Nacre: A Unique Biomaterial Patterned by Liquid Crystals
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1094-DD01-02
Nacre: A Unique Biomaterial Patterned by Liquid Crystals Antonio G. Checa1, Julyan H.E. Cartwright2, Bruno Escribano2, and Ignacio Sáinz-Díaz2 1 Departamento de Estratigrafía y Paleontología, Universidad de Granada, Avenida Fuentenueva s/n, Granada, E-18071, Spain 2 Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas, Edificio Instituto López Neyra, Avenida del Conocimiento s/n, Armilla (Granada), 18100, Spain ABSTRACT The sequence of formation of the organic and inorganic components of nacre in bivalves and gastropods is re-studied. We reach the conclusion that interlamellar membranes are formed well in advance of the other elements. In this way, we support and refine the compartment theory for the formation of nacre. We explain the arrangement of chitin crystallites within a single interlamellar membrane and the layering of interlamellar membranes as a process of formation of a liquid crystal. INTRODUCTION Biomineralized tissues are composites with a variable percentage of organic matter. In the case of mollusc shells, the calcium carbonate (aragonite or calcite) fraction coexists with a percentage of organic matter, which ranges from ~1 to ~7% (pers. obs.). The organic fraction can be intra- or intertabular. In the latter case, the organic matter forms an envelope around every tablet. In any case, the deposition of organic matter is coeval to that of the mineral. All molluscan microstructures are secreted following this pattern, with one exception: nacre. This material is composed of aragonite tablets distributed within lamellae and arranged in a brick-wall pattern [1,2]. In nacre, researchers have traditionally distinguished between two types of organic components: intertabular and interlamellar [3,4]. Intertabular matrices surround tablets throughout their growth, whereas interlamellar membranes only extend horizontally and separate the different lamellae. The extent and distribution of these organic components are found to be responsible of the superior mechanical properties of nacre, particularly when compared to inorganic aragonite (see review in [5]). Intertabular matrices are purely proteinaceous (silk-like) whereas interlamellar membranes have a core of β-chitin fibrils coated on both sides by acidic proteins [6]. A final and important difference is that during nacre deposition, interlamellar membranes precede the formation of tablets and their intertabular envelopes [3,7,8]. Since it was revealed, this crucial fact has received little, if any, attention in the scientific literature dealing with nacre formation. The existence of such preformed interlamellar membranes is what makes nacre unique among molluscan microstructures. Here we re-examine the data that led Bevelander and Nakahara to propose the classical compartment theory [3] and integrate this theory into the present knowledge on nacre. Our conclusion is that the distribution of nacre tablets and thence the unique properties of nacre, are predetermined by the interlamellar membranes. MATERIAL A
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