Soaking Up Rays

As a heavy rod of glass sinks into a searing furnace, a wheel below it whirs. Every second that the drum turns, it pulls almost another meter of glass fiber off the softened end of the translucent shaft.

The fiber then races through lasers that measure its hair-thin diameter and through a cup of liquid polymer, which adds a protective skin. In the cavernous Corning plant in Wilmington, N.C., a row of these fiber-drawing towers operates nonstop, 24 hours per day. They crank out millions of kilometers of optical fiber every year.

Meanwhile in the Antarctic Ocean, a half-world away, another kind of optical fabrication is going on–just as it has for hundreds of millions of years. There, marine sponges of the species Rossella racovitzae silently waft seawater through their tube-like bodies. At some 100 to 200 meters depth, each of the animals is absorbing silicon dioxide from the frigid ocean water. This compound, also known as silica, is the main ingredient of glass.

These creatures, which over a century or more grow to about a meter in length, assemble the silica into a skeleton of glassy spines, or spicules. Some of these spicules form the tubular matrix on which the sponge’s tissue grow, whereas others jut out a finger-length or two from the sponge’s surface.

As different as an Antarctic sponge habitat and a North Carolina optical fiber factory may seem, they create a similar product: thin strands of glass that can conduct light along their length even when curved.

The optical fibers whizzing off the Corning production line will carry gaggles of phone conversations and torrents of digital data. With their remarkable clarity, they can instantly transport vast amounts of information encoded as light pulses across tremendous distances.

In contrast, nobody knows why the flexible, jutting spicules of these Antarctic sponges transmit light. This capability may be part of a light-harvesting adaptation that helps the sponge species survive in the cold, dark depths of Antarctic seas, some researchers have proposed.

Although biologists have been aware for several years that Rossella spicules can guide light as optic fibers do, only recently have materials scientists and engineers taken a closer look at these shafts. They are finding that sponges build glass fibers with a composition and structure markedly different from anything people have created. They’re also learning how unusual details of natural engineering can lead to structures that combine toughness and resilience with what appears to be an exceptional capability to collect light and some potential to pipe it around.

There’s another factor that makes these natural optical devices of interest to engineers. Factory-made optical fibers degrade in water if they don’t have special coatings, but the spicules grow and function in the aquatic realm.

“The spicules apparently are perfectly happy in water,” says Ann M. Mescher, a mechanical engineer and polymer fiber specialist at the University of Washington in Seattle. “It’s fascinating that there’s a creature that produces these fibers at low temperature with these unique mechanical properties and fairly good optical properties.”

Optical fiber specialists aren’t looking to sponges for lessons on transporting light. The spicule investigators say it may take some time before industry discovers what insight the sponge has to offer.

“It’s not something they’re going to put into telecommunications in the next 2 or 3 years,” says University of Washington materials scientist Brian D. Flinn. “It’s something that might be 20 years off.”

Light pipes

Fibers that transmit light take advantage of the reflective boundary between one transparent medium, such as glass, and another, such as air. At most angles, light hitting the boundary refracts, or bends, as it passes through.

At very shallow angles, however, the light reflects off the boundary like a low-flung pebble skipping off a pond’s surface.

In telecommunications systems, optical fibers typically consist of a narrow glass core jacketed in another type of glass, in which light travels more quickly. Most light entering the core repeatedly ricochets off the core-cladding interface all the way to the other end.

Although the best fiber consists of highly purified, chemically modified glass, many short-distance, lower-frequency data transmissions travel through stouter plastic fibers.

Besides the now-ubiquitous data fibers, there’s another class of less exalted fibers that simply guide light from place to place (SN: 5/26/90, p. 335; 12/23 & 30/89, p. 412). Physicians use them to peer inside the body with endoscopes, automakers use them to distribute light to eye-catching dashboard displays, and architects steer daylight through them into the nooks and crannies of buildings.

Light pipes also crop up in the natural world. Not only are the rods and cones in retinas light conduits, but so are certain plant cells and even gray or white hairs (SN: 12/23&30/89, p. 414). One researcher claimed that polar bears use the light-harvesting ability of their fur to stay warm, although other scientists challenged that claim (SN: 3/8/86, p. 153; 4/5/86, p. 211).

Among the natural examples of light-guiding fibers, however, only the sponge spicule is made of glass, like the commercial product.

Glass skeletons

Glass is a long-standing choice for sponges. For unknown reasons, about 600 million years ago, these primitive animals started incorporating silica as the material for their hard parts. In this, they diverged from the rest of the animal world, which makes its skeletal components out of calcium compounds and other materials, says Riccardo Catteneo-Vietti of the University of Genoa in Italy. The once-dominant calcareous sponges slowly dwindled, he says.

Today, nearly all sponges are siliceous. That makes them similar to grasses and some other plants, which toughen their leaves and stems with silica, presumably as a defense against grazers.

The notion that sponges might be using some of their glass to conduct light first occurred to scientists in the early 1990s. It was then that Elda Gaino and Michele Sar, who were both at the University of Genoa in Italy, noticed strands of green algae inside the little, bulbous tropical sponge Tethya seychellensis.

The green strands curled around spicules that radiate from the center of the sponge to the animal’s surface. In a 1994 report, the biologists proposed that the spicules may pipe light into the sponge’s interior where algae use it for photosynthesis. Perhaps, the researchers added, the sponges receive nutrients from the algae in return.

Gaino, now at the University of Perugia, and Sar didn’t see how they could actually test their light-conduction hypothesis on T. seychellensis. Its spicules are shorter than a day-old whisker and a tenth as wide.

Fortunately, the two scientists learned of R. racovitzae, the Antarctica dweller. It has more manageable spicules, about as thick as an office staple and 10 to 20 centimeters long.

