Seagull feather shaft: Correlation between structure and mechanical response

Mechanistic insights into hydration-driven shape memory response in keratinous avian feather structures

Keratinous materials found in the feather shafts of flying birds possess impressive mechanical attributes, combining excellent strength-to-weight balance, toughness, and more. In this study, we investigate the shape memory effect in bird feather shafts, examining its underlying design principles as templates for bioinspired shape memory composites. Through analytical and computational analysis, we aim to uncover the underlying rules and design guidelines based on stimulus-induced softening (pertaining to strength and/or stiffness) and swelling (pertaining to expansion in volume). More specifically, we study a one-dimensional case to examine the synergistic relationship between the matrix and fibers inside the feather structure. We propose three distinct micro-mechanical modeling approaches to evaluate the contribution of each hydration-induced effect-softening, swelling, and the combined action of both. In all models, the matrix is considered to be an elastic-perfectly plastic material that is sensitive to hydration, while the fibers are treated as purely elastic and unaffected by hydration. The findings of the study provide informative insights into the nuanced nature of swelling within the material, highlighting that its desirability is dependent on specific conditions and circumstances. Furthermore, we find that the softening component plays a large pivotal role in driving the process of shape recovery. Using the proposed analytical framework and design principles, we develop a conceptual feather shaft-like composite, followed by demonstrating its tunability in degree of shape recovery and its versatility in selecting constituent base material components. This research offers valuable core framework for exploring and designing advanced bioinspired shape memory materials while eliminating the need for traditionally active shape memory components, holding promising potential for actuation, deployment, and morphing purposes. STATEMENT OF SIGNIFICANCE: This study investigates the shape-memory effect in bird feather shafts, offering bioinspired strategies for designing advanced shape-memory composites. Unlike conventional materials, which often rely on external stimuli or active components, our research focuses on hydration-driven mechanisms-specifically, matrix softening and swelling. Through micro-mechanical modeling, we demonstrate that softening is the key driver of shape recovery, while swelling plays a secondary role under specific conditions. These insights provide new, passive design principles for creating tunable shape-memory composites without the need for traditional active components. The findings have broad implications for applications in actuation, morphing, and reconfigurable systems, where material adaptability is crucial.

Keratin from Animal By-Products: Structure, Characterization, Extraction and Application-A Review

Keratin is a structural fibrous protein and the core constituent of animal by-products from livestock such as wool, feathers, hooves, horns, and pig bristles. This natural polymer is also the main component of human hair and is present at an important percentage in human and animal skin. Significant amounts of keratin-rich animal tissues are discarded worldwide each year, ca. 12 M tons, and the share used for keratin extraction and added-value applications is still very low. An important stream of new potential raw materials, represented by animal by-products and human hair, is thus being lost, while a large-scale valorization could contribute to a circular bioeconomy and to the reduction in the environmental fingerprint of those tissues. Fortunately, scientific research has made much important progress in the last 10-15 years in the better understanding of the complex keratin architecture and its variability among different animal tissues, in the development of tailored extraction processes, and in the screening of new potential applications. Hence, this review aims at a discussion of the recent findings in the characterization of keratin and keratin-rich animal by-product structures, as well as in keratin recovery by conventional and emerging techniques and advances in valorization in several fields.

Lightweight Structural Biomaterials with Excellent Mechanical Performance: A Review

The rational design of desirable lightweight structural materials usually needs to meet the strict requirements of mechanical properties. Seeking optimal integration strategies for lightweight structures and high mechanical performance is always of great research significance in the rapidly developing composites field, which also draws significant attention from materials scientists and engineers. However, the intrinsic incompatibility of low mass and high strength is still an open challenge for achieving satisfied engineering composites. Fortunately, creatures in nature tend to possess excellent lightweight properties and mechanical performance to improve their survival ability. Thus, by ingenious structure configuration, lightweight structural biomaterials with simple components can achieve high mechanical performance. This review comprehensively summarizes recent advances in three typical structures in natural biomaterials: cellular structures, fibrous structures, and sandwich structures. For each structure, typical organisms are selected for comparison, and their compositions, structures, and properties are discussed in detail, respectively. In addition, bioinspired design approaches of each structure are briefly introduced. At last, the outlook on the design and fabrication of bioinspired composites is also presented to guide the development of advanced composites in future practical engineering applications.

