Apart from a few pigments, such as melanin, not many of the molecules in your body absorb much visible light. The main reason why human (and most animal) bodies appear optically opaque is because of scattering, not absorption. The tissues are a hodgepodge of watery cytoplasm and interstitial fluid, with refractive indices around 1.35, and protein- and lipid-based organelles and membranes, with refractive indices of up to 1.5. When light rays try to pass through that optical maze, they get hopelessly tangled up.
Biomedical researchers have developed a toolbox of techniques for making tissues transparent so they can study the structure of an organ or animal without cutting it up. Most, however, are applicable only to post-mortem tissues. (See, for example, Physics Today, June 2013, page 14.) One exception is to replace the water in a tissue with a fluid, such as glycerol, whose high refractive index better matches that of the organelles and membranes. Although displacing the water from a living tissue is not harmless, researchers can use the technique to see through a lab mouse’s skin while the mouse is still alive.
Now an interdisciplinary team of Stanford University researchers, led by Mark Brongersma and Guosong Hong, has found a potentially more practical alternative. Instead of glycerol, the researchers use the yellow food dye tartrazine (known in the US as FD&C yellow 5) to turn tissues transparent, as shown in the images above. Tartrazine absorbs blue light, but over the red–yellow half of the spectrum, a tartrazine solution that’s mostly water has a refractive index that rivals that of pure glycerol. Because the tartrazine doesn’t replace the tissue’s water, the researchers can apply it to live mice without causing acute tissue damage.
It's no coincidence that making tissues absorb light is the key to turning them transparent. A substance’s absorption spectrum and refractive index are the imaginary and real parts of the same function. As such, they’re bound by a pair of equations called the Kramers–Kronig relations, which dictate that when a substance has a strong absorption resonance, as in the plot on the left below, its refractive index must look like the plot on the right, with elevated values across the spectrum. The effect is not unique to tartrazine—glycerol also gets its high refractive index from an absorption resonance, albeit one deep in the UV—but when the researchers screened for the combination of strong absorption, water solubility, and biocompatibility, tartrazine was a clear winner.
The tartrazine concentrations used in the experiments far exceed the amounts you’re likely to encounter in foods, and the method hasn’t yet been tested on humans for safety or effectiveness. But in the future, it could be. The mouse studies so far are promisingly benign: The researchers need only rub tartrazine on a mouse’s shaved skin to see its blood vessels, muscles, and internal organs. And the skin returns to its normal opacity once the dye is rinsed off with water. If approved for use in humans, tartrazine could, among other applications, help phlebotomists collect blood samples. (Z. Ou et al., Science 385, eadm6869, 2024.)
