Hierarchical fibre composite structure and micromechanical properties of phosphatic and calcitic brachiopod shell biomaterials — an overview

Abstract

ABSTR ACTBrachiopods are a phylum of shell-forming sessile marine invertebrates which have existed since the early Cambrian. Two very different biomaterial design strategies for their shells evolved early in their history. Both strategies use hybrid fibre composites, however, one is based on mineral fibres embedded in~2 wt.% of organic biopolymer sheaths and the inorganic fibres are calcite single crystals. In the second strategy the fibres are biopolymers and are reinforced with Ca-phosphate nanoparticles to form a fibrous nanocomposite. Here the organic component (chitin) dominates. The Ca-phosphate nanoparticle-reinforcement strategy is not unlike that in vertebrate bone, however, the microscale structure is laminated with alternating laminae which have a different degree of mineralization.The calcitic shells feature an outer compact layer of calcite micro-and nanoparticles protecting the inner fibrous layer from the outside. Transmission electron microscopy of the fibrous layer reveals intercrystalline and intracrystalline biopolymers. The calcitic shell material is stiff with nanoindentation E-moduli of 63Ô8 GPa and relatively hard (Vickers microhardness up to 400 HV 0.0005/10 and nanohardness 4Ô0.5 GPa). Compared to inorganic calcite the microhardness is doubled and the nanohardness increases by 60%. We attribute this increased hardness to intracrystalline biopolymers. The nano-indentation E-moduli of the chitinophosphatic shells range from 3 to 55 GPa as a result of the varying degree of mineralization between their laminae, and similarly their nanohardness varies between 0.1 and 3 GPa. For brachiopods burrowing inside the sediment, the alternation of nonmineralized laminae with thin, more strongly mineralized laminae provides abrasion-resistance, hardness and longitudinal stiffness while it preserves the flexibility provided by the organic component for bending movements.

Hierarchical structure of marine shell biomaterials: biomechanical functionalization of calcite by brachiopods

“…5a and 6a shows that the primary layer is differentiated into sub-layers, where usually nanostructured material is on the outside, while micrometer-sized columnar or platelet-shaped crystals form a sub-layer towards the inward of the valve (e.g. Schmahl et al 2008;Goetz et al 2009). The large and mineralogically inconspicuous calcite columns (Fig.…”

“…Brachiopods have been populating the shallow sea floors since the Cambrian or even the late Precambrian (Williams et al, 2000). A review of the material architecture of the phosphatic shell forms has recently been published (Schmahl et al, 2008). Here we review the hierarchical architecture of the calcitic rhynchonellide forms and report some complementary observations on the subject.…”

Contrasts between organic participation in apatite biomineralization in brachiopod shell and vertebrate bone identified by nuclear magnetic resonance spectroscopy

“…Differences in inter-and intracrystalline organic matrix organization have implications for properties such as hardness, flexibility and the ability to resist crack propagation [29]. For example, the increased flexibility and reduced brittleness of the Lingula compared with other brachiopod shells is ascribed to the presence of more distinctive and thinner laminated layers of interleaved fibrous organic, and crystalline, matter [30]. It may be that this and similar mechanisms are preferable under some circumstances to employing atomic-level associations with GAG and protein to achieve a similar end.…”

Insight into <i>Emiliania huxleyi</i> coccospheres by focused ion beam sectioning

“…Nevertheless, the FIB-SEM microscope is often used to analyse biological materials which are difficult to cut, such as teeth (Nalla et al, 2005) and bones (Giannuzzi et al, 2007). Another application for FIB-SEM microscopes in biology is the preparation of thin lamellae which can be analysed in a transmission electron microscope (TEM) (De Winter et al, 2009;Schmahl et al, 2008;Kelm et al, 2012). Using backscattered electrons (BSE) instead of secondary electrons (SE) for the image formation allows for discrimination of differently aligned crystals with the same mass contrast due to channelling contrast mechanisms, which depend on the crystallographic orientation of the investigated volume (De Winter et al, 2009).…”