Comportamiento termo-higro-mecánico de las fibras de seda de araña

Las fibras de seda de araña constituyen un material enormemente atractivo. Su elevada resistencia y deformabilidad les permite almacenar gran cantidad de energía de deformación, que pueden después disipar, de modo que son especialmente apropiadas para absorber impactos. Este atractivo ha hecho que se hayan dedicado elevadas sumas de dinero a intentar producir fibras de proteínas similares a las de la seda de araña mediante ingeniería gen ética. El estudio de las propiedades de las fibras de seda de araña y su relación con la microestructura tiene el interés de servir como guía en el diseño y producción de futuros materiales proteicos. EI objetivo de la Tesis ha sido realizar una caracterización lo más completa posible de las propiedades mecánicas de las fibras de seda de araña y su dependencia de las condiciones ambientales, humedad y temperatura. Se ha empleado la especie Argiope trifasciata y se ha estudiado el comportamiento en tracción simple de las fibras de seda tomando como variables la temperatura (T), la humedad relativa (HR), el tiempo y la historia de solicitaciones. Dadas las dificultades de ensayo de estas fibras por su pequeño grosor y por su sensibilidad a las condiciones ambientales, una parte fundamental de la Tesis ha consistido en el desarrollo de dispositivos experimentales suficientemente precisos. Controlando las condiciones de las fibras de seda, se ha estudiado el comportamiento en tracción simple en un amplio rango de valores de temperatura y humedad relativa. Los resultados han permitido caracterizar la influencia de las condiciones ambientales en las propiedades mecánicas y trazar la curva de transición vítrea sobre un diagrama HR-T, que es especialmente interesante para delimitar las posibles aplicaciones estructurales de este material. Además, se ha estudiado el efecto que produce el paso del material desde estado vítreo a estado elastomérico, midiendo las tensiones generadas y la contracción de las fibras. También se ha estudiado el abanico de propiedades mecánicas que el material puede presentar en estado vítreo en función del alineamiento molecular, así como la homogeneidad de la deformación, habiéndose comprobado que la deformación puede ser inhomogénea en ciertas situaciones. Finalmente, se ha estudiado el efecto del envejecimiento físico en las fibras de seda de araña en estado vítreo. Para ello, se ha analizado la evolución a lo largo del tiempo de las propiedades mecánicas en tracción simple, y el rejuvenecimiento del material por paso a estado elastomérico o por deformación

Contraction speed of the actomyosin cytoskeleton in the absence of the cell membrane

The contraction of the actomyosin cytoskeleton, which is produced by the sliding of myosin II along actin filaments, drives important cellular activities such as cytokinesis and cell migration. To explain the contraction velocities observed in such physiological processes, we have studied the contraction of intact cytoskeletons of Dictyostelium discoideum cells after removing the plasma membrane using Triton X-100. The technique developed in this work allows for the quantitative measurement of contraction rates of individual cytoskeletons. The relationship of the contraction rates with forces was analyzed using three different myosins with different in vitro sliding velocities. The cytoskeletons containing these myosins were always contractile and the contraction rate was correlated with the sliding velocity of the myosins. However, the values of the contraction rate were two to three orders of magnitude slower than expected from the in vitro sliding velocities of the myosins, presumably due to internal and external resistive forces. The contraction process also depended on actin cross-linking proteins. The lack of α-actinin increased the contraction rate 2-fold and reduced the capacity of the cytoskeleton to retain internal materials, while the lack of filamin resulted in the ATP-dependent disruption of the cytoskeleton. Interestingly, the myosin-dependent contraction rate of intact contractile rings is also reportedly much slower than the in vitro sliding velocity of myosin, and is similar to the contraction rates of cytoskeletons (different by only 2–3 fold), suggesting that the contraction of intact cells and cytoskeletons is limited by common mechanisms

