MASTER THESIS Biomimetic potential of sponge ... - IAP/TU Wien

spicule is driven by intracellular hydrodynamic forces (Wang et al., 2012b). This principle of
flow-induced formation of biosilica structures has been used in a biomimetic approach
(Wang et al., 2012a).
In the Giant Basal Spicules of the glass sponge Monorhaphis chuni that attracted a lot for
interest for their remarkable optical and mechanical properties as well as their sheer
dimension, several peculiarities have been found. Especially during the later stages of
appositional growth, the deposition of the surface layers, deviations from the mechanism
outlined above have been observed (Wang et al., 2011d) (Wang et al., 2009) (Wang et al.,
2011a). During this stage, cells (or fragments of syncytia) are present in small depressions on
the surface of the spicules. These cells are involved in the deposition of silica in astoundingly
regular, circular ribbons on the surface of the spicule. After synthesis of these ribbons, the
silification proceeds and yields a smooth surface, while the sclerocytes remain attached on
the surface and are “recycled” for the next round of lamellar (ribbon-shaped) silica
deposition. This mechanism lays down a type of biosilica that chemically, mechanically and
optically differs from the type found more centrally. A higher concentration of mono- and
divalent cations (Na + , Mg 2+ , Ca 2+ and Mn 2+ ) is found in the surface lamellae. Mechanically a
higher concentration of these ions makes glass less hard (Wiederhorn, 1969) (Wang et al.,
2011d). Additionally, these ions are desirable dopants for optical fibres since they increase
the refractive index and thus improve the waveguide properties (Sundar et al., 2003)
(Aizenberg et al., 2004).
On the outer surface of the spicule that is exposed to the extracellular space (mesohyl) a
very similar process takes place (Wang et al., 2011b). According to Schröder and colleagues
(2006), extra-spicular biosilica deposition is mediated by a string composed of silicatein and
galectin. Silicatein is provided by sclerocytes that closely surround the spicule and forms
filaments together with the most abundant protein in the mesohyl – galectin – in a calciumdependent
process. These filaments are arranged in a proteinaceous framework around the
spicule (Figure 14h, i). Collagen has been suggested to act in concert with the
galectin/silicatein fibres to establish the framework. Unlike silicatein, however, collagen is
not involved in the catalysis of silica deposition and does not form part of the eventual
structure of the spicules (Wang et al., 2012b). The arising framework serves as catalyser and
template for the biomineralization of silica on both its surfaces (centripetal and centrifugal).
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