Reflectin needs its intensity amplifier: Realizing the potential of tunable structural biophotonics

Abstract

Underwater camouflage and communication in a family of recently evolved squids are controlled, in part, by the reflectin proteins, which regulate dynamic changes in the color and intensity of light reflected from intracellular Bragg reflectors and Mie scatterers. The reflectins act as a signal transducer, precisely tuning the dimensions and refractive index of the membrane-bounded structural reflectors containing them to an extent exactly proportional to neuronal signaling. This calibration between the activating signal and the output color and intensity of reflectance is governed by the physics of the liquid state of reflectin assemblies, which osmotically fine-tune the dimensions and refractive index of the membrane-bounded structural reflectors containing them. While thin-film or genetically engineered assemblies of reflectin deliver changes in the colors of reflection approximating those of the biological system (as do other proteins and polymers), none has delivered the signal-activated increase in the intensity of reflectance characteristic of the native system. The membrane envelopes (Bragg reflector lamellae and Mie-scattering vesicles) containing the reflectins apparently serve as amplifiers, functionally “impedance matched” to the range of tunable sizes of the reflectin assemblies; reflectin films or assemblies lacking such an amplifier fail to deliver signal-activated increases in reflectance intensity.

A Mini-Review on Reflectins, from Biochemical Properties to Bio-Inspired Applications

“…Basic research should be carried out to better understand reflectins’ structure, self-assembly, and their tendency to form aggregates under physiological conditions [ 27 , 65 ]. (2) As membrane envelopes serve as amplifiers to the range of tunable sizes of reflectin assemblies [ 22 ], it is necessary to decipher the mechanism of how reflectins interact with cytomembranes. This is also an interesting but puzzling issue because reflectins are verified to interact with lipid membranes [ 60 ] but without likely transmembrane, alpha-helix, or beta-sheet regions [ 19 ].…”

“…Cephalopods have used controllable iridescence to camouflage and communicate underwater for millions of years. Scientists, admiring their powerful color-change ability, worked hard to identify functional proteins in reflective tissues and explored the biochemical and functional properties of functional proteins [ 13 , 18 , 19 , 20 , 21 , 22 ]. Based on fundamental research, proteins called reflectins have been found and are used to fabricate myriad materials with distinct stimuli-responsiveness or even optical performances [ 5 , 23 , 24 , 25 ].…”

“…However, the relationship between the biological prototypes and these man-made systems is poorly considered. As Morse and Dr. Levenson suggested, the material community sometimes overstates the role of functional proteins in iridocytes but overlooks other equally important components (e.g., membrane envelope) [ 14 , 22 ]. Therefore, the contours of squid dynamic iridescence should be sketched by introducing its material basis, relevant basic studies, and bio-inspired applications to bridge the knowledge of different areas.…”

Multilayered optofluidics for sustainable buildings

“…As a possible source of inspiration, certain biological organisms have evolved multilayered mechanisms within their skin to tune independent optical properties at their interface. In a few species of squid (e.g., Loligo plei ), for instance, active camouflage is achieved through the independent and cooperative action of a pigmentary layer of chromatophore organs and a structural layer of protein cells ( 43 – 45 ), mediating surface color, spectral reflectance, and spatial patterning ( 46 – 50 ) ( Fig. 1 B ).…”

Nacre-inspired layered composite gels with broad tunable mechanical strength and structural color for stress visualization

“…Structural colors, deriving from long/short-range ordered photonic nanostructures, are found on diverse organisms, such as chameleons, 1 nacre shells, 2 cephalopods, 3 etc . Mimicking these natural nanostructures, a large number of artificial-structural-colored materials have been developed based on the assembly of colloidal microspheres, cellulose, block copolymers, or two-dimensional (2D) nanosheets.…”