While scientists unfortunately resorted to tattooing a green pattern on the tails of several defenseless mice (a cruel practice that's still very present in the beauty industry today ) for the study, doing so did lend a few interesting conclusions. First, despite various, non-lethal efforts to kill off the cells that contained the tattoo, the macrophages worked so fast to engulf the color that the ink remained in tact. Second, this likely proves why laser tattoo removal take several sessions to complete. Since lasers break down ink particles , but don't target the macrophages, every time you zap, there's a string of new cells just waiting to swoop in and trap the pigment yet again. The equation for success then becomes, researchers hypothesized, to target both elements.