I like them too, but they get empty quickly. Only a fraction of the cartridge is ink. The majority is plug gel and pressurised air. Apparently that’s required for it to function properly.

The ink from Frixxion pens disappears with time. I found out the hard way when I went to re-read my MLaw notes a few years after writing them.

I've heard that they sometimes reappear when frozen, though not sure if that applies to natural discolorations as well.

the erasing part is what frightens me ; putting your document near a heating device erases your writings (the heat produced by the friction of the eraser is what remove the ink when you do it manually)

The ink gets its color back if you put the paper in the freezer