Squids have long been a source of fascination for humans, providing the stuff of legend, superstition and myth. And it’s no wonder – their strange appearances and strange intelligence, their mastery of the open ocean can inspire awe in those who see them.
Legends aside, squids continue to intrigue people today – people like Professor Daniel Morse of UC Santa Barbara – for much the same, albeit more scientific, reasons. Having evolved over hundreds of millions of years to hunt, communicate, escape predators, and mate in the vast expanses of open water, often without features, squids have developed some of the most sophisticated skins in the animal kingdom.
“For centuries people have been amazed at the ability of squid to change the color and patterns of their skin – which they do wonderfully – for camouflage and underwater communication, signaling to each other. and other species to keep themselves apart, or as an attraction to mating and other types of signaling, ”said Morse, professor emeritus of biochemistry and molecular genetics.
Like their cephalopod cousins the octopus and cuttlefish, squids have specialized pigment cells called chromatophores that expand to expose them to light, resulting in various shades of pigment color. However, the squid’s ability to sparkle and sparkle, reflecting different colors and breaking light onto their skin, is especially interesting to Morse. It’s believed to be an effect that mimics the dappled light of the upper ocean – the only feature of an otherwise austere seascape. By understanding how squid manages to blend in with the simpler backgrounds – or stand out – it may be possible to produce materials with the same light-adjusting properties for a variety of applications.
Morse has worked to unlock the secret of squid skin for the past decade, and with support from the Army Research Bureau and research published in the journal Letters of Applied Physics, he and co-author Esther Taxon are even closer to unraveling the complex mechanisms underlying squid skin.
An elegant mechanism
“What we found is that not only the squid is able to adjust the color of the reflected light, but also its brightness,” said Morse. Research had so far established that certain proteins called reflectins were responsible for the iridescence, but the squid’s ability to adjust the brightness of reflected light was still a mystery, he said.
Previous research by Morse had uncovered structures and mechanisms by which iridocytes – light-reflecting cells – in the opalescent skin of the coastal squid (Doryteuthis opalescens) can take on virtually any color of the rainbow. This happens with the cell membrane, where it folds into accordion-shaped nanoscale structures called lamellae, forming tiny outer grooves that are less than the wavelength in width.
“These tiny grooved structures are like the ones we see on the burned side of a compact disc,” Morse said. The reflected color depends on the width of the groove, which corresponds to certain wavelengths of light (colors). In squid iridocytes, these lamellae have the additional characteristic of being able to metamorphose, widen and narrow these grooves through the actions of a remarkably tuned “osmotic motor”, driven by reflectin proteins condensing or dispersing within. slats.
While material systems containing reflectin proteins were able to approach the iridescent color changes that the squid was capable of, attempts to replicate the ability to intensify the brightness of these reflections have always been unsuccessful, the researchers say. who felt that something had to be coupled with the reflections in the squid skin, amplifying their effect.
That something turned out to be the very membrane enclosing the reflectors – the lamellae, the same structures responsible for the grooves that divide light into its constituent colors.
“Evolution has optimized not only the color adjustment but also the brightness adjustment so well using the same material, the same protein and the same mechanism,” said Morse.
Light at the speed of thought
It all starts with a signal, a neural impulse from the brain of the squid.
“Reflectins are normally very strongly positively charged,” Morse said of the iridescent proteins, which, when not activated, look like a string of beads. Their same charge means they repel each other.
But that can change when a neural signal causes reflectins to bind to negatively charged phosphate groups that neutralize the positive charge. Without the repellency that keeps proteins in their messy state, they fold and attract each other, accumulating in fewer and larger aggregations in the lamellae.
These aggregations exert osmotic pressure on the lamellae, a semi-permeable membrane built to resist only the pressure created by the agglomerating reflections before releasing water outside the cell.
“The water is squashed out of the accordion structure, and it collapses the accordion so that the thickness of the spacing between the pleats is reduced, and it’s like bringing the grooves together on a compact disc.” Morse explained. “Thus, the reflected light can gradually change from red to green to blue.”
At the same time, the collapse of the membrane concentrates the reflectins, causing their refractive index to increase, amplifying the brightness. Osmotic pressure, the motor that drives these optical property adjustments, tightly couples the lamellae to reflectins in a highly calibrated relationship that optimizes output (color and brightness) to input (neural signal). Erase the neural signal and the physics reverse, Morse said.
“It’s a very smart, indirect way of changing color and brightness by controlling the physical behavior of what’s called a colligative property – osmotic pressure, something that isn’t immediately obvious, but reveals the complexity of the evolutionary process, the millennia of mutation and natural selections that have perfected and optimized these processes together. “
Thin films with adjustable brightness
The presence of a membrane may be the vital link for the development of bioinspired thin films with the optical tuning ability of the opalescent coastal squid.
“This discovery of the key role the membrane plays in adjusting the brightness of reflectance has intriguing implications for the design of future buihybrid materials and coatings with adjustable optical properties that could protect soldiers and their equipment,” said Stephanie McElhinny, a program manager at the Army Research Office, an element of the US Army Combat Capability Development Command’s Army Research Lab.
According to the researchers, “This efficient and sophisticated coupling of the reflectin of its osmotic amplifier is closely analogous to the impedance-matched coupling of activator-transducer-amplifier networks in well-designed electronic, magnetic, mechanical and acoustic systems.” In this case, the activator would be the neural signal, while the reflectins act as transducers and the osmotically controlled membranes serve as amplifiers.
“Without this membrane surrounding the reflectins, there is no change in the brightness of these man-made thin layers,” said Morse, who is collaborating with fellow engineers to study the potential of a thin film more like skin. squid. “If we are to capture the power of the biological, we have to include some kind of membrane-shaped enclosure to allow reversible adjustment of the brightness.”