As used in the context of the cosmetics and personal care products industry, a carbomer is any of a series of polymers of acrylic acid used as thickening agents and emulsion stabilizers. Carbomers are used in a variety of skin- and hair-care products, in dentifrices, and in pharmaceutical preparations. By themselves, they are fluffy white powders that readily absorb water to form gels or thick solutions that are non-toxic, stable, and resist spoilage.

Specifically, a carbomer is a homopolymer of acrylic acid with a high molecular weight, which is cross-linked with any of several polyalcohol allyl ethers. It's the polyalcohol portion that confers a carbomer with its high water solubility, unusual in polymers of this size. The name "carbomer" is applied to all of these polymers, the various types indicated by a numerical suffix such as Carbomer 940 or Carbomer 973, which indicates the average molecular weight of the polymer chains.

The consistency and texture of cosmetics and personal care products are of critical importance to the consumer, and different agents are routinely added to these products to control the tactile qualities. Natural plant gums or extracts have been used, but these are often less than ideal. They can vary in quality from batch to batch, may cause allergic reactions in sensitive users, and are often subject to bacterial attack and spoilage, requiring the addition of antibacterials.

Carbomer, on the other hand, is an excellent thickening agent, is consistent from batch to batch, will not support bacterial growth, and is hypo-allergenic. It also has a particularly nice "skin feel," producing solutions and gels that feel rich and luxurious to the touch. As an emulsion stabilizer, carbomers keep oils or creams suspended in water and prevent separation.

The average carbomer molecule is large and highly cross-linked. It therefore forms net-like structures in water capable of supporting fine, insoluble particles better than biologically-derived gels, and at lower concentrations. It's therefore used as a suspension agent when fine solids are to be dispersed in creams or lotions.

Carbomer is listed in the United states Pharmacopeia, and Carbomer 934P/974 is approved for use in oral formulations and ophthalmic solutions, an indication of its low toxicity. Carbomers are not absorbed to any appreciable extent by the skin, and therefore make a good medium for transdermal drug delivery. They are also not absorbed by the body if ingested. Aqueous solutions of carbomer are mildly acidic, however, with a pH of about 3, and the acidity of products containing carbomers is usually adjusted with a base such as triethanolamine, or sodium EDTA, to bring it up to a more neutral value.