Scientists can make mice transparent. But why?

“If you aren’t familiar with physics, it looks like magic,” said a scientist who made a mouse's body see-through.

Researchers at the University of Texas at Dallas have made an unusual discovery: a method for making the skin of a living animal transparent.

The mice's skin was made transparent by rubbing a mixture of water and tartrazine, a common yellow food coloring, on the skull and abdomen of an animal. After the dye had completely diffused into the skin, it became transparent within a few minutes.

What seems pretty bizarre is actually possible, as the skin is a so-called scattering medium. Such materials allow light waves to spread out in different directions when they hit them. Instead of light passing straight through, it gets bounced around. For example, fog scatters light, which is why it cannot be seen through.

Dr. Zihao Ou, assistant professor of physics and lead author of the research admitted, that their discovery “makes sense” for those who understand physics, but for those who don’t, “it looks like a magic trick.”

"We combined the yellow dye, which is a molecule that absorbs most light, especially blue and ultraviolet light, with skin, which is a scattering medium,” explained Dr. Ou.

“Individually, these two things block most light from getting through them. But when we put them together, we were able to achieve transparency of the mouse skin," he added.

The "magic" happens when light-absorbing molecules dissolve in water, changing how the solution bends light to match parts of the skin like fats. This reduces how much light scatters in the skin, similar to clearing up fog and making things easier to see.

"It takes a few minutes for the transparency to appear. It's similar to the way a facial cream or mask works: The time needed depends on how fast the molecules diffuse into the skin," Ou said.

Scientists assured that the mice would not stay forever transparent and that they could go back to normal by simply washing them.

Researchers could directly observe blood vessels on the surface of the brain through the transparent skin of the skull. In the abdomen, they observed internal organs and peristalsis, the muscle contractions that move contents through the digestive tract.

The dye used in the solution, commonly known as FD&C Yellow #5, is often found in orange or yellow snack chips, candy coatings, and other food products. Being a food dye makes it biocompatible and safe for living organisms.

While the experiment worked on mice, the researchers have not yet tested the process on humans, whose skin is about 10 times thicker than a mouse's.

“At this time it is not clear what dosage of the dye or delivery method would be necessary to penetrate the entire thickness,” Ou said.

But why make it transparent?

While the discovery is staggering, the question remains: Why is it needed? The author, however, is not that skeptical.

Ou said one of the first applications of the technique will likely be to improve existing biomedical research methods.

"Optical equipment, like the microscope, is not directly used to study live humans or animals because light can't go through living tissue. But now that we can make tissue transparent, it will allow us to look at more detailed dynamics. It will completely revolutionize existing optical research in biology," he said.

The next steps in the research involve figuring out the best dosage of the dye for human tissue. Researchers are also testing other molecules, including specially designed materials, that might work better than tartrazine.

The initial study was published on September 6th in the journal Science.