Skeleton of Euplectella sp.: Structural Hierarchy from the Nanoscale to the Macroscale
The authors report the robust hierarchical structure of the Euplectella sp. sponge skeleton, and they discuss seven levels of structural hierarchy in the system, from the nano to the macroscale. Comparisons are also drawn to mechanical engineering strategies for strong design that overcomes the limitations of the constituent materials.
Natural material systems can be exceptionally strong and tough despite the inherent mechanical limitations of their constituent materials. Bone, for example, is made of about half organic and half mineral components tightly interconnected at the nanoscale. The hierarchical structure--different structure at many different length scales--of collagen fibers and crystallites of calcium phosphate makes it possible for bone to effectively resist fractures. A similar strategy can be observed in mollusk nacre, wherein soft organic layers deflect cracks in calcium carbonate. The most remarkable use of structural hierarchy in nature is arguably in organisms made almost entirely of glass.
Structural Hierarchy in Euplectella
Euplectella is a deepwater sponge from the Western Pacific whose glassy skeleton is a hollow cylinder. Figure 1A shows the intricate cagelike structure (20 to 25 cm long, 2 to 4 cm in diameter) with lateral openings (1 to 3 mm in diameter). The cylinder is tapered, longer at the top than at the bottom; the basal segment of the cage, which is rigid, is anchored flexibly to the ocean floor.
The assembly of a macroscopic, mechanically resistant cylindrical glass cage is possible in a modular, bottom-up fashion comprising at least seven hierarchical levels, all contributing to mechanical performance. These include silica nanospheres that are arranged into concentric layers separated from one another by alternating organic layers to yield lamellar fibers. Considering Griffith crack theory, we know that biosilica glass as a building material suffers primarily from its brittleness and susceptibility to surface defects. The resulting low strength of glass is balanced at the spicule level. In Figures 1G-H is shown the characteristic laminated architecture of the rod-shaped spicule building block: concentric glass layers that are thinner near the outside and alternate with organic adhesive interlayers that deflect cracks (Figure 3). The variation in layer thickness intelligently gives more tensile-compressive strength to the core and more crack resistance to the surface.
The spicules are in turn bundled and organized within a silica matrix to produce flexurally rigid composite beams at the micron scale (Figure 1D). This is a well-known construction principle in ceramic materials. A bundle of loosely bonded fibers with slightly different strengths will have a much improved defect tolerance and overall strength than a single large fiber. Several composite beams appear to form each macroscopic strut in the cage, as seen in Figure 1C.
Now there is a macroscopic arrangement of these struts in a square lattice with ancillary crossbeams, ideal for resisting tensile and shearing stresses. The chessboard pattern of squares with alternating diagonals is shown in Figure 1B and Figure 2D. This is consistent with theoretical analysis by Dashpande et al, which has shown that mass and metabolic effort can be minimized for a given stiffness by having anti-shear diagonal struts in every other square. When the number of struts per node, Z is at least 6 for two dimensions or 12 for three dimensions, the structure is stable. Above the stability limit, for example, if Euplectella's cage features diagonal struts on every square, the structure is overdesigned.
Finally, there are various structural motifs that provide additional structural benefits to this unique glass skeletal system. As the sponge matures, cementation of its nodes and spicules in the skeletal lattice takes place with silica matrix, as shown in Figures 1E and 2E. This cement is in fact layered itself to inhibit crack propagation! Largest and most visible of the motifs are the characteristic diagonal surface ridges, which provide 3-dimensional stability against ovalization to complement the 2-dimensional strength of the cylinder wall. Form follows function, as the ridges are only present and increase in size at the larger diameter end of the cage (Figure 1A). The flexible anchoring of the rigid cage into the soft sediments of the sea floor completes the structural hierarchy of this remarkable example of natural design.