Dimethicone’s ‘Feel Good’ Role In Cosmetic Formulations

Skin care products must be agreeable to apply and chemically inert. That’s because both the hedonistic and the safety aspects are of importance to consumers and, of course, regulators. That is another reason why chemistry, physical-chemistry, biology, biochemistry and pharmacology are relevant sciences for the personal care industry.

The epidermis contains specialized cells (Merkel cells) able to detect a shear force when it is applied, as in the case of the topical application of a skin care product. Merkel cells can tell when the applied product is water-like or molasses-like or something in between. Experienced chemists know that a topical product can be maintained viscous (to avoid it dripping away) and yet made agreeable to apply. As a matter of fact, they do know that by lowering the surface tension, the force needed to spread a drop of product on the epidermis will be smaller and the strain perceived by Merkel cells will be tinier.

Polymers in general are the result of the reaction of monomers and are considered to be inert… as long as there are no specialized enzymes or other catalyzers to trigger their hydrolysis, but this is only rarely the case.

On the other hand, we know of amorphous substances that are totally inert, such as glass. It is made of SiO2 and it is so chemically inert that glassware objects more than 7,000 years old are found nowadays in archaeological searches. Unfortunately for skin care products, at ordinary temperatures glass is quite difficult to spread on the skin!

What we are looking for, then, is a material as inert as glass but flexible and malleable and dispersible and able to lower the surface tension of oil and water mixtures.

Silicones

Since the 1930s, scientists tried and synthesized silicon-containing molecules to obtain inert materials with low thermal conductivity for industrial uses. The name “silicones” was coined in the 1940s by the English scientist Frederic Stanley Kipping because they were “sticky messes” and because he did not believe that those materials might even have a practical use. Obviously, Kipping was wrong. Scientists succeeded in synthesizing useful silicones, starting in the 1950s, when a toy containing a viscoelastic liquid silicone was put on the market.

This toy, Silly Putty, could bounce but could also be broken by a sharp blow, and could flow like a very viscous liquid. Silly Putty was the result of research undertaken during World War II to find substitutes for rubber, when latex-producing trees in Southeast Asia where under the control of the enemy.

A quarter of a century later, in 1969, Astronaut Neil Armstrong walked on the moon in silicone-rubber soled boots.

The Benefits

Resistance to extreme temperatures, low chemical connectivity, remarkable electrical insulation, adhesive properties—the stuff that makes Post-It notes sticky and the like—are the characteristics of silicones that make them fit for many industrial applications.

Silicones found their way into personal care formulations in 1953, when Revlon put on the market a hand cream called Silicare. The formula was said to form an invisible glove to protect one’s hands.

In the 1980s, silicones were added to antiperspirant products and in the 1990s they were of general use in the personal care market.

Dimethicone

Also called polydimethylsiloxane (PDMS) or dimethylpolysiloxane, dimethicone is a polymer of variable length formed by n repeating monomeric units of [Si(CH3)2O-] capped by two [Si(CH3)3] groups at each one of the extremities, as shown below.

In 2019 the FDA recorded 12,934 dimethicone-containing products. These formulas include moisturizers, shampoos, conditioners, cleansers, bath and baby oils, and self-tanning products. Dimethicone provides an incredibly agreeable velvety touch that adds to the esthetic properties of the products.

Despite such widespread use, Dimethicone gets a bad rap. Detractors call it “non-natural” and its occlusive properties might make it comedogenic and prone to trapping microorganisms that could be harmful to the skin. Since two brands affected by a safety-first paranoia such as Clinique and L’Oréal use dimethicone in their products, I would not worry about the gossips found on the internet.

On the other hand, dimethicone can cause headaches for formulators. The results of the calculation of the theoretical value of its partition coefficient depends on the software used (Osiris versus Molinspiration, for instance) so that the real value of its Log Kow cannot be calculated with accuracy.

This might lead one to formulate dimethicone with actives that have the same partition coefficient and therefore remain associated to it instead of being free to cross the stratum corneum, thereby frustrating the hopes of obtaining products with the expected activity.

Another problem linked to the use of dimethicone is the greasy residue left after topical application.

This greasy feeling may be welcome by female users, particularly those with dry skin, but is dissuasive for male customers who secrete more sebum and sweat than females.

Conclusion

The introduction of dimethicone and other silicones in skin care products is one of the four revolutionary steps that, in the last decade of the twentieth century, led to major changes in the strategy of formulating and marketing skin care products. What are the other three?

• The use of liposomes,
• The formulation of salicylic acid in creams; and
• The introduction of sunscreens in day creams.

As usual, when it comes to successful strategies in cosmetics, it can be expected that the usual “professionals of fear” will invent some danger to create panic and then acquire glory and money by solving a non-existing problem. Such professionals have already manifested their presence by “blahblahing” about the dangers represented by preservatives, exfoliation and sunscreens to name a few.

In these days of general concern about the environment I would not be surprised to learn that “professionals of fear” speak against the cosmetic use of silicones because, being inert, they are not biodegradable and will accumulate in the environment! And yes, that might add up to a problem, but not because of cosmetic products!

Let’s check the data. According to a 2021 EU report, 4.7 tons of “cosmetic” silicones enter surface water every year: this is the equivalent of five bathtubs and contrasts with the 10 million tons of methylsiloxanes that are produced every year for industrial purposes.

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Paolo Giacomoni, PhD
Insight Analysis Consulting
paologiac@gmail.com
516-769-6904
 
Paolo Giacomoni acts as an independent consultant to the skin care industry. He served as executive director of research at Estée Lauder and was head of the department of biology with L’Oréal. He has built a record of achievements through research on DNA damage and metabolic impairment induced by UV radiation as well as on the positive effects of vitamins and antioxidants. He has authored more than 100 peer-reviewed publications and has more than 20 patents.