Clockwerk replaced his organic heart with a metal one to prevent himself from aging. It is unknown, however, what flowed through this heart and the rest of his circulatory system that replaced his organic blood.
Sly 2: Band of Thieves
Rajan utilized half of the heart to accelerate his spice Operation by at least ten times more than normal production, making spices grow at much faster rates than normal. He equipped the other half of the heart to his staff and carried it around with him at all times. Using it, he was able to conjure lightning from the sky and manipulate it for an attack.
Both halves of the heart had been stolen from Rajan by the Cooper Gang. During Murray's time at the Contessa's rehabilitation prison, he kept the half he salvaged safe. He reported that it was hard to hide. The heart was later found by Jean Bison who in turn sold the Heart along with the other Clockwerk Parts to Arpeggio. After the destruction of the hate chip, the heart rusted into nothing, along with the rest of the Clockwerk parts.