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

20.05.2019

Abstract

Silicones not only helps to provide silky feeling in cosmetic products; but also they enable water permeability due to Si-O bonds; moreover support to prevent water loss from skin. In this study, the effects of creams which were obtained by addition of silicones with different viscosities on skin were analyzed by corneometry and transepidermal water loss measurements. The effects of high viscosity silicones on skin moisturization were observed in maxium level.

Introduction

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

Figure 1. Chemical structure of Polydimethylsiloxane (dimethicone)

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

Table 1. The most common INCI names of silicone materials

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

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

The permeability of silicone polymers are verified invivo and in-vitro. In order to decrease the permeability of silicones, organic functionalities like the binding of alkyl groups must be increased. Besides permeability, it has been stated that dimethicone, stearoxy simethicone, simethicone copolyols and cyclomethicone are not comodogenic. The derivatives of dimethicone and alkyldimethicone are accepted to be safe in use of medical/pharmaceutical area. Water reaches skin surface by passive diffusion in sweat vesicles and healthy epidermis. The movement of water via passive diffusion is measured by “transepidermal water loss (TEWL)” and it provides a sensitive evaluation of integrity of stratum corneum (SC). Silicones do not prevent water permeability and they just form a layer that decreases the permeability due to elastic chemical structure when applied on skin. Therefore in the study; the formulations that include different silicones were analyzed with in-vitro measurements of pH, TEWL and skin moisture.

Materials and Methods Experimental Design

One cream formulation without any silicone and 4 cream formulations with different silicones were applied on forearms of 17 people (10 women, 7 men) for only one time. Four measurements were performed at the beginning of the application, 30 minutes later, 1 hour later and 2 hours later.

The Preparation of Creams (Emulsions):

A cream base was prepared to add the different silicones. No humectant was used in the base. Therefore, the formulations composed of three phases were given as in Table 3. Aqueous phase and oil phase were always same, just different silicone materials differed as 5%. Kinematic viscosities were given in Table 3.

Table 3. Cream formulations

Instrumental Measurements:

pH values of cream formulations were measured before application by electrical capacitance corneometry (MPA 5, Couerage and Khazaka, Cologne, Germany) that shows skin hydration level. Also, TEWL measurements were performed by MPA 5 tewameter.

Results pH Values of Cream Formulations

Skin has a pH range of 4-6 and it is acidic. Interior of the body is close to neutral pH (pH 7-9). The skin structure is considered to be acidic to create a defense mechanism against to external factors. Therefore, pH values of creams were measured and the effect of silicone derivatives to pH was investigated. pH values were depicted in Table 4. When formulation 1 which had no silicone was compared with other formulations,it was observed that there was not a specific pH variation with silicone addition.

Table 4. pH values of cream formulations

Corneometer Measurements

Corneometer measurements indicates the moisture level. Hence, mean values of corneometer measurements of both cream applied and unapplied skins of 17 volunteers at the beginning of the application, 30 minutes later, 1 hour later and 2 hours later were given in Table 5 and Figure 2 in detail.

Table 5. Results of corneometry measurements

Cilt Bakımı Figure 2. Time dependent change in skin moisture

According to corneometry measurements; moisture values vary between 60-61 for control and the area for cream formulations to be applied. However, maximum moisture level was seen to have been obtained as 86 with fifth formulation at 30th minute. Same sequence was also observed in 1st hour and 2nd hour. The Dimethicone 350 in formulation 5 has the maximum viscosity and it was analyzed that it provided maximum support on moisturizing. Cyclomethicone in formulation 1 has the minimum viscosity and the most volatile structure so that it decreases the moisturizing level with respect to high viscosity silicones in long term.

TEWL Measurements

TEWL is the indication of skin barrier functions and the increase in TEWL shows the decrease of skin barrier function. Hence, mean values of TEWL measurements of both cream applied and unapplied skins of 17 volunteers at the beginning of the application, 30 minutes later, 1 hour later and 2 hours later were given in Table 6 and Figure 3 in detail.

Table 6. TEWL measurements

Figure 3. Time dependent change in TEWL

TEWL measurements vary between 10-12 (g/m2h) for control and the area for cream formulations to be applied. After 30 minutes and later; unapplied areas has a TEWL range of 10-11 g/m2h while lower TEWL values were observed in applied areas. Minimum decrease was in first formulation and maximum decrease was in fifth formulation. At the end of 120 minutes, TEWL of first formulation was 10.3 g/m2h and TEWL of first formulation was 7.8 g/m2h. The structure of silicones that do no decrease water permeability and increase the water loss got higher the moisturizing effect with viscosity increment.

Conclusion

While silicones give a silky feeling in formulations due to Si-O bond in their structure, they also decrease transepidermal water loss and support the increase of skin moisture. Because low viscosity silicones have more volatile structure, high viscosity silicone derivatives have more effects on skin moisture and transepidermal water loss. Cilt Bakımı