Asymmetry between the dorsal and ventral digging valves of the female locust: function and mechanics.

BACKGROUND: The female locust is equipped with unique digging tools, namely two pairs of valves-a dorsal and a ventral-utilized for excavating an underground hole in which she lays her eggs. This apparatus ensures that the eggs are protected from potential predators and provides optimal conditions for successful hatching. The dorsal and the ventral valves are assigned distinct roles in the digging process. Specifically, the ventral valves primarily function as anchors during propagation, while the dorsal valves displace soil and shape the underground tunnel. RESULTS: In this study, we investigated the noticeable asymmetry and distinct shapes of the valves, using a geometrical model and a finite element method. Our analysis revealed that although the two pairs of valves share morphological similarities, they exhibit different 3D characteristics in terms of absolute size and structure. We introduced a structural characteristic, the skew of the valve cross-section, to quantify the differences between the two pairs of valves. Our findings indicate that these structural variations do not significantly contribute to the valves' load-bearing capabilities under external forces. CONCLUSIONS: The evolutionary development of the form of the female locust digging valves is more aligned with fitting their respective functions rather than solely responding to biomechanical support needs. By understanding the intricate features of these locust valves, and using our geometrical model, valuable insights can be obtained for creating more efficient and specialized tools for various digging applications.

Nanoscale dynamic mechanical analysis on interfaces of biological composites.

Biological composites incorporate structural arrays of rigid-elastic reinforcements made of minerals or crystalline biopolymers, which are connected by thin, compliant, and viscoelastic macromolecular matrix material. The near-interface regions of these biological composites grant them energy dissipation capabilities against dynamic mechanical loadings, which promote various biomechanical functions such as impact adsorption, fracture toughness, and mechanical signal filtering. Here, we employ theoretical modeling and finite-element simulations to analyze the mechanical response of the near-interface in biological composites to nanoscale dynamic mechanical analysis (DMA). We identified the dominating load-bearing mechanisms of the near-interface region and employed these insights to introduce simple semi-empirical formulations for approaching the mechanical properties (storage and loss moduli) of the biological composite from the nanoscale DMA results. Our analysis paves the way for the nanomechanical characterization of biological composites in diverse natural materials systems, which can also be employed for bioinspired and biomedical configurations.

The load-bearing mechanism of plant wings: A multiscale structural and mechanical analysis of the T. tipu samara.

Spinning winged fruits ("helicopter" samaras) generate significant lift forces at relatively low velocities, which enable the wind to disperse them across long distances. The biological material of the samara sustains the aerodynamic loadings and maintains the physical shape of the samara in the air via a yet unknown load-bearing mechanism. Here, positing that this mechanism fundamentally originates from the macro-to-microscale structural and mechanical characteristics of the samara, we use sub-micron computer tomography, electron microscopy, and multi-scale mechanical experiments to map the structural and mechanical characteristics of the tipu tree (Tipuana tipu) samara down to the micrometer length scale. Then, using theoretical models, we characterize the multiscale structural-mechanical principles of the samara and use these principles to disclose the underlying load-bearing mechanism. We found that the structural motifs of the tipu tree samara are closely analogous to various other types and forms of winged fruits, suggesting that this load-bearing mechanism is widespread in plant wings. The structural-mechanical principles governing the samara bear unconventional design concepts, which pave the way toward the development and engineering of small-scale wing elements for miniature aviation platforms with specialized mechanical capabilities. STATEMENT OF SIGNIFICANCE: The biomaterial of plant wings grants them mechanical resistance to flight forces during wind dispersal. "Helicopter seeds" demonstrate an intricate load-bearing mechanism that spans three structure-functional scales of their biomaterial. This mechanism appears widespread in plant wings and may promote novel micro-engineering design guidelines for futuristic flight materials and small-scale aviation platforms.

The effect of structural curvature on the load-bearing characteristics of biomechanical elements.

