The print process uses something called "Thermochromic" inks, and they're coming down in price for print cost. Specifically they're "leucodye thermochromics".
Traditionally the inks turn clear when touched, though they can be bound with a permanent ink. We did a job here at work where we bound a blue leucodye with a red/magenta permanent ink. When it was cool, it was a uniform field of purple. When touched, the blue disappeared, leaving a field of magenta where it had been. We used purple copy ontop of the red, so messages appeared. This was done for a limited access event where the client wanted an "impossible to counterfeit" method of entry.
Cost per unit was under $8, if I recall, for a rather short print run.
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Traditionally the inks turn clear when touched, though they can be bound with a permanent ink. We did a job here at work where we bound a blue leucodye with a red/magenta permanent ink. When it was cool, it was a uniform field of purple. When touched, the blue disappeared, leaving a field of magenta where it had been. We used purple copy ontop of the red, so messages appeared. This was done for a limited access event where the client wanted an "impossible to counterfeit" method of entry.
Cost per unit was under $8, if I recall, for a rather short print run.