living ink
Time-lapse ink
Living Ink from Fort Collins, Colorado,has developed inks made from algae. The inks disappear and reappear when exposed to sunlight, allowing for designs with a touch of magic.
Living Ink uses living algae cells and cyanobacteria for its Living Ink Pen. The cells are diluted in the liquid and can’t be seen by the human eye. What the ink is exposed to sunlight, the cells multiply and become so dense that they become visible to the human eye and the writing or drawing reveals itself. Even after the algae cells have died, the drawings remain intact.

More information: DRUPA