Effects of liquid crystals

The use of an active emulsifier has great options for formulators: stability of the product and benefits on consumer’s skin are brought by one single ingredient. Seppic conducted studies to understand the mechanism of action behind skin benefits. The emulsifier promotes the formation of liquid crystals in emulsions. Two different types of liquid crystals are distinguished: those around oil droplets and those in the continuous aqueous phase. The liquid crystals around the oil droplets are in the form of several lamellar bilayers. The latter forms a barrier against coalescence1. These liquid crystals are anisotropic and visible in the form of Maltese crosses under an optical microscope with polarised light.

Liquid crystals also exist in the continuous aqueous phase in the form of lamellar phases^2,3,4. These net works are viscoelastic and more or less consistent, which prevent the creaming phenomenon. Unlike liquid crystal formed around oil droplets, the lamellar phases in the aqueous phase are not visible under the optical microscope but can be identified using an electron microscope or by means of rheological analysis. Indeed, these lamellar phases increase the elasticity of the emulsion. The complementary mechanisms of the two types of liquid crystals provide stability in emulsions. The new study’s aim is to understand

the influence of time, temperature,transport and formulation composition on creation and stability of lamellar phases and lamellar bilayers. Extensive stability studies have been conducted. Multiple formulations have been made to see the impact of the emulsifier’s nature, of polymers and of the oil phase composition

on those structures. Emulsions have then been submitted for up to six months to various temperatures and to a specific machine mimicking the impact of transportation. A standardised protocol allowed evaluate of the presence or the absence of microscopically visible liquid crystals in the various formulations regularly. The emulsifier with liquid crystals demonstrated great capacity to create stable liquid crystals.

Bioinspired active textures

A focus is then made on the impact of the emulsifier’s texture on skin improvement. The moisturising effect was first measured with a simple formula. To minimise interactions with the other ingredients, skin rugosity and interaction of the stratum corneum were then assessed on an even more basic emulsion, with a reduced oil quantity and without a thickening polymer. Appropriate references were chosen according to the measured parameter for each experimentation. Skin hydration was assessed with classical in vivo corneometry measurements on 20 volunteers with dry skin. The corneometry was monitored for five hours after a single application of the emulsion on legs. A simple emulsion with liquid crystals demonstrates a significant moisturising effect up to five hours after application. This result reinforced a previous result a highlighting moisturising effect with a higher dosage⁵. The effect of the emulsion on the skin texture was evaluated with the ColorFace6 acquisition system. This system allows a 3-Dimensional high-resolution full-face image acquisition in real time. The study was conducted on 20 women with uneven skin texture. The face photography of ¾ profiles were analysed at 0 and 30 minutes, after a single application of each emulsion on half-face. Results are compared using the skin surface roughness index Sdev. The analysis demonstrates a significant decrease in the skin roughness index with emulsion with liquid crystals only.

Mechanism of action

To see what happens to the formulation after application to the skin, the impact of the emulsion on stratum corneum was analysed by x-ray microdiffraction⁷. The profile of the stratum corneum after application of the emulsion with liquid crystals is generally identical to that of the stratum corneum with the same peak positions. Taking into account the intensity scales between profiles, we can see that the lamellar structure is strengthened, and the amount of lipids is increased. These results indicate that the emulsion lipids gently insert between skin lipid molecules resulting in a strengthened skin barrier. X-rays microdiffraction analysis proves that the lamellar bilayer biomimetic structure in the emulsion strengthens the skin barrier function by gently inserting itself within the stratum corneum lipids lamellar structure. As a result, skin roughness is reduced and skin moisturisation increased up to five hours after.