They created different motifs with this ink ranging from a lantern that reveals its light when activated, to multicoloured scales like those in the much-loved children\u2019s book The Rainbow Fish. <\/p>\n
The pigments used in these inks are so-called photochromic dyes<\/a> (from the Greek \u201cphotos\u201d, which means \u201clight\u201d and \u201cchroma\u201d, which means \u201ccolour\u201d). In other words, they are compounds that change colour when exposed to light. These dyes have been used for years in colour-changing sunglasses. <\/p>\n Light carries energy, and photochromic compounds are designed in such a way that this energy is enough to cause a chemical change that alters the shape and therefore the properties of the molecule. <\/p>\n In the case of the \u201cinvisible\u201d tattoo ink, two or three seconds of bright ultraviolet (UV) light is enough to form a new ring in the dye molecule, which switches a colourless pigment to a coloured one. A variety of colours are available depending on the exact structure of the molecule. <\/p>\n A blast of bright white light reverses the reaction, breaking the ring and turning the colour off again. For a long time, the challenge with this chemistry was that the coloured form of the dye was relatively unstable and quickly reverted to the more stable, colourless form \u2013 especially at higher temperatures. <\/p>\n Another issue was fatigue: after a certain number of on-off cycles, the dyes degraded and took on a permanent coloured hue. Have you ever bought transition lens sunglasses and noticed that they became less effective over time? This is due to the same deterioration of photochromic compounds in the lenses. But thanks to recent research efforts, remarkably stable dyes have been designed that reliably hold their colour for months and can undergo over 10,000 activation and deactivation cycles without degrading. <\/p>\n Carson Bruns\u2019 team at the University of Colorado Boulder showed that these pigments could be formulated into tattoo inks by encapsulating them in poly(methyl methacrylate) (PMMA)<\/a>, a polymer approved for medical use in bone cement, dentures and intraocular lenses. This family of inks is known as PhotoTat. <\/p>\n So, when will you be able to get one of these switchable tattoos? A red pigment was already launched in 2024 under the name Magic Ink by Colorado-based biotech company Hyprskn<\/a>. A 2 millilitre (ml) vial will set you back \u00a377, which is about 100 times more expensive than regular tattoo ink due to the specialised dyes and formulation process required. The ink is fully compliant with the EU\u2019s Reach (Registration, Evaluation, Authorisation and Restriction of Chemicals)<\/a> safety regulations. <\/p>\n Now, Bruns\u2019 research team has developed three more PhotoTat inks \u2013 blue, magenta and yellow \u2013 that can combine into 16 other colours. \u201cSo far,\u201d Bruns says<\/a>, \u201cthe only person who\u2019s tested this ink is me. I actually have hundreds of these tattoos on my legs now and they\u2019ve all gone really well.\u201d <\/p>\n Some challenges remain though: direct sunlight contains enough UV radiation to activate the tattoos unless they are protected by sunscreen. Indoor light will slowly switch them off over a period of several days. <\/p>\n![\"PhotoTat\"](\"https:\/\/images.theconversation.com\/files\/766853\/original\/file-20261017-50-10v9na.jpg?ixlib=rb-4.1.1&q=45&auto=format&w=754&fit=clip\")
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\n Matter & Light, Bruns et al.<\/a>, CC BY-SA<\/a><\/span>
\n <\/figcaption><\/figure>\nTime to market<\/h2>\n
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