Miniature, sharped-edge, curved-shape biomechanical elements appear in various biological systems and grant them diverse functional capabilities, such as mechanical defense, venom injection, and frictional support. While these biomechanical elements demonstrate diverse curved shapes that span from slightly curved needle-like elements (e.g., stingers), through moderately curved anchor-like elements (e.g., claws), to highly curved hook-like elements (e.g., fangs)-the curvature effect on the load-bearing capabilities of these biomechanical elements are yet mostly unknown. Here, we employ structural-mechanical modeling to explore the relationships between the curved shapes of biomechanical elements on their local deformation mechanisms, overall elastic stiffness, and reaction forces on a target surface. We found that the curvature of the biomechanical element is a prime modulator of its load-bearing characteristics that substantially affect its functional capabilities. Slightly curved elements are preferable for penetration states with optimal load-bearing capabilities parallel to their tips but possess high directional sensitivity and degraded capabilities for scratching states; contrary, highly curved elements are suitable for combined penetration-scratching states with mild directional sensitivity and optimal load-bearing capabilities in specialized angular orientation to their tips. These structural-mechanical principles are tightly linked to the intrinsic functional roles of biomechanical elements in diverse natural systems, and their synthetic realizations may promote new engineering designs of advanced biomedical injections, functional surfaces, and micromechanical devices.

Principles of elastic bridging in biological materials.

Load-bearing biological materials employ specialized elastic bridging regions to connect material parts with substantially different properties. While such bridging regions emerge in diverse systems of biological systems, their functional-mechanical origins are yet disclosed. Here, we hypothesize that these elastic bridging regions evolved primarily to minimize the near-interface stress effects in the biological material and, supported by experiments and simulations, we develop a simple theoretical model for such stress-minimizing bridging modulus. Our theoretical model describes well extensive experimental data of diverse biomechanical systems, suggesting that despite their compositionally distinct bridging regions, they share a similar mechanical adaptation strategy for stress minimization. The theoretical model developed in this study may directly serve as a design guideline for bio-inspired materials, biomedical applications, and advanced interfacial architectures with high resilience to mechanical failure. STATEMENT OF SIGNIFICANCE: Biological materials exhibit unconventional structural-mechanical strategies allowing them to attain extreme load-bearing capabilities. Here, we identify the strategy of biological materials to connect parts of distinct elastic properties in an optimal manner of stress minimization. Our findings are compatible with broad types of biological materials, including biopolymers, biominerals, and their bio-composite combinations, and may promote novel engineering designs of advanced biomedical and synthetic materials.

The biomechanics of the locust ovipositor valves: a unique digging apparatus.

The female locust has a unique mechanism for digging in order to deposit its eggs deep in the ground. It uses two pairs of sclerotized valves to displace the granular matter, while extending its abdomen as it propagates underground. This ensures optimal conditions for the eggs to incubate and provides them with protection from predators. Here, the direction-dependent biomechanics of the locust's major, dorsal digging valves are quantified and analysed under forces in the physiological range and beyond, considering the hydration level as well as the females' sexual maturation state. Our findings reveal that the responses of the valves to compression forces in the digging and propagation directions change upon sexual maturation to follow their function and depend on environmental conditions. In addition, mature females, which lay eggs, have stiffer valves, up to approximately 19 times the stiffness of the pre-mature locusts. The valves are stiffer in the major working direction, corresponding to soil shuffling and compression, compared with the direction of propagation. Hydration of the valves reduces their stiffness but increases their resilience against failure. These findings provide mechanical and materials guidelines for the design of novel non-drilling burrowing tools, including three-dimensionally printed anisotropic materials based on composites.

Repetitive hygroscopic snapping movements in awns of wild oats.

