Mollusk's Brilliant Blue Stripe Could Inspire Transparent Displays

The brilliant blue stripe of one mollusk, the blue-rayed limpet, may not only protect this tiny marine organism from outside predators, but could also inspire responsive, transparent displays, according to new research.

It is not unusual for various species to boast dazzling colors, whether it's cephalopods like octopuses that use them for camouflage or killifish that use them to find a mate. But while these types of creatures produce colorful displays with organic structures such as scales or plates, limpets are different in that they use two optical structures within their shell.

The structures are configured to reflect blue light while absorbing all other wavelengths of incoming light, resulting in their blue-striped appearance. Depending on the angle at which light hits, a limpet's shell can flash brilliantly even in murky water.

Researchers believe limpets evolved these blue dotted lines to mimic displays on the shells of more poisonous snails, warding off predators. Intrigued by this, a team from MIT and Harvard University led a study to find out more.

The findings are described in the journal Nature Communications.

They performed a detailed structural and optical analysis of the limpet shells. It turns out the blue stripes first appear in juveniles, resembling dashed lines; but as a limpet ages, the stripes grow more continuous and their shade varies, ranging from deep blue to turquoise. So where do these brilliant stripes come from in the first place? Scanning electron microscopy indicates that they likely arose from features embedded deeper in the shell.

To get an even closer looks at the translucent shells, the researchers used a combination of high-resolution 2-D and 3-D structural analysis to reveal the 3-D nanoarchitecture of the photonic structures embedded in the shells. In the regions with blue stripes, the shells' top and bottom layers were relatively uniform, with dense stacks of calcium carbonate platelets and thin organic structures. However, about 30 microns beneath the shell surface the researchers found something they did not expect - regular plates of calcium carbonate morphed into a multi-layered structure with regular spacing between calcium carbonate layers resembling a zigzag pattern.

This distinct zigzag pattern could serve as a design guide for engineering color-selective, controllable, transparent displays that require no internal light source and could be incorporated into windows and glasses.

"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," co-author Mathias Kolle, an assistant professor of mechanical engineering at MIT, said in a news 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 limpet isn't the first creature to inspire future technologies. Scientists are constantly turning to nature for ideas. For example, proteins naturally created by shellfish could lead to stronger waterproof glue for surfboards, and geckoes' sticky feet have led to unique wall-climbing paddles.

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