Common dye turns skin invisible to see the organs inside

A common dye found in snack foods can turn skin invisible so that we can see the organs inside, scientists say.

Researchers found that applying the dye to skin allowed them to see the blood vessels of the brain and watch as the organs and muscles underneath the skin worked.

The researchers conducted the work on mice. But they suggest that the dye is safe for other animals, and that it might be possible to use it for humans.

The dye is a food colouring called tartrazine, used it for its yellowish colour. But that same colour means that it absorbs light, especially blue and ultraviolet light.

We are usually unable to see through skin because it scatters light, in a similar way to fog. But the colour works by changing the way that skin bends light – in a way that stops it scattering so much, and thereby turning it more transparent.

Researchers are able to apply the dye simply: they rubbed a solution of the colouring with water onto the skin of the animal. A few minutes later, it would turn transparent, as it diffused into the skin.

Researchers were then able to see the blood vessels and organs beneath, and even watch the muscle contractions that happened as the mice’s digestive tract works.

When the experiments over, the dye can be washed off and the effect will be reversed. Any that dye that is in the skin will simply be taken in and then excreted as urine.

The dye itself is safe for living organisms – which is good, since it is currently used in candy, snack chips and other foods. It is already certified by the US Food and Drug Administration as being safe for use in food.

It remains unclear whether the process would work on humans, whose skin is 10 times as thick as that of a mouse. It might be possible to use more dye or deliver it in a different way to get all the way through that skin, researchers suggested.

But doing that might help offer new capabilities to doctors and healthcare workers as they try and understand what is happening in a patient’s body.

“In human medicine, we currently have ultrasound to look deeper inside the living body,” Ou said. “Many medical diagnosis platforms are very expensive and inaccessible to a broad audience, but platforms based on our tech should not be.”

The work is described in a new paper, ‘Achieving optical transparency in live animals with absorbing molecules’, published in the journal Science .