Elastic Moduli of Avian Eggshell

Mechanical design principles of avian eggshells for survivability

Biological materials exhibit complex structure-property relationships which are designed by nature's evolution over millions of years. Unlocking the fundamental physical principles behind these relationships is crucial for creating bioinspired materials and structures with advanced functionalities. The eggshell is a remarkable example with a well-designed structure to balance the trade-off as it provides mechanical protection while still being easy for hatching. In this study, we investigate the underlying mechanical design principles of chicken eggshells under various loading conditions through a combination of experiments and simulations. The unique geometry and structure of the eggshell play a critical role in achieving an excellent balance between mechanical toughness and ease of hatching. The effects of eggshell membranes are elucidated to tune the mechanical properties of the eggshell to further enhance this balance. Moreover, a mechanics-based three-index model is proposed based on these design principles, suggesting the optimal eggshell thickness design to improve survivability across a broad range of avian species with varying egg sizes. The survivability-design relationships hold great potential for the development of improved structural materials for applications in sports safety equipment and the packaging industry. STATEMENT OF SIGNIFICANCE: The fundamental physical principles underlying the complex structure-property relationships in biological materials are uncovered in this study, with a particular focus on chicken eggshells as a prime example. Through the investigation of their mechanical design, we reveal the critical role of eggshell geometry and structure in achieving a balance between toughness and ease of hatching. Specifically, the crack resting effect is observed, making the eggshell easier to break from the inside than from the outside. Additionally, we explore the influence of eggshell membranes on this balance, contributing to the enhancement of the eggshell's mechanical properties. For the first time, we propose a three-index model that uncovers the underlying principles governing the evolution of eggshell thickness. This model suggests optimal thickness designs for diverse avian species, with the goal of enhancing egg survivability. These findings can guide the development of improved structural materials with advanced functionalities, enabling greater safety and efficiency in a wide range of applications.

Avian obligate brood parasitic lineages evolved variable complex polycrystalline structures to build tougher eggshells

Avian brood parasites and their hosts display varied egg-puncture behaviors, exerting asymmetric co-evolutionary selection pressures on eggshells' breaking strength. We investigated eggshell structural and textural characteristics that may improve its mechanical performance. Parasitic eggshell calcified layers showed complex ultra- and microstructural patterns. However, stronger parasitic eggshells are not due to lower textural severity (characterized by lower preferred crystallographic orientation, larger local grain misorientation and smaller Kearns factor), but rather to grain boundary (GB) microstructure characteristics within the eggshell outermost layer (palisade layer, PL). Accordingly, the thicker the PL and the more complex the GB pathways are, the tougher the parasitic eggshells will be. These characteristics, which we can identify as a "GB Engineering" driven co-evolutionary process, further improve eggshell breaking strength in those parasitic species that suffer relatively high frequencies of egg-puncturing by congeneric or hosts. Overall, plain textural patterns are not suitable predictors for comparing mechanical performance of bioceramic materials.

Effects of Zinc, Manganese, and Taurine on Egg Shell Microstructure in Commercial Laying Hens After Peak Production

Much strive has been made to improve egg shell quality in laying hens. This study was conducted to evaluate the effects of two microminerals, zinc and manganese, besides taurine semi-essential amino acid on eggshell quality after peak production of Hy-line laying hens. A total of 720 laying hens were assigned to 18 treatments in a completely randomized design (3 × 3 × 2 factorial) at week 71. Experimental period included 8-week adaptation and using 18 dietary treatments for about 6 weeks. Dietary treatments included Zn (0, 80, and 160 mg/kg), Mn (0, 90, and 180 mg/kg), and taurine (0 and 1960 mg/kg). Supplementation of 90 mg Mn and 1960 mg taurine in laying hens' diet after peak of production improved egg shell quality without any negative effect on the internal quality of the egg. Egg specific gravity significantly increased in response to Zn, Mn, and taurine in comparison with control treatment (P < 0.05). Applying 1960 mg taurine/kg diet significantly improved calcite crystal's structure and eggshell strength in comparison with control treatment (P < 0.05). It was concluded that adding Mn and taurine can positively affect eggshell quality of laying hens post peak period.

A Comparative Study on the Microstructures, Mineral Content, and Mechanical Properties of Non-Avian Reptilian Eggshells

We analyze 214 freshly laid eggs belonging to 16 species across three orders of Class Reptilia. Using mechanical compression tests, we measure each egg's absolute stiffness (K, unit: N m-1) and relative stiffness (C number). The effective Young's modulus, E, was obtained by combining experimental and numerical methods. The mineral (CaCO3) content was measured by acid-base titration, the microstructures by scanning electron microscopy (SEM), and the crystallography by electron backscatter diffraction (EBSD). We find that the C number of reptilian eggs is, on average, higher than that of bird eggs, indicating that reptilian eggs are stiffer with respect to the egg mass than birds. However, Young's moduli of the reptilian eggshells (32.85 ± 3.48 GPa) are similar to those of avian eggshells (32.07 ± 5.95 GPa), even though those eggshells have different crystal forms, microstructures, and crystallography. Titration measurement shows that the reptilian eggshells are highly mineralized (>89% for nine Testudines species and 96% for Caiman crocodilus). Comparing the species with aragonite and calcite crystals, we find that calcite shells, including those of the Kwangsi gecko (inner part) and spectacled caiman (outer part), generally have larger grains than the aragonite ones. However, the grain size is not correlated to the effective Young's modulus. Also, as measured by the C number, the aragonite shells are, on average, stiffer than the calcite ones (except for the Kwangsi gecko), primarily due to their thicker shells.