Wild oat (Avena sterilis) is a very common annual plant species. Successful seed dispersion support its wide distribution in Africa, Asia and Europe. The seed dispersal units are made of two elongated stiff awns that are attached to a pointy compartment containing two seeds. The awns bend and twist with changes in humidity, pushing the seeds along and into the soil. The present work reveals the material structure of the awns, and models their functionality as two-link robotic arms. Based on nano-to-micro structure analyses the bending and twisting hygroscopic movements are explained. The coordinated movements of two sister awns attached to one dispersal unit were followed. Our work shows that sister awns intersect typically twice every wetting-drying cycle. Once the awns cross each other, epidermal silica hairs are suggested to lock subsequent movements, resulting in stress accumulation. Sudden release of the interlocked awns induces jumps of the dispersal unit and changes in its movement direction. Our findings propose a new role to epidermis silica hairs and a new facet of wild oat seed dispersion. Reversible jumping mechanism in multiple-awn seed dispersal units may serve as a blueprint for reversibly jumping robotic systems. STATEMENT OF SIGNIFICANCE: The seed dispersal unit of wild oats carries two elongated stiff awns covered by unidirectional silica hairs. The awns bend and twist with changes in humidity, pushing the seed capsule along and into the ground. We studied structures constructing the movement mechanism and modeled the awn as a two-link robotic arm. We show that sister awns, attached to the same seed capsule, intersect twice every drying cycle. Once the awns cross each other, the epidermal silica hairs are suggested to lock any subsequent movements, causing stress accumulation. Sudden release of the interlocked awns may cause the dispersal unit to jump and change its direction. Our findings suggest a new role to silica hairs and a new dispersal mechanism in multiple-awn seed dispersal units.

Assessing the Interfacial Dynamic Modulus of Biological Composites.

Biological composites (biocomposites) possess ultra-thin, irregular-shaped, energy dissipating interfacial regions that grant them crucial mechanical capabilities. Identifying the dynamic (viscoelastic) modulus of these interfacial regions is considered to be the key toward understanding the underlying structure-function relationships in various load-bearing biological materials including mollusk shells, arthropod cuticles, and plant parts. However, due to the submicron dimensions and the confined locations of these interfacial regions within the biocomposite, assessing their mechanical characteristics directly with experiments is nearly impossible. Here, we employ composite-mechanics modeling, analytical formulations, and numerical simulations to establish a theoretical framework that links the interfacial dynamic modulus of a biocomposite to the extrinsic characteristics of a larger-scale biocomposite segment. Accordingly, we introduce a methodology that enables back-calculating (via simple linear scaling) of the interfacial dynamic modulus of biocomposites from their far-field dynamic mechanical analysis. We demonstrate its usage on zigzag-shaped interfaces that are abundant in biocomposites. Our theoretical framework and methodological approach are applicable to the vast range of biocomposites in natural materials; its essence can be directly employed or generally adapted into analogous composite systems, such as architected nanocomposites, biomedical composites, and bioinspired materials.

Interfacial indentations in biological composites.

Biocomposites comprise highly stiff reinforcement elements connected by a compliant matrix material. While the interfacial elastic properties of these biocomposites play a key role in determining the mechanical properties of the entire biocomposite, these properties cannot be measured directly from standard nanomechanical experiments. Developing a method for extracting the interfacial elastic properties in biocomposites is, therefore, a major objective of cutting-edge biomaterials science. Here, using mechanical modeling and Finite-Element simulations, we analyze the interfacial force-depth relationships, stress distribution, and indentation modulus of standard nanoindentation testing in biocomposites, and we establish an analytical framework that connects these results to the elastic properties of the underlying matrix and reinforcement components. The resulting analytical framework is general and holds for a broad range of biocomposites, thus enabling a deeper understanding of the mechanical characteristics of functional interfaces in various biomaterials. Moreover, this framework can be adapted to account for synthetic, microscale, and nanoscale composite materials, and thereby promotes the development of advanced interfacial configurations with specialized mechanical capabilities.

Solanales Stem Biomechanical Properties Are Primarily Determined by Morphology Rather Than Internal Structural Anatomy and Cell Wall Composition.

