When the tattoo artist makes a tattoo, he has to introduce the ink into the dermis (just after the basal layer), since, if they only did it in the epidermis, the drawing would not last in time (due to the renewal of the skin, deterioration of the sun, external agents, etc.) Once introduced into the skin, although our immune system sends a troop of white blood cells, to heal the “wound” (which our body interprets just had) the ink molecules are too large to be removed in its entirety, so the drawing remains intact.
With the laser and its photoacoustic energy these pigment molecules are minimized that our body is not able to eliminate by itself, by destroying them and making them much smaller, we can eliminate them through our lymphatic and vascular system.