Snail Shells Are Inspiring Tomorrow's Toughest Materials
Abalone shells combine the strength of glass with the toughness of plastic, and they've fascinated biologists and engineers for decades.
Image: Francois Barthelat/McGill University
At McGill University's Laboratory for Advanced Materials and Bioinspiration, researchers are shaping hexagonal plates into tough, bendable armour, like the scales of a fish, and making glass that doesn't shatter, but bends and absorbs force.
Across the Atlantic, at the Leibniz Institute for Interactive Materials in Aachen, Germany, another group is developing flexible nanoclay sheets—thinner than paper and completely translucent—that are impervious to flame and gas.
What both teams have in common is an interest in sea snails— abalone snails, specifically—whose shells have fascinated biologists and engineers since the mid-1970s. The shells are made of a material called nacre, which is composed almost entirely of a brittle mineral. But because of the way the mineral is organized, in sheets with tiny amounts of plastic-like proteins creating a complex system of sliding plates, nacre is over 3,000 times tougher than the mineral on its own.
"Nature takes materials that are not appealing in terms of strength or toughness and puts them together in way that produces high performance. This is the sort of thing we are after. Purely through architecture we can completely change a material's behaviour," said Francois Barthelat, head of the McGill lab. His team has made huge advances in toughening up one of humanity's most familiar breakable materials—glass.
With glass, the challenge is finding a balance between strength and toughness. Glass is strong. It doesn't bend or deform over time, but is easily shattered by a sudden impact. Plastic, on the other hand, is tough. It can survive sudden impacts, but it bends and scratches easily. But abalone shells, amazingly, combine the best features of both.
There are experiments underway to create glass that can absorb direct impacts from steel balls, and is more bendable than ever before.
The sliding plate structure of nacre allows the brittle mineral to shift when force is applied, adding to its toughness—but it's the channels between the plates that interest Dr. Barthelat's group most. When a shell is hit, force is deflected away from the site of impact down microscopic pathways of low resistance and taken on what scientists call a "tortuous route." Energy is split off at angles, nudged into a vast internal labyrinth of interlocking paths, and sent around the whole structure until it's too weak to crack anything.
Using computer programs that can model impacts on glass, the scientists designed their own system of paths. They used a laser to etch their configurations onto existing sheets of glass, and found that the glass with channels was 200 times tougher than before.
The results caused a stir when the group published them a year ago. The concept of strengthening existing glass with a simple and elegant laser-etching process proved appealing. But for Dr. Barthelat, the initial work was simply proof of principle: by introducing micro-architectural features based on natural models, they could change the properties of existing materials.
"We have to tailor the architecture to the application. We found this type of jigsaw architecture [in the first study] that works well for pulling forces," he told me. "If you want to resist impact from dropping a steel ball, the optimal architecture will be different. And these new architectures we can find in nature, also."
In fact, there is no shortage of natural structures that Dr. Barthelat's team can use as inspiration; they recently developed an interlocking hexagonal system of glass plates to resist punctures. It is a flexible armour that is based off of fish scales and armadillo hides. There are also experiments underway to create glass that can absorb direct impacts from steel balls, and glass that is more bendable than ever before—all by incorporating features that first evolved in hides and shells millions of years ago. "Every week we have new patterns we want to try, inspired by lobsters or from glass sponge, all kinds of creatures," said Dr. Barthelat. The possibilities seem endless.
But they haven't forgotten nacre, the original inspiration for the group's first glass breakthrough. In a forthcoming paper, the scientists describe a new type of glass that is over 900 times tougher than untreated sheets. The interlocking brick-like maze etched into the glass is filled with flexible polyurethane, and closely resembles natural abalone shell.
Dr. Barthelat describes his work as "top-down", taking an assembled material and altering it to be more like a seashell. But if that's the case, then scientists such as Dr. Andreas Walther and his team at the Leibniz-Institute for Interactive Material are working from the bottom up, taking new synthetic materials and building a nacre homage.
One of the most appealing aspects of bio-inspired materials is the way in which nature assembles building blocks into complex structures—without factories and huge amounts of power.
The group has been working for years on a cheap and easy way to assemble thin films of nanoclay—tiny layers of super-strong minerals—held together by flexible polymers like a seashell. The resulting material is transparent, bendable, and totally resistant to heat. Scientists project that it could be used everywhere from fire and gas barriers on aeroplanes and electronics, to food packaging.
Previous attempts at mimicking nacre have been labor-intensive, high-energy multi-step processes. But in a paper from January the group revealed that they had developed a system to coat the minerals in a flexible polymer and guide the mixture to assemble itself into paper-like sheets.
"Mussels grow nacre in a lengthy process. For our nanocomposites, we instead apply a rapid self-assembly process," Dr. Walther said, and noted in the paper that producing a sheet of the artificial nacre-like composite took less than a day.
The new process is also a significant step towards simple, sustainable manufacturing of these materials. One of the most appealing aspects of bio-inspired materials is the way in which nature assembles building blocks into complex structures—without factories and huge amounts of power.
"All the materials that nature uses are sustainable. They are recycled from other things. It's an idea that's very attractive for engineering," Dr. Bathelat told me. "Structurally, in terms of pure mechanical performance, [these materials] are not yet as good as oil based polymers. But we can learn from nature how to amplify properties, without changing the chemistry."
It shouldn't come as a surprise that the prospect of tough, high-tech composites created from cheap, plentiful materials is catching the attention of manufacturers. Dr. Barthelat said he is already working with companies who want to apply his group's etching process to their glass and ceramic products, and test new applications that take advantage of naturally-inspired designs.
And it wouldn't be the first time we've learned a thing or two from the natural world. Velcro was inspired by the simple hooks on plant burrs, and early aeronautics was driven by painstaking studies of birds in flight. But it's funny to think that we've come full-circle, back to using shells and bone as the basic components of our tools—albeit, in a highly advanced and engineered way.
This story is part of The Building Blocks of Everything, a series of science and technology stories on the theme of materials. Check out more here: http://motherboard.tv/building-blocks-of-everythin...
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