Light-absorbing dye turns live mouse temporarily transparent

By topically applying a common food dye that strongly absorbs light to a live mouse, Zihao Ou and colleagues were able to turn its tissues transparent, allowing them a look into the blood vessels of the scalp, the movement of organs laying under the skin of the abdomen, and tiny contractile units of muscle at work. Achieving optical transparency in live animals depends on some interesting physics, where strongly absorbing dye molecules enhance light transmission through a medium usually characterized by substantial light scattering. This scattering is the result of a low refractive index in aqueous parts of tissue and a high refractive index of its protein and fat-based component. Typical methods of tissue clearing can involve processes such as clearing proteins and fats, which wouldn’t work in a live animal. Ou et al. found that a common food dye called tartrazine (mixed with water, it’s called FD&C Yellow 5) topically applied can modify the refractive index of the aqueous parts of tissue by absorbing light in the near ultraviolet and blue regions of the spectrum, to better match the refractive index of nearby high-refractive materials. This allows the non-absorbing part of the spectrum, namely the red/orange part, to transmit deeper through the tissue. The result is a temporary transparency effect that can be undone with a quick wash, and does not harm living animals, unlike other approaches used to enhance transparency. Christopher Rowlands and Jon Gorecki discuss the imaging possibilities of the technology in a related Perspective.