The function of copper ions in copper releasing IUD's is

$C{u}^{2+}$ interfere in the sperm movement, hence suppressing the sperm motility and fertilizing capacity of sperms. The copper T is a simple intra-uterine device (IUD) made of a flexible, "T" shaped piece of plastic wrapped with a thin copper-containing wire. This shape is chosen as it fits the area around the uterus, allowing for the copper T to sit in place for years without moving around.

Example: CuT, Multiload 375, Cu7, etc.