In very simple terms, we see objects because light bounces off them. Like a mirror that reflects an image back to us. And then the colors we see them with come in. This directly depends on the color of the light that illuminates them. If we illuminate an object with white light, we will see the color that reflects the most, while if we see it black, it is because it has absorbed all the colors. And, If it is green, it will absorb all colors… except green. Like a mirror that swallows everything except that specific color.
But it’s not that simple. Scattering is the reason we can’t see through our bodies: fats, fluids inside cells, proteins and other materials each have a different refractive index, a property that dictates how much an incoming light wave will bendlike many superimposed mirrors that absorb light to different “degrees.”
Grasp how light is refracted is therefore key to searching for an invisible materialone that allows light to pass through them without reflecting the “mirror” back to us. Thanks to this, a team of scientists at Stanford University has developed a new way of seeing organs inside a body by making the superimposed tissues transparent to visible light.
The process, published in the journal Science, is based on topical application of a food-safe dye and was reversible in tests on animal subjects and may ultimately be applied to a wide range of medical diagnoses, from locating lesions to monitoring digestive disorders or identifying cancers.
“Looking to the future,” explains Guosong Hong, leader of the study, “this technology could make the veins more visible for blood drawsfacilitate laser tattoo removal or aid in the early detection and treatment of cancers. For example, certain therapies use lasers to eliminate cancerous and precancerous cells, but they are limited to areas close to the surface of the skin. This technique could improve light penetration.”
To master the new technique, the researchers developed a way to predict how light interacts with stained biological tissuesHong’s team realized that if they wanted to make biological material (skin, veins, muscles, etc.) transparent, they had to find a way to match the different refractive indices so that light could travel through them without barriers.
Based on fundamental knowledge in the field of opticsresearchers realized that dyes that are most effective at absorbing light can also be very effective at directing light evenly across a wide range of refractive indices.
One dye that researchers predicted would be particularly effective was Tartrazinethe food coloring most commonly known as FD&C Yellow 5 (used in cola drinks, desserts, mustard-based sauces or French fries). When dissolved in water and absorbed into tissues, tartrazine molecules are perfectly structured to match refractive indices and prevent light from scattering, resulting in transparency.
The researchers first tested their predictions on thin slices of chicken breast. As concentrations of tartrazine increased, the refractive index of the fluid inside the muscle cells increased until it matched the refractive index of the muscle proteins: the slice became transparent.
They then gently rubbed a temporary solution of tartrazine into the mice. First, they applied the solution to the scalp, turning the skin transparent to reveal the blood vessels that run through the brainThey then applied the solution to the abdomen, which faded within minutes to show intestinal contractions and movements triggered by heartbeats and breathing.
The technique showed micron-scale features and even improved microscopic observations. When the dye was rinsed out, the fabrics quickly returned to their normal shade. Tartrazine did not appear to have any long-term effects and any excess was excreted in the waste within 48 hours.
Researchers suspect that the injection of the dye should lead to even deeper insights into organismswith implications for both biology and medicine.
With methods based in the fYofundamental physics, the researchers hope their approach will launch a new field of study that combines dyes with biological tissueseitherbasic gicostogoing into properties eitheroptics, which could potentially lead toto to a wide range of applications mandtips.
“As an optics expert, I’m amazed at how much they’ve been able to exploit with this study,” concludes Adam Wax, program director at the National Science Foundation, which supported Hong’s work. “Every optics student knows about refraction, but this team used the equations to figure out how a highly absorbent dye can make skin transparent. This has given us a new insight into how to make skin transparent.” a step forward in a bold new directiona great example of how fundamental knowledge of optics can be used to create new technologies, even in biomedicine.”