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INTRODUCTION

Activities:

Occlusive, emollient, skin protectant, antifoaming agent

Important Chemical Components:

The chemical formula for dimethicone (also known as polydimethylsiloxane or PDMS) is CH3[Si(CH3)2O]n Si(CH3)3 where n is the number of repeating monomer units [molecular formula = (C2H6OSi)n]. Silicon dioxide (silica) is the base material from which silicones are derived. Silicon is element number 14 on the atomic chart (see Figure 18-1).

Origin Classification:

Laboratory made from natural ingredients

Personal Care Category:

Hair conditioner, moisturizer

Recommended for the following Baumann Skin Types:

All 16 skin types can use these ingredients.

FIGURE 18-1

Chemical structure of dimethicone.

SOURCE

Dimethicone is derived from silicon. It is a hypoallergenic, noncomedogenic, and nonacnegenic silicone-based polymer second only to petrolatum in terms of frequency of use as an ingredient in moisturizers.1 Silicones are derived from silica (silicon dioxide), found in sandstone, beach sand, granite, and quartz, and represent the source of all oil-free moisturizers.2

HISTORY

Silicon was discovered by the Swedish chemist Jöns Jacob Berzelius in 1824. Today it is produced by heating sand (SiO2) with carbon to temperatures approaching 2200°C. For this reason it is difficult to categorize silicones as natural or laboratory made. Dimethicone was suggested as a protective ingredient for the skin barrier as early as the late 1950s, and is covered in the United States Food and Drug Administration (USFDA) monograph for skin protectants.3–5

CHEMISTRY

According to Nair, “Dimethicone is a fluid mixture of fully methylated linear siloxane polymers end-blocked with trimethylsiloxy units.”6 The potent siloxane bonds that unite silica and oxygen in these compounds account for the noted thermal and oxidizing stability of silicones.7 The silicone used in topical skin care products is odorless, colorless, as well as nontoxic and is ideal for water-resistant formulations because it is also immiscible and insoluble in water.2

ORAL USES

Dimethicone and silicone products are found in some foods and are not considered to be toxic when taken orally. However, they have no oral activity that would justify their use for skin conditions.6

TOPICAL USES

Dimethicone and silicones exert a protective effect on the skin by forming a barrier that hinders the penetration of irritants and allergens. They also decrease transepidermal water loss (TEWL), but not as effectively as petrolatum (Table 18-1). Whereas petrolatum lowers TEWL by 98 percent, dimethicone and other silicones typically reduce TEWL by 20 to 30 percent.8–10

TABLE 18-1Pros and Cons of Dimethicone and Silicones

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