![\"\"](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2024\/09\/05115104\/SEI_220041122.jpg\")
<\/div>The yellow food coloring tartrazine changes the speed at which light travels through tissues<\/p>\n
University of Texas at Dallas<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
Massaging a common food dye onto the skin<\/a> of living mice turned their tissues transparent, allowing us to see their blood vessels and organs at work. The technique could one day help doctors look deeper into our bodies to diagnose conditions.<\/p>\n Monitoring the internal environment of a living animal isn\u2019t easy. When it is dead, we can take slices of its tissues or use chemicals to remove proteins and fats from them to get a better view. With live animals, some things can be viewed via scans and endoscopies, but to monitor living tissue, it often has to be cut out.<\/p>\n Now, Zihao Ou<\/a> at the University of Texas at Dallas and his colleagues have turned tissues transparent in living mice by rubbing the food dye tartrazine, also known as E102 or Yellow 5, onto their skin. When the skin absorbs the dye molecules, this modifies the tissue\u2019s refractive index \u2013 the speed at which light travels through it.<\/p>\n The dye made organs visible in living mice<\/p>\n Zihao Ou et al. 2024<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n The mice then became transparent, which enabled the researchers to see peristalsis, the muscle contractions that move food<\/a> through the digestive tract, and to look into the blood vessels on the surface of their brains.<\/p>\n <\/p>\n To understand how the technique works, think about fizzy water, says Ou. Light<\/a> shining through the fluid changes direction every time it goes from water to a gas bubble or vice versa, he says. This means light scatters in all directions and can\u2019t penetrate the fluid as easily as it can in just air or water that isn\u2019t fizzy. Biological tissue behaves similarly because it contains a lot of water, but also other molecules like lipids and proteins, which usually have a higher refractive index than the water.<\/p>\n Adding the dye changes the refractive index of the water inside tissues to more closely match that of the lipids and other molecules, reducing how much the light scatters. \u201cThat means you can see deeper, you can probe deeper,\u201d says Ou.<\/p>\n The dye can be washed off and didn\u2019t seem to harm the mice.<\/p>\n The work strikes at the heart of one of the biggest problems in microscopy, says Christopher Rowlands<\/a> at Imperial College London. \u201cIf you wanted to see anything more than a millimetre or so from the surface of the tissue, forget it, that wasn\u2019t happening \u2013 and now suddenly that\u2019s a possibility,\u201d he says. \u201cYou\u2019re seeing through a centimetre, whereas you could previously see a millimetre. That centimetre makes a difference for a lot of applications.\u201d<\/p>\n Tartrazine could potentially be toxic if a lot of it is put on skin, says Rowlands, but neurobiologists routinely stick probes and lenses into the brain<\/a> or remove chunks of the cortex. Using a dye on skin that is widely accepted as safe for consumption would probably still be less harmful, he says.<\/p>\n But although the technique makes skin more transparent, it won\u2019t give doctors a totally clear of view of a person\u2019s insides. \u201cIt\u2019s not going to be Harry Potter\u2019s invisibility cloak,\u201d says Rowlands. \u201cIt\u2019ll be a thing where it looks like the skin is more glassy than it should be.\u201d Even if the effect happened throughout the body, you would still be seeing bones and specialised structures called organelles inside cells, he says.<\/p>\n Topics:<\/p>\n<\/section><\/div>\n![\"\"](\"https:\/\/images.newscientist.com\/wp-content\/uploads\/2024\/09\/05115345\/SEI_220063387.jpg\")
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