Scientists at Stanford University have made live mice transparent, easing biomedical research and saving their lives. They used an FDA-approved food coloring dye called tartrazine, which makes the mice’s skin transparent. They used this dye on the mouse brain, abdomen, and muscle for imaging. Below is the figure of the mouse’s abdomen before and after applying tartrazine to their skin. After imaging, the dye can be removed easily by rinsing with water. It showed no toxicity to the mice even when used for the long term.
Figure 1: Generated by MetaAI
Physics behind it
The biological material is difficult to see through due to unwanted light scattering and absorption. Light scattering is more challenging as compared to non-specific absorption. The scattering occurs due to differences in the low refractive index of aqueous components such as cytosol to the high refractive index of lipid and protein-based components such as plasma membrane, myelin, etc. The refractive index measures the bending of light in a medium as compared to a vacuum. Thus, each medium has a specific refractive index value. Most of the current imaging methods focus on reducing the differences in the refractive indexes of biological material. It does so by either replacing the low refractive index aqueous solution with high refractive index material which might be toxic or removing the high refractive index medium. In this study, they found that a dye such as tartrazine, has a sharp absorption in the visible spectrum (400-700 nm). It can increase the refractive index of water in red and infrared parts of the spectrum and thus reduce the difference, achieving transparency. They claim that this transparency is obtained by reducing the scattering events in the tissue and not affecting the absorption.
Figure 2: Transparent mice, figure extracted from the reference.
What next
Based on their findings with tartrazine, they hypothesized that dye molecules with strong absorption and superior solubility can increase the refractive index of the aqueous solution. They developed a model to find more such dyes that could reduce light scattering and be more efficient than tartrazine. However, this needs to be tested.
