All that and a bag of chips

Tartrazine, a dye commonly found in Doritos, has the ability to render tissues transparent in living mice. A team of researchers at Stanford University used theoretical physics to study how this dye changes how opaque body tissues interact with light, making them see-through.

The components that make up living tissues have different refractive indices, which is the ratio of the velocity of light in a vacuum to its velocity in a given medium. Lipids and proteins have higher refractive indices, while aqueous materials have lower ones. Due to the differences in refractive indices across these components, light is scattered differently, causing tissues to appear opaque. To produce transparency in these tissues, each component must have a very similar refractive index. The scientists hypothesized that adding a dye that strongly absorbs light to such tissues could achieve this goal.

After reaching this prediction, the team of researchers began studying different materials that could render the refractive indices nearly the same. The group settled on tartrazine, an FDA approved dye found in many processed foods. Tartrazine was then compared to glycerol, another agent currently used to produce transparency in mice. Each material was placed in a cuvette filled with an opaque suspension, and the transmission of the liquid was measured in a spectrophotometer. Glycerol had limited transmission at all absorption wavelengths, so the liquid remained opaque. However, tartrazine achieved complete optical transparency at wavelengths around 720nm, or the red region of the visible light spectrum. This indicated tartrazine could potentially produce transparency in mice tissue, leading the researchers to test this dye on the animals.

When the dye was massaged into the head of the mice, the scientists were able to observe cerebral blood vessels via laser speckle contrast imaging (LSCI). Typically, LSCI requires the scalp to be removed for visualization. However, with the use of the dye, the mouse remained living while this technique was applied. Furthermore, the researchers applied tartrazine to the abdomen of the mice. The dye rendered the skin transparent, especially in the red window. After application, internal organs such as the liver and intestines were viewable through the skin with the naked eye. Scientists were also able to observe the heartbeat of the mice and peristalsis, the contraction of muscles in the digestive system, without any additional technology. Phillip Keller, a biologist at the Howard Hughes Medical Institute, speculates this technique may be used to better understand the nervous system and neurodegenerative diseases.

Tartrazine enables a minimally invasive approach to visualize the internal structures of living mice. Since tartrazine is a food dye, it is safe to use on living mice without harming them. Once analysis is complete, the dye may simply be washed off and the skin will return to being opaque. This enables the mice to remain living during and after observation, something that is not possible with current techniques. Moreover, because the mice do not need to be euthanized for results, scientists can complete longer term experiments including offspring of the mice. This can be useful in observing the heritability and progression of internal abnormalities, such as heart defects.

“This enables the mice to remain living during and after observation, something that is not possible with current techniques.”

At present, tartrazine is only capable of providing tissue transparency for depths of 3 millimeters, indicating its use in larger animals with thicker tissues is limited. Moreover, this approach currently relies on matching the refractive index of tartrazine with the components of the tissue. However, different tissue components have varying refractive indices, making it challenging to find an optimal concentration of tartrazine to obtain transparency. Despite these limitations, this technique opens the door for a plethora of new research and observation in mouse models.