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[Image above] The odd shape of tiny structural rods found in a species of sea sponges turns out to be optimal for resistance to buckling. Credit: Kesari Lab; Brown University

Humans make a lot of glass, in a variety of forms, for a variety of applications.

But we’re not the only ones.

Silica, the raw material used to makes glass, is so abundant on earth that nature also occasionally makes glass—for instance, during volcanic lightning.

And some living creatures form their own glass, too. Sea sponges can precipitate nanoscale silica from silicic acid in seawater and use the material to build glass structures.

And these aren’t just simple glass structures—they’re engineered designs.

The aptly named glass sea sponge, also called a Venus flower basket, builds glass spicules with concentric rings that are optimized for load capacity, giving these thin glass threads impressive strength that the sponges use to anchor themselves to rocks.

Other types of sponges make glass, too. Orange puffball sponges (Tethya aurantia) also extract silica from seawater to build their own version of glass spicules, called strongyloxea, which provide internal structural support to these otherwise squishy creatures.

Orange puffball sponges have evolved bundles of toothpick-shaped spicules that help the sponge maintain its shape. While a sponge skeleton may sound like a silly thing, these silica spicules are no joke—the sponges depend on their porous structures to survive, because the sponges filter food from the churning tidal seawaters that they call home.

But how can squishy sponges maintain their structural integrity with only thin filaments made of glass?

The same researchers at Brown University who studied the strength of Venus flower basket silica spicules—Haneesh Kesari and Michael Monn—apparently have a penchant for less-than-creatively-named sea creatures.

The team has now taken a closer look at orange puffball spicules and found that they, too, have evolved a precisely engineered design that provides the structures with maximal strength.

The researchers’ close examination of orange puffball sponges showed that their monolithic, axially symmetric, tapered silica spicules—which are about 35 μm thick and just 2 mm long—are “remarkably uniform,” the authors write in an open-access Scientific Reports paper describing the work.

But orange puffballs don’t use the same concentric layers as the Venus flower basket for strength—they have a different engineering trick to make extremely strong support structures out of glass.

While examining the spicules, the Brown researchers noticed that in addition to their uniformity, the structures had an unmistakable toothpick-like shape—thicker in the middle and tapered at the ends.

Why, the team wondered, would the sponges use such tapered structures rather than simple columns?

Kesari and Monn found their answer in an old engineering theory devised by German scientist Thomas Clausen in 1851. Clausen thought that a column structure that was tapered at the ends would be most resistant to buckling, the primary failure mechanism in slender structures—and he was right.

Since Clausen first proposed this theory, mathematical calculations have shown that a Clausen column is in fact the optimal shape to prevent buckling. Calculations show that a Clausen column is 33% more resistant to buckling than a cylinder and 18% more resistant than an ellipse.

So the design of the orange puffball sponge’s spicules provide them with resistance to buckling, despite the fact that they’re made of glass.

“We use a structural mechanics model to better understand the function of the strongyloxea spicules within the sponge and to show that they are well tuned for performing that load bearing function,” Monn explains via email.

Beyond providing some basic science behind sea sponges, however, what does this mean for modern structural engineering?

Clausen column structures have been historically difficult to engineer via traditional manufacturing, which is why most architectural designs incorporate simple columns instead. “However, 3-D printing and other digital manufacturing techniques could totally shift this balance and allow for practical designs that leverage optimal shapes like the Clausen column,” Monn notes in the email.

According to Monn, optimized Clausen columns might be able to improve and strengthen nanoscale and microscale truss materials in particular, such as these light and deformable ceramic structures developed by Julia Greer’s group at CalTech.

“High-resolution 3-D printing has allowed us to design the interior of components so they don’t need to be solid,” Monn explains. “This makes the materials’ specific stiffness and strength much higher by replacing solid material with a nanoscale or microscale truss lattice. But the downside is that you introduce a new and important failure mode—truss element buckling.”

And because Clausen columns are optimized to resist buckling, this small structural change could help make such truss structures even stronger.

“So we hope that our work reinvigorates interest in column optimization in the broader mechanics and materials community—I would really like to see nanostructures or microstructures in which the truss elements are shaped like Clausen columns.”

The open-access paper, published in Scientific Reports, is “A new structure-property connection in the skeletal elements of the marine sponge Tethya aurantia that guards against buckling instability” (DOI: 10.1038/srep39547).