In-vitro Evaluation of Silicones in Skin Care Applications and Effects on Skin Moisture

Abstract

Silicones not only provide a silky feel in cosmetic products; they also enable water permeability due to Si-O bonds; moreover they help prevent water loss from skin. In this study, the effects of creams containing silicones with different viscosities on skin were analyzed using corneometry and transepidermal water loss measurements. The effects of high viscosity silicones on skin moisturization were observed at maximum levels.

Introduction

Silicones define a large chemical group consisting of silicon-based materials. They have a polymeric chemical structure; Si-O bonds are bonded to metal groups in the backbone. Their general chemical structure is given in Figure 1.

Figure 1. Chemical structure of Polydimethylsiloxane (dimethicone)

Silicones may vary due to the addition of different radical groups and changes in molecular weight. More than 500 silicone derivatives are listed in the INCI dictionary documents but the common classification of silicones is given in Table 1.

Table 1. The most common INCI names of silicone materials

The reason silicones are preferred is their sensory properties. They provide a silky feel during and after application in cosmetic products without greasiness. Because they are noncomedogenic, they do not cause clogging in skin pores due to the long, stable and non-oxidizable properties of Si-O bonds. Si-O bonds have a smoother structure than C-C and C-O bonds, which provides elasticity. While the C-C bond has a length of 0.154 nm and an angle of 112°; the Si-O bond has a length of 0.163 nm and an angle of 130°. The elasticity ensures the formation of a diffusion layer and a breathable film. The water vapor permeability of some materials is given in Table 2.

Table 2. Water vapor permeability of different materials (Payne Cup Method)

The permeability of silicone polymers has been verified in vivo and in vitro. To decrease the permeability of silicones, organic functionalities such as the binding of alkyl groups must be increased. Besides permeability, it has been stated that dimethicone, stearoxy dimethicone, dimethicone copolyols and cyclomethicone are not comedogenic. The derivatives of dimethicone and alkyldimethicone are considered safe for use in the medical and pharmaceutical area. Water reaches the skin surface through passive diffusion in sweat vesicles and healthy epidermis. The movement of water via passive diffusion is measured by "transepidermal water loss (TEWL)" and provides a sensitive evaluation of stratum corneum (SC) integrity. Silicones do not prevent water permeability; they form a layer that decreases permeability due to their elastic chemical structure when applied on skin. Therefore, in this study, formulations containing different silicones were analyzed using in vitro measurements of pH, TEWL and skin moisture.

Materials and Methods Experimental Design

One cream formulation without silicone and four cream formulations with different silicones were applied to the forearms of 17 people (10 women, 7 men) one time only. Four measurements were performed at the beginning of application, 30 minutes later, 1 hour later and 2 hours later.

Preparation of Creams (Emulsions):

A cream base was prepared to which different silicones were added. No humectant was used in the base. Therefore, the formulations composed of three phases are given as in Table 3. The aqueous phase and oil phase were always the same; only different silicone materials differed at 5%. Kinematic viscosities are given in Table 3.

Table 3. Cream formulations

Instrumental Measurements:

pH values of cream formulations were measured before application using electrical capacitance corneometry (MPA 5, Courage and Khazaka, Cologne, Germany) which indicates skin hydration level. TEWL measurements were also performed using the MPA 5 tewameter.

Results pH Values of Cream Formulations

The skin has a pH range of 4–6 and is acidic. The interior of the body is close to neutral pH (pH 7–9). The skin structure is considered acidic to create a defense mechanism against external factors. Therefore, pH values of the creams were measured and the effect of silicone derivatives on pH was investigated. pH values are shown in Table 4. When formulation 1, which contained no silicone, was compared with other formulations, no specific pH variation with silicone addition was observed.

Table 4. pH values of cream formulations

Corneometer Measurements

Corneometer measurements indicate the moisture level. Mean values of corneometer measurements on both cream-applied and unapplied skin of 17 volunteers at the beginning of application, 30 minutes later, 1 hour later and 2 hours later are given in Table 5 and Figure 2 in detail.

Table 5. Results of corneometry measurements

Figure 2. Time-dependent change in skin moisture

According to corneometry measurements, moisture values ranged from 60–61 for the control and the area to which cream formulations were to be applied. However, the maximum moisture level was obtained as 86 with the fifth formulation at the 30-minute mark. The same sequence was also observed at 1 hour and 2 hours. Dimethicone 350 in formulation 5 has the maximum viscosity and was found to provide maximum support for moisturizing. Cyclomethicone in formulation 1 has the minimum viscosity and the most volatile structure, so it decreases the moisturizing level compared to high viscosity silicones over the long term.

TEWL Measurements

TEWL is an indication of skin barrier function and an increase in TEWL indicates a decrease in skin barrier function. Mean values of TEWL measurements on both cream-applied and unapplied skin of 17 volunteers at the beginning of application, 30 minutes later, 1 hour later and 2 hours later are given in Table 6 and Figure 3 in detail.

Table 6. TEWL measurements

 

Figure 3. Time-dependent change in TEWL

TEWL measurements ranged from 10–12 (g/m²h) for the control and the area to which cream formulations were to be applied. After 30 minutes and later, unapplied areas showed a TEWL range of 10–11 g/m²h while lower TEWL values were observed in applied areas. The minimum decrease was in the first formulation and the maximum decrease was in the fifth formulation. At the end of 120 minutes, the TEWL of the first formulation was 10.3 g/m²h and the TEWL of the fifth formulation was 7.8 g/m²h. The structure of silicones that do not decrease water permeability but increase the water-loss-reducing effect achieved higher moisturizing effects with increasing viscosity.

Conclusion

While silicones provide a silky feel in formulations due to the Si-O bond in their structure, they also decrease transepidermal water loss and support increased skin moisture. Because low viscosity silicones have a more volatile structure, high viscosity silicone derivatives have greater effects on skin moisture and transepidermal water loss. Bahar Kafadar Senior R&D Engineer Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.           Merve Yılmazer R&D Engineer Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.       Burak Saka R&D Category Manager Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.       Özlem Esen R&D and Quality Group Manager Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.