Scientists have discovered a common food colorant has a remarkable property - making the skin of live mice transparent, so the organs beneath become visible.

The dye - tartrazine, aka E102 - is a mostly-harmless dye used to make food yellow. In a paper published in the journal Science last week, scientists describe how the dye’s interaction with light allowed them to see through a living mouse’s skin and observe its organs. In another experiment, they made a thin slice of chicken breast temporarily transparent.

“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,” said Zihao Ou, assistant professor of physics at the University of Texas at Dallas.

"But when we put them together, we were able to achieve transparency of the mouse skin," Ou stated. "For those who understand the fundamental physics behind this, it makes sense; but if you aren’t familiar with it, it looks like a magic trick.”

The physics Ou referred to is the “refractive index” - the measure of how much a substance bends light. If a substance bends most light, it scatters on a surface.

“The ‘magic’ happens because dissolving the light-absorbing molecules in water changes the solution’s refractive index … in a way that matches the refractive index of tissue components like lipids,” Ou and team explained.

Dissolving the dye in water and applying it to a mouse’s skin therefore means light passes through the skin.

The effect is easily stopped - just wash off the dye. Any colorant absorbed by the body will be expelled in the urine, the paper states.

“As soon as we rinsed and massaged the skin with water, the effect was reversed within minutes,” said Guosong Hong, assistant professor of materials science and engineering at Stanford University and senior author on the paper. “It’s a stunning result.”

By now, you're probably wondering if this could be part of the ultimate Halloween costume, a point raised immediately by one Register hack. But the researchers haven't tested it on humans yet, pointing out that some dyes can be toxic if used incorrectly, and that human skin is around ten times thicker than that of a mouse. However, you can still try it out using 3 mm slices of chicken breast as the National Science Foundation has produced a guide [PDF] for home experimenters.

The discovery has many potential practical applications. Medicine has numerous ways to look beneath our skin - from X-rays to MRI scanning, but these can carry potential health risks and are usually expensive. Having a cheap, easily available dye do the job could be a game changer.

“Our research group is mostly academics, so one of the first things we thought of when we saw the results of our experiments was how this might improve biomedical research,” Ou said.

“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.”

The next stage in the research is to try this out with thicker human skin and get the dosage right for this to achieve the same effects. Ou also wants to try other ingredients to see if he can get an even clearer look inside the body. ®

https://www.theregister.com//2024/09/09/food_dye_skin_transparent/