Polymethylsilsesquioxane is a type of silicone that is used in cosmetics and personal care products as a film-forming agent, skin-conditioning agent, and anti-aging ingredient.
Polymethylsilsesquioxane is a polymer formed by the hydrolysis and condensation of the silicone methyltrimethoxysilane. Silicones are synthetic polymers with a backbone composed of repeating units of siloxane (elemental silicon and oxygen), which is why silicones may also be referred to as polysiloxanes. Often, the terms “silicone” and “silicon” are mistakenly used interchangeably, when they are actually quite different. Silicon is the 14th element on the periodic table and the second most abundant element in the earth’s crust, after oxygen. In contrast, silicones are always synthetically produced.
Specifically, polymethylsilsesquioxane is a spherical fine particle resin that is composed of very small particles (typical particle size 4 to 6 microns). The small particle size allows polymethylsilsesquioxane to be easily distributed in formulas.
As a class, silicones improve the feel, appearance, and performance of cosmetic products. These ingredients act as silky moisturizers, conditioners, solvents, and delivery agents for other skin care ingredients. Silicones are able to help with skin redness and irritation due to their low surface tension, which enables them to spread easily across the surface of skin and form a protective covering.
Polymethylsilsesquioxane functions as a film-forming agent by forming an invisible film on the surface of skin. The film creates a lubricating, smooth, silky feel that does not feel greasy or sticky. It still allows oxygen, nitrogen, and other important nutrients to pass through, but blocks water from evaporating. Ultimately, this leads to skin that looks and feels hydrated and smooth. Additionally, the film formed by polymethylsilsesquioxane has excellent water-repellency. This property is useful for the formulation of sun protection products.
Polymethylsilsesquioxane is also frequently used in anti-aging skin care products. As mentioned above, this ingredient helps skin to retain moisture better. This is important because one of the major factors that contributes to aging skin is loss of moisture. By keeping the skin moisturized, fine lines and wrinkles are less likely to form. Another reason to use polymethylsilsesquioxane in anti-aging products is due to its ability to temporarily fill in fine lines and wrinkles, giving the skin a more plump appearance.
The safety of polymethylsilsesquioxane and its derivatives has been assessed by the Cosmetic Ingredient Review (CIR) Expert Panel. The Panel evaluated the scientific data and concluded that polymethylsilsesquioxane and its derivatives are safe for use in cosmetics and personal care products.
Even though polymethylsilsesquioxane and other silicones have been proven both safe and effective for cosmetic purposes, a multitude of rumors exist claiming they are unsafe for topical use. For instance, there are claims that silicones as a class can cause or worsen skin concerns, cause sensitization, and “suffocate” skin. However, these claims have not been proven in any published research.
To dispel these false claims, it’s first important to understand that the large molecular size of silicones prevents them from being absorbed by the skin. If a substance cannot penetrate the skin, it cannot react with cells of the immune system. Thus, silicones are not allergens. Additionally, claims that silicones can bioaccumulate (build up) in our bodies are also false. Once again, this is because their large size prevents them from penetrating the skin. If silicones cannot penetrate the skin, they certainly cannot pass through cell membranes, a key requirement for bioaccumulation.
Lastly, it is incorrect to say that the film formed by silicones somehow “suffocates” the skin. As mentioned above, the soft, invisible film that polymethylsilsesquioxane and other silicones form after topical application allows oxygen and other gases to pass through. Furthermore, the film does not block the natural skin secretion. Thus, the skin can still “breathe” after topical application of a product containing silicones.
References: Truth In Aging, “Polymethylsilsesquioxane”, Making Cosmetics, “Polymethylsilsesquioxane”, Cosmetic Ingredient Review, “Safety Assessment of Polysilsesquioxanes as Used in Cosmetics”, 2017.