US Patent 7513437 - CNT security mark for gaming card

http://www.freepatentsonline.com/7513437.html

In order to prevent cheating in casinos it is useful to mark card decks with a security mark that is transparent to the players but is easily verifiable. This patent teaches providing such security marks using a carbon nanotube coating to modify the resistance level of gaming cards. Claim 1 reads:

1. A gaming card having a security feature incorporated therein, said gaming card comprising:

a gaming card; and

a conductive material comprising carbon nanotubes located on said gaming card, said conductive material being formed as a patterned layer comprising identifying indicia;

wherein said patterned layer of identifying indicia is contactable by and readable by a conductivity meter to provide a resistance value of at least a portion of said identifying indicia to impart a security feature to said gaming card.