a container of orange powder
Pure Yellow #5 food dye is actually orange and gives many foods, including Doritos, their bright color.
Someday, doctors might be able to see inside patients without using fancy imaging machines or cutting through the skin. A new solution temporarily turns skin transparent.
The secret ingredient is a food dye found in Doritos, Gatorade and many types of candy. When applied to a mouse’s skin, it allows researchers to see what’s happening inside the animal’s body.
Explainer: What is skin?
Researchers have shared their findings in Science.
Being able to peer through skin so easily could help us “better understand biology and medicine,” says Lydia Kisley. She did not take part in the new work. She does, however, develop new methods of imaging at Case Western Reserve University in Cleveland, Ohio.
Right now, this imaging “can be done in real time and without requiring invasive surgeries,” Kisley says. “I think this will mainly be used in animals and in the lab first.” In the future, she says, it might also find its way into human medicine.
This video shows how the dye molecules allow light to penetrate deeper into the tissues.
Letting the light shine through
We see objects when light bounces off of them and into our eyes. Something transparent — such as a window — lets light through. That light can now bounce off objects on the other side of the window and back to our eyes. This is why we can see outdoor objects through the glass.
That is, unless it’s foggy. Light travels at a different speed through through air than through the tiny droplets that make up fog. So instead of light traveling from a tree or building to our eyes, it now bounces off fog droplets in all directions. This turns the air a milky white that’s difficult to see through.
For the same reason, skin is typically not transparent.
Skin contains a lot of water. It also contains fats. If light moved through water and fat at the same speed, we would be able to see through skin. But it doesn’t, so the light scatters. As a result, we can see the skin’s surface, but not what’s beneath it.
The key to turning skin transparent is to allow light to move through the tissue as it does through a window. And that’s where the dye comes in.
a diagram showing how light is usually scattered by the properties of a mouses skin, making it opaque, and a diagram showing how light moves in a straight line after dye is applied
Applying dye to a mouse’s skin changes how light scatters inside the animal’s tissues. Instead of bouncing around, making the skin look opaque (before), light travels in a straight line, which lets us peer inside (after).
Dyes absorb certain hues, or wavelengths, of light. The ones they don’t absorb reflect back to our eyes. That’s the color that we see. To find something that could make skin see-through, researchers tested 21 different types of orange and red dyes. Each absorbs blue wavelengths of light. Removing those blue hues could reduce light scattering.
Materials scientist Zihao Ou led this study while at Stanford University in California. (He now works at the University of Texas at Dallas.) Ou “conducted a comprehensive screening of a large number of common absorbing molecules,” says Guosong Hong. He’s a neuroengineer at Stanford who also took part in the work.
Tartrazine (TAR-truh-zeen) worked especially well. This orangey-red dye colors foods brightly. Since it absorbs bluer wavelengths, it helped light pass through water and fat molecules at similar speeds. That turned skin transparent.
Educators and Parents, Sign Up for The Cheat Sheet
Weekly updates to help you use Science News Explores in the learning environment
E-mail Address*
Food dye makes skin see-through
The team mixed tartrazine with water, then tested it on a thin slice of chicken. The meat lay on a light table over lines and words. (A light table is a transparent work surface with a light shining up from below.) As the researchers rubbed the solution into the meat, they soon began to see the words below. When they washed the solution off, the chicken became opaque again and the words disappeared.
a series of photos showing how greater amounts of tartrazine rubbed into chicken skin made it increasingly more transparent.
This series of images shows the word “Stanford” starting to show through a slice of chicken after tartrazine was rubbed into the meat.
The researchers then tried the solution on a sedated mouse. After removing the animal’s fur, they rubbed the dye on its head, belly and leg. Each time, they were able to see inside the mouse’s body.
But the solution didn’t stay only on the mouse’s skin. As it seeped into the animal’s muscles and other tissues, it turned them transparent, too. This allowed the team to spy internal organs, such as a tiny pumping heart and inflating lungs. The scientists needed no special equipment to do this — just the liquid dye and a bright light.
“As soon as we rinsed and massaged the skin with water, the effect was reversed within minutes,” Hong said in a Stanford University press release. “It’s a stunning result.”
a before and after image of a mouse stomach showing that after dye is applied the stomach skin becomes transparent. in the after image the mouse's organs, intestines and various blood vessels are visible
Rubbing dye on this mouse’s belly lets us peer inside. We can see blood vessels and organs, including inflating lungs and a tiny, beating heart.
The technique is not yet ready for use in people. Our skin is far thicker than a mouse’s. So the dye may not work to peer inside us. Plus, it’s not yet known whether the dye is safe enough to use on human skin, Hong says. But if it is, one day this dye might guide lasers for tattoo removal. It even might replace some X-rays or other body scans to let doctors detect skin cancer without having to cut through the skin.
You can see how well the dye works on a piece of meat at home. But pause for a minute before you mix Dorito dust with water. Instead, carefully follow these instructions and safety guidelines (provided by the National Science Foundation).
Dye may be okay to consume in the small amounts applied to foods. But handling pure food coloring is different. Indeed, Hong notes, the pure dye “should not be [eaten or] applied to people or animals.”
Do you have a science question? We can help!
Submit your question here, and we might answer it an upcoming issue of Science News Explores
Power Words
More About Power Words
biology: The study of living things. The scientists who study them are known as biologists.
cancer: Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
develop: To emerge or to make come into being. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size, mental maturity or sometimes even shape.
fat: A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.
invasive: An adjective that refers to something that can invade some environment (such as an invasive species) or alter some environment (such as invasive medical procedures).
laser: A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
materials scientist: A researcher who studies how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.
molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
muscle: A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.
National Science Foundation: The U.S. Congress created this independent federal agency in 1950 to promote the advancement of science; national health, prosperity and welfare; and the nation’s defense. In many fields such as mathematics, computer science and the social sciences, NSF is the major source of government funding.
opaque: Unable to see through, blocking light.
organ: (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.
real time: A term that connotes immediacy; something is being studied, recorded and/or reported at the very time it is happening.
sedate: (n.) A term for the state of being calm, sleepy or very relaxed. (v.) To make an individual calm, sleepy or relaxed (often through use of some drug).
transparent: Allowing light to pass through so that objects behind can be distinctly seen. Or information that lets others understand the otherwise hidden people, processes and impacts behind some product, action or proposal.
wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.
Citations
Journal: Z. Ou et al. Achieving optical transparency in live animals with absorbing molecules. Science. Vol. 385, September 6, 2024, p. 1061. doi: 10.1126/science.adm6869.