When we look at a person, it is difficult to imagine them as transparent. It seems impossible for our gaze to penetrate such a solid body. However, glass is just as solid and we see what is behind it. When changing medium, between air and glass, light changes its speed and direction, depending on the refractive index. In this change of pace, its path can twist a little, but, as the material is homogeneous, it reaches the other side without problem and we can see the street behind the glass. However, the body is made of heterogeneous materials and light, every time it passes from one material to another, faces very different refractive indexes and alters its course. This makes bodies opaque and in order to clearly see what is inside a person, it is necessary to cut them first. But it does not have to be this way. A group of scientists has just presented a dye that makes the skin or skulls of living mice transparent.
The achievement is signed by in the magazine Science A team of researchers from Stanford University (USA) applied some fundamental ideas to reach a conclusion that goes against intuition. Some dyes that are especially effective at absorbing light and making objects opaque could also be used to match the refractive indices of different materials and make them transparent. “Something very creative about this article is that, to achieve transparency, they use molecules that absorb a lot of light and block it. But only in the blue range. In the red range, they change the refractive index of other materials, such as skin, and make them transparent,” explains Martín López, a researcher at the CSIC Institute of Optics, who explores this type of phenomena, but has not participated in the Stanford work. “We combined the yellow dye, which is a molecule that absorbs most of the light, with skin, which is a scattering medium. Individually, these two things prevent most light from passing through, but together, they allowed us to make mouse skin transparent,” Zihao Ou, a physics professor at the University of Texas at Dallas and co-author of the study, said in a statement from his institution.
One of the dyes that he thought would be particularly interesting in producing this effect, which is very opaque in blue and capable of providing transparency in red, was tartrazine, which is used in many foods to achieve a yellowish colour. When the researchers introduced it into water, its molecules were structured in such a way that they matched the refractive indices of the environment and prevented the dispersion of light, producing transparency.
They then began testing this ointment on thin chicken breasts. As they increased the concentration of tartrazine, the refractive index of the fluids inside the muscle cells increased until it matched that of the muscle proteins. At that point, as if by magic, the breasts became transparent.
The ointment was then tested on live mice, first on the skull. After a few minutes, when it had fully penetrated the skin, it became transparent, revealing the blood vessels that supply the brain. Tartrazine was then applied to the animals’ abdomens, which became discoloured until the contractions of the intestine could be seen. When the skin and skull of the rodents were rinsed, their opacity returned and the researchers observed no harm from the substance. “It is very relevant that this is done with very well-known molecules that are used as food colouring, because it is known that they are not toxic,” explains López. “In our field, when we work with these advanced optical properties, we normally do so with very toxic materials,” he adds.
The new technique, which still needs to be developed, has great potential, both from a research point of view and for its medical applications, to access the inside of the body without the need for painful, dangerous or bothersome interventions. The worms C. elegans They became animal models that revolutionized biology, in part, because they were transparent and allowed us to observe what was happening inside them while they were alive. Tartrazine would expand the number of species in which this advantage could be taken into account. “Zebrafish larvae are transparent and are used because it is possible to see the neurons in their brain while they are alive,” says Juan Lerma, a researcher at the Alicante Institute of Neurosciences of the CSIC. “Here, they are able to see the enteric neurons [que controlan el hambre o los movimientos del intestino] through the skin. These cells are between two layers of muscle and here you don’t need to open the gut to see them. I find this surprising and an important starting point with applications in research and non-invasive medicine,” he concludes.
In addition to medical applications, there will be no shortage of people asking whether smearing or injecting tartrazine ointment, which the authors claim would improve its effectiveness, can make one invisibly invisible. For the moment, according to Martín López, it seems complicated. “They, for example, have to shave the skulls of mice before applying the ointment. The reason is that the refractive index of bone is different from hair and if I want to make them transparent at the same time I would have to apply two different molecules. If I add more tissues, I have to add more refractive indices, and it would be impossible to equalize the refractive indices of all the materials in the body.” Science, for the moment, leaves the business of invisibility cloaks to magic.
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