What if you could make your skin transparent and then turn it opaque again? Soon enough, that might be a possibility. And if you’re a lab mouse, it’s already possible. A team of researchers applied tartrazine, a yellow synthetic dye found in Cheetos, Kool-Aid, and Doritos, to make mice’s skin transparent. Tartrazine is approved by the FDA for use as a colored additive in foods. Researchers simply rubbed the dye on shaved mice skin, and then they could directly see into the mouse. After rinsing the dye off with water, the mice’s skin quickly returned to normal, and no mice appeared to be affected long-term by the dye.
Skin is typically opaque because the tissue scatters light, preventing light from passing through one side of skin to the other in a way we can see. Everything transparent has a refractive index, which is the ratio of speed of light in a vacuum to the actual speed of light when passing through that material. Tissue has a variety of refractive indexes because it contains various materials. Water’s refractive index is 1.33, yet cell membranes have an index of 1.4. Cell membranes are surrounded by water, so materials with differing refractive indices are directly next to each other.
This difference in refractive indices is enough to scatter light in various directions, making skin opaque. If one were to somehow raise the refractive index of the water to nearly that of cell membranes, then the minimal difference in refractive indexes would make the skin transparent — and so that’s exactly what the researchers in this study did. And all it took was applying tartrazine, the food dye, to skin.
This technique allows red and orange light to penetrate the skin, resulting in an orange-tinted view of the mice’s insides due to the absorption of blue light by the skin. There was also still some light scattering present because of the challenge of exactly matching refractive indices, meaning the images were slightly blurry. Researchers were able to create views into the mice’s bodies in various locations, and they were even able to view inside brains and see neurons firing thanks to the natural translucency of the mice’s skulls. The researchers from this study claim that this is the first time anyone has been able to view a living rodent’s internal organs without an invasive procedure.
University of Tokyo biologist Hiroki Ueda, who was not involved in the research, told Science the technique is “kind of a dream in our field” and said it could have immediate uses in other fields of scientific research.
And if this technique is safe in humans, it could have even wider implications, replacing some X-rays and CT scans. It could also simplify blood draws by making veins easier to locate, and could similarly aid in tattoo removal by making the pigments more detectable.
“This could have an impact on health care and prevent people from undergoing invasive kinds of testing,” researcher Guosong Hong told Stanford Engineering. “If we could just look at what’s going on under the skin instead of cutting into it, or using radiation to get a less than clear look, we could change the way we see the human body.”
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