An Ingenious Microstructure Arrangement in Deep-Sea Nautilus Shell against the Harsh Environment

Interfacial Mechanical Behavior in Nacre of Red Abalone and Other Shells: A Review

Interfaces between nacreous tablets are crucial to the outstanding mechanical properties of nacre in natural shells. Excellent research has been conducted to probe the effect of interfaces on strength and toughness of nacre, providing critical guidelines for the design of human-made laminated composites. This article reviews recent studies on interfacial mechanical behavior of nacre in red abalone and other shells, including experimental methods, analytical and numerical modeling. The discussions focus on the mechanical properties of dry and hydrated nacreous microstructures. The review concludes with discussions on representative studies of nacre-like composites with interfaces tuned using multiple approaches, and provides an outlook on improving the performance of composites with better interfacial controls.

Wet shells and dry tales: the evolutionary 'Just-So' stories behind the structure-function of biominerals

The ability of evolution to shape organic form involves the interactions of multiple systems of constraints, including fabrication, phylogeny and function. The tendency to place function above everything else has characterized some of the historical biological literature as a series of ‘Just-So’ stories that provided untested explanations for individual features of an organism. A similar tendency occurs in biomaterials research, where features for which a mechanical function can be postulated are treated as an adaptation. Moreover, functional adaptation of an entire structure is often discussed based on the local characterization of specimens kept in conditions that are far from those in which they evolved. In this work, environmental- and frequency-dependent mechanical characterization of the shells of two cephalopods, Nautilus pompilius and Argonauta argo, is used to demonstrate the importance of multi-scale environmentally controlled characterization of biogenic materials. We uncover two mechanistically independent strategies to achieve deformable, stiff, strong and tough highly mineralized structures. These results are then used to critique interpretations of adaptation in the literature. By integrating the hierarchical nature of biological structures and the environment in which they exist, biomaterials testing can be a powerful tool for generating functional hypotheses that should be informed by how these structures are fabricated and their evolutionary history.

Distinctive Impact of Heat Treatment on the Mechanical Behavior of Nacreous and Crossed-Lamellar Structures in Biological Shells: Critical Role of Organic Matrix

As biological ceramic composites, mollusk shells exhibit an excellent strength-toughness combination that should be dependent on aragonite/organic matrix interfaces. The mechanical properties and fracture mechanisms of the nacreous structure in the Cristaria plicata (C. plicata) shell and crossed-lamellar structures in the Cymbiola nobilis (C. nobilis) shell were investigated, focusing on the critical role of the organic matrix/aragonite interface bonding that can be adjusted by heat treatments. It is found that heat treatments have a negative impact on the fracture behavior of the nacreous structure in the C. plicata shell, and both the bending and shear properties decrease with increasing heat-treatment temperature because of the loss of water and organic matrix. In contrast, for the crossed-lamellar structure in C. nobilis shell, the water loss in heat treatment slightly decreases its bending properties. When the organic matrix is melted after an appropriate heat treatment at 300°C for 15 min, its bending properties can be greatly enhanced; in this case, remarkable toughening mechanisms involving crack deflection and the fiber pull-out are exhibited, although the interfacial bonding strength reduces. Therefore, an appropriate heat treatment would bring about a positive impact on the fracture behavior of crossed-lamellar structure in the C. nobilis shell. The major research findings have provided an important inspiration that the inducement of moderately weak interfaces rather than all strong interfaces might enhance the comprehensive mechanical properties of fiber-reinforced ceramic composites.