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

Summary

Silicones help provide a soft feel in cosmetic products while, thanks to their Si-O bonds, do not prevent water passage; in fact, they help prevent water loss. In the study, the effects of cream bases containing silicones of different viscosities on skin were analyzed through corneometry and transepidermal water loss measurements. The moisturizing effect of high-viscosity silicones was observed at maximum levels.

Introduction

Silicones are a broad chemical family referring to silicon-based materials. Their structure is polymeric; the main chain of the polymer contains Si-O bonds attached to metal groups. The general chemical structure is as shown in Figure 1.

Figure 1. Polydimethylsiloxane (dimethicone) structure

Silicone types can be highly diversified by adding different radical groups and varying molecular weight. The INCI dictionary documents contain more than 500 silicone material derivatives, but in their most general form, the classification of different silicone substances is as shown in Table 1:

Table 1. Most common INCI names for silicone materials

Silicones are preferred because of their sensory properties. In cosmetic products, they provide softness during and after application without leaving a greasy feel. Since they are not comedogenic, they do not clog skin pores because the Si-O bonds in silicones are long, stable, and not easily oxidized. It has a flatter structure than C-C and C-O bonds and this property provides elastic structure. While the C-C bond has a length of 0.154 nm and an angle of 112°C, the Si-O bond has a length and angle of 0.163 nm and 130°C. This flexibility allows diffusion of substances of different sizes and creates a breathable film layer. The water vapor permeability values of some silicones and other materials are given in Table 2.

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

The permeability of silicone polymers has been verified both in vivo and in vitro. To reduce the permeability of silicones, it is necessary to increase organic functions such as binding alkyl groups. Besides permeability, it has been reported that dimethicone, stearoxy simethicone, simethicone copoliols, and cyclomethicone are not comedogenic. The use of dimethicone and alkyldimethicone, hydroxypropyl dimethicone and similar derivatives in cosmetic and medical/pharmaceutical fields has been recognized as safe. Water reaches the skin surface through sweat channels and healthy epidermis via passive diffusion. The movement of water from the epidermis via passive diffusion is measured by "transepidermal water loss (TEWL)" and provides a sensitive assessment of the integrity of the stratum corneum (SC). When silicones are applied to the skin, due to their flexible chemical structure, they create a layer that does not prevent but only reduces water permeability. For this reason, in the study, formulas containing different silicones were evaluated in vitro using pH, transepidermal water loss (TEWL), and skin moisture measurements to investigate the effect of silicones on moisture in the stratum corneum.

Materials and Methods Experimental Design

One negative control and four cream formulas containing silicone substances were applied once to areas on the forearms of 17 volunteers (10 women, 7 men). Four measurements were performed: at the start of application, 30 minutes after application, 1 hour after, and 2 hours after.

Preparation of Creams (Emulsion):

A cream base was prepared suitable for the integration of different silicone substances at 5% concentration. No substance with moisturizing effect was used in the formulation. Therefore, the contents of the 3-phase cream formulations are as shown in Table 3. The water phase and oil phase remained constant in each formula and only different silicone substances were added at 5% concentration. The kinematic viscosities of the silicone derivatives are as shown in Table 3.

Table 3. Cream formulations

Instrumental Measurements:

Before application, pH values of the cream formulations were measured. Skin hydration level was measured with an electrical capacitance corneometer (MPA 5, Courage and Khazaka, Cologne, Germany). Subsequently, TEWL measurements were obtained with the MPA 5 tewameter device.

Results pH Values of Cream Formulations

Skin normally has a pH range of 4-6 and is acidic. The body's internal environment, however, has a pH range close to neutral (pH 7-9). It is thought that the skin structure is acidic because it creates a defense mechanism against external factors. For this reason, the 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 the pH values of formula 1 without silicone and other formulas were compared, it was observed that the silicone types in the study did not cause significant pH change.

Table 4. pH values of cream formulations

Corneometer Measurements

Corneometer measurements indicate the degree of skin moisture. Therefore, the average values of corneometer measurements on both cream-applied and non-applied skin in 17 volunteers at the start of application and 30 minutes after application, 1 hour after, and 2 hours after are shown in detail in Table 5 and Figure 2.

Table 5. Corneometer measurement results

Figure 2. Time-dependent change in skin moisture

According to corneometer measurements, without cream application and initial moisture values of the areas where cream would be applied were between 60-61. However, by the 30-minute mark, the highest moisture content was achieved with formula 5 at 86. The lowest moisture was obtained with formula 1. In the 1-hour and 2-hour measurements, the rankings observed at the 30-minute mark were also observed. Dimethicone 350 in formula 5 is the highest-viscosity silicone, and it was determined that high-viscosity silicones support moisturization to the greatest extent. Cyclomethicone in formula 1 has the lowest viscosity and most volatile structure, which reduces the degree of moisture over a long period compared to high-viscosity silicones.

TEWL Measurements

TEWL is an indicator of skin barrier functions, and an increase in TEWL indicates a decrease in skin barrier function. The average values of transepidermal water loss (TEWL) measurements in 17 volunteers at the start of application, 30 minutes after application, 1 hour after, and 2 hours after are shown in Table 6 and Figure 3.

Table 6. TEWL measurement results

Figure 3. Time-dependent change in TEWL

Initially, the TEWL measurement values in areas without cream application and where cream would be applied ranged between 10-12. After application, while TEWL values in areas where cream was not applied remained in the 10-11 g/m²h range, decreases in TEWL values were observed in cream-applied areas from the 30-minute mark onward. The smallest decrease was in formula 1, and the largest decrease was in formula 5. After 120 minutes, the TEWL value in formula 1 was 10.3 g/m²h while in formula 5 it was 7.8 g/m²h. The structure of silicones that reduces water loss rather than preventing water passage increased the moisturizing effect in formulations together with increased viscosity.

Discussion

Thanks to the Si-O bonds in their chemical structure, silicones provide a soft feel in formulations while reducing transepidermal water loss and contributing to increased skin moisture. Low-viscosity silicones have a more volatile structure, so high-viscosity silicone derivatives have greater effects on skin moisture and transepidermal water loss.   Bahar Kafadar Senior Research and Development Engineer Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.         Merve Yılmazer Research and Development Engineer Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.         Burak Saka Research and Development Category Manager Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.         Özlem Esen Research and Development and Quality Group Manager Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.