Helge-Otto Fabritius

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

Correlation of structure, composition and local mechanical properties in the dorsal carapace of the edible crab Cancer pagurus

Abstract The exoskeleton of crustaceans is formed by the cuticle, a chitin-protein-based nano-composite with hierarchical organization over at least eight levels. On the molecular level, it consists of chitin associated with proteins forming fibres, which are organized in the form of twisted plywood. On the higher levels, the twisted plywood organization is modified and forms skeletal elements with elaborate functions. The load-bearing parts of crustacean cuticle are reinforced with both crystalline and amorphous biominerals. During evolution, all parts of the exoskeleton were optimized to fulfill different functions according to different ecophysiological strains faced by the animals. This is achieved by modifications in microstructure and chemical composition. In order to understand the relationship between structure, composition, mechanical properties and function we structurally characterized cuticle from the dorsal carapace of the edible crab Cancer pagurus using light and scanning electron microscopy (SEM). The local chemical composition was investigated using energy dispersive X-ray spectroscopy (EDX) and confocal m-Raman spectroscopy. Nanoindentation tests were performed to study the resulting local mechanical properties. The results show local differences in structure on several levels of the structural hierarchy in combination with a very heterogeneous mineralization. The distal exocuticle is mineralized with calcite, followed by a layer containing a magnesium, phosphate and carbonate rich phase and ACC in the proximal part. The endocuticle contains magnesian calcite and ACC in special regions below the exocuticle. Structure and mineral phase are reflected in the local stiffness and hardness of the respective cuticle regions. The heterogeneity of structural organization and mechanical properties suggests remarkable consequences for the mechanical behaviour of the bulk material.

Ultrastructural organization and micromechanical properties of shark tooth enameloid

The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80nm) and long (>1μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites.

Characterization of crocodile teeth: Correlation of composition, microstructure, and hardness

Structure and composition of teeth of the saltwater crocodile Crocodylus porosus were characterized by several high-resolution analytical techniques. X-ray diffraction in combination with elemental analysis and infrared spectroscopy showed that the mineral phase of the teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: Dentin, enamel, and cementum. The fluoride content in the three tissues is very low (<0.1 wt.%) and comparable to that in human teeth. The mineral content of dentin, enamel, and cementum as determined by thermogravimetry is 71.3, 80.5, and 66.8 wt.%, respectively. Synchrotron X-ray microtomography showed the internal structure and allowed to visualize the degree of mineralization in dentin, enamel, and cementum. Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin (100-200 μm). The crystallites in the enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases, and the crystallites do not display an ordered structure as in the enamel. The microhardness was 0.60±0.05 GPa for dentin, 3.15±0.15 GPa for enamel, 0.26±0.08 GPa for cementum close to the crown, and 0.31±0.04 GPa for cementum close to the root margin. This can be explained with the different degree of mineralization of the different tissue types and is comparable with human teeth.

Function-related adaptations of ultrastructure, mineral phase distribution and mechanical properties in the incisive cuticle of mandibles of Porcellio scaber Latreille, 1804

In terrestrial isopods the mandibles consist of a corpus carrying strong muscle tissue, and a pars incisiva (PI) that cuts dry leaves into smaller ingestible pieces. We studied the cuticle of the PI of Porcellio scaber in order to understand region-dependent differences in its ultrastructure, composition, and the resulting mechanical properties, employing several microscopic and analytical techniques as well as nanoindentation experiments. The cuticle of the incisive tip is not mineralized and consists of an unusually thick epicuticle containing thin fibrils, two subjacent cuticular layers, and a central core containing fibrils of different orientation, either longitudinal or circumferential. A thick epicuticle of the middle region just behind the tip projects long epicuticular extensions into the subjacent endocuticle, likely to prevent delamination. A distinct exocuticular layer is lacking in the middle region. Most chitin-protein fibrils within the endocuticle are oriented in parallel pointing towards the tip. Surprisingly, the middle region is mineralized by amorphous calcium phosphate (ACP) only. Near the base, ACP is successively replaced by amorphous calcium carbonate and calcite is restricted to a distal layer in the base. At the transition between middle and base, the epicuticle forms a hybrid material containing fibrils of the exocuticle. Nanoindentation experiments reveal an increase of the stiffness and hardness from the tip towards the base and significantly higher values on transversal in comparison to longitudinal planes. The results suggest that ultrastructure and composition are adapted for conveying high forces from a rather thin cutting edge to the stable base of the PI.

