Scientists have found that massaging tartrazine—aka "Yellow 5," aka the food dye used in Doritos—into the skin of mice can temporarily make the mice transparent. The Washington Post explains that in a recent study, researchers found that about five minutes after applying the dye, "the opaque skin of the mice transformed temporarily into a living window, revealing branching blood vessels, muscle fibers and contractions of the gut." Popular Science describes the process:
The method is simple. In short, massaging tartrazine solution into hairless mouse skin over the course of a few minutes or using microneedling achieves "complete optical transparency in the red region of the visible spectrum," per the study. Wash the dye away, and the skin returns to its natural, opaque state.
Scientist Christopher Rowlands, who Popular Science interviewed, was blown away by the discovery:
"I instantly looked at it and went, 'my god, this is revolutionary,'" says Christopher Rowlands, a senior lecturer and biophotonics researcher at Imperial College London. Rowlands wasn't involved in the study, but wrote an accompanying perspective article on it after he acted as a reviewer for the research. "In optics, we spend an awful lot of time trying to increase [how well we can see into tissue] by 20 percent or 50 percent. These guys come along and they annihilate the boundary by a factor of 10." The most advanced current optical imaging (i.e. non-invasive, and not reliant on radiation) techniques might allow a scientist to see a couple of millimeters into live tissue, but the new method could make detail more than a centimeter deep visible to the naked eye, says Rowlands. "It's not magic, but it's still very powerful," he adds . . .
The physics concepts involved in the process are so fundamental that, "I was kicking myself that I hadn't thought of it," Rowlands tells Popular Science. "It's one of these moments where it's blindingly obvious, but you need somebody else to tell you it first. It makes so much sense in retrospect." It's a "triumph of fundamental understanding," he adds, which demonstrates what happens when a deep understanding of a theory comes together with real-world experimentation.
The Washington Post addresses the physics of the phenomenon:
How does bright yellow food coloring turn tissue transparent? To understand why, it's essential to consider the reason things look opaque in the first place. The bits of our body — cell membranes, proteins, fluids — all cause light to refract, or bend.
If light bends just once — think of a beam of sunlight hitting a sheet of glass — the image it carries is still mostly clear. But as light refracts over and over, off fluids, proteins and other cellular miscellany, it scatters in lots of directions. All that scattered light, Rowlands said, makes it hard to see through — "like watching TV through a glass of milk." . . .
By applying textbook physics principles, the researchers were able to screen for molecules that they predicted would, when absorbed by the body, change how biological tissues refract light. They hit on tartrazine, dissolved in water. But the proof was in the experiment. They soaked a slice of raw chicken in a tartrazine solution and found that the chicken turned clear as they increased the amount of tartrazine. When they rubbed that solution onto the skin of mice, they saw internal organs come into view. The tartrazine reduced the amount of refraction, the light scattered less and the tissue appeared clear.
Popular Science further describes the relevant physics concept, the "Kramers-Kronig relations":
"Biological tissues, like skin, are usually not see-through because light gets scattered as it passes through them," says Guosong Hong, co-senior study author and a bioengineer at Stanford University. Animal flesh is a matrix of different materials, mostly water and fats, and these two types of compounds refract light at different angles, he explains. A light particle, or photon, traveling through tissue under normal circumstances moves from water particle to lipid particle, being bounced around, taking a long, winding path, and oftentimes being absorbed by one of the many molecules it collides with along the way.
But tartrazine dye, through its powerful absorption of blue wavelengths of light, changes the refractive index of water to be much closer to that of fat, Hong says. This happens through a basic physical principle called the Kramers-Kronig relations, which dictates that waves (like those of light–which is both a particle and a wave) are the result of predictable signals. As a result, a photon can pass through skin almost as if the tissue were homogenous. It takes a shorter path, avoiding all of the bouncing and angle changes that increase the likelihood of light absorption, ultimately illuminating the inside of a mouse.
The effect is temporary, as the tissues return to normal when the dye is washed off with, as The Washington Post reports, "minimal systemic toxicity" for the mice.
So, besides the cool sci-fi factor, what's the point? Again, The Washington Post:
The technique may help scientists answer long-standing questions in biology — for example, allowing researchers to observe a mouse's brain activity, including in the deepest parts of the brain. It could be used to diagnose deep-seated tumors without surgery, help locate a vein for a blood draw or make cosmetic procedures like tattoo removal more precise, said Guosong Hong, a materials scientist at Stanford and one of the study's leaders.
Video below or click through to this Popular Science article to see an image of the transparent mice. And no, I'm pretty sure rubbing your belly with Doritos dust won't suddenly make you transparent.
