Invisibility, like flying, superstrength, and other superpowers, has long resided in human imagination. However, it is no longer an impossible feat. In fact, it has never been entirely unfeasible, with many animals having developed intrinsic methods of achieving near-invisibility.
Light manipulation is vital to achieve transparency; in the end, visibility is determined by how light strikes an object and bounces back into our retina, which then converts the light to neural signals that are transported to our brains. If the light that initially strikes the object fails to reach our eyes, then we would not be able to see the object. This is the logic behind cellular invisibility: maximizing the light that passes through the cells while also minimizing the amount of light that reaches our eyes through diffusively reflecting light rays.
This is why natural invisibility is more frequently observed in aquatic organisms than is in terrestrial ones—the refractive index of water is greater than that of air, and is thereby much closer to the refractive index of living tissue. A greater refractive index disparity leads to a greater surface reflection, discouraging the evolution of transparency in animals. Greater amounts of surface reflection mean that the light bouncing off the animals are more likely to reach our retina, thereby making them visible to us.
Moreover, terrestrial animals require cellular pigment for protection against ultraviolet radiation, another hindrance for their invisibility. This pigment—melanin, to be exact—is necessary to convert radioactive rays into heat through a very rapid chemical process, preventing the ultraviolet ray from harming cells. While land animals that are frequently exposed to direct light require this protective pigment, aquatic animals are subject to a smaller amount of light given that it is difficult for sunlight to penetrate through water, and therefore are able to sacrifice skin pigment for a better chance at survival.
Generally, cephalopods—the animal class including squid, octopus, and cuttlefish—able to mimic transparency do so with leucosomes, which are protein structures with the ability of increasing the scattering visible light. These leucosomes contain reflectins, proteins with unique amino acid sequences that give them a high refractive index and the ability to self-assemble. These two characteristics give the animals the ability to turn partially transparent at their will: the high refractive index helps them scatter more light, while self-assembly allows these animals to change the orientation of their proteins in response to external threats, such as predators, turning them invisible. However, the structural differences between human cells and those of invisibility-possessing animals has been a major obstacle in the step toward human invisibility, as human cells do not possess leucosomes.
Figure 1: A close-up view of the reflectin structures that make up leucosomes, protein structures in cephalopods that serve to scatter visible light. Reflectins contain unique amino acid sequences that give them high refractive indices and the ability to self-assemble, allowing cephalopods to turn partially transparent at will.
Source Credit: Nature.com (LINK)
Another vital factor of transparency is the random orientation of the nanostructures responsible for reflecting and changing the direction of light. The glasswing butterfly, for example, has transparent wings caused by nanopillars covering its body. With random heights and widths, these pillars contribute to creating the optical illusion of invisibility for the animal. The primary function of these nanopillars is to minimize the light that bounces off the wings of the butterfly, and for the light that does, vary the direction of the rays such that a minimal number of them travel to our retinas. Despite being a terrestrial animal, the glasswing butterfly is able to achieve partial transparency through employing random nanostructuring. However, given the difference between human skin and the thin membrane of a butterfly wing, drawing inspiration from this form of transparency-generation posed yet another challenge.
Figure 2: A picture of the glasswing butterfly, which exhibits uniquely transparent wings. This is largely due to their low absorption levels of light and minimal scattering of light that passes through its cells.
Source Credit: Prince Edward Island Preserve Company (LINK)
A team of researchers was able to overcome these biological shortcomings, modifying human cells by giving them a protein-based, photonic structure to make it reconfigurable, allowing these cells to appear transparent through light manipulation. Moreover, by designing these human cells to be responsive to stimuli, the scientists were able to have them change their appearance and interactions with light through organized cues, similar to how transparency-achieving cells work in animals.
This design was primarily influenced by cephalopods, which have pigment organs that allow them to alter how their skin receives, absorbs, and reflects light. With these methods of camouflage and transparency, these animals have developed unique survival techniques through generations of evolution. It is generally believed that the pigment organs in these animals consist of tunable leucophores, which contain the aforementioned leucosomes in disorderly arrangement. Varying diameters and widths have further improved the light-scattering abilities of these leucosomes, leading scientists to experiment with the implementation of similar designs in human cells.
