In 1897, British author H.G. Wells published his most famous novel, The Invisible Man. Its main character invents a serum that renders his body transparent. The principle is simple: control the refractive index of his cells to bring it closer to that of air. Light rays thus pass through the different tissues without encountering the slightest obstacle. And so, Griffin becomes invisible. Pure science fiction, of course.

However, based on the same theory, an American team has just developed a method for making a mouse partially transparent. In an article published on September 5, in the journal Science, researchers from two Stanford University laboratories announced they had succeeded in seeing through the rodents' skin thanks to the local application of a simple dye.

Undoubtedly, you have never heard of tartrazine. Yet you have been ingesting it for a long time. The yellow-orange color in sodas, candies, chips, and tacos is tartrazine, code-named E102. Researchers in California discovered that, when they applied the substance to the skin of a previously shaved mouse, the skin not only took on a reddish hue, as one might expect, but also became transparent.

This "apparent magic trick," as described by physicist Zihao Ou, the first author of the paper, a postdoctoral fellow at Stanford during the research and now senior lecturer at the University of Texas in Dallas, revisited Wells' idea. "Skin is a complex mixture of water – with a low refractive index – and lipids and proteins – with a high refractive index", he explained in detail. Remember that the refractive index measures a medium's ability to slow down or deflect a ray of light. Depending on whether a ray hits water or lipids and proteins, it takes a different direction. Just as in a thick fog (air and water) or a mixture of water and oil, it is impossible to see through it. "By adding the dye, the refractive index of the aqueous medium increases to match that of the lipids and proteins," said the researcher. And there, the trick is done.

The counterintuitive result is that to obtain transparency, the researchers reduced the scattering of light in water. It's as if they had added ink to the water to render it lighter... "Until now, we had tried to do the opposite, purify the water by removing the lipids", said Alain Chédotal, a researcher at the Institut de la Vision in Paris, who has developed methods for studying embryos using transparency. "But that only worked on tissue samples. What they are proposing is both highly original and very simple."

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