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| Guest Article | A new method for contactless in vivo quantitative measurement of stratum corneum gloss attributes — influence of active ingredients | Véronique Gillon, G. Perie, S. Schnebert, A. Charbonnelle, G. Pauly |
1. Introduction
Skin optical properties play an important role in both cosmetology and dermatology to enhance or modulate the skin appearance.
Several parameters interfere on skin optical properties. 80 to 90% of skin color comes from light reflected from within the skin. Skin which is too transparent or which has too much pigmentation may show exaggerated redness or become spotted. On the other hand, the skin becomes thicker, rougher and loses its transparency and becomes dull when the stratum corneum is dehydrated. Finally, early aged skin is characterized by roughness or squamous state when its surface integrity is altered. When cutaneous microrelief becomes unidirectional, skin loses its brightness and its velvet aspect.
The gloss appearance of human skin is related to the proportion of the diffuse reflection comparative to the specular reflection at the stratum corneum surface. The specular reflection represents a very slight quantity of light and is mainly influenced by skin roughness, hydration state and the presence of the hydro-lipidic film.
Gloss measurement is a perfectly standardized procedure in the industry. Specific methods and adapted devices for an objective assessment of skin optical properties, however, are quite rare in cosmetology.
The aim of our research was to develop a new device for studying complete skin reflection in vivo in order to evaluate the influence of natural active ingredients on skin complexion. The effects of a treatment with a pearly cosmetic cream including Vegeseryl HGP LS 8572B * on the skin optical properties have been studied.
2. The principle of skin optical measurements
a) Fresnels's law
Fresnel's law demonstrates that the amplitude of the reflected light on a dielectric surface is expressed by a mathematical formula (Figure 1).
θ1 is the incident angle
θ2 is the angle of the refracted light inside the dielectric material
rs is the amplitude of the s-polarized reflected light
rp is the amplitude of the p-polarized reflected light
s corresponds to a polarization perpendicular to the plane of incidence and p to a polarization parallel to the plane of incidence
n is the refractive index.
Figure 1a and b:
Specular reflection of a 1.5 refractive index dielectric material in both polarization directions (1). Specular reflection is very constant near the normal direction.
| Figure 1a |
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b) Application of Fresnel's law to the skin
Skin can be considered as dielectric material. According to Fresnel's law about specular light intensity, the variation of the reflected beam in function of the angle of incidence is negligible near the normal direction (Figure 1a). Under these conditions it is possible to precisely measure the specular reflection regardless of exact angle incidence value or of skin geometry.
Because the skin is a very sensitive material, especially concerning its optical properties, where the slightest contact applied on its surface during measurement modifies its optical properties, it is preferable to evaluate the skin gloss without any contact.
A small proportion of the light is reflected at the stratum corneum/air interface, due to specular reflectance, this manifestation giving a shining skin radiance if well-moisturized and tightened.
3. Materials and methods
As the different apparatus available on the market did not correspond to our needs, we have developed a new device which is based on an adapted in vivo contactless determination of both specular and diffuse light reflections continuously in numerous directions. This apparatus named "brillanometer" has been patented; it allows fast and precise measurement of the skin gloss.
| Figure
2 Presentation of the Brillanometer (patented by Laboratoires Sérobiologiques). Simultaneous visualization of skin microrelief images by videomicroscopy |
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The tested cream contained various active ingredients including Vegeseryl HGP LS 8572B from Laboratoires Sérobiologiques. It consists of native botanical proteins, combining natural albumins and globulins with a high molecular weight. Thanks to its multiple benefits, Vegeseryl HGP LS 8572B has many applications, particularly moisturizing, anti-age, anti-wrinkle efficacy, tightening and smoothing care.
For skin optical measurements, an incident polarized light beam is sent on the skin surface to be analyzed according to one direction, and the reflected beam is simultaneously measured in the same direction (Figure 3). Specular reflected light keeping the same polarization as incident light is studied, representative for skin shine (2).
| Figure
3 Presentation of the two components of skin reflected light measurement. Variation of optical head positioning in numerous directions |
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Specular reflected light keeps the same polarization as incident light. On the contrary, diffuse reflected light is depolarized. This optical property allows to separate both components of the light (Figure 4), only specular reflected light is studied (3).
| Figure
4 Reflected light vs angle. Example after application of petrolatum |
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Light intensity 10² (A.U.)
