Researchers make mouse skin transparent using a common food dye

From the article:

Published in Science on Sept. 5, the research details how rubbing a dye solution on the skin of a mouse in a lab allowed researchers to see, with the naked eye, through the skin to the internal organs, without making an incision. And, just as easily as the transparency happened, it could be reversed.

“As soon as we rinsed and massaged the skin with water, the effect was reversed within minutes,” said Guosong Hong, assistant professor of materials science and engineering and senior author on the paper. “It’s a stunning result.”

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To match the refractive indices of different tissue components, the team massaged a solution of red tartrazine – also known as the food dye FD&C Yellow 5 – onto the abdomen, scalp, and hindlimb of a sedated mouse. The skin turned red in color, indicating that much of the blue light had been absorbed due to the presence of this light-absorbing molecule. This increase in absorption altered the refractive index of the water at a different wavelength – in this case, red. As a result of the absorption of the dye, the refractive index of water matches that of lipids in the red spectrum, leading to reduced scattering and making the skin appear more transparent at the red wavelength.

This research is a new application of decades-old equations that can describe the relationship between absorption and refractive index, called the Kramers-Kronig relations. In addition to this food dye, several other light-absorbing molecules have demonstrated similar effects, thereby confirming the generalizability of the underlying physics behind this phenomenon.

Researchers were able to see, without special equipment, the functioning internal organs, including the liver, small intestine, cecum, and bladder. They were also able to visualize blood flow in the brain and the fine structures of muscle fibers in the limb. The mouse’s beating heart and active respiratory system indicated that transparency was successfully achieved in live animals. Furthermore, the dye didn’t permanently alter the subject’s skin, and the transparency disappeared as soon as the dye was rinsed with water.

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Right now, the study has only been conducted on an animal. If the same technique could be translated to humans, it could provide a range of biological, diagnostic, and even cosmetic benefits, Hong said.

If approved for human use, Goth dance parties in 2030 might be pretty disturbing.