June 24, 2017 | David F. Coppedge

Sea Sponge Makes Flexible Glass

How do you make glass that bends without breaking? Learn from a lowly sponge. Then look around for other ideas.

We’ve seen the Venus Flower Basket before. It’s the amazing ‘primitive’ form of life that makes fiber optic cables (see 7/08/05). Now, researchers at Brown University found another marvel about this sponge: it makes bendable glass (see Phys.org). The glass, composed of strands half the width of a human hair called spicules, are part of its ‘holdfast’ mechanism that anchors it to the sea floor.

Venus Flower Basket, Credit: Kesari Lab/Brown University

The spicules, each about half the diameter of a human hair, are made of a central silica (glass) core clad within 25 thin silica cylinders. Viewed in cross-section, the arrangement looks like the rings in a tree trunk. The new study by researchers in Brown University’s School of Engineering shows that compared to spicules taken from a different sponge species that lacks the tree-ring architecture, the basalia spicules are able to bend up to 2.4 times further before breaking….

When study co-author Haneesh Kesari, assistant professor in Brown’s School of Engineering, first saw the internal architecture of the basalia spicules, he was immediately intrigued by the consistency and regularity of the pattern. “It looked like a figure from a math book,” he said.

The secret appears to be in the arrangement of the concentric cylinders. They become thinner toward the outside. The arrangement is “mathematically optimal for maximizing the spicules’ strength” but at the same time offers flexibility. The strands bent so far, engineers were unable to apply the standard Euler-Bernoulli beam theory to measure it. They had to change their analysis approach for this biological design that allows the organism to twist its fibers into the silt for a firm anchor. And so, this humble sea creature is set to advance scientific analysis and to inspire engineers.

Monn hopes that studies like this one will provide the data needed to devise proper models to explain the properties of these natural structures, and eventually make use of those structures for new human-made materials.

Not for Suckers: Other Biological Designs

Octopus suckers have inspired a new adhesive patch that works underwater. In “How to suck like an octopus” (Nature), Jonathan J. Wilker says, “Rubber sheets that reversibly bind and release substrates have been made by copying a subtlety in the shape of octopus suckers.” Wilker makes it clear that the octopus is not the only organism creating excitement in science:

Steve Lodefink/Flickr, Wikimedia Commons

Characterizing and mimicking biological attachment strategies is a booming research area. The two most prominent strategies are wet adhesion and dry adhesion. Mussels, seagrasses and bacteria belong to the wet-adhesive community of organisms: they deposit glue, and use it to stay in place for long periods, if not their entire lives. The underwater bonding achieved by such species cannot be matched by most synthetic adhesives, although some biomimetic compounds now exhibit adhesion strengths similar to those of their natural counterparts.

Dry adhesion is more typical of insects and geckos, which use hardened, hair-like or pad-like structures on their feet to walk up walls. Such adhesion is temporary, used for locomotion and often employed in dry environments. Efforts to mimic natural dry adhesives have also yielded high-performance, hard-structured adhesives in the past few years ….

Starfish use dry adhesion, he goes on to say, but are very slow for underwater applications to imitate. The octopus is fast. “The grace of an octopus moving across the sea floor is captivating, combining the advantages of a soft body with water-jet propulsion and an enviable coordination of body motion,” he says. “Their suckers contribute by enabling fast cycles of attachment and detachment.” Indeed, the article’s photo of an octopus, its arms studded with suckers, looks like a work of art.

It’s hard to imitate the muscular attachment the octopus uses, but a Korean team, publishing in the same issue of Nature, succeeded in mimicking the protuberances to create a wet-tolerant adhesive patch.  The protuberances resemble a dome in a cup. It worked, but as Wilker says, it’s just a starting point. If you can forgive his tip to Darwin, he shows why biomimetics is not slowing down any time soon:

Researchers developing biomimetics often find themselves playing catch-up with evolution. The current work is a starting point — perhaps the addition of further biomimetic features, such as synthetic muscles, would improve the function of octopus mimics. If fully functioning mimics can be made at multiple size scales, it might open up applications such as locomotion strategies for robots and biomedical devices (and maybe even better toys). Applications aside, understanding and mimicking the fundamental science of attachment strategies used by sea creatures can just be plain fun.

Phys.org says the Korean test patch can be attached and removed a thousand times, and offers hope for pain-free bandages that don’t pull when removed, and can work wet or dry. Kids and their pediatricians will be happy about that. Noteworthy: the Korean team didn’t say anything about evolution in their paper. Only Wilker did. What do you expect from a guy from Purdue?

