Common food coloring makes skin and muscles temporarily transparent | Medical research

Researchers have looked into the brains and bodies of living animals after discovering that a common food coloring can temporarily make skin, muscles and connective tissue transparent.

By applying the dye to the abdomen of a mouse, the liver, intestines and bladder became clearly visible through the abdominal skin. By applying it to the rodent's scalp, the scientists were able to see the blood vessels in the animal's brain.

According to researchers at Stanford University, the treated skin returned to its normal color after the dye was washed off. They believe the procedure has numerous potential applications in humans, from locating injuries and finding veins for blood sampling to monitoring digestive disorders and detecting tumors.

“Instead of relying on invasive biopsies, doctors could diagnose deep-seated tumors by simply examining a person's tissue, without the need for invasive surgical removal,” said Dr. Guosong Hong, a lead researcher on the project. “This technique could potentially make blood draws less painful by helping phlebotomists easily find veins under the skin.”

The trick is reminiscent of Griffin's approach in HG Wells' 1897 novel The Invisible Man, in which the brilliant but doomed scientist discovers that the secret of invisibility lies in matching an object's refractive index, or its ability to bend light, to that of the surrounding air.

When light enters biological tissue, it is mostly scattered because the structures inside, such as fatty membranes and cell nuclei, have different refractive indices. As light moves from one refractive index to another, it bends, making the tissue opaque. The same effect makes a pencil look bent when dropped into a glass of water.

Dr. Zihao Ou and his colleagues at Stanford counterintuitively theorized that certain dyes could make certain wavelengths of light pass more easily through skin and other tissues. Highly absorbing dyes change the refractive index of the tissues that absorb them, allowing scientists to match the refractive indices of different tissues and suppress any scattering.

In a series of experiments described in Science, researchers show how a fresh chicken breast became transparent to red light just minutes after being immersed in a solution of tartrazine, a yellow food dye used in US Doritos, SunnyD drinks and other products. The dye reduced light scattering in the tissue, allowing the rays to penetrate deeper.

The team then smeared the yellow dye on a mouse's lower abdomen, making the abdominal skin transparent and revealing the rodent's intestines and organs. In another experiment, they applied the dye to a mouse's shaved head and used a technique called laser speckle contrast imaging to see blood vessels in the animal's brain.

“The most surprising thing about this study is that we normally expect dye molecules to make things less transparent. For example, if you mix blue ballpoint pen ink in water, the more ink you add, the less light can pass through the water,” Hong said. “In our experiment, when we dissolve tartrazine in an opaque material like muscle or skin, which normally scatters light, the more tartrazine we add, the clearer the material becomes. But only in the red part of the light spectrum. This goes against what we normally expect from dyes.”

The researchers describe the process as “reversible and repeatable,” with the skin returning to its natural color once the dye is washed away. Currently, transparency is limited to the depth the dye penetrates, but Hong said microneedle patches or injections could deliver the dye deeper.

The procedure has not yet been tested on humans and researchers need to prove that it is safe to use, especially when the dye is injected under the skin.

Others will also benefit from this breakthrough. Many scientists study naturally transparent animals such as zebrafish to see how organs and diseases such as cancer develop in living beings. Using transparent dyes would allow a much larger number of animals to be studied in this way.

In an accompanying article, Christopher Rowlands and Jon Gorecki of Imperial College London say the technique will attract “extremely broad interest.” Combined with modern imaging techniques, it could enable scientists to image the entire mouse brain or detect tumors under tissue several centimeters thick. “HG Wells, who studied biology under TH Huxley, would surely approve of this as a student,” they write.