Most of today's disc implants in use are made of metal alloys and plastic. Titanium is the metal most highly favored so far. Nickel can't be used because so many people have a nickel sensitivity.
Plastic is becoming more popular because it wears well and is less stiff than metal. It also eliminates the problem of wear debris seen with metal implants. Bits of metal get shaved off the implant with daily movement. Metal debris can cause pain from inflammation and implant loosening.
Researchers have found that what works well in one area of the body doesn't always hold up elsewhere. For example, one type of plastic used in hip replacements wears extremely well but doesn't hold up in the knee. It isn't used in the spine either.
Find out what kind of implant your doctor is using. Ask him or her this question, too. A combination of titanium and plastic might work best in your situation.