Applications aside, understanding and mimicking the fundamental science of attachment strategies used by sea creatures can just be plain fun

Mussel muscle: Speaking of mussels, a team from Rice University created nanofibers for medical applications inspired by the “feet” of muscles. “The Rice lab of chemist Jeffrey Hartgerink had already figured out how to make biocompatible nanofibers out of synthetic peptides,” Science Daily reports. “In new work, the lab is using an amino acid found in the sticky feet of mussels to make those fibers line up into strong hydrogel strings.” Who should they credit for this idea? “Rice University chemists can thank the mussel for putting the muscle into their new macroscale scaffold fibers.”

Bamboo without bamboozle: Wonder how bamboo, with its hollow trunks, avoids being snapped in the wind? Like the Venus Flower Basket sponge, it uses concentric cylinders for flexibility, but this time they get thicker toward the outside. First, the ‘wow’ factoids from Phys.org:

Light and tough, bamboo is widely used as a natural, functional material in Japan and other Asian countries. Bamboo is light because of its hollow structure, which allows the plant to grow faster with small amounts of woody parts and expose itself to sunlight above other trees. But this lightness also leaves bamboo vulnerable to strong crosswinds and can make it difficult for the plant to support its own weight. To overcome this shortcoming, the woody parts of bamboo are reinforced with thin but robust fibers (vascular bundles). Each fiber is as rigid as steel.

Now that we see why engineers would like to imitate bamboo to create better building materials, we learn about a Japanese team measuring the distribution of the fibers. “Surprisingly, the real bamboo data displayed almost the same fiber distribution as the one with the theoretical, optimal fiber distribution.” But should anyone be surprised? Life often hits the bull’s-eye in optimization. Not only that, “bamboo precisely adjusts the distribution of fibers so flexural rigidity is maximized with the smallest volume of wood material possible.” No wonder, then, that the lead author of the study remarked, “Imitating the systems of animals and plants which have survived harsh conditions, an approach called biomimetics has proved successful in solving many problems in the development of materials in recent years.”

Silk night-vision goggles: Another Korean team has created “inverse opals” that may be useful for increasing human vision at night or the ability to see in the infrared. Inverse opals refer to 3-D photonic crystals. Their inspiration was silk from silkworms. In their PNAS paper, they explain, “By exploiting the favorable material traits of silk, our deformable silk-based optical nanostructure adds a dimension at the interface between nanooptics and biology.”

Pitcher perfect: The Chinese are not going to be left behind in the biomimetics bonanza. The pitcher plant has inspired a Chinese team to make “shape-memory graphene film with tunable wettability.” Their paper in Science Advances says, “Inspired by nature’s Nepenthes pitcher plant, we present a novel slippery film with tunable wettability based on a shape-memory graphene sponge.”

Lizard wiggle digger: Some desert lizards bury themselves in the sand on hot days to where it’s cooler. They do this with a “fascinating strategy,” says Phys.org, by quickly by wiggling with “lateral undulations” that give the normally resistant sand a fluid-like property. Hey; idea. Humans need machines that dig into the sand sometimes. “The researchers think that such a strategy can be implemented to reduce the energy required by engines digging into soil.” The idea is only in its infancy, but we can thank the desert horned lizard for the inspiration.

Imitating the systems of animals and plants which have survived harsh conditions, an approach called biomimetics has proved successful in solving many problems

Compute like a brain: The ultimate biomimetics prize will be to design a computer that works as well as the human brain. Science Daily reports on progress at Georgia Tech, where engineers seek to harness the power of collective computing the way the brain uses neural networks. Instead of trying to improve the single-file methods of traditional computers, these engineers look to “natural forms of computers” where “dynamical systems with complex interdependencies evolve rapidly and solve complex sets of equations in a massively parallel fashion.” The word evolution in this sentence refers to a goal-directed form of intelligently designed strategy of making many processors converge toward a solution. “Our goal is to reach a system with hundreds of oscillators, which would put us in striking distance of developing a computing substrate that could solve graph coloring problems whose optimal solutions are not yet known to mankind,” a team member said. That’s been happening inside human skulls ever since mankind appeared on the planet.

The future is in biological design! Don’t get left in the dust clinging to the quaint Victorian creation myth of the Bearded Buddha. Humans know design; humans see design; humans follow design. Science will advance by leaps and bounds once academic institutions and journals realize that Darwin provides them only with a ball and chain.

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