While looking for a way to create a more eco-friendly printer ink using algae, the folks at Living Ink Technologies stumbled upon invisible ink that grows after a few days exposure to sunlight.
They are now marketing pens filled with this algae-based ink on Kickstarter that use photosynthesis to let the drawing appear. These Living Ink pens contain cyanobacteria, algae, and chlorophyll in small amounts invisible to the naked eye. When exposed to sunlight, the algae and bacteria reproduce quickly, growing in number to the point where they finally appear green.
Don’t fret about not being able to see your crazy drawing and wondering if your lines are even straight, the Living Ink material contains disappearing ink so you can see your design. There are two magic pens in the set, a pink one that contains fast ink which then grows in the first one or two days, and a blue pen containing slow ink that will appear a few days later, allowing you to create a two-stage secret reveal over the course of about a week.
After you develop your art, you just stick in in the special greenhouse containing nutrient-rich agar to help it grow, plop it in the base and put it in the sun.