Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

A ubiquitous biological material, keratin represents a group of insoluble, usually high-sulfur content and filament-forming proteins, constituting the bulk of epidermal appendages such as hair, nails, claws, turtle scutes, horns, whale baleen, beaks, and feathers. These keratinous materials are formed by cells filled with keratin and are considered 'dead tissues'. Nevertheless, they are among the toughest biological materials, serving as a wide variety of interesting functions, e.g. scales to armor body, horns to combat aggressors, hagfish slime as defense against predators, nails and claws to increase prehension, hair and fur to protect against the environment. The vivid inspiring examples can offer useful solutions to design new structural and functional materials. Keratins can be classified as α- and β-types. Both show a characteristic filament-matrix structure: 7 nm diameter intermediate filaments for α-keratin, and 3 nm diameter filaments for β-keratin. Both are embedded in an amorphous keratin matrix. The molecular unit of intermediate filaments is a coiled-coil heterodimer and that of β-keratin filament is a pleated sheet. The mechanical response of α-keratin has been extensively studied and shows linear Hookean, yield and post-yield regions, and in some cases, a high reversible elastic deformation. Thus, they can be also be considered 'biopolymers'. On the other hand, β-keratin has not been investigated as comprehensively. Keratinous materials are strain-rate sensitive, and the effect of hydration is significant. Keratinous materials exhibit a complex hierarchical structure: polypeptide chains and filament-matrix structures at the nanoscale, organization of keratinized cells into lamellar, tubular-intertubular, fiber or layered structures at the microscale, and solid, compact sheaths over porous core, sandwich or threads at the macroscale. These produce a wide range of mechanical properties: the Young's modulus ranges from 10 MPa in stratum corneum to about 2.5 GPa in feathers, and the tensile strength varies from 2 MPa in stratum corneum to 530 MPa in dry hagfish slime threads. Therefore, they are able to serve various functions including diffusion barrier, buffering external attack, energy-absorption, impact-resistance, piercing opponents, withstanding repeated stress and aerodynamic forces, and resisting buckling and penetration. A fascinating part of the new frontier of materials study is the development of bioinspired materials and designs. A comprehensive understanding of the biochemistry, structure and mechanical properties of keratins and keratinous materials is of great importance for keratin-based bioinspired materials and designs. Current bioinspired efforts including the manufacturing of quill-inspired aluminum composites, animal horn-inspired SiC composites, and feather-inspired interlayered composites are presented and novel avenues for research are discussed. The first inroads into molecular-based biomimicry are being currently made, and it is hoped that this approach will yield novel biopolymers through recombinant DNA and self-assembly. We also identify areas of research where knowledge development is still needed to elucidate structures and deformation/failure mechanisms

Teologiczne aspekty starotestamentalnego posłuszeństwa

Die in den letzten Jahren beständigen Aufschwung nehmende Christologie stellt den Exegeten die deutliche Aufgabe, ihre Beweismittel in der Quelle der Offenbarung zu suchen. Die vorliegende Abhandlung soll eine Teilantwort auf diese Forderung sein. Der Verfasser geht auf die alttestamentliche Frage des Gehorsans in der Beziehung Got – Mensch und Mensch – Mensch ein. In der vorliegenden Abhandlung wurde die Aufmerksamkeit auf die Folge rung gerichtet, die sich aus der Annahme des Wortes Gottes ergibt Wie aufgezeigt, ist die Forderung des Gehorsams ein an die Gläubi gen gerichteter Aufruf

