Shutterstock | Andrea Izzotti
The intricate patterns of biological organisms, such as chameleons and plants, and their ability to perform a multitude of complicated functions inspires material scientists to create new materials which attempt to mimic these properties.
This field, also known as biomimetic engineering, takes the principles of biological organisms and adapts them to design and develop new materials and technologies. A recent advance in biomimetic engineering has drawn its inspiration from a unique, camouflaged Hawaiian squid.
Researchers have mimicked the protein reflectin, which is found in the squid, to create optical films and fibers, which could be woven together to form an invisibility cloak.
Reflectin: The Reflective and Self-Organizing Protein
As apparent in its name, the reflectin protein plays a vital role in the reflection of light by the nocturnal Eupyrmna scolopes, a Hawaiian Bobtail squid native to the central Pacific ocean. Reflectin proteins are the key components which allow for these unique cephalopods to exhibit adaptive iridescence upon exposure to sunlight.
Present within both the light organs and skin tissues of E. scolopes, reflectins are insoluble proteins which are found as layers of flat and stacked platelets throughout the squid's tissues, and between these layers are alternating high and low refractive indices. As light penetrates through the skin and organs of the E. scolpes, it is reflected and scattered in a multitude of directions, which facilitates rapid changes in the squid’s external coloration.
The reflectin proteins are especially unique in their chemical composition and assembly, as they exhibit the highest refractive index of any other type of protein measured to date.
Reflectin for Films, Fibers and Invisibility Cloaks
Through a process known as flow-coating, in which small amounts of a reflectin protein solution was added onto a silicon wafer substrate, a group of researchers at the Air Force Research Laboratory in Dayton, Ohio have successfully developed a series of reflective films of various thicknesses. Once the protein solution was spread across the surface of the substrate, the researchers altered the concentration of the solutions to acquire different thicknesses of the film, which subsequently determined the amount of light reflected by the manufactured film.
For example, any exposure of the film solution to water dramatically increased its thickness from approximately 120 nanometers (nm) to about 207 nm, which thereby also increased the wavelength of the reflected light of the film from 760 nm to 400 nm. By manipulating the film thickness through adding and removing water from the substrate and solution, the researchers determined that they were able to successfully reflect every visible color of the electromagnetic spectrum off of the film.
To further analyze the properties of the reflectin solution on the silicon wafer substrate, the researchers then dipped the fully coated substrate into an ionic solution called BMIM, which resulted in a higher regular spacing pattern on the substrate that extended for several millimeters (mm). The spacing pattern of the substrate was determined to be dependent upon the velocity of the dipping, in which a greater velocity of dipping resulted in smaller spaces, or diffraction gratings, between each space. The diffraction gratings are capable of splitting incident light into constituent wavelengths that can be useful for a variety of optical devices in the future.
As the researchers gain a better understanding of the mechanisms by which the reflactin molecules are able to assemble themselves into various spheres and stripes, they can manipulate these nanostructures for a wide variety of technological uses in the future, such as the development of invisibility cloaks for military personnel.
The visibility of an object is completely dependent upon the way in which any type of applied light bounces off of the object, therefore a material, such as that which is composed of carefully manufactured reflectin nanostructures, could potentially cause incident light to pass around the object, thereby rendering any covered object virtually invisible. Current research on developing a reflectin invisibility cloak is currently being conducting by the United States Military with funding supported by the Defense Advanced Research Projects Agency.
References & Further Reading
Kramer, R. M., Crookes-Goodson, W. J., & Naik, R. R. (2007). The self-organizing properties of squid reflectin protein. Nature Mater. DOI: 10.1038/nmat1930.