Thanks to Charlotte Lin
https://www.nature.com/articles/d41586-026-03210-z
A group of nanoscientists has developed tattoo inks that become visible only under ultraviolet light, and can be hidden again using bright visible light. The inks are made from tiny spheres of acrylic glass that contain photochromes — compounds that undergo a reversible colour change on exposure to a specific wavelength of light. The products could be used in medical tattooing, such as markers to direct radiotherapy to specific parts of the body.