Of course, this is not a common tattoo in the simple sense of the word.
Biotechnologists have specifically designed human cells, which, in fact, constitute a "tattoo".
These cells contain a calcium-sensitive receptor.
The same cells are "packaged" in a system that interferes with the skin.
The system can monitor the level of calcium in the blood.
If the level of calcium in the blood exceeds the threshold value and remains at this level for a long time, a signal is activated that leads to the production of melanin in genetically modified cells.
That is, when the cells "feel" the increase in calcium level, appear on the skin in the form of a dark spot, similar to a birthmark.