Medicine Researcher's trick: food coloring makes skin transparent

Food coloring is usually used to dye things - researchers now used it to make skin transparent. They applied a yellow-orange color to the skin of mice and were able to see the blood vessels in the scalp or the movement of organs in the abdomen.

The method could probably also be used in humans and is conceivable for many medical fields, writes the team from Stanford University in the journal "Science".

The synthetic dye used, called tartrazine, absorbs long-wave ultraviolet light (UV-A) and blue light. Red and orange light can penetrate the tissue. The researchers write that the transparency effect works on skin, muscles and connective tissue. The dye can be washed off with water after the experiments, making the skin opaque again. The substance also does not harm the animals.

Just don't try it on humans

Tartrazine (E 102) is approved in the USA and the European Union. The colorant is used, for example, in baked goods, sweets, mustard, cheese and medicines. It is also used in the color powders at Holi festivals.

The team has not yet tested the dye on humans. Co-author Guosong Hong, Assistant Professor of Materials Science and Engineering at Stanford, explains that the application is also more complex than in mice. "Human skin is much thicker." The outermost layer of the epidermis represents a considerable barrier, so that the molecules cannot effectively penetrate the dermis.

The dye would therefore actually have to be injected into humans - but this would first have to be comprehensively tested and evaluated in terms of safety. The researcher believes that under no circumstances should anyone simply use the freely available dye on humans. "We strongly advise against trying this on human skin, as the toxic effects of dye molecules in humans, especially when applied externally, have not yet been fully investigated."

Blob of color for looking inside the body?

However, Hong believes that the dye could possibly be used in medicine at some point. "In the future, this technology could make veins more visible for blood sampling, simplify the removal of tattoos using lasers or help in the early detection and treatment of cancer." It may also be possible to localize cancer cells and precancerous lesions under the skin in order to remove them with lasers.

In order to research transparency, the Stanford team focused primarily on the scattering and refraction of light as it passes from one material to another. Scattering is the main reason why we cannot normally see through the body: Cells, proteins and other materials all had a different refractive index. This means that when light passes from one material to another, it is refracted to different degrees, i.e. scattered.

Physical effect

The researchers tried to find a way to equalize the different refractive indices so that the light is not scattered so strongly. The idea: dyes that absorb light most effectively could also equalize the refractive indices particularly effectively. The researchers identified tartrazine, also known as FD & C Yellow 5, as being particularly effective.

When they dissolved tartrazine in water and added it to tissue that absorbed the dye, light scattering was reduced, leading to transparency. They first tested this on a thin slice of chicken breast. As the concentration of the dye increased, the muscle cells became more transparent. An image, which they had placed under the glass dish with the chicken, became visible.

View of the intestine and heart

The researchers then rubbed mice with the solution. The scalp became so transparent that the blood vessels in it became visible. They also applied the solution to the mice's stomachs, where the movements of the intestines, heartbeat and breathing became visible within minutes. The researchers suspect that injecting the dye will lead to even deeper insights into the organisms.

SDA