Polymeric fibers with tunable properties Lessons from spider silk

Making artificial fibers inspired in spider silks is considered as one of the milestones in the field of biomimetics. The interest is usually justified by the outstanding tensile properties of natural fibers, but it is usually overlooked that spider silk is endowed with a number of related properties – supercontraction, recovery and the existence of a ground state – that impart the material with additional desirable features, such as the possibility of tuning its mechanical behaviour. In this work we present a review on the experimental analysis and significance of these properties, stressing the contributions of our research group to the field. It is also demonstrated how the knowledge gained in the basic study of the natural material has been essential for the improvement of the properties exhibited by artificially processed bio-inspired silk fiber

The hidden link between supercontraction and mechanical behavior of spider silks

The remarkable properties of spider silks have stimulated an increasing interest in understanding the roles of their composition and processing, as well as in the mass-production of these fibers. Previously, the variability in the mechanical properties of natural silk fibers was a major drawback in the elucidation of their behavior, but the authors have found that supercontraction of these fibers allows one to characterize and reproduce the whole range of tensile properties in a consistent way. The purpose of this review is to summarize these findings. After a review of the pertinent mechanical properties, the role of supercontraction in recovering and tailoring the tensile properties is explained, together with an alignment parameter to characterize silk fibers. The concept of the existence of a mechanical ground state is also mentioned. These behaviors can be modeled, and two such models–at the molecular and macroscopic levels–are briefly outlined. Finally, the assessment of the existence of supercontraction in bio-inspired fibers is considered, as this property may have significant consequences in the design and production of artificial fibers

Study of the influence of actin-binding proteins using linear analyses of cell deformability

The actin cytoskeleton plays a key role in the deformability of the cell and in mechanosensing. Here we analyze the contributions of three major actin cross-linking proteins, myosin II, a-actinin and filamin, to cell deformability, by using micropipette aspiration of Dictyostelium cells. We examine the applicability of three simple mechanical models: for small deformation, linear viscoelasticity and drop of liquid with a tense cortex; and for large deformation, a Newtonian viscous fluid. For these models, we have derived linearized equations and we provide a novel, straightforward methodology to analyze the experiments. This methodology allowed us to differentiate the effects of the cross-linking proteins in the different regimes of deformation. Our results confirm some previous observations and suggest important relations between the molecular characteristics of the actin-binding proteins and the cell behavior: the effect of myosin is explained in terms of the relation between the lifetime of the bond to actin and the resistive force; the presence of a-actinin obstructs the deformation of the cytoskeleton, presumably mainly due to the higher molecular stiffness and to the lower dissociation rate constants; and filamin contributes critically to the global connectivity of the network, possibly by rapidly turning over crosslinks during the remodeling of the cytoskeletal network, thanks to the higher rate constants, flexibility and larger size. The results suggest a sophisticated relationship between the expression levels of actinbinding proteins, deformability and mechanosensing

Efecto de la Longitud de onda de la radiación UV sobre la seda de araña

En el presente trabajo se continúa el análisis de la influencia de la radiación UV sobre las propiedades mecánicas de las fibras de seda de araña. Para ello se ha empleado la seda producida por la glándula ampollácea mayor de la especie Argiope trifasciata y se ha estudiado el comportamiento en tracción simple de fibras de seda sometidas a diferentes tiempos de irradiación con luz UV de longitudes de onda de 254, 312 y 365 nm. Se ha encontrado que la radiación ultravioleta disminuye la tensión de rotura y la deformación de rotura modificando, en algunos casos, el aspecto general de las curvas tensión-deformación. Además se ha hecho uso de la radiación UV de longitud de onda de 254 nm para introducir daño en las fibras con objeto de realizar un análisis fractográfico sistemático. El estudio se complementa con la evaluación del efecto producido por la irradiación en el fenómeno de supercontracción

Material properties of evolutionary diverse spider silks described by variation in a single structural parameter

