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From Doritos to Gatorade, food coloring is ubiquitous. The bright and artificial dyes make our favorite foods and drinks more exciting, but their widespread use in recent decades has stirred controversy. Nevertheless, a research group from Stanford University has proposed a surprising new use for food coloring: turning skin and other live tissue transparent for biomedical research.

The Challenges of Imaging Live Tissue

Scientists studying a variety of live tissues, including the brain, gut, and muscles, often encounter difficulty visualizing internal anatomy due to light scattering and refraction, the bending of light as it travels from one medium to another. In particular, different refractive indexes (RIs) between aqueous solutions and lipids make visualizing through skin, muscles, and connective tissues challenging. Previously, researchers have attempted to circumvent this barrier by replacing water with chemicals that have high RIs and removing lipids altogether. However, these substances are toxic and scientists cannot use these methods in live animals.

Above: 2D chemical structure of tartrazine. Image courtesy of PubChem.

Until now, refining microscopy and tissue-clearing techniques have been at the forefront of the race to achieve better optical results for biomedical research, particularly studies that require imaging of live animal models. However, new research suggests that achieving tissue transparency may not require a two-photon microscope or other expensive equipment. A group led by Zihao Ou, a postdoc working under the direction of Dr. Guosong Hong in the Department of Materials Science and Engineering at Stanford University, has explored the potential applications of tartrazine, also known as FD&C Yellow 5. Their study demonstrated that tartrazine can reversibly clear the skin, muscles, and connective tissue of rodents for internal visualization in live animals.

How does Tartrazine Work?

According to the paper, water-soluble dyes can reduce the contrast in biological tissue by raising the RI in the red and infrared parts of the electromagnetic spectrum when dissolved in water. Ou and colleagues selected tartrazine due to its characteristically vibrant yellow-orange color, water solubility, and high absorption of blue visible light wavelengths. Importantly, their research relies upon the reduction of the scattering of light by the tissue rather than the absorption of photons by dye molecules.

Testing the Effects in Live Mice

After testing several different tartrazine solution concentrations in the red visible spectrum, the researchers found that the 0.6 molar concentration achieved the best transparency in living tissue. They applied it to the shaven scalp, abdomen, and legs of live mice to visualize its effects in different tissues. Ou and colleagues also compared the results to common tissue-clearing agents, such as glycerol, and found that tartrazine improved the optical results.

Above: Cerebral artery visualization using LSCI before and after applying tartrazine to the shaven mouse scalp. Image courtesy of Ou et al.

When applied topically or injected into the tissue, the tartrazine solution achieved remarkable transparency in the rodents. Both application methods achieved similar results, but Ou found that injection proved more effective for penetrating thicker tissues. To test the efficacy of tartrazine for improving microscopic imaging resolution, the researchers visualized cerebral arteries using laser speckle contrast imaging (LSCI). This method normally requires the removal of the scalp. However, with tartrazine application, clear visualization was possible without surgical removal. Additionally, when applied to the abdominal skin tissue, the internal organs and veins were visible to the naked eye—no microscope needed. Rinsing the skin and massaging the area gently reversed the effects and washed away the tartrazine Thus, tartrazine was effective not only in improving imaging resolution but also allowed for gross visualization of internal organs.

Above: Internal organ visualization before and after applying tartrazine to the shaven mouse abdomen. Image courtesy of Ou et al.

Future Applications

Ou’s team has demonstrated the power of highly absorbed water-soluble dyes in altering RIs of aqueous solution, improving imaging resolution. This technique holds particular promise in fields such as neurobiology, where optimizing imaging methods to be high-resolution yet minimally invasive is critical. The paper highlights that the transparency can allow clear visualization of fluorescently labeled enteric neurons and track their activity. Additionally, the dye concentration is compatible with fluorescent proteins such as tdTomato, illustrating this new technique's compatibility with widely used tools in biomedical research.

Next time you eat a Dorito, remember that the molecules that give the chip its signature color might be key ingredients to uncovering new discoveries in medicine.