Self-supporting plants and climbers exhibit differences in their structural and biomechanical properties. We hypothesized that such fundamental differences originate at the level of the material properties. In this study, we compared three non-woody members of the Solanales exhibiting different growth habits: (1) a self-supporting plant (potato, Solanum tuberosum), (2) a trailing plant (sweet potato, Ipomoea batatas), and (3) a twining climber (morning glory, Ipomoea tricolor). The mechanical properties investigated by materials analyses were combined with structural, biochemical, and immunohistochemical analyses. Generally, the plants exhibited large morphological differences, but possessed relatively similar anatomy and cell wall composition. The cell walls were primarily composed of hemicelluloses (~60%), with α-cellulose and pectins constituting ~25% and 5%-8%, respectively. Immunohistochemistry of specific cell wall components suggested only minor variation in the occurrence and localization between the species, although some differences in hemicellulose distribution were observed. According to tensile and flexural tests, potato stems were the stiffest by a significant amount and the morning glory stems were the most compliant and showed differences in two- and three-orders of magnitude; the differences between their effective Young's (Elastic) modulus values (geometry-independent parameter), on the other hand, were substantially lower (at the same order of magnitude) and sometimes not even significantly different. Therefore, although variability exists in the internal anatomy and cell wall composition between the different species, the largest differences were seen in the morphology, which appears to be the primary determinant of biomechanical function. Although this does not exclude the possibility of different mechanisms in other plant groups, there is apparently less constraint to modifying stem morphology than anatomy and cell wall composition within the Solanales.

The exoskeleton of scorpions' pincers: Structure and micro-mechanical properties.

Since scorpions exist almost all over the world, some expected body differences exist among the species: undoubtedly, the most evident is the shape and size of their pincers or chelae. The scorpion chela is a multifunctional body component (e.g. attack/defense, mating and protection from the environment) that leads to the development of different stresses in the cuticle. How such stresses in the cuticle are accommodated by different chelae shape and size is largely unknown. Here we provide new comparative data on the hierarchical structure and mechanical properties of the chela cuticle in two scorpion species: Scorpio Maurus Palmatus (SP) that has a large chela and Buthus Occitanus Israelis (BO), with a slender chela. We found that the SP exocuticle is composed of four different sublayers whereas the BO exocuticle displays only two sublayers. These structures are different from the exocuticle morphologies in crustaceans, where the Bouligand morphology is present throughout the entire layer. Moreover, the scorpion chela cuticle presents an exclusive structural layer made of unidirectional fibers arranged vertically towards the normal direction of the cuticle. Nanoindentation measurements were performed under dry conditions on transversal and longitudinal planes to evaluate the stiffness and hardness of the different chela cuticle layers in both scorpions. The chela cuticle structure is a key factor towards the decision of the scorpion whether to choose to sting or use the chela for other mechanical functions. STATEMENT OF SIGNIFICANCE: Many arthropods such as lobsters, crabs, stomatopods, isopods, and spiders have been the subject of research in recent years, and their hierarchical structure and mechanical properties extensively investigated. Yet, except for a limited number of pre-1980 publications, comparatively little work has been devoted to the terrestrial scorpion. The scorpion chela is a multifunctional part of the body (e.g. attack/defense, mating and protection from the environment) that involves the development of various stresses in the cuticle. How these stresses in the chela cuticle are managed by different chelae shape and size is still unknown. The lack of a single study that integrates morphological characterization of the entire hierarchical structure of the scorpion chela cuticle, and local mechanical properties, significantly affects the scientific knowledge regarding important structural approaches that can be used by nature to maximize functionality.

Autotomy in plants: organ sacrifice in Oxalis leaves.

