Applying the dye to lab mice made their skin temporarily transparent, allowing Stanford University researchers to observe the rodents’ digestive system, muscle fibers, and blood vessels, according to a study published in Science.
“It’s a stunning result,” said senior author Guosong Hong, PhD, who is assistant professor of materials science and engineering at Stanford University in California. “If the same technique could be applied to humans, it could offer a variety of benefits in biology, diagnostics, and even cosmetics.”
The work drew upon optical concepts first described in the early 20th century to form a surprising theory: Applying a light-absorbing substance could render skin transparent by reducing the chaotic scattering of light as it strikes proteins, fats, and water in tissue.
A search for a suitable light absorber led to FD&C Yellow 5, also called tartrazine, a synthetic color additive certified by the Food and Drug Administration (FDA) for use in foods, cosmetics, and medications.
Rubbed on live mice (after areas of fur were removed using a drugstore depilatory cream), tartrazine rendered skin on their bellies, hind legs, and heads transparent within 5 minutes. With the naked eye, the researchers watched a mouse’s intestines, bladder, and liver at work. Using a microscope, they observed muscle fibers and saw blood vessels in a living mouse’s brain — all without making incisions. Transparency faded quickly when the dye was washed off.
Someday, the concept could be used in doctors’ offices and hospitals, Dr. Hong said.
“Instead of relying on invasive biopsies, doctors might be able to diagnose deep-seated tumors by simply examining a person’s tissue without the need for invasive surgical removal,” he said. “This technique could potentially make blood draws less painful by helping phlebotomists easily locate veins under the skin. It could also enhance procedures like laser tattoo removal by allowing more precise targeting of the pigment beneath the skin.”
From Cake Frosting to Groundbreaking Research
Yellow 5 food dye can be found in everything from cereal, soda, spices, and cake frosting to lipstick, mouthwash, shampoo, dietary supplements, and house paint. Although it’s in some topical medications, more research is needed before it could be used in human diagnostics, said Christopher J. Rowlands, PhD, a senior lecturer in the Department of Bioengineering at Imperial College London, England, where he studies biophotonic instrumentation — ways to image structures inside the body more quickly and clearly.
But the finding could prove useful in research. In a commentary published in Science, Dr. Rowlands and his colleague Jon Gorecki, PhD, an experimental optical physicist also at Imperial College London, noted that the dye could be an alternative to other optical clearing agents currently used in lab studies, such as glycerol, fructose, or acetic acid. Advantages are the effect is reversible and works at lower concentrations with fewer side effects. This could broaden the types of studies possible in lab animals, so researchers don’t have to rely on naturally transparent creatures like nematodes and zebrafish.
The dye could also be paired with imaging techniques such as MRI or electron microscopy.
“Imaging techniques all have pros and cons,” Dr. Rowlands said. “MRI can see all the way through the body albeit with limited resolution and contrast. Electron microscopy has excellent resolution but limited compatibility with live tissue and penetration depth. Optical microscopy has subcellular resolution, the ability to label things, excellent biocompatibility but less than 1 millimeter of penetration depth. This clearing method will give a substantial boost to optical imaging for medicine and biology.”
The discovery could improve the depth imaging equipment can achieve by tenfold, according to the commentary.
Brain research especially stands to benefit. “Neurobiology in particular will have great use for combinations of multiphoton, optogenetics, and tissue clearing to record and control neural activity over (potentially) the whole mouse brain,” he said.
Refraction, Absorption, and The Invisible Man
The dye discovery has distant echoes in H.G. Wells’ 1897 novel The Invisible Man, Dr. Rowlands noted. In the book, a serum makes the main character invisible by changing the light scattering — or refractive index (RI) — of his cells to match the air around him.
The Stanford engineers looked to the past for inspiration, but not to fiction. They turned to a concept first described in the 1920s called the Kramers-Kronig relations, a mathematical principle that can be applied to relationships between the way light is refracted and absorbed in different materials. They also read up on Lorentz oscillation, which describes how electrons and atoms inside molecules react to light.
They reasoned that light-absorbing compounds could equalize the differences between the light-scattering properties of proteins, lipids, and water that make skin opaque.
With that, the search was on. The study’s first author, postdoctoral researcher Zihao Ou, PhD, began testing strong dyes to find a candidate. Tartrazine was a front-runner.
“We found that dye molecules are more efficient in raising the refractive index of water than conventional RI-matching agents, thus resulting in transparency at a much lower concentration,” Dr. Hong said. “The underlying physics, explained by the Lorentz oscillator model and Kramers-Kronig relations, reveals that conventional RI matching agents like fructose are not as efficient because they are not ‘colored’ enough.”
What’s Next
Though the dye is already in products that people consume and apply to their skin, medical use is years away. In some people, tartrazine can cause skin or respiratory reactions.
The National Science Foundation (NSF), which helped fund the research, posted a home or classroom activity related to the work on its website. It involves painting a tartrazine solution on a thin slice of raw chicken breast, making it transparent. The experiment should only be done while wearing a mask, eye protection, lab coat, and lab-quality nitrile gloves for protection, according to the NSF.
Meanwhile, Dr. Hong said his lab is looking for new compounds that will improve visibility through transparent skin, removing a red tone seen in the current experiments. And they’re looking for ways to induce cells to make their own “see-through” compounds.
“We are exploring methods for cells to express intensely absorbing molecules endogenously, enabling genetically encoded tissue transparency in live animals,” he said.
A version of this article first appeared on Medscape.com.