The researchers Stanford University Aqueous solutions of tartrazine, a common food coloring approved by the U.S. Food and Drug Administration, have been shown to have the effect of reversibly transparentizing the skin, muscle, and connective tissue of live rodents.

“We combined a yellow dye, a molecule that absorbs most light, especially blue and ultraviolet light, with skin, a scattering medium; these two individually block most light from passing through,” said study lead author Dr. Zhihao Ou, who conducted the research with colleagues during his postdoctoral research at Stanford University before joining the University of Texas at Dallas in August 2024.

“But when we combined them, we were able to achieve skin transparency in mice.”

To master this new technique, Dr. Ou and his colleagues developed a way to predict how light would interact with stained biological tissue.

These predictions required a deep understanding not only of light scattering, but also of the process of refraction, how light changes speed and bends as it passes from one material to another.

Scattering is why we can't see through the body: fats, fluids within cells, proteins, and other substances all have different refractive indices, properties that determine how much incoming light waves bend.

In most tissues, these materials are so densely packed that differences in refractive index cause light to scatter as it passes through them, resulting in what our eyes perceive as opaque, colored biological material.

The researchers realized that if they wanted to make biological materials transparent, they had to find a way to match the different refractive indices so that light could pass through unimpeded.

Drawing on fundamental insights from optics, the researchers realized that the dyes that are most effective at absorbing light are also highly effective at directing light evenly through a wide range of refractive indices.

One dye that scientists predicted would be particularly effective was Tartrazinecommonly known as FD&C Yellow 5, is a food coloring.

As it turns out, they were right: when dissolved in water and absorbed into tissue, the tartrazine molecule becomes perfectly structured to match the refractive index, preventing light scattering and resulting in transparency.

The authors first tested their predictions on thin slices of chicken breast.

As the concentration of tartrazine increased, the refractive index of the fluid inside the muscle cells increased, matching the refractive index of muscle proteins, causing the sections to become transparent.

Next, the researchers gently applied the temporary tartrazine solution to the mice.

First, the researchers applied a solution to the scalp, making the skin transparent to reveal the blood vessels crisscrossing the brain.

The researchers then applied the solution to the abdomen, where it disappeared within minutes and demonstrated intestinal contractions and movement due to heartbeat and breathing.

This technique allows for the resolution of micron-scale features and improved microscopy.

Once the dye was washed off, the tissue quickly returned to its normal opacity.

Tartrazine appears to have no long-term effects and excess is excreted within 48 hours.

“It's important that the dye is biocompatible and safe for living organisms,” Dr. Ou said.

“Plus, it's very cheap and efficient. You don't need that much of it to work.”

The team has yet to test the process on humans, whose skin is about 10 times thicker than that of mice.

“At this point, it's unclear how much dye or the delivery method is needed to penetrate the entire skin,” Dr. Ou said.

“In human medicine, we now have ultrasound that can see much deeper into the body.”

“Many medical diagnostic platforms are prohibitively expensive and inaccessible to a wide range of users, but this shouldn't be the case for a platform based on our technology.”

of study Published in this week's journal Science.

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Wu Zhihao others2024. Achieving optical transparency in living animals using absorbing molecules. Science 385 (6713); doi: 10.1126/science.adm6869

This article is an edited version of an original release from the National Science Foundation and the University of Texas at Dallas.