Researchers Turn Skin Invisible Using Food Dye

And now for something completely different: A team of researchers has managed to make living tissue transparent using a common food dye. The discovery could yield a range of innovations in medicine, from managing muscle injuries to spotting cancers.

The team’s research, published today in Science, makes use of a concept once relegated to science fiction: making soft tissues like skin transparent safely and temporarily.

How did they do it? Well, the reason we can’t see through most objects is because light scatters off them. To overcome this, the team needed to develop a technology that would match up the refractive indices of different soft tissues, allowing light to pass through those structures unimpeded. The team used a food dye called tartrazine, or FD & C Yellow 5 to do this; the dye’s molecules are particularly good at matching up those refractive indices. When light hit objects affected with the dye, instead of scattering off those structures, it carried on its way, thereby rendering the tissues transparent.

The team first dyed chicken breast and found that increasing the concentration of tartrazine absorbed by the muscle cells raised their refractive indices. When those indices matched those of the breast’s muscle proteins, the chicken became effectively see-through.

″Looking forward, 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,″ said Guosong Hong, a materials scientist at Stanford University and co-author of the paper, in a National Science Foundation release. ″For example, certain therapies use lasers to eliminate cancerous and precancerous cells, but are limited to areas near the skin’s surface. This technique may be able to improve that light penetration.″

After their chicken run (so to speak), the research team tested their solution on the scalps of living mice. According to the release, the tartrazine-treated scalps then revealed blood vessels moving across the rodents’ brains. Applying the solution to the mices’ abdomens revealed their intestinal activity and motions caused by their heartbeats and breathing.

We must stress, despite the top illustration showing a semi-transparent human hand, the effect has only been tested on animals to date. Do not try this at home! Even though the team found their method was non-invasive and reversible, you will not be successful.

“Ou et al.’s insight is that the absorption spectrum of a material can be used to not only infer the refractive index but also control it—and not just by a small amount,” wrote Christopher Rowlands and Jon Gorecki, optical experts at Imperial College London who were not affiliated with the recent team, in an accompanying Perspectives article.

As Rowlands and Gorecki point out, the team’s approach is remarkably similar to the technology developed by the protagonist in H.G. Wells’ 1897 work The Invisible Man. The character manages to become (irreversibly) invisible by matching up the refractive indices of his body’s cells to the surrounding medium: air.

“This suppression of tissue scattering is no mere magic trick,” they added. “Tissue scattering almost always presents the biggest obstacle to deep optical imaging in vivo [in a living organism]. It is therefore of extremely broad interest across the fields of biology and medicine.”

So there you have it. Dying tissue can, counterintuitively, open a window deeper into the body. It’s yet to be tested on humans, but the nascent technology—grounded in old ideas—could substantially change the ways we do medicine.