[mybodyparts]

They are normally made from hard Silicone :

"As an implantable biomaterial it is most commonly used in the form of the dimethylsiloxane polymer which is a large molecule of repetitive units containing silicon, oxygen and methane, ((CH3)3SiO-[(CH3)2SiO]x-Si(CH3)3). The extent of polymerization determines the viscosity of dimethylsiloxane and hence its physical state: liquid, gel or solid. Short, linear chains of polymethylsiloxane result in oils or gels...

"...Heat and catalysts increase branch length and form solids. Silicone polymers are easily contaminated with heavy metals, low-chain-length polymers, and other impurities during the manufacturing process and their refinement requires specialized filtration and sterilization. Only Medical grade silicone is approved for implantation in the human body". Grand Rounds - Advanced Biomaterials

Although, many medical device companies are manufacturing newer types (both solid implants and injectables) which are made from expanded polytetrafluoro- ethylene (ePTFE), rigid polyethylene, and hydroxyapatite (processed coral). The rigid polyethylene implants can be screwed into place into the bones of the face to prevent shifting of the implant post-operatively. The (ePTFE) implants are designed to be porous so that the tissues can attach themselves to the implant to prevent shifting. There are Mersilene implants, carvable blocks, PMMA, MMA and more. Please refer to the Augmentation Options section to read of these materials. Discuss with your doctor which implants he or she prefers and why.