Are squid the key to invisibility?

Original paper: Adaptive infrared-reflecting systems inspired by cephalopods


While many today would associate a “cloak of invisibility” with Harry Potter, the idea of a magical item that renders the wearer invisible is not a new one. In Ancient Greek, Hades was gifted a cap of invisibility in order to overthrow the Titans, whereas, in Japanese folklore, Momotarō loots a straw-cloak of invisibility from an ogre, a story which is strangely similar to the English fairytale Jack the Giant-Slayer. Looking to the future in Star Trek, Gene Roddenberry imagined a terrible foe known as the Klingons, a war-driven race that could appear at any moment from behind their cloaking devices – indeed, any modern military would bite your arm off to get hold of this kind of device. Clearly, invisibility is a concept that has captured minds across many cultures, genres, and eras, so it should be no wonder that scientists are working on making it a reality.

As is often the case in materials science, a good starting point for inspiration is to look to biology, after all, life has had billions of years of competition-driven evolution to craft its tools. To that end, Gorodetsky and his team at of the University of California, Irvine, have been attempting to replicate the master of disguise: the cephalopod. From this class of mollusk, squid and octopuses in particular excel at adaptively altering the color, texture, and patterning of their skin to camouflage against a wide variety of oceanic backdrops (see Figure 1). They accomplish this primarily by stretching and contracting pigment-containing skin cells. More importantly to this research, some species of squid have additional skin cells called ‘iridocytes’, which are structured reflective cells that resemble a microscopic comb or folded pleats. These folds reflect light at specific wavelengths that correspond to the fold size. By actively stretching and contracting these cells on-demand, the squid effectively becomes a self-modifying bioelectronic display.

SquidCamouflage (1)
Figure 1: Squid before (left) and after (right) deploying camouflage. Images reproduced from
a video by H. Steenfeldt under the YouTube Creative Commons Attribution license.

Following this technique of stretch-induced camouflage, the authors devise a technique for replicating some squid-like properties in an artificial material. The procedure consists of using electron-beam evaporation[1] to deposit an aluminum layer onto a stretched polymer film held under strain. When they release the strain, the metallically coated material shrinks and buckles to form microscopic wrinkles that are analogous to the structures in squid iridocyte cells. The polymer film of choice is an excellent proton conductor, so when mounted to electrodes, it can be stretched back to the flat state simply by applying a voltage.

When stretched, the aluminum coated film will reflect infrared light like a perfect mirror, whereas when wrinkled, the incoming light is reflected diffusively in multiple directions, like sunlight hitting the moon. To see this effect, the scientists shine infrared light – a heat source – at the material and position an infrared camera at a specific angle so that when flat, it reflects all the incoming radiation toward the camera and appears hot; when relaxed and wrinkled, much less of the light is scattered towards the camera, making the material appears to take on the thermal properties of the background and disappear from view.

Perhaps as a head-nod to their biological inspiration, the team then recreate this material in the likeness of a squid and watch as its infrared silhouette disappears from the camera’s view (Figure 2).

InfraInvisibleSquidShape_Fixed (1)
Figure 2: The flexible squid-shaped material, viewed under an infrared camera. When relaxed (left), very little infrared is reflected towards the camera, and it appears cold. When stretched (right), it reflects most incoming infrared light towards the camera and appears hot. Image edited from Fig. 5 in the manuscript.

The authors conclude that this ready-for-manufacturing material will have immediate applications in heat-regulating technology, and while it is currently limited to the infrared part of the spectrum, they also note that there is no reason why this technique couldn’t be adapted to the optical range.

Squid haven’t solved our desires for a cloak of invisibility just yet, but these mysterious creatures may hold more secrets than we realize. We would be wise to keep an eye on them … if we can.


[1]  Electron beam evaporation is the technique of bombarding a solid metal with energetic electrons, causing it to evaporate. This metal vapor then cools and condenses uniformly on all nearby surfaces, forming a uniform metallic coating.

 

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