Question about writing on PDMS

Greetings,
This may seem like a silly question, but for our application we are using
polydimethylsiloxane (PDMS) for microcontact printing of cells. We
are busily generating a large quantity of PDMS stamps with various patterns and
we could like to keep them straight by marking them
appropriately, so a user can readily tell what's on it.
Unfortunately, etching it with a razor is time-consuming and most markers don't
seem to work very well. We have found that ultra-fine
Sharpie marker works the best, but since we store the stamps flat (marked) side
down in a petri dish, the ink gradually comes off onto the
dish.
Does anyone know of a means to mark PDMS permanently?
TIA,
-Mark