What it is
Trimethylsiloxysilicate is a three-dimensional silicone resin recognized by the EU CosIng database as a reaction product of silica with trimethylsilane. In a finished product it dries down into a permeable, non-tacky film over the skin, which is why it is used to make makeup and sunscreen formulas more long-lasting. It is typically dissolved in a volatile silicone or other carrier so it spreads easily and then leaves the resin behind as the carrier evaporates.
Safety & warnings
Trimethylsiloxysilicate is a large, inert silicone resin that sits on the surface of the skin rather than penetrating, and cosmetic references describe it as forming a non-tacky dried film well tolerated in leave-on products. Because it is film-forming, people who prefer easily removable products or who find silicone resins difficult to cleanse may wish to use a thorough cleanser.
Origin & sourcing
It is a wholly synthetic silicone material. According to the EU CosIng cosmetic ingredient database, trimethylsiloxysilicate is produced as the reaction product of silica with trimethylsilane, building a branched siloxane resin from silicon-oxygen units capped with trimethylsilyl groups.
Questions everyone asks
What does trimethylsiloxysilicate do in a sunscreen?
It is a silicone resin that dries into a thin film on the skin, helping the formula feel smooth and stay in place longer rather than feeling tacky.
Is trimethylsiloxysilicate natural or synthetic?
It is synthetic. CosIng describes it as the reaction product of silica with trimethylsilane, forming a branched silicone resin.
Does it leave a sticky finish?
No. Cosmetic references note it leaves a non-tacky dried film, which is part of why it is used in long-wear formulas.