C. Sachs

Revealing the Design Principles of High‐Performance Biological Composites Using Ab initio and Multiscale Simulations: The Example of Lobster Cuticle

In the course of evolution nature developed materials based on organic–inorganic nanocomposites with complex, hierarchical organization from A u ngstroms to millimeters tailored via molecular self-assembly. [1–3] Such materials possess outstanding stiffness, toughness, and strength related to their low density, while the mechanical characteristics of their underlying constituents are rather modest. [2,4] This remarkable performance is a consequence of their hierarchical structure, the specific design at each level of organization, and the inherent strong heterogeneity [4] resulting in the accommodation of macroscopic loadings bydifferentdeformationmechanisms at differentlength scales. Therefore, to understand the macroscopic mechanical properties of the tissue, one should take into account its structure–property relations at all length scales down to the molecular level. To date, this key challenge has been only partly addressed due to severe obstacles in obtaining mechanical and structural data at the nanometer scale. The mechanical properties of important proteins and biominerals as well as some details about their exact structure are still unknown. A powerful tool to overcome these difficulties and to better understand the structure–property relationships in biomaterials is multiscale modeling encompassing all length scales. [3,5] Some progress in the development of multiscale structure–property relationships for mineralized tissues has been achieved by combined modeling and experimental approaches applied to bone, [4] nacre, [6] and fish skin armor. [7] However, these approaches do not explicitly integrate a molecular-level description and use continuum mechanics at the meso- and macroscale (e.g., finite element analysis) coupled with experimental data obtained, for example, by nanoindentation. A

Robustness and optimal use of design principles of arthropod exoskeletons studied by ab initio-based multiscale simulations

Recently, we proposed a hierarchical model for the elastic properties of mineralized lobster cuticle using (i) ab initio calculations for the chitin properties and (ii) hierarchical homogenization performed in a bottom-up order through all length scales. It has been found that the cuticle possesses nearly extremal, excellent mechanical properties in terms of stiffness that strongly depend on the overall mineral content and the specific microstructure of the mineral-protein matrix. In this study, we investigated how the overall cuticle properties changed when there are significant variations in the properties of the constituents (chitin, amorphous calcium carbonate (ACC), proteins), and the volume fractions of key structural elements such as chitin-protein fibers. It was found that the cuticle performance is very robust with respect to variations in the elastic properties of chitin and fiber proteins at a lower hierarchy level. At higher structural levels, variations of design parameters such as the volume fraction of the chitin-protein fibers have a significant influence on the cuticle performance. Furthermore, we observed that among the possible variations in the cuticle ingredients and volume fractions, the experimental data reflect an optimal use of the structural variations regarding the best possible performance for a given composition due to the smart hierarchical organization of the cuticle design.

Structure and Crystallographic Texture of Arthropod Bio-Composites

In this study we present experimental investigations on the microscopic structure, constituent phases, and crystallographic textures of the exoskeleton of three types of decapod crustaceans, namely, lobster, crab, and horseshoe crab. The carapace of such animals is a biological multiphase nano-composite consisting of an organic matrix (crystalline chitin and non-crystalline proteins) and biominerals (calcite, phosphate). The synchrotron measurements of the crystalline chitin and of the biominerals which are embedded in the chitin-protein matrix (in case of lobster and crab) reveal strong textures. The horseshoe crab does not seem to contain notable amounts of crystalline minerals. The Debye-Scherrer images of the lobster specimen suggest that the biominerals form clusters of crystals with similar crystallographic orientation. TEM images support this suggestion. The crystallographic texture of the chitin is arranged with its longest cell axis parallel to the normal of the surface of the exoskeleton.

Mesostructure of the Exoskeleton of the Lobster Homarus Americanus

The exoskeleton of the lobster Homarus americanus is a multiphase bio-composite which consists of a fibrous organic matrix (crystalline α-chitin and various types of non-crystalline proteins) and embedded biominerals (mainly calcite). In this study we present experimental data about the microscopic and mesoscopic structure of this material.

Multi‐scale Modelling of a Biological Material: The Arthropod Exoskeleton

From the viewpoint of materials science, the vast majority of biological materials are organic–inorganic composites with a hierarchical organization spanning over multiple levels from the molecular scale to the macro‐scale. What makes them interesting as materials is that they have been optimized during evolution to perform vital functions within the specific eco‐physiological constraints imposed on living organisms. These functions are very diverse and can be e.g. of mechanical, locomotive, optical or sensory nature, and frequently combinations of them. The required diversity of physical properties is caused by structural and chemical alterations at different hierarchical levels utilizing the morphological and genetic prerequisites available to the organism. In order to understand the design principles of such materials with specific functions, it is necessary to study the relationship between their structure, composition and the resulting physical properties. This is usually done using an experimental approach, where materials science offers a large variety of methods to study microstructure, chemical composition and mechanical properties and behaviour. In practice, however, it is frequently not possible to establish and validate the overall structure–property relationships for a biological material owing to the complex structural hierarchy and methodological constraints. Numerical multi‐scale models are very elegant and versatile tools to overcome these inherent shortcomings since they can systematically describe materials properties from the atomic up to the macroscopic scale.