Functional adaptation of crustacean exoskeletal elements through structural and compositional diversity: a combined experimental and theoretical study

The crustacean cuticle is a composite material that covers the whole animal and forms the continuous exoskeleton. Nano-fibers composed of chitin and protein molecules form most of the organic matrix of the cuticle that, at the macroscale, is organized in up to eight hierarchical levels. At least two of them, the exo- and endocuticle, contain a mineral phase of mainly Mg-calcite, amorphous calcium carbonate and phosphate. The high number of hierarchical levels and the compositional diversity provide a high degree of freedom for varying the physical, in particular mechanical, properties of the material. This makes the cuticle a versatile material ideally suited to form a variety of skeletal elements that are adapted to different functions and the eco-physiological strains of individual species. This review presents our recent analytical, experimental and theoretical studies on the cuticle, summarising at which hierarchical levels structure and composition are modified to achieve the required physical properties. We describe our multi-scale hierarchical modeling approach based on the results from these studies, aiming at systematically predicting the structure-composition-property relations of cuticle composites from the molecular level to the macro-scale. This modeling approach provides a tool to facilitate the development of optimized biomimetic materials within a knowledge-based design approach.

Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals.

We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect.

The structure and dynamics of chitin nanofibrils in an aqueous environment revealed by molecular dynamics simulations

Chitin is one of the most abundant structural biomolecules in nature, where it occurs in the form of nanofibrils that are the smallest building blocks for many biological structural materials, such as the exoskeleton of Arthropoda. Despite this fact, little is known about the structural properties of these nanofibrils. Here, we present a theoretical study of a single chitin molecule and 10 α-chitin nanofibrils with different numbers of chains in an aqueous environment that mimics the conditions in natural systems during self-assembly. Our extensive analysis of the molecular dynamics trajectories, including free energy calculations for every model system, reveals not only the structural properties of the nanofibrils, but also provides insight into the principles of nanofibril formation. We identified the fundamental phenomena occurring in the chitin nanofibrils such as their hydrogen bonding pattern and resulting helical shape. With increasing size, the nanofibrils become increasingly stable and their structural properties approach those of crystalline α-chitin if they consist of more than 20 chains. Interestingly, this coincides with the typical size of chitin nanofibrils observed in natural systems, suggesting that their evolutionary success was at least partially driven by these specific structure–property relations.

Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks

Teeth represent the hardest tissue in vertebrates and appear very early in their evolution as an ancestral character of the Eugnathostomata (true jawed vertebrates). In recent vertebrates, two strategies to form and mineralize the outermost functional layer have persisted. In cartilaginous fish, the enameloid is of ectomesenchymal origin with fluoroapatite as the mineral phase. All other groups form enamel of ectodermal origin using hydroxyapatite as the mineral phase. The high abundance of teeth in the fossil record is ideal to compare structure and composition of teeth from extinct groups with those of their recent successors to elucidate possible evolutionary changes. Here, we studied the chemical composition and the microstructure of the teeth of six extinct shark species, two species of extinct marine reptiles and two dinosaur species using high-resolution chemical and microscopic methods. Although many of the ultrastructural features of fossilized teeth are similar to recent ones (especially for sharks where the ultrastructure basically did not change over millions of years), we found surprising differences in chemical composition. The tooth mineral of all extinct sharks was fluoroapatite in both dentin and enameloid, in sharp contrast to recent sharks where fluoroapatite is only found in enameloid. Unlike extinct sharks, recent sharks use hydroxyapatite as mineral in dentin. Most notably and hitherto unknown, all dinosaur and extinct marine reptile teeth contained fluoroapatite as mineral in dentin and enamel. Our results indicate a drastic change in the tooth mineralization strategy especially for terrestrial vertebrates that must have set in after the cretaceous period. Possibly, this is related to hitherto unconsidered environmental changes that caused unfavourable conditions for the use of fluoroapatite as tooth mineral.

Effects of homogenization technique and introduction of interfaces in a multiscale approach to predict the elastic properties of arthropod cuticle

In this paper the mechanical response of the arthropod cuticle is evaluated by means of a multiscale approach including interface effects. The cuticles elastic behavior is modeled at the nano and the micro scales by mean-field homogenization techniques. With respect to the work of Nikolov et al. (2011), the idea has been extended to study, at different scales of the structure, the effect of the used homogenization technique as well as the interface effect on the global elastic properties. First results revealed the sensitivity of the used homogenization technique on the global predicted elastic properties of the arthropod cuticle. To account for the interface between the fillers and the matrix of the composite structure of the arthropod cuticle, interphases are assumed at different scales of the structure with the same shape and topological orientation as the fillers. The approaches are based on few parameters directly related to the mechanical properties, the volume fraction and the morphology of the interphase. Results of the predicted elastic properties using the multiscale model including interphases are in good agreement with the experimental results. We show that the introduction of interphases leads to an improvement of the global elastic response in comparison to the multiscale model without interphases.