Light-absorbing dye turns live mice’s skin invisible, reveals internal organs

A common food dye combined with a solution has made mice’s skin “transparent.”

Researchers from the universities of Texas and Stanford conducted this breakthrough research, which could advance medical imaging.

To make the skin transparent, scientists used a simple solution of water and tartrazine, a popular food coloring dye.

In the experiment, the mixture was applied to the mice’s skulls and abdomens. The dye quickly diffused into the skin, making these spots transparent.

“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,” said Dr. Zihao Ou, assistant professor of physics at The University of Texas at Dallas and lead author.

This temporary effect is reversible. Moreover, the dye is “biocompatible” and poses no harm to the animals.

If adopted, this simple technique could open a new way of observing organs within a body.

Experts envision this technology being used to make veins more visible for blood draws, simplify laser tattoo removal, and even aid in the early detection and treatment of cancers.

Reducing light scattering

Living skin scatters light, similar to fog, making it difficult to see through.

The key was finding a combination that would reduce light scattering in the skin tissue.

“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. 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,” explained Ou.

The “magic” happens when the light-absorbing molecules dissolve in water and change the refractive index of the solution. This corresponds to the refractive index of skin tissue components such as lipids. Interestingly, this decreases light scattering in the skin, making it look transparent.

“In essence, the dye molecules reduce the degree to which light scatters in the skin tissue, like dissipating a fog bank,” the press release noted.

It showcased mice’s internal structures

The transparent skin provided researchers with a unique opportunity to observe various internal structures of the mice.

Researchers directly viewed blood vessels on the brain through the transparent skull. On the other hand, the transparent abdomen revealed internal organs and the muscular contractions that move food through the digestive system.

While the process has been successful in mice, its effectiveness in humans remains to be tested. The human skin is roughly 10 times thicker than a mouse’s, requiring a different dye dose or delivery technique for penetration.

Future research will focus on determining the optimal dye dosage for human tissue. Moreover, the team aims to explore alternative molecules, including engineered materials, that may be more effective than tartrazine.

Ultrasound is the most commonly used method for viewing internal structures in living beings. A technology based on this solution could be a less expensive option for such examinations. Moreover, it can be used with existing optical imaging tech.

“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,” Ou added.

The findings were published in the journal Science.