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What is the difference between humans and jellyfish? The most intuitive difference is that our bodies are opaque.

But now, a groundbreaking study shows that mammalian skin can actually become transparent. Even just by applying a mixture of water and a common food coloring, FD&C Yellow #5, to the body, and then waiting for a few minutes, you can see the criss-crossing blood vessels, the beating heart and the continuous peristalsis of the intestines in the body. After washing off the dye, the skin, which is like a "skylight", can return to normal.

It may sound a little crazy, but this almost magical discovery was published in the recent issue of Science magazine.

Make the skin "transparent"

The skin is really a barrier

Before we try to explain the "magic", we probably have to understand why mammals' bodies cannot be as transparent as jellyfish.

In essence, the fundamental reason why the body is opaque is that skin tissue is a scattering medium. When light from the outside world shines on it, its first reaction is to block, intercept, and reflect the light instead of allowing it to pass through unimpeded.

Even if some light can "escape this disaster" by chance, the composition of human tissue is so complex - composed of water, fat, protein and other substances, it is difficult for light to "take advantage" in such a complex situation - the different refractive indices of these components will slow down the propagation speed of light in them in different ways. For example, for visible light, the refractive index of water is about 1.33, while fat and protein are higher. In most tissues of the human body, these components are closely intertwined, so that when visible light passes through, it must be scattered. The ultimate biggest impact of this effect is to tell our eyes that the skin is an opaque colored biological material.

But at the same time, this also gave researchers an idea: If the refractive index of cells could be precisely controlled to perfectly match that of the surrounding medium, thereby avoiding the scattering of light at the boundary between the two, wouldn't it be possible to make the skin transparent?

Theoretically, this is correct. But it is extremely difficult to find a way to reduce the difference in refractive index between different components. What the research team from Stanford University in the United States did not expect was that the material that finally realized their vision was actually right before their eyes.

Gifted lemon yellow

How close? Well, as one of the nine food color additives certified by the U.S. Food and Drug Administration (FDA), tartrazine is widely added to food and beverages such as chicken essence, puffed food, soda, and blended alcohol. Not only on the dining table, but also in cosmetics and daily necessities such as lipstick, toothpaste, shampoo, and detergent on the dressing table and sink, it is not difficult to find it.

The study's first author holds a tube of tartrazine solution

So, how does this pigment, which is omnipresent and hidden around you and me, "change our destiny"?

First, we need to introduce an important optical concept, the Kramers-Kronig relationship. It shows that the colors of light are not independent of each other, but are related to each other: if a material absorbs more of a certain color of light, then this absorption will change the way another color of light passes through it. And lemon yellow happens to absorb most visible light, especially blue light and ultraviolet light.

According to the Kramers-Kronig relationship, the absorption of blue light and ultraviolet light will change the way other visible light, such as red light, passes through. In the experiment, this is reflected in the increase of the refractive index of water for red light from 1.33 to about 1.45, which is equivalent to fat and protein. At this time, red light will no longer be scattered as before, but will propagate smoothly, and the propagation method in water will be the same as in fat. In this way, the entire tissue looks like a single, uniform material and becomes visually transparent.

Schematic diagram of the principle of achieving optical transparency by adjusting the refractive index

At present, the relevant theory has been verified by in vivo experiments. The researchers applied a solution containing lemon yellow to different parts of the mouse's body. When they applied it to the mouse's scalp, they could see the blood vessels in the mouse's brain; when they applied it to the mouse's abdomen, they could see the mouse's intestines; and when they applied it to the mouse's limbs, they saw the muscle fibers on the limbs. More importantly, once the dye completely diffuses into the skin, the skin becomes transparent, and the whole process is non-invasive and reversible. Not only does it not involve any tissue removal or surgery, but once the residual dye is washed away with water, or the dye diffused into the skin is metabolized and excreted from the body through urine, the skin can also return to normal opacity, just like a magician's curtain call, with a beginning and an end.

Where should “transparent technology” go?

"Making the body transparent" sounds like a bit useless except for curiosity, and even a bit strange and "perverted" if you think about it carefully, but if you create some special life scenes, it is not difficult to imagine its bright future. For example, the operating table, the blood collection place, and the moment when expensive instruments are used to peek at the lesion. When the transparent skin technology is promoted, we may hear less crying in these places in the future.

But there is no doubt that the future is still far away and the years are still long. Researchers said that they cannot yet predict how it will work on human skin. Because human skin is 10 times thicker than that of mice, it is still unknown how much dose is needed and what delivery method is used to penetrate human skin. In addition, although lemon yellow is an approved food dye, it may still pose certain health risks, such as causing allergic reactions.

In this regard, the researchers also expressed their efforts and determination. They are stepping up their efforts to improve this new technology so that it can be applied to human tissues. In the next step of research, they plan to focus on understanding what dose of dye molecules is most effective in human tissues, and whether there are other molecules that can work more effectively than lemon yellow, etc., hoping that one day it will inspire new technologies that will help improve existing medical imaging research methods.