It sounds like a piece of science fiction—a food-safe dye that turns your skin transparent. And yet, scientists at Stanford University have done just that.
The technique, described by experts as "groundbreaking," exploits the behavior of light as it moves through biological tissues and has a range of applications in medical diagnostics and treatment.
"This technology could make veins more visible for the drawing of blood, make laser-based tattoo removal more straightforward, or assist in the early detection and treatment of cancers,″ Guosong Hong, Stanford University assistant professor of materials science and engineering who helped lead this work, said in a statement.
When light passes through an object, it changes speed and bends depending on the material through which it travels. This process is called refraction, and the extent to which a material causes light to bend is known as its refractive index.
Our tissues are made up of various materials, like fats, proteins, and cellular fluids, that are all closely packed together. Each causes light to bend slightly differently, scattering the incoming light and bouncing off all of these materials at different angles.
When our eyes detect this scattering effect, they interpret it as opaque, colored biological material, i.e. skin. However, if were able to find a way to match the different refractive indexes of all of these different materials, light would be able to travel through unimpeded, essentially making the material see-through.
In a new study, published in the journal Science, Hong and colleagues found that a common food dye, known as FD & C Yellow 5, was able to effectively direct light through a range of refractive indexes, preventing light scattering and causing the skin to become transparent.
The researchers first tested their technique on thin slices of chicken breast before trying it out on mice. After gently rubbing the solution on the mice's scalps, a complex network of blood vessels became visible through the rodents' skin.
Next, they trialed the solution on the mice's abdomen, revealing the contractions and movements of their intestines.
When the dye was rinsed off, the tissues returned to normal opacity with no observable long-term effects. However, the safety profile of this technology has yet to be tested on humans.
The researchers suspect injecting the dye would enable even deeper views into the tissues, enabling noninvasive imaging for various medical applications, from investigating digestive issues in real time to identifying cancers.
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References
Ou, Z., Duh, Y.-S., Rommelfanger, N. J., Keck, C. H. C., Jiang, S., Brinson, K., Jr., Zhao, S., Schmidt, E. L., Wu, X., Yang, F., Cai, B., Cui, H., Qi, W., Wu, S., Tantry, A., Roth, R., Ding, J., Chen, X., Kaltschmidt, J. A., Brongersma, M. L., & Hong, G. (2024) Achieving optical transparency in live animals with absorbing molecules. Science, 385(6713). https://doi.org/10.1126/science.adm6869
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