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

Diagonal struts are important factors of the skeletal stability, supporting torsional, bending
and shear loads (Weaver et al., 2007).
The external helical ridges (level 6 according to Aizenberg et al.) have specifically been
explained as structures avoiding the ovalization of the tube like skeleton. Deformation of the
circular shape of the skeleton to an oval cross section would otherwise be a likely failure
mechanism. The circular ridges prevent this, although at the cost of making failure due to
torsion more likely, which is counteracted by helical ridges in opposite direction (Weaver et
al., 2007). Epitomizing the benefit of differentially arranged structures at different length
scales, the torsional strain can also be absorbed by the diagonal struts at level 4 (Currey,
2005) (Deshpande et al., 2001).
At the level of the individual spicules, the aforementioned crack arresting mechanism
(Currey, 2005) is brought to bear. Since the organic interlayers between the silica layers are
much weaker than these, any crack that might occur is arrested at the interlayers. Failure
therefore is usually not due to one catastrophic event, but a series of crack nucleation,
propagation over a single silica layer and arrest at the interlayer (Chai & Lawn, 2002)
(Seshadri et al., 2002). Cracks are also prevented from penetrating deep inside the spicule by
decreasing width of the silica-lamellae towards the periphery (Weaver et al., 2007). A
revised, more detailed mechanism is presented by Johnson and co-workers (2010), see
below).
At the ultrastructural level (nanoscale) individual silica spheres can be made visible by
etching while usually each lamella appears as amorphous, bulk silica. Accordingly, an
individual lamella is as brittle as normal glass and fractures yielding planar surfaces (Weaver
et al., 2007) For the optical properties of silica spicules, Rayleigh scattering due to
inhomogeneities is an important factor as it causes transmission losses. The presence of
distinct nanospheres would increase the scattering, as at the boundary of every nanosphere
the density fluctuates. Therefore, the high condensation to amorphous glass achieved here
is probably essential for the optical qualities of spicules (Aizenberg et al., 2004).
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