nanoscale mapping of the local dichroism of individual fibers by polarized near-field optical microscopy unveils for the first time the presence of an internal spatial variation of the molecular order, namely the presence of a core with axially aligned molecules and a sheath with almost radially oriented molecules. These results allow important and specific fiber properties to be manipulated and tailored, as here demonstrated for the polarization of emitted light.

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Plasmodium gallinaceum ookinetes adhere specifically to the midgut epithelium of Aedes aegypti by interaction with a carbohydrate ligand.

M Shahabuddin, J. Nawrocki, H Zieler (1999)

During the course of its development in the mosquito and transmission to a new vertebrate host, the malaria parasite must interact with the mosquito midgut and invade the gut epithelium. To investigate how the parasite recognizes the midgut before invasion, we have developed an in vitro adhesion assay based on combining fluorescently labelled ookinetes with isolated midgut epithelia from blood-fed mosquitoes. Using this assay, we found that Plasmodium gallinaceum ookinetes readily adhered to midguts of Aedes aegypti, mimicking the natural recognition of the epithelium by the parasite. This interaction is specific: the ookinetes preferentially adhered to the lumen (microvillar) side of the gut epithelium and did not bind to other mosquito tissues. Conversely, the binding was not due to a non-specific adhesive property of the midguts, because a variety of other cell types, including untransformed P. gallinaceum zygotes or macrogametes, did not show similar binding to the midguts. High concentrations of glycosylated (fetuin, orosomucoid, ovalbumin) or non-glycosylated (bovine serum albumin) proteins, added as non-specific competitors, failed to compete with the ookinetes in binding assays. We also found that the adhesion of ookinetes to the midgut surface is necessary for sporogonic development of the parasite in the mosquito. Antibodies and other reagents that blocked adhesion in vitro also reduced oocyst formation when these reagents were combined with mature ookinetes and fed to mosquitoes. Chemical modification of the midguts with sodium periodate at pH 5.5 destroyed adhesion, indicating that the ookinete binds to a carbohydrate ligand on the surface of the midgut. The ligand is sensitive to periodate concentrations of less than 1 mmol l-1, suggesting that it may contain sialic-acid-like sugars. Furthermore, free N-acetylneuraminic acid competed with the ookinetes in binding aasays, while other monosaccharides had no effect. However, in agreement with the current belief that adult insects do not contain sialic acids, we were unable to detect any sialic acids in mosquito midguts using the most sensitive HPLC-based fluorometric assay currently available. We postulate that a specific carbohydrate group is used by the ookinete to recognize the midgut epithelium and to attach to its surface. This is the first receptor-ligand interaction demonstrated for the ookinete stage of a malaria parasite. Further characterization of the midgut ligand and its parasite counterpart may lead to novel strategies of blocking oocyst development in the mosquito.