Living skin is a scattering medium. Despite its key role in biology and medicine, light scattering in living tissue hinders optical imaging.
The desire to see inside biological tissue and uncover the fundamental processes of life has spurred extensive research into deep-tissue optical imaging methods. However, existing methods lack sufficient penetration depth and resolution or are unsuitable for living animals.
In a pioneering new study, scientists from the University of Texas at Dallas have made the skin on the skulls and abdomens of live mice transparent. They used a mixture of water and tartrazine, a common yellow food coloring.
This yellow dye absorbs most of the light with the skin. Individually, these two things block most of the light from entering. But when they are combined, scientists achieve transparency in the mouse skin.
Doesn’t it look like a magic trick?
This magic happens after dissolving the light-absorbing molecules in water, which changes the solution’s refractive index (RI). The resulting RI matches the refractive index of tissue components like lipids, helping light scatter less in the skin.
Scientists applied their mixture to the mice’s abdomen and skull skin in the experiment. After the mixture completely penetrated the skin, the skin became transparent, and the transparent areas took on an orangish color.
The process is reversible. Any remaining dye is washed off, and the dye diffused into the skin is metabolized and excreted through urine.
Dr. Zihao Ou, an assistant professor of physics at the University of Texas at Dallas, said, “It takes a few minutes for the transparency to appear. It’s similar to how a facial cream or mask works: The time needed depends on how fast the molecules diffuse into the skin.”
The transparent skin allowed the scientists to see blood vessels on the surface of the brain. They also observed internal organs and peristalsis, the muscle contractions that move contents through the digestive tract in the abdomen.
The dye is biocompatible, hence safe for living organisms. Also, it is inexpensive and efficient.
However, scientists haven’t tested it on humans yet. Moreover, it is not clear what dosage of the dye or delivery method would be necessary to penetrate the entire thickness.
Ou said one of the technique’s first applications will likely be improving existing research methods in optical imaging.
“Our research group is mostly academics, so one of the first things we thought of when we saw the results of our experiments was how this might improve biomedical research,” he said.
Journal Reference:
- Zihao Ou, Mark Brongersma et al. Achieving optical transparency in live animals with absorbing molecules. Science. DOI: 10.1126/science.adm6869