“Shabbat Pen”

Good week
In the army, it is customary to use a “Shabbat pen,” I assume you are familiar with it. The ink disappears.
I wanted to look into the possibility of using this at home. Could I ask for your opinion on some aspects of the issue?
First of all, do you think this is forbidden by Torah or rabbinic law? Can you detail the sources of the prohibition, either way?

If it is a rabbi – what can justify it anyway? For example, can a very clear economic need, or an acute emotional need, justify it, even if it is not really a matter of saving one’s life?
If at all, of course