Initial experiments were conducted on Human Embryonic Kidney 293 cells, given their high tolerance for foreign biomolecules and their desirable production of recombinant proteins. Moreover, the scientists chose the reflectin A1 isoform to build the protein structures out of, given the material’s large refractive indices. Arranging these proteins into strength-responsive structures, the scientists were able to mimic the behavior of squid skin cells. Additionally, upon interaction with chemical stimuli, these modified cells were also able to reconfigure their internal architectures to scatter and transmit different amounts of visible light, thereby altering the transparency level of the cells.
Although having long been solely part of our imagination, human invisibility is now on the horizon. While this signifies a great step in modern scientific development and heralds even more radical breakthroughs in the future, we must keep in mind that with new technologies come new responsibilities. Harry Potter using the Cloak of Invisibility to explore Hogwarts after curfew might have seemed fairly exciting, but substituting the situation with a criminal infiltrating government headquarters makes it anything but fun. Invisibility is a new power we may develop; but it is, in the end, a mere tool. The hands that hold it will determine its lasting impact on human society, and whether it is used for good or evil is entirely up to them.
Q&A:
Sally: If animals have both melanin and leucosomes, can they possibly be invisible?
Yes: in fact, most animals have both melanin and leucosomes; the melanin causes the animal to be visible, and the leucosomes minimize this effect by scattering light. However, this invisibility is partial invisibility—the animals just become as transparent as physically possible.
Xavier: Are there some other future implications for “invisible” human cells other than superpowers? How near in the future is this ability projected to be possible?
There are a number of future implications, in particular for military operations (which is quite possibly a very bad happening). In fact, scientists from the University of Rochester have already created a gadget to aid human invisibility. While this is not necessary the same technology described in this article, a similar logic of minimizing light reflection is at play in this human cell-altering scenario. Given the advancements being made in this field and the general technological growth of the human race, I would give it another 10 to 20 years for this technology to be perfected. Honestly though, who knows?
Eric: How successful is invisibility/camouflage as a self-defense mechanism in the wild? What other ways do the example animals protect themselves if invisibility fails to work?
Invisibility is relatively successful as a self-defense mechanism in the wild, especially for aquatic animals such as jellyfish, squid, octopi, and cuttlefish. While some squid create ink clouds to hinder the visibility of predators and buy time to escape, for other species, invisibility is the predominant method of protection; if it fails to work, then these animals will most likely be consumed by predators.
Wooseok: Can individual cells containing reflectin achieve invisibility? (e.g. unicellular organisms)
Given that scientists are modifying individual human cells to achieve partial transparency, I think the answer would be yes. However, the unicellular organism would have to be artificially modified.
Hugh: What is the “reflectin A1 isoform?”
The reflectin A1 isoform is a type of reflectin that is able to replicate the growth of human cells due to its optical, electrical, and assembly properties. This protein plays a key role in cephalopod coloration.
John: How do the dimensions of the leucosome affect its light-scattering ability?
If by dimensions you mean its ability to self-assemble, then this property of the leucosomes will affect their light-scattering ability through allowing them to modify their photonic architectures to scatter more light in some situations than others.
Works Cited:
Chatterjee, A., Cerna Sanchez, J.A., Yamauchi, T. et al. Cephalopod-inspired optical engineering of human cells. Nat Commun 11, 2708 (2020). https://doi.org/10.1038/s41467-020-16151-6
Davis, Nicola, and India Rakusen. “How Cephalopod Cells Could Take Us One Step Closer to Invisibility - Podcast.” The Guardian, Guardian News and Media, 18 June 2020, www.theguardian.com/science/audio/2020/jun/18/how-cephalopod-cells-could-take-us-one-step-closer-to-invisibility-podcast.
Lund University. "Skin pigment renders sun's UV radiation harmless using projectiles." ScienceDaily. ScienceDaily, 26 September 2014. <www.sciencedaily.com/releases/2014/09/140926085818.htm>.
Siddique, R., Gomard, G. & Hölscher, H. The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly. Nat Commun 6, 6909 (2015). https://doi.org/10.1038/ncomms7909
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