Biomimicry - from sharks to sea urchins and seashells

Materials World magazine
,
1 Feb 2016

Jay Harman discusses how lessons learned from nature can solve human problems, such as mimicking shark skin to reduce hull friction and sea urchin spines to strengthen concrete.

Why do we need biomimicry right now? Because most of our environmental and economic problems result from an outdated way of doing business. Industry has continued to depend on the same old ‘heat, beat, and treat’ methods that were mechanised in the industrial revolution but, I believe, these methods simply aren’t sustainable. Nature, on the other hand, has solved virtually every problem facing humanity. Nature is clean, green, and sustainable, using the least amount of materials and energy for maximum result.

Modern biomimicry is more than just copying nature’s shapes. It includes systematic design and problem-solving processes that anyone can learn and apply. Materials science is an ideal field for bio-inspiration, with university laboratories from Quebec to Singapore dedicated to biomimetic materials research. Emerging commercial examples include a new textile that mimics the way pinecones respond to humidity, a substitute for glass fibre developed from carrots and self-hardening materials based on blowfly maggot skin. Here are several exciting developments – from just two groups of animals – sharks and seashells.

Designed for speed

Sharks have been around, biting things, for almost a half a billion years. It’s more than a little ironic that while they’re notorious for it, sharks very rarely eat humans – or even bite them. Only around 75 shark bites are reported worldwide every year, with an average of four deaths. But humans eat sharks – up to one hundred million of them per year. Now they’re also being used as inspiration for a number of valuable products. 

Sharks are prime examples of extraordinary design. Alongside their streamlined shape, they reduce drag resistance thanks to their cleverly evolved skin, made of tiny vertical scales known as placoid scales, or dermal denticles – ridges that make shark skin so rough that carpenters used to use it for sanding wood.

It seems counterintuitive that a rough surface offers less drag, or resistance, than a smooth one. However, nature proves this to be true in countless examples. No matter how smooth its hull is, a ship can drag its own weight in water as it travels. Essentially, by roughing up the water in a precise way at the intersection of the shark and its surroundings, less of the water sticks to a shark to slow it down. This feature is currently being exploited by a number of businesses. 

For example, researchers at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research in Germany have developed a paint that, when applied with a particular stencil, creates ridges in a pattern that improves surface fluid dynamics. The German researchers were able to reduce hull friction by more than 5% in a test conducted with a shipbuilding facility. Extrapolated over just one year, it would mean a potential saving of two thousand tonnes of fuel for a large container ship typically travelling thousands of nautical miles – and there are fifty thousand large ships in the world. If applied to the world’s aircraft, the total savings could be more than four million tonnes of fuel per year. 

Fast-moving fish, like tuna, rarely have creatures like algae, worms, or barnacles attached to them. Slower moving animals, such as turtles and some whales, do attract their share of these freeloaders, which start with omnipresent oceanic populations of tiny microorganisms or bacteria attaching to the animal’s outer skin. Despite being known for their speed, sharks actually spend most their time cruising in low gear to conserve energy, so one would think they’d attract their share of these microscopic creatures. It turns out, though, that the shark’s dermal denticles – the same feature of their skin that produces less drag – also have the added benefit of attracting fewer surface passengers. Which is exactly what led to Sharklet Technologies, based in Colorado, developing a thin film that mimics these denticles and resists the colonisation of microorganisms.

Dr Anthony Brennan, an engineering and materials science professor at the University of Florida, USA, is an expert in the life cycle of microorganisms. As many of these tiny creatures can grow and divide every 20 minutes, a single cell can become more than eight billion cells in less than 24 hours. Once these cells attach to a surface, they rapidly colonise into so-called biofilms, which then lead to algae formation, weed growth, and other forms of fouling on a hull. Dragging all these extra passengers through the water has a negative impact on fuel use – up to 40% more fuel if it is fouled, so boat owners around the world must regularly haul their boats out of the water to clean underwater surfaces. These dry dock sessions are not only expensive in labour and materials but represent downtime for the boat owner. It is an even more critical concern for military ships.

