From the moment a needle deposits ink deep in the skin, the immune system recognizes the these particles as foreign intruders, dispatching armies of white blood cells to engulf them. The white blood cells then escort small ink particles to the liver, where they are processed and excreted. Tattoo removal lasers, also called Q-switch lasers or ultra-short pulse lasers, are extremely hot, operate in a very narrow frequency, and are very, very fast. The tattoo removal laser in the video, called the PicoLaser, works on a scale of picoseconds, or a trillionth of a second. This speed and heat is crucial to cracking the ink particles apart. To break up an ink particle, you need to heat it to make it expand due to thermal expansion, but the zap has to be quick enough so that half of the particle remains cool. The opposing cool and hot forces then rip the ink particle apart. This process, called photothermolysis, is also used in laser hair removal. Once the lasers break the ink particles apart into bite-sized chunks, the white blood cells can absorb them for transportation to the liver. Now this may sound relatively easy, but the expensive process of tattoo removal often takes several rounds of treatments and can leave scarring. Darker pigments are eliminated more easily while lighter and more reflective inks are less responsive to the lasers.