Autotomy is a self-defence strategy of sacrificing a body part for survival. This phenomenon is widespread in the animal kingdom (e.g. gecko's tail) but was never reported in plants. In this study, we characterize the autotomy mechanism in the leaves of an invasive plant of South African origin, Oxalis pes-caprae. When the leaves and flowers of this plant are pulled, they break easily at their base, leaving the rest of the plant intact. Microscopic observations of the leaves reveal an area of small cells and a marked notch at this designated breaking point. Mechanical analysis showed that the strength statistics of the petioles follow Weibull's function. A comparison of the function parameters confirmed that strength of the tissue at that point is significantly smaller than at other points along the petiole, while the toughness of the tissue at the notch and at mid-petiole are approximately the same. We conclude that leaf fracture in Oxalis is facilitated by an amplification of the far-field stress in the vicinity of local, but abrupt, geometrical modification in the form of a notch. This presents an autotomy-like defence mechanism which involves the sacrifice of vital organs in order to prevent the uprooting of the whole plant.

On the form and bio-mechanics of venom-injection elements.

A wide variety of animals-from insects to snakes-crucially depend on their ability to inject venom into their target, be it their prey or their predator. To effectively deliver their venom, venomous animals use a specialized biomechanical element whose tip must penetrate through the integument of the target. During this process, the tip of the venom-injection element (VIE) is subject to local forces, which may deform it and cause considerable structural damage to the VIE, with devastating consequences for the survival of the animal or, in the case of eusocial insects, to the colony. Hence, it is plausible that millions of years of evolution have carefully 'shaped' the architecture of VIEs across different taxa toward a similar mechanical function, namely, to effectively resist the mechanical forces exerted on the tip. The present study aims to identify such a common architecture by analyzing the form-function relationships in various biological VIEs. A universal structural modeling, which quantifies the fundamental geometrical characteristics of a wide range of VIEs is constituted, and a theoretical mechanical framework that analytically correlates these characteristics with the material stress fields is introduced. This investigation reveals that the architecture of biological VIEs reduces the magnitude of applied stresses and confines the maximal stress to the near-tip region of the element. The presented analytical approach and modeling can be straightforwardly applied to various other types of bio-mechanical elements and can potentially be employed for developing a new class of microscopic injection elements for bio-medical and engineering applications. STATEMENT OF SIGNIFICANCE: Venomous animals-both vertebrate and invertebrate-use an extremely wide variety of venom-injection elements to incapacitate their prey or predator. Despite the clear differences in their typical dimensions, shapes, and evolutionary paths, all venom-injection elements have evolved to perform a single mechanical function, namely, to penetrate a target surface. Accordingly, the architecture of many such elements appears to follow similar principles and their material exhibits similar stress characteristics upon biologically relevant mechanical loadings. The current study introduces a theoretical model that draws connections between the 'universal' structural characteristics of such elements and their bio-mechanical functions. It is found that all examined venom-injection elements provide extreme load-bearing capabilities and unusual post-failure functionalities, which are in good agreement with the wide range of numerical and experimental findings from the literature. The emerging theoretical insights from this study thus shed light on the biomechanical origins of the naturally evolved forms of various biological organisms, including bee and wasp stingers, spider and snake fangs, porcupine fish spines, and scorpion stingers.

Plant and algal structure: from cell walls to biomechanical function.

Plant and algal cell walls are complex biomaterials composed of stiff cellulose microfibrils embedded in a soft matrix of polysaccharides, proteins and phenolic compounds. Cell wall composition differs between taxonomic groups and different tissue types (or even at the sub-cellular level) within a plant enabling specific biomechanical properties important for cell/tissue function. Moreover, cell wall composition changes may be induced in response to environmental conditions. Plant structure, habit, morphology and internal anatomy are also dependent on the taxonomic group as well as abiotic and biotic factors. This review aims to examine the complex and incompletely understood interactions of cell wall composition, plant form and biomechanical function.

Stomatal cell wall composition: distinctive structural patterns associated with different phylogenetic groups.

