Methicone is a type of linear silicone. Silicones are synthetic polymers with a backbone composed of repeating units of siloxane (elemental silicon and oxygen). Methicone has a simple molecular structure compared to other larger, more complex silicones. Even though methicone has a simple structure, it still has a high molecular weight that makes this ingredient too large to penetrate through the surface of skin.
In cosmetics and personal care products, methicone functions as a skin-conditioning agent and film-forming agent. Upon topical application, methicone forms a film on the surface of skin that is extremely hydrophobic, which means it repels water. This film prevents water from evaporating from the skin, which helps to keep skin moisturized. Furthermore, the film improves the staying power of makeup because it prevents sweat and oils from coming through underneath which would normally cause makeup to wear off.
While there is controversy about the safety of topical silicones, scientific studies have proven that silicones, including methicone, are safe for topical use. These ingredients cannot cause or worsen diseases or cause allergic reactions because their size prevents them from being absorbed by the skin.