Researchers peered into the brains and bodies of living animals after discovering that a common food dye could temporarily make skin, muscle and connective tissue transparent.
Applying the dye to the stomach of a mouse made its liver, intestines and bladder clearly visible through the abdominal skin, while smearing it on the rodent’s scalp allowed scientists to see blood vessels in the animal’s brain.
Treated skin regained its normal color when the dye was washed off, according to researchers at Stanford University, who believe the procedure opens up a host of applications in humans, from detecting injuries and finding veins to draw blood to monitoring digestive disorders and the detection of tumors.
“Instead of relying on invasive biopsies, doctors may be able to diagnose deep-seated tumors simply by examining a person’s tissue without the need for invasive surgical removal,” said Dr Guosong Hong, a senior researcher on the project. . “This technique could potentially make blood draws less painful by helping phlebotomists easily locate veins under the skin.”
The trick echoes the approach used by Griffin in HG Wells’ 1897 novel The Invisible Man, in which the brilliant but doomed scientist discovers that the secret to invisibility lies in an object’s refractive index, or ability to refract light bend, to match that of the surrounding air.
When light enters biological tissue, much of it is scattered because the structures inside, such as fatty membranes and cell nuclei, have different refractive indices. As light moves from one index of refraction to another, it bends, making tissue opaque. The same effect makes a pencil appear bent when dropped into a glass of water.
Dr. Zihao Ou and his colleagues at Stanford counterintuitively theorized that specific dyes could allow certain wavelengths of light to pass more easily through skin and other tissues. Strongly absorbing dyes change the refractive index of tissues that absorb them, allowing scientists to match the refractive indices of different tissues and suppress any scattering.
In a series of experiments described in Scienceshow the researchers how a fresh chicken breast became transparent to red light minutes after being immersed in tartrazine solution, a yellow food coloring used in American Doritos, SunnyD drink and other products. The dye reduced light scattering inside the tissue, allowing the rays to penetrate deeper.
The team then applied the yellow dye to a mouse’s lower abdomen, which made the abdominal skin translucent and exposed the rodent’s intestines and organs. In another experiment, they applied dye to a mouse’s shaved head and, using a technique called laser speckle contrast imaging, saw blood vessels in the animal’s brain.
“The most surprising part of this study is that we normally expect dye molecules to make things less transparent. For example, if you mix blue pen ink in water, the more ink you add, the less light can pass through the water,” Hong said. “In our experiment, when we dissolve tartrazine in an opaque material like muscle or skin, the more tartrazine we add, the clearer the material becomes. But it’s inconsistent with what we usually expect with dyes.”
The researchers describe the process as “reversible and repeatable”, with the skin returning to its natural color once the dye is washed away. Right now, transparency is limited to the depth the dye penetrates, but Hong said microneedle spots or injections could deliver the dye deeper.
The procedure has not yet been tested on humans and researchers will have to show it is safe to use, especially if the dye is injected under the skin.
Others will benefit from the breakthrough. Many scientists study naturally transparent animals, such as zebrafish, to see how organs and features of diseases, such as cancer, develop in living beings. With transparent dyes, a much wider range of animals could be studied in this way.
In a accompanying articleChristopher Rowlands and Jon Gorecki, of Imperial College London, say there will be “extremely broad interest” in the procedure, which, when combined with modern imaging techniques, could allow scientists to image an entire mouse brain or tumors under centimeters thick can see. tissues. “HG Wells, who studied biology under TH Huxley, as a student would surely approve,” they write.
