There are expensive removal services and coin-machine temporaries, but for the most part, a tattoo is forever. A Harlem-based startup is trying to change that by developing a tattoo ink that lasts roughly a year. The ink works by reverse-engineering the system that makes most tattoos permanent: when you puncture the skin with a tattoo needle, white blood cells rush in and try to absorb any invading particles. Because particles of ink are too big for the cells to absorb, they stay put. Ephemeral has created smaller ink molecules encapsulated inside bigger spherical structures, so they stay in your skin the same way regular tattoo ink does. But once you use the separate removal solution that's designed to break down that structure, the body's defenses flush away the ink and the tattoo disappears.