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.
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. 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. This is a well-known construction principle in ceramic materials. The macroscopic arrangement of these beams in a rectangular lattice with ancillary crossbeams is ideal for resisting tensile and shearing stresses. Finally, there are various structural motifs that provide additional structural benefits to this unique glass skeletal system.