WASHINGTON — For the strongest glass you can imagine, look for sponges at the bottom of the ocean. If you find cartoon superstar SpongeBob Square Pants, keep looking; he’s a bath sponge with a soft skeleton and no glass in his pants. Some of Bob’s distant sponge relatives, however, build glass cages that have biologists and materials scientists oohing, ahhing and taking notes for future bio-inspired engineering projects and materials.
These glass cages have at least seven levels of structural organization, many of which follow basic principles of mechanical engineering, according to new research in Friday's issue of the journal Science, published by AAAS, the nonprofit science society.
The “glass sponges” use many of the textbook principles of mechanical engineering in their efforts to transform brittle glass into a strong building material. Fiber-reinforced cements, bundled beams and diagonal beams running at 45-degree angles to reinforce structural columns can be seen in both modern architecture and glass sponge skeletons, explained Science author Joanna Aizenberg from Bell Labs/Lucent Technologies in Murray Hill, N.J.
These reinforced glass cages rise 8 to 12 inches (20 to 30 centimeters) above the sea floor, but you need to think in terms of millionths and billionths of a meter to understand how they are put together.
The individual needle-like glass beams that make up the basic structure of the skeleton are 10 to 100 millionths of a meter in diameter and are far more complex than your average glass rod. The beams are composed of alternating layers of glass and glue. Each glass layer is made of fused glass particles that are smaller still: We’re talking bits of glass with dimensions in the billionths of meters. The glue between each glass layer adds strength to the whole skeleton by stopping cracks from passing from one glass layer to the next.
Why they're so tough
Aizenberg described the toughness of some of these glass fibers.
“You can bend them, twist them, and they probably won’t break because the energy of the force you apply is dissipated in the glue.”
Every other square in this lattice work is reinforced by diagonal beams that help the sponges get the strongest cage at the minimum cost. The number and placement of the diagonal beams fits an equation engineers use to calculate the minimum number of reinforcements needed to achieve the maximum stability.
“The sponge uses exactly what’s needed but nothing more,” said Aizenberg, who mentioned that the diagonal-beam design strategy is commonly used to reinforce human-made structures, from bookshelves to buildings.
Next, the sponge cage is wrapped in spiraling surface ridges that protect it from being squeezed like an empty can of soda.
Finally, the sponges are anchored to the soft sediments of the sea floor in such a way that they do not break off due to the stress and strains of ocean currents.
"It’s quite remarkable how many basic engineering design principles the sponge uses in construction of this skeletal system," said Science author James Weaver, from the University of California at Santa Barbara.
Building glass cages
How the complex cylindrical cage itself forms is a mystery.
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“It puzzles me. In my wildest dreams I can’t imagine how these fibers are assembled to make the nearly perfect, highly regular square cells, diagonal supports and surface ridges of the cage,” said Aizenberg.
Researchers have a little better idea about how the individual fibers form. The protein at the center of each glass filament is thought to play an important role in the formation of the individual reinforced glass fibers.
Many of the new insights into the structure of the glass cages are possible because of images taken with electron microscopes that help reveal structural details invisible using standard light microscopes.
Soak up some sponge facts
Sponges are an extremely ancient group of animals whose “footprints” have been in the fossil record for more than half a billion years. Some scientists think of sponges as a group of collaborating individual cells instead of one unified animal. Sponges do not form tissues, which means they don’t have hearts, lungs or other organs.
“Despite the simplicity of their anatomy, sponges are capable of creating some of the most complex and diverse skeletal systems known,” Weaver said.
A thin veneer of living material covers the deep water sponge’s glass cage. Some of these cells help to create water currents that increase the amount of water flowing to their filter-feeding cells. The sponges live on the ocean floor, commonly at depths of 325 to 1,000 feet (100 to 300 meters), well beyond scuba diving depth. These sponges are often called “Venus’ flower basket” as a reference to artwork depicting the Roman goddess Venus holding a cornucopia full of flowers.
© 2013 American Association for the Advancement of Science