The Armor of the Chinese Sturgeon: A Study of the Microstructure and Mechanical Properties of the Ventral Bony Plates

Benefiting from their unique morphological characteristics and structural properties, the ventral bony plates of the Chinese sturgeon are excellent biological protective tissue. In this work, we studied the micro- and macro-morphology and mechanical properties of the ventral bony plates of the Chinese sturgeon to elucidate the special protective mechanisms of the bony plates. Experiments involving scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that the bony plates possess a hierarchical structure and a ridge-like shape. This structure comprises a surface layer containing mineralized nanocrystals and an internal layer containing mineralized collagen fibers. From the surface layer to the internal layer, the degree of mineralization decreases gradually. Nanoindentation, tension, and compression tests demonstrated that the bony plates feature excellent mechanical properties and a high specific tensile strength comparable to that of stainless steel. Moreover, water can significantly improve the fracture toughness and deformability of the bony plates and effectively enhance the damage tolerance of the structures. The obtained results concerning the microstructure-property-function relationships of the ventral bony plates of the Chinese sturgeon may provide novel insights for designing protective structures that are both lightweight and high strength.

Effect of different factors on treatment of oily wastewater by TiO2/Al2O3-PVDF ultrafiltration membrane

An ultrafiltration membrane developed by our research group was applied to treat simulated emulsified oil wastewater. ATR-FTIR, SEM, TEM, and Zeta potential analyzes demonstrated that the modified ultrafiltration membrane (MM) has excellent stability and anti-fouling capacity than origin membrane (OM), which possesses a pure water flux of 260 L·m^-2·h^-1 and oil/water (o/w) rejection of 98.5 ± 0.33%. Inorganic salt CaCl₂ has more considerable influence than MgSO₄ and NaCl under the same mass concentration in the two membranes UF process. Along with concentration increasing, flux sharply reduces; meanwhile, the rejection has an opposite trend. Moreover, permeation flux has a maximum value, and the rejection also gets its optimal state under neutral conditions during the pH value of 2-12. The membrane also exhibits excellent anti-fouling performance and anti- o/w adsorption properties with an adsorption rate below 0.8% compared with OM, which has an adsorption rate of nearly 2.1%, respectively. A kind of new UF membrane developed by our research group was applied to treat simulated o/w. ATR-FTIR, SEM, TEM, and Zeta potential analyzes demonstrated that PVDF-Al₂O₃/TiO₂ material has excellent stability and anti-fouling capacity. CaCl₂ has the greatest influence than MgSO₄ and NaCl under the same mass concentration. Moreover, permeation flux has maximum value and the rejection also gets its optimal state under neutral conditions during pH 2-12. The membrane also exhibits excellent anti-fouling performance and anti-O/W adsorption properties with adsorption rate below 0.8% compared with OM which has an adsorption rate nearly 2.1%, respectively.

Microscopy imaging and modeling study on the mechanical properties of the primary flight feather shaft of the bean goose, Anser fabalis

Avian flight feathers have the unique advantages of lightweight and high strength, which play a key role in their flight capacity. In this article, the rachis of the bean goose's primary flight feather was used as the research object. Its compressive properties were analyzed and the 3D microscale was observed by 3D microscope system with a super wide depth of field. The distribution of mechanical properties, section variation of fiber and internal microstructure of rachis were obtained by micro-CT technique. Based on these results, a 3D reconstructed model was established for structure mechanical simulation. The simulation results were close basically to the compressive strength of the actual sample. These results show that the synergistic effect of cortex and medulla can improve mechanical resistance of the rachis. Therefore, the best position (N3) of the primary flight feather shaft can be applied to the bionic design of thin wall structures for energy absorption. This research can provide some guidance for the application of lightweight structural design.

Anisotropic Composition and Mechanical Behavior of a Natural Thin-Walled Composite: Eagle Feather Shaft

Flight feather shafts are outstanding bioinspiration templates due to their unique light weight and their stiff and strong characteristics. As a thin wall of a natural composite beam, the keratinous cortex has evolved anisotropic features to support flight. Here, the anisotropic keratin composition, tensile response, dynamic properties of the cortex, and fracture behaviors of the shafts are clarified. The analysis of Fourier transform infrared (FTIR) spectra indicates that the protein composition of calamus cortex is almost homogeneous. In the middle and distal shafts (rachis), the content of the hydrogen bonds (HBs) and side-chain is the highest within the dorsal cortex and is consistently lower within the lateral wall. The tensile responses, including the properties and dominant damage pattern, are correlated with keratin composition and fiber orientation in the cortex. As for dynamic properties, the storage modulus and damping of the cortex are also anisotropic, corresponding to variation in protein composition and fibrous structure. The fracture behaviors of bent shafts include matrix breakage, fiber dissociation and fiber rupture on compressive dorsal cortex. To clarify, ‘real-time’ damage behaviors, and an integrated analysis between AE signals and fracture morphologies, are performed, indicating that calamus failure results from a straight buckling crack and final fiber rupture. Moreover, in the dorsal and lateral walls of rachis, the matrix breakage initially occurs, and then the propagation of the crack is restrained by ‘ligament-like’ fiber bundles and cross fiber, respectively. Subsequently, the further matrix breakage, interface dissociation and induced fiber rupture in the dorsal cortex result in the final failure.

