Tattoo ink is trapped in the deepest layer of the skin. When tattoos are applied, ink particles are inserted with a needle into varying depths of this bottom layer.
When tattoo removal laser light is applied to the skin, it heats up and shatters the ink that it reaches first within the skin. In other words, the shallowest layers of ink are removed before the deepest layers.
It takes multiple treatments to penetrate through all of the varying depths of ink in a tattoo. If a tattoo is particularly dark or bold, it will require more treatments to remove the ink compared to shaded tattoos with a lower density of ink.