Using red laser light, Gaino, Sar, Cattaneo-Vietti, and their colleagues demonstrated in 1996 that an R. racovitzae spicule could indeed transmit the light, albeit with far less efficiency than a commercial optical fiber does. The scientists also noted that a peculiar four-pointed spike that caps the outer end of each spicule boosts the amount of light absorbed.

Although they saw no green algae colonizing the spicules, the researchers noted that unicellular, siliceous plants, called diatoms, “tend to gather just along the spicules,” Gaino says.

Just how biological structures develop seemingly industrial traits–like that light-guiding capability of spicules–is the kind of puzzle that appeals to Mehmet Sarikaya and his biomimetics group at the University of Washington. Biomimetics researchers strive to understand how organisms produce marvelously engineered constructions such as shells, teeth, glues, and even the functional equivalent of compass needles. Then they apply those lessons to artificial creations (SN: 9/26/98, p. 198).

“So many things we really dream about in industry are already there in nature,” says Joanna Aizenberg, a biomimetics investigator at Lucent Technologies’ Bell Labs in Murray Hill, N.J. “It’s just a matter of seeing how [organisms] do it and applying the same mechanism.”

After reading the Italian scientists’ report, the scientists in Seattle obtained some spicules from Gaino and set out to reveal what’s behind their light-piping feats.

The biomimetics specialists weren’t surprised to find that the spicules have a finely layered structure; that’s also the way seashells are made. But it’s completely unlike the structure of commercial optical fibers. In the spicules, the layers form a jellyroll structure. Between the spicules’ 50 to 200 layers of glass–the cake–are thinly spread organic molecules–the jelly–that are probably primarily protein. So far, the researchers have been unable to discern whether the layers form a continuous spiral or concentric tubes.

Laboratory tests

Researchers have yet to study the spicules’ optical properties in their natural marine settings, but they’ve subjected the structures to laboratory tests.

In one set of experiments, Mescher removed each spicule’s star-shaped cap and measured the refractive index of the remaining shaft. A material’s refractive index is the ratio of the speed of light in a vacuum to the speed of light in the material.

As it turns out, the refractive index of the spicule, which was determined to be 1.49, is nearly the same as that of the cores of commercial telecommunications fibers. Their indexes range from 1.44 to 1.48. The refractive index of window glass is 1.51 and that of leaded glass exceeds 2. Seawater has a rather low refractive index of 1.37.

For sponge spicules, seawater serves as the cladding. A big difference in refractive indexes between two materials is a plus for collecting and confining light within a fiber. The greater the difference in refractive index between the fiber’s core and cladding, the larger the angle of incoming light that can be confined by the fiber. In water, the polished ends of cut spicules can accept light at angles up to 52 away from straight-on, the researchers find. That matches the upper end of the commercial-fiber range.

The spicule “is a very efficient collector of light,” says Mescher.

Once spicules gather light, however, they don’t carry it very far, at least in the laboratory tests. The dried-out, cracked spicules that Mescher and her colleagues have studied are such poor light conductors that the team hasn’t even bothered to measure their transmission. Mescher suspects spicules do better when they’re fresh and in their natural settings.

“If you took a [commercial] glass fiber and produced the same number of microscopic cracks in it, the transmission would [also] be absolutely horrible,” she notes.

Her colleague Flinn has taken on the task of measuring the spicules’ mechanical properties. He and his coworkers reported their results in the May Journal of Materials Research.

Most noteworthy, Flinn found, is the spicules’ response to sharp bending. They break in stages–a consequence of their layered structure, he says. Scanning electron micrographs of the shattered spicules show that a crack starting in the outermost silica layer doesn’t go more than a few layers before it stops in the organic matter between silica tubes. Then, tension must build again in order for a new crack to penetrate deeper.

In contrast, commercial optical fibers can be very strong, but once a crack starts, the fiber goes “tink” and breaks all the way through immediately, he says.

Nature vs. technology

So far, the optical fiber industry has not beat a path to the Seattle group’s door. In a sector where the pace of technological progress is about as fast as it gets, engineers and product designers don’t pause long to consider such an oddity as R. racovitzae and its little light pipes.

Some industry researchers are intrigued when they hear that an exotic undersea creature makes glass fibers. However, their enthusiasm wanes when they realize that the fraction of light lost within the 15 centimeters of a spicule is greater than what gets lost through a kilometer of ordinary commercial fiber. Moreover, since sponge spicules contain lots of water, they would automatically block the infrared light most commonly used for telecommunications.

What’s more, breakage and water damage were problems for optical fiber only when the technology was new. With today’s manufacturing methods, “those issues are well resolved,” says Martin G. Drexhage, who heads the Center for Advanced Fiberoptic Application.

The Seattle researchers readily concede that sponge spicules can’t hold a candle to products of Corning and other optical fiber makers. “There’s no deep-sea critter that’s going to revolutionize the way commercial fibers are going to be produced,” says Mescher.

Even so, the sponge fiber may be useful “as a model of a material we may want to make,” adds Flinn.

Mescher agrees. She contends that the spicules’ capability to accept light coming in from a wide range of angles might suggest ways to make fibers that are easier to connect. That could help bring down the now-high cost of running optical fibers all the way into private homes. Such fibers might also benefit solar power, she suggests, particularly in space applications, where light can be scarce.

Flinn notes that light-based communications and computing are just getting started. Perhaps some aspect of the sponge spicules may play a role in emerging technologies, he says.

The biologists who first drew attention to the spicules don’t care much about technological potential or lack thereof. For them, the spicules embody nature at its most wondrous and perplexing: Did the interplay of the life histories of algae and sponges really drive the evolution of these natural light guides? Or could the spicules’ way with light merely be a glimmering artifact that comes with making one’s skeleton out of glass?