Study on the structural features and geometric parameters affecting the axial mechanical properties of the primary feather rachis

The seagull feather shaft is an important part of the feather, which provides a good mechanical support for the excellent flight performance of seagull, and has the characteristics of lightweight and high strength. In this paper, the microstructure of the seagull feather rachis was observed firstly. Then, based on the structure of feather rachis, combined with the cortex that plays the main load-bearing role, a model with the characteristics of the cortex was proposed and its finite element model was established. Through analyzing the simulation, the effect of section shape of cortex on mechanical properties of feathers under axial impact was revealed. And the conclusion that the section shape with groove structure and non-equal wall thickness could have different effects on mechanical properties was drawn. Then, parameterized cortical models were studied, including different impact velocities and different cortical heights, to reveal the differences in mechanical properties of cortical models.

Correlation between wing bone microstructure and different flight styles: The case of the griffon vulture (gyps fulvus) and greater flamingo (phoenicopterus roseus)

Flying is the main means of locomotion for most avian species, and it requires a series of adaptations of the skeleton and of feather distribution on the wing. Flight type is directly associated with the mechanical constraints during flight, which condition both the morphology and microscopic structure of the bones. Three primary flight styles are adopted by avian species: flapping, gliding, and soaring, with different loads among the main wing bones. The purpose of this study was to evaluate the cross-sectional microstructure of the most important skeletal wing bones, humerus, radius, ulna, and carpometacarpus, in griffon vultures (Gyps fulvus) and greater flamingos (Phoenicopterus roseus). These two species show a flapping and soaring flight style, respectively. Densitometry, morphology, and laminarity index were assessed from the main bones of the wing of 10 griffon vultures and 10 flamingos. Regarding bone mineral content, griffon vultures generally displayed a higher mineral density than flamingos. Regarding the morphology of the crucial wing bones involved in flight, while a very slightly longer humerus was observed in the radius and ulna of flamingos, the ulna in griffons was clearly longer than other bones. The laminarity index was significantly higher in griffons. The results of the present study highlight how the mechanics of different types of flight may affect the biomechanical properties of the wing bones most engaged during flight.

Structure of Keratin

Keratins, as a group of insoluble and filament-forming proteins, mainly exist in certain epithelial cells of vertebrates. Keratinous materials are made up of cells filled with keratins, while they are the toughest biological materials such as the human hair, wool and horns of mammals and feathers, claws, and beaks of birds and reptiles which usually used for protection, defense, hunting and as armor. They generally exhibit a sophisticated hierarchical structure ranging from nanoscale to centimeter-scale: polypeptide chain structures, intermediated filaments/matrix structures, and lamellar structures. Therefore, more and more attention has been paid to the investigation of the relationship between structure and properties of keratins, and a series of biomimetic materials based on keratin came into being. In this chapter, we mainly introduce the hierarchical structure, the secondary structure, and the molecular structure of keratins, including α- and β-keratin, to promote the development of novel keratin-based biomimetic materials designs.

Morphological characterization of flight feather shafts in four bird species with different flight styles

Variation in rachis (central shaft) morphology in individual remiges (flight feathers) within and among species reflects adaptations to requirements imposed by aerodynamic forces, but the fine-scale variation of feather morphology across remiges is not well known. Here we describe how the shape of the rachis, expressed by the height/width ratio, changes along the longitudinal and lateral axis of the wing in four bird species with different flight styles: flapping-soaring (white storks), flapping-gliding (common buzzards), passerine-type (house sparrows) and continuous flapping (pygmy cormorants). Overall, in each wing feather, irrespective of species identity, rachis shape changed from circular to rectangular, from the base towards the feather tip. The ratio between the height and width of the calamus was similar across remiges in all species, whereas the ratio at the base, middle and tip of the rachis changed among flight feathers and species. In distal primaries of white storks and common buzzards, the ratio decreased along the feather shaft, indicating a depressed (wider than high) rachis cross section towards the feather tip, whereas the inner primaries and secondaries became compressed (higher than wide). In house sparrows, the rachis was compressed in each of the measurement points, except at the distal segment of the two outermost primary feathers. Finally, in pygmy cormorants, the width exceeds the height at each measurement point, except at the calamus. Our results may reflect the resistance of the rachis to in-plane and out-of-plane aerodynamic forces that vary across remiges and across study species. A link between rachis shape and resistance to bending from aerodynamic forces is further indicated by the change of the second moment of areas along the wing axes.

