Hierarchical Design and Nanomechanics of the Calcified Byssus of Anomia simplex
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Hierarchical Design and Nanomechanics of the Calcified Byssus of Anomia simplex Jakob R. Eltzholtz1, Marie Krogsgaard1 and Henrik Birkedal1 1 Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), 140 Langelandsgade, Aarhus University, DK-8000 Århus, Denmark ABSTRACT Biology has evolved several strategies for attachment of sedentary animals. In the bivalves, byssi abound and the best known example being the protein-based byssus of the blue mussel and other Mytilidae. In contrast the bivalve Anomia sp. has a single calcified thread. The byssus is hierarchical in design and contains several different types of structures as revealed by scanning electron microscopy images. The mechanical properties of the byssus are probed by nanoindentation. It is found that the mineralized part of the byssus is very stiff with a reduced modulus of about 67 GPa and a hardness of ~3.7 GPa. This corresponds to a modulus roughly 20% smaller than that of pure calcite and a hardness that is about 20% larger than pure calcite. The results reveal the importance of microstructure on mechanical performance. INTRODUCTION Underwater adhesion is an important technological problem that has been solved in biology by several different organisms. Amongst the bivalves, sedentary lifestyles are often secured through a byssus. In the Myilidae, comprising the common blue mussel, a bunch of byssus threads is used that are protein-based [1-6]. Due to recent advances of the Waite group and coworkers, we now have a good picture of how proteins are used as adhesives [5, 6] and take part in extensible coatings that imbue the threads with abrasion resistance [4]. In contrast to the Mytilidae, the bivalves Anomiidae have only a single thread that is highly mineralized [7-11] with the exception of the species Enigmonia [7]. Only the jingle shell, Anomia sp., has been investigated beyond standard morphology [8-11]. The animals attach to small stones or shells through the byssus [8]. The left shell is pressed towards the substrate and is thus referred to as the ventral shell and the byssus extends through a hole in this shell. The thread is mineralized by CaCO3, over 90% by weight [9]. Both aragonite and calcite are present but there are widely differing opinions as to the ratio of the two phases due to the qualitative nature of the diffraction investigations performed [8-10]. We have recently investigated the hierarchical design of the byssus by extensive scanning electron microscopy (SEM) investigations and used Rietveld refinement of X-ray diffraction data to show that the two polymorphs occur in roughly equal proportions [11]. We also showed that there are distinct spatial distributions of magnesium using energy dispersive X-ray analysis; this resulted in clear shoulders on the calcite diffraction peaks [11]. These results showed that Anomia displays an astonishing control over crystal phase, composition, shape and placement in a three dimensional hierarchical structure. Here we present further structural data based on optical mi
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