Strength from the sea

In a paper published in

, a

researcher suggests that a tropical deep-sea sponge can teach engineers and architects a lot about building remarkably strong structures from extremely fragile materials.

"Nature has found a way to perfect inherently fragile materials by employing standard engineering principles from the nano to the macro scale," said Bell Labs materials scientist Joanna Aizenberg, the lead researcher on the team. "This creature's skeleton is a textbook lesson in mechanical engineering, offering valuable knowledge that could lead to new concepts in materials science and engineering design."

Aizenberg's team studied Euplectella, commonly called a Venus's Flower Basket, a cylindrical sponge made of a natural glass called biosilica that generally grows to about six inches in length. It lives in the depths of Pacific Ocean and is usually home to a pair of mating shrimp, which are protected from predators by the sponge's strong skeleton.

Tufts of glass fibres about the thickness of a human hair grow at the base of the sponge. In August 2003, Aizenberg led a research team in studying the optical properties of those fibres, discovering that the sponge uses multiple layers of glass held together by an organic glue to cover this natural "optical fibre," making it extremely resistant to cracking and breaking. This discovery led to unique insights in the production of commercial fibre optic strands.

In their latest studies of Euplectella, Aizenberg and her team at Bell Labs, working with collaborators at the Institute for Collaborative Biotechnologies and Materials Research Laboratory at the University of California, Santa Barbara, and the Max Planck Institute of Colloids and Interfaces in Potsdam, Germany, identified seven different levels of structural hierarchy in the sponge. Each of the structural levels corresponds to a fundamental construction principle commonly used in civil engineering and sometimes product design, but on a scale 1,000 times smaller than a building.

For example, the fibres that comprise the sponge's skeleton are arranged in a lattice, or open criss-cross pattern, reinforced by fibres that run diagonally in both directions inside alternate squares in the lattice. This construction technique is often found in high-rise buildings and free-standing bookshelves to counteract shear stress, which can easily collapse a non-reinforced square structure.

The team also discovered that when the diameter of the sponge's skeleton increases beyond a certain point the outer structure is reinforced by ridges in a spiral pattern. The ridges counteract an effect known as "ovalisation," which makes cylindrical structures more prone to collapse, as demonstrated by the fact that it's fairly easy to twist or flatten an empty aluminium can. By stabilising its skeleton with external ridges, the sponge makes itself difficult to crush. Other hierarchical levels include highly stable laminated glass beams, fibre-reinforced cements, and bundled structural elements, to name a few.

"These discoveries illustrate that Nature often demonstrates simple solutions to complex scientific problems," said Elsa Reichmanis, director of Materials Research, Bell Labs. "These sponges are formed perfectly, with exactly the right amount of material needed to optimise the design. Understanding how these structures evolve could help improve the synthetic materials and design processes we use today and in the future."