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

efore syneresis, a space-filling arrangement of 30 silica monomers occupies 30% more
space than a condensated arrangement of the same molecules after syneresis (cf. Figure 15c
vs. g). Through the mechanism exemplified here, the diameter has been found to decrease
from 10 µm in a developing spicule (Figure 15b) to only 2-3 µm in a mature,
polycondensated spicule (Figure 15f) (Schröder et al., 2006) (Müller et al., 2006b). Compared
to the rapid synthesis (steps i and ii) the maturation of spicules is far slower.
Interestingly this exact mechanism is also likely causing more complex shapes in spicules.
It has been proposed that bending of spicules can be induced by apposition of a cell to the
spicule. This would trigger the increased removal of water from that area as outlined above
and thus locally reduce the volume of silica. Since the volume on the side of the spicule not
attached to the cell remains constant, the spicule bends towards the direction where the cell
resides (Wang et al., 2012b).
Even the rendering of complex shapes and ornaments found in many spicules may well
depend on localized removal of water. Examples in hexactinellid spicules include the spikes
found on some megascleres (Figure 6c). In demosponges, this might well be exemplified by
the spiny, curved isochela microscleres of the genus Guitarra (Figure 6a). The process of
forming these ornaments is not only of interest because of their biological (and aesthetical)
value, but also because it has been found that light guided through a spicule is out-coupled
via spines (Aizenberg et al., 2004). Thus, spines serve as illumination points and might be
taken into consideration for biomimetic applications where localized release of light from a
waveguide is desired.
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