Yellow dye makes living mice temporarily transparent

Why isn’t your body transparent? Some animals, such as jellyfish, zebra fish and some glass frogs, have transparent bodies. But most mammals, including humans, are not transparent.

The idea of ​​a transparent body might sound weird or a little scary, but it could be really helpful for doctors. If our body was transparent, doctors could easily see inside and diagnose diseases in organs like the liver, spleen and brain. They wouldn’t need invasive procedures like biopsies or expensive machines like CT scanners and MRIs.

I’m a materials scientist, and my team and I work on how new materials can aid biomedicine. My colleagues and I have wondered if it’s possible to make living tissue temporarily transparent to aid medical treatments and other uses.

We found that by dissolving certain dye molecules in water, including a food coloring commonly used in snacks called FD&C Yellow 5, we can change the way light passes through water. We used this phenomenon to make biological tissue – specifically the thin skin of mice – transparent, our study published in Science in September 2024.

Refractive index

Our bodies, like those of most mammals, are not transparent, mainly because of the way light interacts with our tissues. Normally, light travels in a straight line through the air. But when light hits the human body, it doesn’t go very far before its path is scattered. The light bounces off in different directions instead of traveling straight through. If light passed through us without scattering, our tissues would be transparent.

This scattering occurs because human tissues are made up of many different components, including water, fat, and protein. Each of these components slows light in a different way, known as the refractive index.

For example, the refractive index of water is about 1.33, while the refractive index of fats and proteins is about 1.45 in the visible spectrum. Therefore, light travels more slowly in lipids than in water.

The key to making living tissues transparent will be to minimize the differences in light fluxes between different parts of the tissue — especially between water and fats and proteins.

Kramers–Kronig relation

A principle in physics known as the Kramers-Kronig relation states that if a substance absorbs more light of one color, such as blue, this increased absorption will change the way light of another color, such as red, passes through it. The Kramers-Kronig relation states that the colors of light are not independent of one another but are linked.

FD&C Yellow 5 absorbs blue light very strongly, which changes its color from orange to red when dissolved in water. This occurs because the blue portion of the light is absorbed, leaving only the orange to red portion visible. As a result of the Kramers-Kronig relations, this absorption of blue light increases the refractive index of water for red light. The refractive index of water increases from 1.33 to about 1.45, comparable to that of fats and proteins.

When the refractive indices match, red light no longer scatters as much. It travels in water the same way as it does in fat in tissue. Therefore, the entire tissue appears as a single, uniform substance. This process can make tissue transparent, even if it is normally opaque.

making tissue transparent

My research team applied this idea in an experiment that used a scattering phantom, a material designed to mimic the opacity of human skin. As we added more FD&C Yellow 5 dye to the phantom, it became more orange-red, just as we expected.

However, something else happened. It became more transparent to red light. This increased transparency allowed us to see the grid pattern on the table beneath the phantom.

The more yellow we added, the more transparent the scattered ghost became. Guosong Hong We then decided to test this idea on a piece of chicken breast bought from the grocery store. Chicken meat usually looks opaque unless it’s sliced ​​very thin.

When we soaked the chicken breast in a solution containing FD&C Yellow 5 dye, something amazing happened. It became more transparent, allowing us to clearly see the Stanford logo underneath.

Finally, we used this idea to make the mouse’s skin optically transparent. We applied FD&C Yellow 5 dye to different parts of the mouse’s body. When we inserted it into the mouse’s skull, we could see the blood vessels in its brain. When we inserted it into the mouse’s abdomen, we could see its intestines. When we inserted it into the mouse’s organs, we could see its muscle fibers.

All that was needed for this experiment was to gently massage the dye solution onto the rat’s skin and have some patience.

This process is non-invasive as it does not require tissue removal or surgery, and the skin returns to its normal opacity once you wash the color off with water. Although this is a fascinating technique, we strongly advise against trying it on yourself.

Although the FDA has approved the use of Yellow 5, some people have raised concerns about its potential health risks. These include allergic reactions – especially in people with asthma – hyperactivity in children and a possible link to cancer. But researchers will need to do more testing to determine if there is any risk.

future use

So what can this method be used for? Right now, it works best on very thin layers of skin, like rat skin.

Unfortunately, human skin is very thick, so this method is not yet ready for practical use on people. Also, the red color of the dye means that the color balance is not quite right and the transparency is not perfect across the entire visible spectrum. The dye still blocks blue light.

My colleagues and I are working on improving this technique to make it more effective for human tissues. We are also trying to shift the absorption of the dye towards the ultraviolet spectrum, which would produce a more balanced transparency effect across all visible colors.

Looking to the future, this technology could one day make veins more visible, making it easier to perform venipuncture (the procedure of withdrawing blood or injecting fluids through a needle) — especially in elderly patients whose veins are difficult to see.

It may also aid in the early detection of skin cancer, help deliver light to deeper tissues for photodynamic and photothermal treatments, and simplify laser-based tattoo removal.

In photodynamic and photothermal therapy, doctors use lasers to kill cancer and precancerous cells. But the light from the laser penetrates only a short distance into tissue, so these therapies are not yet suitable for deeper organs in the body.

All of these applications could benefit from a reversible, on-demand transparency window in the body.

Guosong Hong is an assistant professor of materials science and engineering at Stanford University. This article is republished from Conversation,