EIn 1897, British author HG Wells published his most famous novel, The Invisible Man. The main character invents a serum that makes his body transparent. The principle is simple: control the refractive index of his cells to bring it closer to that of air. The light rays therefore pass through different environments without detecting the slightest boundary. And Griffin becomes invisible. Pure science fiction, naturally.
Yet it is based on the same theory that an American team has just developed a method for making a mouse partially transparent. Coming from two laboratories at Stanford University, it announces, in an article published on September 5 in the journal Sciencehaving managed to see through the skin of rodents thanks to the local application of a simple dye.
You probably don’t know about tartrazine. However, you’ve been swallowing it for a long time. The orange-yellow in sodas, candy, chips and tacos, that’s it, hidden under the code name E102. Californian researchers noticed that when they smeared it on the skin of a previously shaved mouse, it took on a red tint, as you might expect, but it also became transparent.
Ce “apparent magic trick”as described by physicist Zihao Ou, first author of the paper, a postdoctoral fellow at Stanford during the research and now a lecturer at the University of Texas at Dallas, takes up Wells’ idea. “The skin is a complex mixture of water – with a low refractive index – and lipids and proteins – with a high refractive index”he explains. Remember that the refractive index measures the ability of a medium to slow down or deflect a light ray. Depending on whether a ray hits water or lipids and proteins, it therefore takes a different direction. As in a thick fog (air and water) or a mixture of water and oil, it is impossible to see through. “By adding the dye, the aqueous medium sees its refractive index increase to approach that of lipids and proteins”the researcher continues. And that’s it.
A spectacular and reversible operation
So we arrive at this counter-intuitive result, to say the least, that to obtain transparency, the researchers reduced the diffusion of light in the water. A bit like adding ink to the water to lighten it… “Until now, we have tried to do the opposite, purify the water by removing the lipidsexplains Alain Chédotal, a researcher at the Vision Institute in Paris, who has developed methods for studying embryos through transparency. But it only worked on tissue samples. What they are proposing is both very original and very simple.”
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