Implanted materials must be biocompatible and "get along with" your internal chemistry — it's not like you can easily take out an, so you absolutely need to be sure that the material that the implant is made of is safe for long-term implantation.
The good news is that the medical industry has done an enormous amount of research for us into this subject and we know that we have a number of options; the most common ones include metals such as implant grade steels, Titanium and titanium/aluminum/Niobium alloys, as well as polymers such as Teflon and Silicone. An important thing to note is that it is often not entirely legal to buy (let alone implant) proper medical grade plastics for non-doctors — that's not to say that they aren't available through a multitude of gray market suppliers (or suppliers who don't ask questions, or supply raw materials), but do be aware that there are no guarantees that high quality materials are being used on you, and there is no reasonable way for a customer to tell the difference between even low grade industrial polymers and higher grades.
Ensuring that the materials being used on you are of a suitable grade can minimize this risk. Most artists should have no problem providing you with some documentation as to the makeup of the material being used.