Turning transparent with food dye: a breakthrough in non-invasive imaging

This breakthrough, recently published in Science, could change how researchers and doctors look inside living animals – and eventually humans – without needing to cut or use harmful radiation. The key to this discovery? A simple, FDA-approved, food dye known as tartrazine, commonly found in everyday products like sweets and beverages.

Why can’t we see through skin?

To understand why this discovery is so significant, it helps to know why skin is normally opaque.

When light hits the skin, it scatters in different directions because of the varying structures and materials within the tissue. This scattering effect is caused by differences in the refractive index of the tissue’s components. The refractive index refers to how much light bends when passing through a material. In skin, materials like water and fat have very different refractive indices, causing light to scatter rather than pass straight through.

Visible light, which is the range of wavelengths that humans can see, forms only a small part of the electromagnetic spectrum. The spectrum includes other types of radiation, like ultraviolet light (which causes sunburn) and infrared light (used in night-vision cameras). Visible light ranges from around 380 to 700 nanometres (nm) in wavelength. Different colours of light correspond to different wavelengths: blue light has shorter wavelengths (around 450nm), while red light has longer wavelengths (about 700nm). Light scattering in biological tissues occurs because different molecules within the skin affect these wavelengths unevenly, especially in the visible range​.

This is why existing imaging technologies, like X-rays, are used to look inside the body. X-rays are higher-energy wavelengths that can pass through tissues but come with risks due to radiation exposure. Magnetic resonance imaging (MRI) provides detailed images without radiation but is expensive and time-consuming. This new technique, which uses simple food dyes to turn skin temporarily transparent, offers a much safer and cheaper alternative.

How does the dye make skin transparent?

The key to this discovery lies in manipulating how light interacts with the skin. The researchers applied a solution of red tartrazine (also known as FD&C Yellow 5) to the skin of a sedated mouse. Tartrazine is a light-absorbing molecule, meaning it can soak up specific wavelengths of light. By absorbing certain colours in the visible spectrum, the dye changes how light behaves as it moves through the skin. This reduces the amount of light that scatters and allows more of it to pass straight through.

The dye works by altering the refractive index of water within the tissue. Water normally has a much lower refractive index than lipids (fat molecules), which are abundant in the skin. This difference causes visible light to scatter, but when the dye is applied, it increases light absorption and reduces scattering, making the tissue more transparent at certain wavelengths​.

Once the dye is applied, the researchers could see the mouse’s internal organs, such as the liver and intestines, with the naked eye. They could even observe fine details like blood flow in the brain and muscle fibres in the limbs, all without the need for invasive surgery or special imaging equipment​.

Easy on, easy off

The process is safe and the transparency effect is not permanent. The researchers found that simply rinsing the mouse’s skin with water was enough to wash away the dye and return the skin to its normal opaque state. Within minutes of being rinsed, the skin stopped being transparent, with no harm done to the tissue​.

This reversibility makes the process ideal for applications in both research and medicine. Since the dye doesn’t cause any long-lasting changes to the skin, it could be used multiple times on the same subject. Researchers believe this is the first time a non-invasive method has been developed to achieve real-time visibility of living internal organs in animals​.

What next?

Although this method has so far only been tested on mice, the potential for human applications is enormous. If the technique can be adapted for humans, it could replace some invasive procedures like biopsies, where a small sample of tissue is removed for testing. Instead, doctors could look directly at tissues under the skin, such as for diagnosing skin cancers like melanoma, without cutting into the body.

This technology could also reduce the need for medical imaging techniques that expose patients to radiation, like X-rays and CT scans. Phlebotomists (professionals who draw blood) could benefit from the ability to easily see veins beneath the skin, making the process of taking blood samples less painful. Additionally, it could help in procedures like laser tattoo removal, by allowing technicians to focus the laser beams more precisely on the pigment below the skin​.

Light's role in medicine

The study of how light interacts with biological tissues is a growing field with many exciting possibilities. Techniques like this are part of a broader effort to find non-invasive ways to look inside the body. By controlling light’s behaviour with molecules like tartrazine, scientists can now peer deeper into living tissues than ever before. This approach is grounded in the Kramers–Kronig relations, a set of equations that describe the relationship between how much a material absorbs light and how it bends (or refracts) it. This mathematical foundation helped researchers predict how tartrazine could be used to make skin temporarily transparent​.

The use of light-absorbing molecules to achieve optical transparency in living tissue offers a glimpse into the future of medical imaging. As researchers continue to refine this technique, they aim to develop better dye formulations and more efficient methods of applying the solution.