Background and Aims: Stomatal morphology and function have remained largely conserved throughout ∼400 million years of plant evolution. However, plant cell wall composition has evolved and changed. Here stomatal cell wall composition was investigated in different vascular plant groups in attempt to understand their possible effect on stomatal function. Methods: A renewed look at stomatal cell walls was attempted utilizing digitalized polar microscopy, confocal microscopy, histology and a numerical finite-elements simulation. The six species of vascular plants chosen for this study cover a broad structural, ecophysiological and evolutionary spectrum: ferns ( Asplenium nidus and Platycerium bifurcatum ) and angiosperms ( Arabidopsis thaliana and Commelina erecta ) with kidney-shaped stomata, and grasses (angiosperms, family Poaceae) with dumbbell-shaped stomata ( Sorghum bicolor and Triticum aestivum ). Key Results: Three distinct patterns of cellulose crystallinity in stomatal cell walls were observed: Type I (kidney-shaped stomata, ferns), Type II (kidney-shaped stomata, angiosperms) and Type III (dumbbell-shaped stomata, grasses). The different stomatal cell wall attributes investigated (cellulose crystallinity, pectins, lignin, phenolics) exhibited taxon-specific patterns, with reciprocal substitution of structural elements in the end-walls of kidney-shaped stomata. According to a numerical bio-mechanical model, the end walls of kidney-shaped stomata develop the highest stresses during opening. Conclusions: The data presented demonstrate for the first time the existence of distinct spatial patterns of varying cellulose crystallinity in guard cell walls. It is also highly intriguing that in angiosperms crystalline cellulose appears to have replaced lignin that occurs in the stomatal end-walls of ferns serving a similar wall strengthening function. Such taxon-specific spatial patterns of cell wall components could imply different biomechanical functions, which in turn could be a consequence of differences in environmental selection along the course of plant evolution.

Surface protection in bio-shields via a functional soft skin layer: Lessons from the turtle shell.

The turtle shell is a functional bio-shielding element, which has evolved naturally to provide protection against predator attacks that involve biting and clawing. The near-surface architecture of the turtle shell includes a soft bi-layer skin coating - rather than a hard exterior - which functions as a first line of defense against surface damage. This architecture represents a novel type of bio-shielding configuration, namely, an inverse structural-mechanical design, rather than the hard-coated bio-shielding elements identified so far. In the current study, we used experimentally based structural modeling and FE simulations to analyze the mechanical significance of this unconventional protection architecture in terms of resistance to surface damage upon extensive indentations. We found that the functional bi-layer skin of the turtle shell, which provides graded (soft-softer-hard) mechanical characteristics to the bio-shield exterior, serves as a bumper-buffer mechanism. This material-level adaptation protects the inner core from the highly localized indentation loads via stress delocalization and extensive near-surface plasticity. The newly revealed functional bi-layer coating architecture can potentially be adapted, using synthetic materials, to considerably enhance the surface load-bearing capabilities of various engineering configurations.

Stomatal Opening: The Role of Cell-Wall Mechanical Anisotropy and Its Analytical Relations to the Bio-composite Characteristics.

Stomata are pores on the leaf surface, which are formed by a pair of curved, tubular guard cells; an increase in turgor pressure deforms the guard cells, resulting in the opening of the stomata. Recent studies employed numerical simulations, based on experimental data, to analyze the effects of various structural, chemical, and mechanical features of the guard cells on the stomatal opening characteristics; these studies all support the well-known qualitative observation that the mechanical anisotropy of the guard cells plays a critical role in stomatal opening. Here, we propose a computationally based analytical model that quantitatively establishes the relations between the degree of anisotropy of the guard cell, the bio-composite constituents of the cell wall, and the aperture and area of stomatal opening. The model introduces two non-dimensional key parameters that dominate the guard cell deformations-the inflation driving force and the anisotropy ratio-and it serves as a generic framework that is not limited to specific plant species. The modeling predictions are in line with a wide range of previous experimental studies, and its analytical formulation sheds new light on the relations between the structure, mechanics, and function of stomata. Moreover, the model provides an analytical tool to back-calculate the elastic characteristics of the matrix that composes the guard cell walls, which, to the best of our knowledge, cannot be probed by direct nano-mechanical experiments; indeed, the estimations of our model are in good agreement with recently published results of independent numerical optimization schemes. The emerging insights from the stomatal structure-mechanics "design guidelines" may promote the development of miniature, yet complex, multiscale composite actuation mechanisms for future engineering platforms.