Unveiling the secret of ancient Maya masons: Biomimetic lime plasters with plant extracts

Ancient Maya produced some of the most durable lime plasters on Earth, yet how this was achieved remains a secret. Here, we show that ancient Maya plasters from Copan (Honduras) include organics and have a calcite cement with meso-to-nanostructural features matching those of calcite biominerals (e.g., shells). To test the hypothesis that the organics could play a similar toughening role as (bio)macromolecules in calcium carbonate biominerals, we prepared plaster replicas adding polysaccharide-rich bark extracts from Copan's local trees following an ancient Maya building tradition. We show that the replicas display similar features as the organics-containing ancient Maya plasters and demonstrate that, as in biominerals, in both cases, their calcite cement includes inter- and intracrystalline organics that impart a marked plastic behavior and enhanced toughness while increasing weathering resistance. Apparently, the lime technology developed by ancient Maya, and likely other ancient civilizations that used natural organic additives to prepare lime plasters, fortuitously exploited a biomimetic route for improving carbonate binders performance.

Microstructural and crystallographic evolution of palaeognath (Aves) eggshells

The avian palaeognath phylogeny has been recently revised significantly due to the advancement of genome-wide comparative analyses and provides the opportunity to trace the evolution of the microstructure and crystallography of modern dinosaur eggshells. Here, eggshells of all major clades of Palaeognathae (including extinct taxa) and selected eggshells of Neognathae and non-avian dinosaurs are analysed with electron backscatter diffraction. Our results show the detailed microstructures and crystallographies of (previously) loosely categorized ostrich-, rhea-, and tinamou-style morphotypes of palaeognath eggshells. All rhea-style eggshell appears homologous, while respective ostrich-style and tinamou-style morphotypes are best interpreted as homoplastic morphologies (independently acquired). Ancestral state reconstruction and parsimony analysis additionally show that rhea-style eggshell represents the ancestral state of palaeognath eggshells both in microstructure and crystallography. The ornithological and palaeontological implications of the current study are not only helpful for the understanding of evolution of modern and extinct dinosaur eggshells, but also aid other disciplines where palaeognath eggshells provide useful archive for comparative contrasts (e.g. palaeoenvironmental reconstructions, geochronology, and zooarchaeology).

Designing Bioinspired Composite Structures via Genetic Algorithm and Conditional Variational Autoencoder

Designing composite materials with tailored stiffness and toughness is challenging due to the massive number of possible material and geometry combinations. Although various studies have applied machine learning techniques and optimization methods to tackle this problem, we still lack a complete understanding of the material effects at different positions and a systematic experimental procedure to validate the results. Here we study a two-dimensional (2D) binary composite system with an edge crack and grid-like structure using a Genetic Algorithm (GA) and Conditional Variational Autoencoder (CVAE), which can design a composite with desired stiffness and toughness. The fitness of each design is evaluated using the negative mean square error of their predicted stiffness and toughness and the target values. We use finite element simulations to generate a machine-learning dataset and perform tensile tests on 3D-printed specimens to validate our results. We show that adding soft material behind the crack tip, instead of ahead of the tip, tremendously increases the overall toughness of the composite. We also show that while GA generates composite designs with slightly better accuracy (both methods perform well, with errors below 20%), CVAE takes considerably less time (~1/7500) to generate designs. Our findings may provide insights into the effect of adding soft material at different locations of a composite system and may also provide guidelines for conducting experiments and Explainable Artificial Intelligence (XAI) to validate the results.

Analysis of ultrastructure and microstructure of blackbird (Turdus merula) and song thrush (Turdus philomelos) eggshell by scanning electron microscopy and X-ray computed microtomography

The unique structure of the egg allows for efficient reproduction on land. Although the functions of the egg are ensured by the concomitant cooperation of all its structures, the eggshell also plays a significant role. Apart from maintaining an aqueous environment within the egg along with controlled gas exchange, the color and pigmentation pattern of eggshell contributes to identification and protection. As a result of all these functions, the structure, shape, and pigmentation of eggshell greatly vary across the class of birds, and understanding these three variability-determining factors may aid in better interpretation of evolutionary mechanisms. In this study, we analyzed for the first time the structure, mineral composition, and characteristics of the pigmentation of blackbird (Turdus merula) and song thrush (Turdus philomelos) eggshells. The shell of blackbird eggs is much thicker compared to the shell of song thrush eggs which is due to a much thicker crystalline and palisade layers. In both species, strongly elongated mammillary knobs are observed, which create a large space between the mineralized shell and the egg membranes. The blackbird egg shell has a higher water vapor conductivity which is due to the larger diameter of the circle and the surface area of individual pores. The primary compound entering the mineral composition of the shell in both species is CaCO₃ however, the thrush egg shells contained more Mg in all layers except the crystalline layer, and S in the crystalline and palisade layers. The two species clearly differ in the size and distribution of pigment spots on the eggshell. We suppose that the differences in shell structure and pigmentation presented in this study may in the future provide a basis for explaining the reasons for the much lower reproductive efficiency of song thrush compared to blackbird.