Mice got transparent skin from exposure to yellow food dye, opening up more questions in physics and medicine.
By Helen Petre
A common yellow food dye, tartrazine, or FD&C (Food, Drugs, and Cosmetics) Yellow #5, is used in snack chips (specifically Doritos), candy, soda, and cosmetics. The dye gives products that bright yellow-orange color. Dr. Zihao Ou and his research team at Stanford University published a study that shows that this yellow food dye, applied to the skin of genetically hairless mice, gives them transparent skin. This is not magic. It is physics.
If you cover yourself with Doritos, will you disappear?
Don’t try this at home, because it won’t work. You can’t just smear yourself with Doritos and disappear, like Harry Potter. First, Doritos have other ingredients, such as corn, which interfere with the optical properties of the yellow dye. Also, the researchers did this with the skin of mice. Mice skin is about 10 times thinner than human skin, and so far as they have reported, they haven’t even tried this on humans. The mice were genetically hairless, which also contributes to the successful results. People have hair.
How can a yellow dye lead to transparent skin?
Why do you see the sky as blue, or your veins as blue, or the sky red at sunset? Why do you see things differently under water, or through a glass of water? Different substances scatter light differently. Green plants absorb all the colors in light except green. Yellow dyes absorb most light, but not yellow light. Water changes the refractive index, or the way light is bent. The researchers mixed yellow dye with water and put it on mouse skin. The skin absorbed the dye-water mixture. Water is transparent, but changing the refraction by adding the dye changes the light-scattering properties, which changes the way light is absorbed through the absorbed dye and makes the thin mouse skin transparent.
The reason this happens is the same reason we think the sky is blue. It isn’t really. The sun makes white light. The atmosphere (mostly nitrogen) scatters the particles of light, and blue light, which has a shorter wavelength, is scattered more, so we see blue. At sundown, the sun is at the horizon, so the light passes through more of the atmosphere, which scatters the light more, allowing the red longer wavelengths to be visible.
This is actually the same reason veins look blue through light-colored skin. Blood is red. Think of red meat. The meat is red. You see veins as blue for the same reason you see the sky as blue. The light passes through the skin, and since blue wavelengths are short, they are scattered more, and you see blue.
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When you see a rainbow, you are seeing white light, but it passes through water in the atmosphere, which refracts, or bends, the light into all the colors contained in white light. Physics, not magic.
Yellow dye absorbs blue and violet wavelengths. When the dye is absorbed into the skin, the light you see passes through the dye and through the skin. Skin contains fats and water, which refract light and, in perfect conditions, the combination of the dye and the skin temporarily creates transparent skin.
Is this safe?
The FDC has decided that FD&C Yellow #5 is safe, in small quantities. When applied to skin in order to make it transparent, a large quantity is used. To do this with human skin, with hair, very large quantities must be used. Who knows if it is safe?
The dye is absorbed through skin, into the blood, and is broken down into molecules that can be eliminated by the body through urine. If you eat a bag of Doritos, your body can break down the dye and get rid of it easily and safely. Can your body do this with enough dye to give you transparent skin? Questionable.
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Approval of a color additive for one intended use does not mean approval for other uses
According to the United States Food and Drug Administration (USFDA), no dyes or color additives are completely safe. The USFDA says there is a reasonable certainty of no harm when additives are used properly, or as they are proposed to be used. Obviously, no one has approved to spread large quantities of dye on human skin. It may be safe, but we don’t yet know whether that is the case.
Many substances we consume are safe in small quantities but toxic in large quantities. Salt, for example, is necessary for normal blood pressure and nerve function, but who would sit down and eat a cup of salt? That would certainly be bad. Spices are another example. While oregano is nice to flavor spaghetti sauce, it might be bad to eat a pound of it. Even water is toxic in large enough quantities.
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Significance of the study
The researchers are physicists. The purpose they propose is that transparent skin could be an efficient way to visualize internal blood vessels or organs in a way that would improve biomedical research. Since we cannot see into humans, this could be a noninvasive way to see into living bodies. They propose that this technique could be used to view inside human bodies without using imaging equipment or surgery. They even suggest that the dye could be used to make veins more visible for blood testing, or inserting an IV more accurately and efficiently. If refined, it could replace MRIs, CT scans, and even ultrasound.
While this technique still has a ways to go before it is actually used in medicine, it is an exciting use of optical properties and great physics.
This study was published in the peer-reviewed journal Science.
References
Ou, Z., Duh, Y.-S., Rommelfanger, N. J., … Hong, G. (2024). Achieving optical transparency in live animals with absorbing molecules. Science, 385(6713), eadm6869. https://doi.org/10.1126/science.adm6869
United States Food and Drug Administration. (2023, July 13). How safe are color additives? https://www.fda.gov/consumers/consumer-updates/how-safe-are-color-additives
About the Author
Helen Petre is a retired biologist who continues to learn and teach science.
