The skin on scars (marks left on your skin after injury) and stretch marks (small tears in the skin) doesn't rejuvenate at the same rate as clear skin.
Our studies on scar tissue lead us to not recommend application on scars or stretch marks, as the ink could get trapped in the skin tissue in these areas.
As well, scar tissue tends to be thicker. Because our ink needs to seep into the first layer of skin to develop, thicker skin can act as a barrier, preventing proper development.
If you do apply over a scar and ink becomes trapped in the scar tissue, the ink is biodegradable, so it should fade over time.