How does tattoo ink work?

Kat Arney put this to fellow Naked Scientist, Chris Smith...

Chris - When you do tattooing, you're putting in big molecules of a dye or ink.

It goes in under the top layer of the skin, because the surface layers of the skin are continuously being replaced. You have very high cell turnover in the skin. Your stem cells that make skin slowly grow upwards through the skin and it takes about a month from the time the stem cell produces a new skin cell for that skin cell to work its way up to the top then get worn off, fall off, and it's replaced by one from below.

The tattoo is going in just in under that layer. The ink is being taken up by cells in the skin, probably macrophages and long lived cells, which eat the dye molecules and it stains the cells. As a result, you end up with that pigment sitting there.

But that's why tattoos blur as you get older because, slowly, the dye gets carted off to your lymph nodes; it also breaks apart and spreads out a bit more as some of those long live cells die. But you're basically staining the skin!