In the past, a variety of toxic paints were applied to kill or resist microorganisms, but the environmental effects from these are so harmful that the most effective paints are now banned. Brennan’s breakthrough came as he and his colleagues watched a submarine with a fouled hull return to Pearl Harbor in Hawaii. He commented that it looked like an algae-covered whale, and asked himself and his team if there were any slow-moving sea animals that weren’t victim to the same fate. Sharks captured his attention.

The Sharklet surface is composed of millions of microscopic, diamond-shaped denticles. Each is 20 µm thick and 2 µm deep, so the pattern can’t be seen with the naked eye. Like shark’s skin, each denticle has a particular height-to-width ratio, which creates just the right amount of surface roughness to discourage microorganisms from attaching and colonising. The spacing and height of tiny ribs on sharks’ denticles, mirrored in this technology, also interfere with the bacteria’s ability to signal one another. The company states that ‘green algae settlement on the surface is reduced by 85% compared to smooth surfaces. Sharklet is now the first nontoxic, long-lasting, no-kill surface proven to control the growth of bacteria'. Such properties open up many commercial opportunities for the technology in marine applications, though after analysing the potential markets, Sharklet started with a range of low-cost biomedical products that could be easily installed by consumers. Its adhesive-backed plastic films can be attached to doorplates, bathroom stalls, locker room benches, and other sites to reduce the spread of bacteria. The company is now developing other applications, including catheters to diminish the rate of urinary tract infections in hospitals.

Sharp ideas 

The chiton mollusc is the product of around 500 million years of nature’s ‘R&D’, and countless prototypes. Researcher Derk Joester at Northwestern University, USA, studies its self-sharpening teeth. Chiton teeth, like seashells, are created in water at ambient temperature, so they don’t require foundries with massive amounts of heat and pressure to manufacture. Yet they cling tenaciously to rock and grind it away with their teeth to extract their algae supper. The promise of this research? Just for starters – superior dental implants and artificial hips.

While on the subject of toughness, the beautiful angelwings seashell looks fragile, but its surfaces are so hard that by grinding back and forth, the up-to-seven-inch-long animal can tunnel into solid rock, where it takes up permanent residence protected by a stone fortress. The US$1 trillion mining industry might learn something from this strategy.

Sea urchins are soft animals sheltered by a globular shell and razor-sharp spines. They’re closer cousins to starfish than to sea snails, but they also munch on rock to make safe caves for protection from predators. Scientists at the University of Wisconsin, USA, have discovered that urchin teeth are among the most complicated structures in the natural world. They’re not only super sharp but also made of interlocking, curved plates and fibres that are self-sharpening. Parts of the continuously growing tooth are designed to break away, leaving the remaining structure razor sharp – a little like chipping obsidian to make arrowheads. The researchers have confirmed that the sea urchin’s strategy outperforms man-made cutting tools. 

Researchers at the University of Konstanz, Germany, are studying the strength and fracture resistance of sea urchin spines themselves. As defensive weapons, they’re necessarily very hard but also shock absorbing. This seeming contradiction has made them one of the most studied of all biomaterials. One potential opportunity is to create much tougher, fracture-resistant concrete.

Deep and meaningful

You’ve probably heard how little we know about the ocean depths and their inhabitants. One snail, Chrysomallon squamiferum, which is found thousands of feet down in the Indian Ocean, surprised researchers. Many mollusks have a defensive front door for their shells called an operculum. This deep ocean snail, affectionately called scaly foot, has a unique operculum reinforced with layers of iron pyrite and gregite. No other animal on earth is known to incorporate these minerals. 

Researchers at MIT, USA, analysed the mechanical properties of the scaly foot’s shell and discovered that its unique, three-layered, structure dissipates mechanical energy, which helps the snails fend off attacks from crabs that try to break the shell with their claws. The shell possesses a number of additional energy-dissipation features compared to typical mollusk shells that are primarily composed of calcium carbonate. Industrial opportunities anticipated include superior helmets, armour, and new structural materials. Pyrites and gregite are also being evaluated as an alternative to silicone for the creation of cheap, abundant solar cells.

These are just a few examples of the potential economic and environmental benefits of biomimetic materials. With nature as our teacher, the future looks bright.

Jay Harman, author of The Shark’s Paintbrush, is a scientist, inventor, and entrepreneur dedicated to creating breakthrough technologies through biomimicry to radically reduce energy consumption worldwide.