“For those who understand the fundamental physics behind this, it makes sense; but if you aren’t familiar with it, it looks like a magic trick,” said the study’s first author, Zihao Ou, assistant professor of physics at the University of Texas at Dallas, in a statement.

Time-lapse images of blood vessels in the brain just beneath the skull of a sedated mouse are revealed by dyeing the tissues with FD&C Yellow No. 5, a food coloring also known as tartrazine. The process doesn’t involve any surgery, incisions or damaging of the mouse’s bone or skin.

US NSF

Light-absorbing dye molecules

The “magic” uses insights from the field of optics. Light-absorbing dye molecules enhance the transmission of light through the skin by suppressing the tissue’s ability to scatter light.

The dye, when mixed with water, modifies the refractive index — a measure of the way a substance bends light — of the aqueous part of the tissue to better match the index of proteins and fats in the tissue. The process is akin to a dissipating cloud of fog.

“We combined the yellow dye, which is a molecule that absorbs most light, especially blue and ultraviolet light, with skin, which is a scattering medium,” said Ou, who conducted the study as a postdoctoral researcher at Stanford University in California.