The structure and mechanical performance of teleost fish skin

The skin of teleost fish is a natural material that exhibits a range of properties that are considered desirable for the biomimetic design of flexible and protective materials and systems. Teleost integument functions to enhance the resistance of the skin to sharp penetration and also to promote the efficient swimming of the fish. Using the common teleost striped bass (Morone saxatilis), the puncture mechanics of individual fish scales were first explored and specialized mechanisms of the scale contributing to the penetration resistance of the skin were revealed. The puncture resistance of the intact scaled skin was then investigated, and additional collective scale mechanisms were demonstrated, which further increase the penetration resistance of the skin. The possible mechanical role of teleost skin to function as an external tendon during undulatory locomotion and promote swimming efficiency was also investigated by examining the contribution of both the scales and the underlying dermal stratum compactum to the bending mechanics of the fish. Although the scales were found not to serve an exotendon role during locomotion, the results supported the s. compactum as a tendon-like energy storage device that improves swimming performance. This characterization of the puncture and bending mechanics of teleost fish skin will be applied to the design of biomimetic protective materials that exhibit similar properties as teleost skin, such as high puncture resistance and energy storage capability.La peau du poisson téléostéen est un matériau naturel qui présente une gamme de propriétés remarquables pour le domaine de l’étude et de la fabrication biomimétique des matériaux et systèmes flexibles et protecteurs. Le tégument du poisson téléostéen a pour rôle d’augmenter la résistance à la pénétration de la peau ainsi que d’améliorer l’efficacité de la nage du poisson. En étudiant le bar rayé commun (Morone saxatilis), la mécanique de la perforation d’écailles de poisson isolées a été en premier explorée et les mécanismes particuliers de l’écaille contribuant à la résistance à la pénétration de la peau ont été révélés. La résistance à la perforation de la peau écaillée intacte a été étudiée, et des mécanismes collectifs additionnels ont été démontrés, augmentant de ce fait la résistance à la pénétration de la peau. La possible fonction mécanique de la peau du poisson téléostéen comme tendon externe durant la locomotion ondulatoire améliorant l’efficacité de la nage a aussi été étudiée en examinant la contribution à la fois des écailles et du stratum compactum dermique sous-jacent du poisson. Bien qu’il ait été constaté que les écailles n’avaient pas la fonction d’exo-tendon pendant la locomotion, les résultats décrivent le s. compactum comme un appareil tendineux de stockage de l’énergie qui augmente la performance de la nage. Cette caractérisation de la perforation et de la mécanique de la flexion de la peau du poisson téléostéen sera appliquée à la fabrication biomimétique de matériaux protecteurs présentant des propriétés similaires à celles de la peau du poisson téléostéen, comme la haute résistance à la perforation ou la capacité de stockage d’énergie

Toward Accurate Modeling of Galaxy Clustering on Small Scales: Halo Model Extensions and Lingering Tension

This paper represents an effort to provide robust constraints on the galaxy–halo connection and simultaneously test the Planck ΛCDM cosmology using a fully numerical model of small-scale galaxy clustering. We explore two extensions to the standard Halo Occupation Distribution model: assembly bias, whereby halo occupation depends on both halo mass and the larger environment, and velocity bias, whereby galaxy velocities do not perfectly trace the velocity of the dark matter within the halo. Moreover, we incorporate halo mass corrections to account for the impact of baryonic physics on the halo population. We identify an optimal set of clustering measurements to constrain this “decorated” HOD model for both low- and high-luminosity galaxies in SDSS DR7. We find that, for low-luminosity galaxies, a model with both assembly bias and velocity bias provides the best fit to the clustering measurements, with no tension remaining in the fit. In this model, we find evidence for both central and satellite galaxy assembly bias at the 99% and 95% confidence levels, respectively. In addition, we find evidence for satellite galaxy velocity bias at the 99.9% confidence level. For high-luminosity galaxies, we find no evidence for either assembly bias or velocity bias, but our model exhibits significant tension with SDSS measurements. We find that all of these conclusions still stand when we include the effects of baryonic physics on the halo mass function, suggesting that the tension we find for high-luminosity galaxies may be due to a problem with our assumed cosmological model

Calcification Provides Mechanical Reinforcement to Whale Baleen Alpha-Keratin

Hard a-keratins such as hair, nail, wool and horn are stiff epidermal appendages used by mammals in a variety of functions including thermoregulation, feeding and intraspecific competition. Hard a-keratins are fibre-reinforced structures consisting of cytoskeletal elements known as ‘intermediate filaments’ embedded in an amorphous protein matrix. Recent research has shown that intermediate filaments are soft and extensible in living keratinocytes but become far stiffer and less extensible in keratinized cells, and this stiffening may be mediated by air-drying. Baleen, the keratinous plates used by baleen whales during filter feeding, is an unusual mammalian keratin in that it never air dries, and in some species, it represents the most heavily calcified of all the hard a-keratins. We therefore tested the hypothesis that whale baleen is stiffened by calcification. Here, we provide, to our knowledge, the first comprehensive description of baleen material properties and show that calcification contributes to overcoming the shortcomings of stiffening this hard a-keratin without the benefit of air-drying. We also demonstrate striking interspecies differences in the calcification patterns among three species of baleen whales and provide novel insights into the function and evolution of this unusual biomaterial