Mother-of-Pearl: An Engineering Gem

Mollusca are a phylum of animals composed of at least 50,000 living species (Bunje, The Mollusca, http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php, 2003). They encompass ancient life forms that first appeared on Earth about 545 million years ago at the beginning of the Cambrian Period, including conch shells, abalone, clams, mussels, and oysters, among many more. While the bodies of Mollusca are soft, they are usually covered by hard shells that accomplish many important functions (Espinosa et al., Prog Mater Sci 54(8):1059–1100, 2009). As the primary barrier between a Mollusc’s soft body and the outer environment, these shells must provide protection to damage from predators, remain intact without shattering under tidal waves, and resist pressures around hydrothermal vents found in the deep ocean where many Molluscs reside, among many other functions (Kaplan, Curr Opin Solid State Mater Sci 3(3):232–236, 1998). Over time, numerous types of shells have developed—foliated and cross lamellar, prismatic, and columnar and sheet nacreous structures (Espinosa et al., Prog Mater Sci 54(8):1059–1100, 2009)—but due to the high strength of nacreous structures, they have become very popular among researchers. Many studies have confirmed the outstanding mechanical properties of the material, and this chapter highlights these properties while exploring the structural reasons for such excellence. Nacreous shells consist of a hierarchical structure that features an armor system on one level and brick-and-mortar architecture on another. The layered structure of tablets and soft protein enhances the mechanical properties of nacre by allowing sliding, which contributes to nacre’s high toughness (Denkena et al., J Mater Proc Technol 210(14):1827–1837, 2010). Tablet waviness is another important mechanism found in nacreous shells that distributes inelastic deformations so as to prevent failure (Espinosa et al., Prog Mater Sci 54(8):1059–1100, 2009). Finally, interlocking mechanisms between tablets encourage deformation and progressive failure, increasing toughness and reducing the chance of catastrophic failure (Katti and Katti, Mat Sci Eng C 26(8):1317–1324, 2006). Engineers have been inspired to create novel nano-composites that mimic the structure and mechanisms of nacreous shells in order to achieve superior mechanical properties (Luz and Mano, Philos Trans Math Phys Eng Sci 367(1893):1587–1605, 2009). For example, Tang et al. succeeded in creating a nano-scale version of nacre using organic and inorganic layers consisting of polyelectrolytes and clays (Tang et al., Nat Mater 2:413–418, 2003). In addition, Zhu and Barthelat created a prototype of a nacre-like material composed of poly-methyl-methacrylate (PMMA) tablets (Zhu and Barthelat, A novel biomimetic material duplicating the structure and mechanics of natural nacre. In: Proulx T (ed) Mechanics of biological systems and materials, vol 2. Springer, New York, 2011). Engineering applications also include using nacre itself as bone implants due to its biocompatibility (Denkena et al., J Mater Proc Technol 210(14):1827–1837, 2010). Based on the sheer amount of useful applications and innovations that nacre has bioinspired, it truly stands out as an engineering gem.