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Two parameters are appraised:
- maximum of specular reflected light, representative for skin shine,
- mean specular reflected light, representative for skin radiance.
10 female healthy volunteers with dull, rough and dry skin presenting alterations of cutaneous microrelief on the internal side of the forearms were recruited and treated bidaily for 3 weeks with 2 creams: cream containing pearly components and active ingredients including Vegeseryl HGP LS 8572B on one side, and cream containing pearly components alone on the other side.
The skin optical properties were measured quantitatively by brillanometry after a last standardized application at the end of the 3 weeks treatment. Simultaneously, the skin hydration state was controlled and skin surface changes were visualized by videomicroscopy.
4. Results
a) Skin optical properties
After a last standardized application at the end of the 3 weeks of treatment, skin shine and skin radiance were significantly increased with the cream containing pearly components and active ingredients including Vegeseryl HGP LS 8572B in comparison with a cream containing pearly components alone (Figure 5). These pearly components alone had mainly an influence on the skin shine.
b) Skin hydration state
After a last standardized application at the end of the 3 weeks of treatment, the skin hydration state was significantly increased with cream containing pearly components and active ingredients including Vegeseryl HGP LS 8572B, in comparison with the cream containing pearly components alone (Figure 6). The moisturizing efficacy was significantly demonstrated after 3 weeks of treatment, the last application being 12 hours before. After each application, the moisturizing state is obviously prolonged.
c) Skin tightening efficacy
Tightening active ingredients including Vegeseryl HGP LS 8572B were in favor of a microrelief recovering in both directions with more turgescent and larger plates (see results visualized by videomicroscopy in Figure 7). Under these conditions, the efficacy of pearly components was largely valorized. Skin shine (maximum of gloss) and skin radiance (harmonious distribution of gloss at the skin surface) were increased. Videomicroscopy results were completely in accordance with skin optical and skin hydration state measurements.
5. Conclusion
A new contactless device has been built for measuring and studying complete skin reflection in vivo in order to evaluate the influence of natural active ingredients on the skin complexion. A combination of active ingredients including Vegeseryl HGP LS 8572B was added to pearly components in a cosmetic cream which was compared to the cream containing pearly components alone.
Quantitative skin shine (maximum of gloss) and skin radiance (harmonious distribution of gloss at the skin surface) were significantly increased by the active ingredients, improving skin microrelief with more turgescent and larger plates and emphasizing pearly components function.
* Vegeseryl HGP LS 8572B is a registered trademark of Laboratoires Sérobiologiques, Pulnoy, France. INCI name = Glycine soja protein
6. References:
[1] Smith, Warren. In J. Modern Optical Engineering. 2nd Ed Mc Graw-Hill Inc NY, 1990
[2] Hundevadt, Andersen P. Reflectance spectroscopic analysis of selected experimental dermatological models. Chapter 2. Optical properties of skin. Skin Research & Technology, 3: 8-15, 1997
[3] Anderson, R.R. Polarized light examination and photography of the skin.
Archives of Dermatology, 127: 1000-1005, 1991
Author:
Véronique Gillon is Doctor of Pharmacy. She is in charge of clinical and sensory evaluation laboratory at Laboratoires Sérobiologiques (Division of Cognis France) - R&D Department. Her main activity is the evaluation of the efficacy of active ingredients in vivo. She takes part in the research for new cosmetic concepts and in the developement of new biophysical devices, closely collaborating with the Biophysical Department.
Dr. Véronique Gillon
Laboratoires Sérobiologiques,
Division de Cognis France, Pulnoy
5-7 rue de Seichamps
F-54425 Pulnoy
France
E-Mail: Veronique.Gillon@Cognis.com
Internet: http://www.laboratoires-serobiologiques.com
This article was published as a poster at the Stratum Corneum III Congress, in Basel (September 2001), and in "Stratum corneum", Ed. R. Marks, J.-C. Lévêque, R. Voegeli, 2002, pp. 325.
The studies were performed in cooperation with S. Schnebert, LVMH, Branche Parfums & Cosmétiques, St Jean de Braye, France.
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