Invisibility dye: they create an ointment that makes mice transparent

Tartrazine allows light to pass through the skull of mice (NSF)

The advance of new technologies is increasingly narrowing the border between science fiction and reality. From Harry Potter’s invisibility cloak to films like The Man Without a Shadow, the possibility of being invisible has been a recurring theme in works of fiction and fantasy and has captivated the interest of many. And although we are not yet driving flying cars, as imagined in Back to the Future, we may be closer than ever to making matter incorporeal.

A team of scientists from Stanford University (United States) has managed to make mice transparent using a dye-based ointment. This type of dye makes the skin transparent by modifying the light-scattering capacity of the fluids surrounding the cells. To create the ointment, they used tartrazine, a yellowish dye found in many foods and which is capable of absorbing all light and blocking it.

“We combined the yellow dye, which is a molecule that absorbs most of the light, with the skin, which is a scattering medium. Individually, these two things prevent most of the light from passing through, but together, they allowed us to make the mouse skin transparent,” explains Zihao Ou, a physics professor at Stanford University and senior author of the study.

The results of the research have already been published in the journal Science, where they explain that when light passes through materials with different refractive properties, it is dispersed in all directions. This results in an opaque material. When these materials share the same refractive index, the light rays pass through the clarity boundary at deeper levels and this transparency occurs. Some animals, such as glass frogs or zebrafish, have this characteristic naturally.

Researchers found that tartrazine, used in prepared soups, sweets and soft drinks, had the ability to absorb a proportion of light of the appropriate colour, which made it possible to change the refractive index of the fluid surrounding cells and significantly reduce the scattering of light, making it more transparent.

Abdomen of a mouse before and after using the ointment (Stanford University)

Stanford scientists rubbed a mixture of water and tartrazine dye onto the mouse’s skin. Within minutes, they were able to see detailed blood vessels, organs and even the contraction of muscles in the digestive tract.

After this, the dye is washed off and the skin becomes dull again. If the dye penetrates deeper, the body is able to eliminate it with urine.

Beyond the fascinating nature of the discovery, this scientific finding could be very useful for medicine. “In the future, this technology could make veins more visible for blood draws, facilitate laser tattoo removal, or help in the early detection and treatment of cancer,” concludes Ou.