Microstructure and compression resistance of bean goose (Anser fabalis) feather shaft

The bean goose Anser fabalis, noted for its excellent flying ability, has feathers composed of keratinized products derived from epidermal cells, which play a crucial role in flight. The feather shaft is an important connective unit, made of a lightweight material, which also contributes to aiding flight. The shaft can withstand loads from different directions and has outstanding compression resistance. In this study, the microstructure and composition of the A. fabalis feather shaft were observed by scanning electron microscopy and Fourier transform infrared spectrometry, and its compression resistance was studied by compression testing. The results indicated that the mechanical property of the shaft is related to its microstructure. Compression testing verified that the primary feathers had the strongest mechanical properties, followed by the secondaries, and finally the alulae. Under the same conditions, the specific energy absorption of the three feather types was 5.96, 5.02, and 3.17 J/g, respectively. With increasing moisture content, the rachis was softened and the energy absorption was reduced. At low moisture content, the specific energy absorption of the primaries was reduced to 1.03 J/g, that of the secondaries was reduced to 1.72 J/g, and that of the alulae to 0.39 J/g. The feather shafts have the advantage of light weight while maintaining the required mechanical properties. These results provide a theoretical and experimental basis for crashworthiness in bionic designs based on the requirements of light weight.

On the correlation between the curvature of the human eyelash and its geometrical features

Although human eyelashes are generally curved, the cause of the natural curvature of eyelashes has not yet to be clarified elsewhere. Related with this, this paper reports our discovery of a correlation between the curvature of the eyelash and its geometrical features. Eyelashes can be divided into root, middle and tip sections. Because the curvature at the root is larger than that at the tip, we expected that the root section could be more easily deformed by bending compared with the tip section. However, the structural elasticity in bending, which is the flexural rigidity without depending on the external dimensions, at the root was found to be greater than that at the tip, contrary to our initial expectations. Next we examined the internal dimensions of cross sections of the eyelashes, and found that the thicknesses of the cuticle layer at the root were different for the convex and concave sides of the curved eyelash, although these were almost the same at the tip. Theoretical analysis of this variation in thickness of the outer cuticle layer shows that this displaces the neutral axis. Finally, we found that there is a good correlation between the displacement of the neutral axis and the curvature of the eyelash.

STATEMENT OF SIGNIFICANCE

Why are human eyelashes naturally curved? To find a hint for this question, the mechanical and geometrical properties of human eyelash were investigated. Although the curvature at the root of the eyelash was larger than that at the tip, this was not related to the deformability of the eyelash by bending. From the cross-sectional observation of eyelash, we noticed that the thickness of the outer cuticle layer was non-uniform depending on the position, and this brought the displacement of the neutral axis of the eyelash for bending. Finally, a good correlation between the curvature and the change in the neutral axis was discovered. With practically using this findings, the curvature of the eyelash might be controlled artificially in the future.

A review of terrestrial, aerial and aquatic keratins: the structure and mechanical properties of pangolin scales, feather shafts and baleen plates

Keratinous materials, omnipresent as the hard and durable epidermal appendages of animals, are among the toughest biological materials. They exhibit diverse morphologies and structures that serve a variety of amazing and inspiring mechanical functions. In this work, we provide a review of representative terrestrial, aerial and aquatic keratinous materials, pangolin scales, feather shafts and baleen plates, and correlate their hierarchical structures to respective functions of dermal armor, flight material and undersea filter. The overlapping pattern of pangolin scales provides effective body coverage, and the solid scales show transverse isotropy and strain-rate sensitivity, both important for armor function. The feather shaft displays a distinct shape factor, hierarchical fibrous structure within the cortex, and a solid shell-over-foam design, which enables synergistic stiffening and toughening with exceptional lightness to fulfill flight. Baleen plates exhibit a sandwich-tubular structure that features anisotropic flexural properties to sustain forces from water flow and remarkable fracture toughness that ensures reliable undersea functioning. The latest findings regarding the structural design principles and mechanical properties are presented in order to advance current understanding of keratinous materials and to stimulate the development of new bioinspired materials.