Darien Fawkes' Tattoo - On the sci-fi series THE INVISIBLE MAN/SYN/2000-2002, Darien Fawkes (Vincent Ventresca), a thief and conman, offered his body for experimentation to elude a life sentence in prison. The project? Implant an artificial "quicksilver" gland into his head to turn him invisible. When the artificial gland releases a quicksilver solution into Darien's body, it causes light to pass through him. Unfortunately, the quicksilver gland has a large side effect. After an exposure to the quicksilver, the brain's higher functions degrade and the host's brain reverts to a primal state of savagery filled with dark and menacing impulses. If Darien is not kept in check with an antidote, he will go ballistic and kill.

To keep his sanity, Darien agrees to help the government out on various secret assignments in exchange for the serum that keeps him from going bonkers. An attractive blond scientist known as The Keeper administers the precious counteragent to Darien.

To alert Darien when he needs a counteragent, The Keeper placed a small chemically sensitive tattoo on his wrist (a green snake biting its tail) to let Darien know how long he can go before his next fix. Before the colors on the tattoo completely fade, Darien must be injected with the antidote to stay sane.   

 
Antidote almost gone