Novel controlled releasing polythiourea microcapsules were prepared from terephthaloyl diisothiocyanate and o-phenylenediamine by interfacial polycondensation.
Each sheet contains millions of microcapsules, about as wide as a human hair, filled with negatively-charged white pigments and positively-charged black pigments.
Well, you could also put heat-sensitive dyes inside of microcapsules and you could make different tattoo inks that change color at different temperatures.