The Hygroscopic Opening of Sesame Fruits Is Induced by a Functionally Graded Pericarp Architecture.

To enhance the distribution of their seeds, plants often utilize hygroscopic deformations that actuate dispersal mechanisms. Such movements are based on desiccation-induced shrinkage of tissues in predefined directions. The basic hygroscopic deformations are typically actuated by a bi-layer configuration, in which shrinking of an active tissue layer is resisted by a stiff layer, generating a set of basic movements including bending, coiling, and twisting. In this study, we investigate a new type of functionally graded hygroscopic movement in the fruit (capsule) of sesame (Sesamum indicum L.). Microscopic observations of the capsules showed that the inner stiff endocarp layer is built of a bilayer of transverse (i.e., circumferential) and longitudinal fiber cells with the layers positioned in a semi-circle, one inside the other. The outer mesocarp layer is made of soft parenchyma cells. The thickness of the fibrous layers and of the mesocarp exhibits a graded architecture, with gradual changes in their thickness around the capsule circumference. The cellulose microfibrils in the fiber cell walls are lying parallel to the cell long axis, rendering them stiff. The outer mesocarp layer contracted by 300% as it dried. Removal of this outer layer inhibited the opening movement, indicating that it is the active tissue. A biomechanical hygro-elastic model based on the relative thicknesses of the layers successfully simulated the opening curvature. Our findings suggest that the sesame capsules possess a functionally graded architecture, which promotes a non-uniform double-curvature hygroscopic bending movement. In contrast to other hygroscopic organs described in the literature, the sesame capsule actuating and resisting tissues are not uniform throughout the device, but changing gradually. This newly described mechanism can be exploited in bio-inspired designs of novel actuating platforms.

Ordering of protein and water molecules at their interfaces with chitin nano-crystals.

Synchrotron X-ray diffraction was applied to study the structure of biogenic α-chitin crystals composing the tendon of the spider Cupiennius salei. Measurements were carried out on pristine chitin crystals stabilized by proteins and water, as well as after their deproteinization and dehydration. We found substantial shifts (up to Δq/q=9% in the wave vector in q-space) in the (020) diffraction peak position between intact and purified chitin samples. However, chitin lattice parameters extracted from the set of reflections (hkl), which did not contain the (020)-reflection, showed no systematic variation between the pristine and the processed samples. The observed shifts in the (020) peak position are discussed in terms of the ordering-induced modulation of the protein and water electron density near the surface of the ultra-thin chitin fibrils due to strong protein/chitin and water/chitin interactions. The extracted modulation periods can be used as a quantitative parameter characterizing the interaction length.

Gelatin yarns inspired by tendons--structural and mechanical perspectives.

Tendons are among the most robust structures in nature. Using the structural properties of natural tendon as a foundation for the development of micro-yarns may lead to innovative composite materials. Gelatin monofilaments were prepared by casting and spinning and small yarns--with up to ten filaments--were assembled into either parallel or 15° twisted yarns. The latter were intended as an attempt to generate mechanical effects similar to those arising from the crimp pattern in tendon. The mechanical properties of parallel and 15° twisted gelatin yarns were compared. The effect of an increasing number of filaments per yarn was also examined. The mechanical properties were mostly affected by the increasing number of filaments, and no benefit arose from twisting small yarns by 15°. However, since gelatin filaments are elasto-plastic rather than fully elastic, much increased toughness (by up to a factor of five for a ten filament yarn) can be achieved with yarns made of elasto-plastic filaments, as demonstrated by experiments and numerical simulations. The resulting effect shows some resemblance to the effect of crimp in tendons. Finally, we developed a dependable procedure to measure the toughness of single filaments based on the test of a yarn rather than on a large number of individual filament tests.