silicone, polymer in which atoms of silicon and oxygen alternate in a chain; various organic radicals, such as the methyl group, CH3, are bound to the silicon atoms. Silicones, which are unusually stable at extreme temperatures (both high and low), may occur as liquids, rubbers, resins, or greases. Silicones are prepared from halides of organic silicon compounds by decomposition. Such compounds are chosen and used in mixtures that allow the desired molecular weight and degree of cross-linking to be obtained in the final polymer. Water repellent, chemically inert, and stable at extreme temperatures, silicones are used as protective coatings and electrical insulators and in caulk.

Implants consisting of silicone gel surrounded by hard silicone were used in reconstructive and cosmetic breast surgery until 1992. The safety of inserting silicone prostheses into the body was questioned in a large product-liability case involving breast implants, but an Institute of Medicine panel concluded in 1999 that there was no evidence linking such implants with cancer, autoimmune diseases, and other serious illnesses. The use of such silicone implants for reconstructive surgery and for cosmetic surgery has been again approved by the FDA since 1998 and 2006 respectively.

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