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Sea Sponge’s "Structural Principles" Inspire "Design Strategy"


Sponges are among the earliest multicellular animals in the fossil record. They appear at first glance to be randomly organized, full of holes, asymmetric, and lacking complex organs. Yet one particular sea sponge has secrets that make engineers drool with envy. The Venus Flower Basket (Euplectella aspergillum) mastered fiber optics and force-absorbing building materials long before physicists and architects dreamed of such things.

Researchers at Brown University and Harvard University’s Wyss Center for Biologically-Inspired Engineering have published a paper in PNAS with new findings about this sponge:

To adapt to a wide range of physically demanding environmental conditions, biological systems have evolved a diverse variety of robust skeletal architectures. One such example, Euplectella aspergillum, is a sediment-dwelling marine sponge that is anchored into the sea floor by a flexible holdfast apparatus consisting of thousands of anchor spicules (long, hair-like glassy fibers). Each spicule is covered with recurved barbs and has an internal architecture consisting of a solid core of silica surrounded by an assembly of coaxial silica cylinders, each of which is separated by a thin organic layer. The thickness of each silica cylinder progressively decreases from the spicule’s core to its periphery, which we hypothesize is an adaptation for redistributing internal stresses, thus increasing the overall strength of each spicule. [Emphasis added.]

The researchers designed a mathematical model of this concentric arrangement, seeing what sequence of radii and thicknesses would produce the maximum force transmission to the rest of the skeleton. Then they scored the sponge by their model:

Compared with measurements of these parameters in the native sponge spicules, our modeling results correlate remarkably well, highlighting the beneficial nature of this elastically heterogeneous lamellar design strategy. The structural principles obtained from this study thus provide potential design insights for the fabrication of high-strength beams for load-bearing applications through the modification of their internal architecture, rather than their external geometry.

When engineers design buildings, they typically think of external geometry. The steel I-beam, the truss, and the buttress are common examples. What if the materials were designed, instead, from the inside out? Imagine how skyscrapers built on this "design strategy" could withstand earthquakes. A press release from Brown describes the sponge’s habitat:

Life may seem precarious for the sea sponge known as Venus’ flower basket. Tiny, hair-like appendages made essentially of glass are all that hold the creatures to their seafloor homes. But fear not for these creatures of the deep. Those tiny lifelines, called basalia spicules, are fine-tuned for strength, according to new research led by Brown University engineers.

The news item reveals the "Aha!" moment when a member of the engineering team looked at the structure under an electron microscope:

When Haneesh Kesari, assistant professor of engineering at Brown, first saw this structure, he wasn’t sure what to make of it. But the pattern of decreasing thickness caught his eye.

"It was not at all clear to me what this pattern was for, but it looked like a figure from a math book," Kesari said. "It had such mathematical regularity to it that I thought it had to be for something useful and important to the animal."

Kesari got together with James Weaver and Joanna Aizenberg from Harvard’s Wyss Institute for Biologically Inspired Engineering, "who have worked with this sponge species for years." Years ago, the two found that the Venus Flower Basket’s glass fibers act as perfect little fiber optic strands, performing even better than man-made ones. Their paper in 2004 stated:

The spicules can function as single-mode, few-mode, or multimode fibers, with spines serving as illumination points along the spicule shaft. The presence of a lens-like structure at the end of the fiber increases its light- collecting efficiency. Although free-space coupling experiments emphasize the similarity of these spicules to commercial optical fibers, the absence of any birefringence, the presence of techno-logically inaccessible dopants in the fibers, and their improved mechanical properties highlight the advantages of the low-temperature synthesis used by biology to construct these remarkable structures.

So here, more than a decade later, another spectacular design feature of these microscopic fibers comes to light. "The researchers say this is the first time to their knowledge that anyone has evaluated the mechanical advantage of this particular arrangement of layers," the press release says. Thus, these tiny little fibers that could so easily be dismissed by the casual onlooker as "simple" or "primitive" actually perform two independent functions — light transmission and anchoring — exceptionally well. It’s like watching a strong man pull a semi-truck and solve differential equations at the same time.

Remarkable Conclusions

It’s interesting how many times the word "remarkable" is used in the press release and the paper. The fibers are "remarkably strong." They have a "remarkable internal structure" giving them "remarkable properties." The results of the mathematical model correspond to the actual sponge spicules "remarkably well." Since this sponge is so remarkable, let’s add some remarks.

All of the scientific data, observations, and models in these studies depend entirely on design thinking. The sponge spicules possess a mathematical structure that fine tunes them for two independent functions, which they perform to specs that human engineers cannot yet reach. Why on earth would anyone attribute these traits to blind operations of chance? Remarkably, that’s what these scientists and reporters do:

The lives of these sponges depend on their ability to stay fixed to the sea floor. They sustain themselves by filtering nutrients out of the water, which they cannot do if they’re being cast about with the flow. So it would make sense, Kesari thought, that natural selection may have molded the creatures’ spicule anchors into models of strength — and the thickness pattern could be a contributing factor. [From the press release.]

Even more importantly, many biological skeletal elements are inherently multifunctional and have evolved the ability to perform a variety of tasks in addition to their mechanical ones. [From PNAS.]

Those are throwaways that gloss over the details. The useful lines are those that provide inspiration, insight, and application due to observation of remarkable design. The press release ends:

It could add to the list of useful engineered structures inspired by nature.

"In the engineered world, you see all kinds of instances where the external geometry of a structure is modified to enhance its specific strength — I-beams are one example," Monn said. "But you don’t see a huge effort focused toward the internal mechanical design of these structures."

This study, however, suggests that sponge spicules could provide a blueprint for load-bearing beams made stronger from the inside out.

Add to this that the Venus Flower Basket is not unique. Other creatures, unrelated by evolution, produce similar models of fine-tuned strength, like the nacre in oyster shells, the scales on fish, and the structure of our own bones and teeth. Nature’s designs have led to institutes that devote themselves to the study of "Biologically Inspired Design."

How does one evaluate a good science? Sir Francis Bacon said you would know it by its fruits. If inspiration, understanding, and application are good fruits, intelligent design is ripe for the picking.

Image by NOAA Office of Ocean Exploration (NOAA) [Public domain], via Wikimedia Commons.

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