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

The adaption in commensurate applications, like 3D-MEMS, circumvent these potential
problems (Gebeshuber et al., 2009) (Gebeshuber, 2012c). Thus, the similarity in scale,
referring to the diameter of spicules of course, allows for a rather direct adaptation of
spicules for production of optical fibres. This holds great promise for a successful
implementation.
Optical properties of siliceous sponge spicules and conventional glass fibres, in contrast
to their mechanical properties, are similar indeed. Not surprisingly, this observation was
picked up by researchers at the renowned Bell laboratories, a leader in research on
telecommunication technology.
Specifically, (Sundar et al., 2003) investigated the basal spicules of Venus’ Flower Basket
(Euplectella aspergillum) that protrude 15 cm approx. in a crown-like formation from the
base of the cage-shaped skeleton. These spicules have a 40-70 µm wide cross section that
appears homogeneous when breaking the spicule. When the broken surface is treated
(mechanical stress or etching), though, a distinct zonation becomes visible (Figure 19j). A
homogeneous core including the organic axial filament (OF) is inserted into an equally
smooth central cylinder (CC) while the exterior cladding is striated (SS). The chemical
composition between the three regions varies. The 2 µm central core consists of pure silica,
while the central cylinder has a remarkably high organic content. In the outer shells again
the organic content decreases in centrifugal direction. This distinct chemical and structural
profile of the spicular cross-section is indeed reflected by a tripartite profile of the refractive
index n. The central core has a high refractive index exceeding that of vitreous silica, while
the central cylinder is far less refringent (lower n). In the outer cladding, the layered
structure is reflected by an oscillating refractive index that is higher than in the central
cylinder and furthermore increases notably towards the surface (Sundar et al., 2003). The
reason for the increased refractive index of the spicule core compared to vitreous is the
inclusion of sodium ions as refractive index-raising dopant (Aizenberg et al., 2004).
The profile of a high index core with a lower index cladding is found in all technical
optical fibres (cf. Figure 19a,b,d,e,g,h). Depending of the diameter of the high-index core as
well as the continuous vs. stepwise decrease of the refractive index from the core to the
cladding different classes of optical waveguides can be distinguished. The distinction
between these different classes of waveguides is important for communication fibres, since
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