User:Tylerlorenzi/Hierarchical Structure of Glass Sponges

The skeletal structure of the glass sponge Euplectella aspergillum displays a remarkable multi-level hierarchical design that gives it superior structural properties. Research efforts headed by Joanna Aizenberg and James C. Weaver have led to a detailed understanding of this complex but significantly ordered system.

Spicules
Individual spicules are composed of a central proteinaceous core with a rectangular or circular cross-section (found in Demosponges). This organic inner filament is surrounded by spherical silica nanospheres (50-200 nm in diameter) that increase in diameter further from the central axis. This region of spheres is referred to as the central cortex and has been shown to trap and transport light better than commercial fiber optic wire (link).

This central cortex is surrounded by laminated layers of alternating protein and silica sheets. The organic layers are approximately 2-5 nm thick and are believe to dissipate stress by preventing crack propagation. These organic layers alternate with inorganic layers of Hydrated silica 0.1 to 0.2 um thick.

Building Blocks
The lattice is composed of two types of spicules:

Underlying Quadrate Structure
Both types of spicules are bent at an oblique (obtuse) angle at their midpoint to allow for an overall cylindrical shape. The stauractine (vertical) spicules form the outer layer while the horizontal components form an inner structure. These spicules are offset and overlapped (initially about 50%) to form the general cross-hatched lattice with holes approximately 2.5 mm wide. This design allows for an increase in the diameter of the body from the base (2-3 cm up to 3-5 cm) by varying the degree of overlap. During initial construction the lack of connection between these lattices gives the sponge a greater degree of strength by enabling the sponge to dissipate stress independently.

Exterior Spicule Bundles
The existing lattice structure described above is then surrounded by spicule bundles 200-500 um in diameter. These bundles are themselves composed of smaller spicules 5-50 um in diameter bonded laterally to yield a higher damage tolerance. This construction allows one fiber to fail while neighboring fibers hold and for the crack to be deflected toward other fibers or the surrounding material. Vertical bundles line the exterior of the skeleton while horizontal bundles line the interior of the structure. This method of construction sandwiches the underlying quadrate structure and offers additional support. About 2-3 cm from the top of the sponge skeleton the bundles split and approximately double in number to aid in attaching the sieve (top) plate.

Diagonal Bundles
Pairs of similarly sized bundles encircle the existing skeleton at 45 degrees in oblique spirals. These intersect in such a pattern as to leave a modified quadrate pattern of open/closed/open holes in all directions. This diagonal construction aids significantly increases the sponges' ability to deal with bending, shear and torsional loads.

Diagonal Ridges
On top of these existing diagonal bundles a ridge system develops. Short beams formed radially are then connected by tangentially oriented long spicules to form arched protrusions every 4 openings on the lattice. These arches fuse into ridges that vary in height but rarely exist for more than a 1/2 rotation about the sponge, either turning back or ending. This discontinuous construction aids in preventing ovalization and further distributes torsional loads more effectively.

Sieve Plate (top cover)
The top of the sponge skeleton is composed of a variety of interdigiated spicules (2-point, 4-point and 6-point) connected to the branched ends of the vertical bundles. This structure offers strength to the skeleton and prevents collapse of the cylindrical structure.

Anchors
Sponges of the class Hexactinellida are found most often at substantial depths and on sediment surfaces necessitating an effective sea floor anchor system. Nearly 2000 fibrillar spicules exist at the sponge's base and extend about 1/3 up the body. Each of these spicules contains a barb at its terminus which is believed to act as an anchor on the sea floor. These specific lengthy fibers have been investigated extensively for their optical properties and are believed to aid in light transmission to the sponge body at the significant depths these sponges reside.

Inter-Spicule Mortar
Intertwined with the framework described above can be found smaller spicules (5-15 um in diameter) which are thought to help in the matrix cementation process and contribute to increased strength. It is believed that a silica/organic composite cement forms this inter-spicule mortar.