Spider major ampullate gland silks (MAS) vary greatly in material properties among species but, this variation is shown here to be confined to evolutionary shifts along a single universal performance trajectory. This reveals an underlying design principle that is maintained across large changes in both spider ecology and silk chemistry. Persistence of this design principle becomes apparent after the material properties are defined relative to the true alignment parameter, which describes the orientation and stretching of the protein chains in the silk fiber. Our results show that the mechanical behavior of all Entelegynae major ampullate silk fibers, under any conditions, are described by this single parameter that connects the sequential action of three deformation micromechanisms during stretching: stressing of protein-protein hydrogen bonds, rotation of the ?-nanocrystals and growth of the ordered fraction. Conservation of these traits for over 230 million years is an indication of the optimal design of the material and gives valuable clues for the production of biomimetic counterparts based on major ampullate spider silk

The variability and interdependence of spider viscid line tensile properties

True stress-true strain curves of naturally spun viscid line fibers retrieved directly from the spiral of orb-webs built by Argiope trifasciata spiders were measured using a novel methodology. This new procedure combines a method for removing the aqueous coating of the fibers and a technique that allows the accurate measurement of their cross sectional area. Comparison of the tensile behaviour of different samples indicates that naturally spun viscid lines show a large variability, comparable to that of other silks, such as major ampullate gland silk and silkworm silk. Nevertheless, application of a statistical analysis allowed identifying two independent parameters that underlie the variability and characterize the observed range of true stress-true strain curves. Combination of this result with previous mechanical and microstructural data suggested the assignment of these two independent effects to the degree of alignment of the protein chains and to the local relative humidity which, in turn, depends on the composition of the viscous coating and on the external environmental conditions

Influencia de la radiación UV en las propiedades mecánicas y en el comportamiento en fractura de un polimero artificial bioinspirado en la seda de araña

En el presente trabajo se estudia la influencia de la radiación UV sobre las propiedades mecánicas y las superficies de fractura de un polímero artificial bioinspirado en la seda de araña. Las fibras de seda de araña constituyen un material enormemente atractivo ya que su elevada resistencia y deformabilidad lo convierten en el material con mayor trabajo hasta rotura de los conocidos hasta el momento. Además se ha encontrado que posee una elevada biocompatibilidad y un comportamiento biodegradable. Debido a estas excelentes propiedades se han dedicado importantes esfuerzos a intentar producir fibras inspiradas en la seda de araña. Fruto de estos esfuerzos es el polímero artificial estudiado en este trabajo. Dicho polímero presenta una secuencia de aminoácidos inspirada en la spidroína 1, que es una de las dos proteínas que conforman la seda de araña natural. Uno de los factores más perjudiciales para los polímeros es la radiación ultravioleta (UV), de presencia ubicua en aplicaciones al aire libre, ya que puede provocar la modificación de sus enlaces covalentes y, como consecuencia, modificar sus propiedades mecánicas. Para evaluar el efecto de la radiación UV sobre el material bioinspirado se ha estudiado el comportamiento a tracción simple de fibras sometidas a diferentes tiempos de irradiación con luz UV de longitud de onda de 254 nm. Se ha observado que la radiación UV de 254 nm modifica considerablemente las propiedades mecánicas de este material a tiempos de exposición elevados (a partir de 3 días de irradiación). Además se ha estudiado el comportamiento a fractura de este material cuando es irradiado con luz UV. Se ha observado que a medida que aumenta el tiempo de irradiación las superficies de fractura comienzan a ser cada vez más planas, obteniéndose un aspecto extremadamente especular para muestras irradiadas durante 16 día

Correlation Between Processing Conditions, Microstructure and Mechanical Behavior in Regenerated Silkworm Silk Fibers

Regenerated silkworm fibers spun through a wet-spinning process followed by an immersion postspinning drawing step show a work to fracture comparable with that of natural silkworm silk fibers in a wide range of spinning conditions. The mechanical behavior and microstructure of these high performance fibers have been characterized, and compared with those fibers produced through conventional spinning conditions. The comparison reveals that both sets of fibers share a common semicrystalline microstructure, but significant differences are apparent in the amorphous region. Besides, high performance fibers show a ground state and the possibility of tuning their tensile behavior. These properties are characteristic of spider silk and not of natural silkworm silk, despite both regenerated and natural silkworm silk share a common composition different from that of spider silk