The latest creature to inspire materials science research? Limpets.
Although their name may sound more like a baked breakfast good, limpets are mollusks. More specifically, they’re a kind of gastropod, i.e., a snail. And limpets are somewhat notable, as scientists recently discovered that their teeth are the strongest natural material known to man.
MIT researchers recently discovered another unique feature that these unassuming gastropods are hiding within their shells. Their work revealed that some limpet shells contain unique biological photonic structures that are the first known to be made from inorganic, mineralized structures. The researchers published their findings in Nature Communications.
The research zoomed in on one particular variety of fingernail-sized snail, the blue-rayed limpet. Blue-rayed limpets feature a drab shell that is adorned with brilliant blue stripes, which make the tiny snails stand out even in murky water of the kelp beds they call home along the coasts of Norway, Iceland, the United Kingdom, Portugal, and the Canary Islands.
While other creatures—such as beetles, butterflies, and birds—also display bright hues, those colors come from photonic structures within the creatures’ shells, scales, or feathers that are composed primarily of organic materials.
“The majority of structurally diverse, functional biophotonic architectures in different species are mainly comprised of highly ordered organic materials, including chitin, guanine, collagen, keratin, reflectin, pterin, melanin, or carotenoids,” the authors explain in the paper. “Being the most prominent example of a biologically produced mineral-based iridescent material, nacre’s diverse colour palette originates from light interference within its layered composite structure of microscopic aragonite tablets. The structural colour is only apparent in the interior of the shell with little to no external visibility.”
Credit: Massachusetts Institute of Technology (MIT); YouTube
To investigate the interesting color of blue-rayed limpets’ stripes, the researchers first examined the shells from the outside. Scanning electron micrographs of the shells’ surface showed no structural differences in the striped regions, so the researchers dug deeper.
Using focused ion beam milling, they cut out a cross-section of the shell and examined the underlying nanoarchitecture using 2-D and 3-D structural analyses.
Their findings show that about 30 micrometers below the shells’ striped surface, the shell-standard uniform composition of stacked calcium carbonate platelets changed. There, the researchers found wider zigzagged layers of calcium carbonate that were underlaid with randomly arranged spherical colloidal particles.
Further analysis of the zigzags’ spacing and angles revealed that they were optimally arranged to reflect blue and green light, acting as an optical interference filter. And the colloidal spheres served to “absorb transmitted light that would otherwise desaturate the reflected blue color,” according to an MIT press release. So the zigzags reflect only blue light, while the remaining light spectrum travels through the shell, where it is absorbed by the colloidal particles, making the blue look bluer.
“Therefore the absorbing particles underneath the limpet’s multilayer architecture are not the direct origin of the blue colour, which is caused by the multilayer architecture,” the authors write in the paper. “The particles rather provide an absorbing background for the multilayer filter to enhance the spectral purity of the reflected blue light.”
According to the paper, the chemical composition of the colloidal particles remains unknown.
In addition to representing the first known biological photonic structures that are composed of inorganic structures, the authors make another interesting point: the limpet shells are able to incorporate photonic structures without compromising structural integrity.
“It’s all about multifunctional materials in nature: Every organism—no matter if it has a shell, or skin, or feathers—interacts in various ways with the environment, and the materials with which it interfaces to the outside world frequently have to fulfill multiple functions simultaneously,” Mathias Kolle, co-author and MIT mechanical engineering professor, says in the press release. “[Engineers] are more and more focusing on not only optimizing just one single property in a material or device, like a brighter screen or higher pixel density, but rather on satisfying several … design and performance criteria simultaneously. We can gain inspiration and insight from nature.”
The researchers speculate that their findings could be used to develop similar smart architectures in custom-designed materials.
“Let’s imagine a window surface in a car where you obviously want to see the outside world as you’re driving, but where you also can overlay the real world with an augmented reality that could involve projecting a map and other useful information on the world that exists on the other side of the windshield,” Kolle says in the release. “We believe that the limpet’s approach to displaying color patterns in a translucent shell could serve as a starting point for developing such displays.”
The open-access paper, published in Nature Communications, is “A highly conspicuous mineralized composite photonic architecture in the translucent shell of the blue-rayed limpet” (DOI: 10.1038/ncomms7322).