19 A Review on the Structure and Mechanical Properties of Mollusk Shells – Perspectives on Synthetic Biomimetic Materials

Natural materials can exhibit remarkable combinations of stiffness, low weight, strength, and toughness which are in some cases unmatched by manmade materials. In the past two decades significant efforts were therefore undertaken in the materials research community to elucidate the microstructure and mechanisms behind these mechanical performances, in order to duplicate them in artificial materials [1, 2]. This approach to design, called biomimetics, has now started to yield materials with remarkable properties. The first step in this biomimetic approach is the identification of materials performances in natural materials, together with a fundamental understanding of the mechanisms behind these performances (which has been greatly accelerated by recent techniques such as scanning probe microscopy). The mechanical performance of natural materials is illustrated in Fig. 19.1, a material properties map for a selection of natural ceramics, biopolymer, and their composites [3]. The upper left corner of the map shows soft and tough materials such as skin, with a mechanical behavior similar to elastomers. The lower right corner of the chart shows stiff but brittle minerals such as hydroxyapatite or calcite. Most hard biological materials incorporate minerals into soft matrices, mostly to achieve the stiffness required for structural support or armored protection [4]. These materials are seen in the upper right part of the map and show how natural materials achieve high stiffness by incorporating minerals while retaining an exceptional toughness. Alternatively, one can consider how natural materials turn brittle minerals into much tougher materials, in some cases only with a few percent additions of biopolymers. These materials have in general relatively complex structures organized over several length scales (hierarchical structures [1, 2]) with mechanisms operating over several length scales, down to the nanoscale [5, 6]. Mollusk shells, the topic of this chapter, are an excellent example of such highperformance natural materials. Mollusks are composed to at least 95% of minerals