Skin is composed of two main layers, the dermis and epidermis, which are in turn composed of sub-layers. The surface of the skin is the outermost layer of the epidermis and is called the Stratum Corneum. It is composed mainly of dead cells that lack nuclei and contain keratin, a protein that helps keep the skin hydrated by preventing water evaporation. In addition, these cells can also absorb water.
The standard method for measuring skin hydration in the Stratum Corneum is to measure a change in the electrical properties of the skin (specifically the capacitance), which is related to the degree of hydration. The apparatus commonly used for this measurement is called a Corneometer® (available from Courage & Khazaka).
Water has a very high dielectric constant, closely followed by glycerine, both higher than the dry skin values. Therefore it is assumed that an increase in Corneometer values can be equated with an increase in moisturization (mainly due to a water effect, however contributions from glycerine cannot be ruled out). However the scale is not linear, and therefore the technique is not quantitative. Also the depth over which the data is collected is poorly defined. Although the depth range is believed to cover the entire Stratum Corneum it may well cover a portion of the viable epidermis as well. With this technique it is also not possible to get information on the distribution of water within the stratum corneum, with only a single number being generated per reading. In addition, alternative ways of making such measurements must often be made by invasive methods.
The present invention uses a different technique to measure skin moisturization, based on Confocal Raman Spectroscopy. Raman spectroscopy is the measurement of the wavelength and intensity of inelastically scattered light from molecules. Raman scattered light occurs at wavelengths that are shifted from the incident light by the energies of molecular vibrations. The mechanism of Raman scattering is different from that of infrared absorption, and Raman and IR spectra provide complementary information. For further background information on Raman spectroscopy, see for example “Fundamentals of Molecular Spectroscopy”, C. N. Banwell, McGraw Hill, 1983.
Two of the major advantages of Raman Spectroscopy are the non destructive nature of this technique and the virtually no need for sample preparation that it requires, which may provide significant cost and time savings.
Confocal optics relate to the illumination of a sample with a diffraction limited spot such that the illuminating spot is imaged on an ideally point-like detector, the point-like detector being realised with an adjustable pinhole called ‘confocal hole’ in front of the real detector (entrance slit). An advantage of confocal sampling is the ability to separate the signal from each layer of a layered sample. In the case of skin measurement, Confocal Raman Microspectroscopy allows to measure the property of skin as a function of depth.
It has been recently proposed to apply Raman Microspectroscopy to the analytical determination of skin moisturization, see:                “Automated depth-scanning Confocal Raman microspectrometer for rapid in vivo determination of water concentration of water concentration profile in human skin”, P. J. Caspers, G. W. Lucassen, H. A. Bruining and G. J. Puppels, J Raman Spectrosc. 31, 813-818 (2000);        “In vivo Confocal Raman Microspectroscopy of the Skin: Noninvasive Determination of Molecular Concentration Profiles”, P. J. Caspers, G. W. Lucassen, E. A. Carter, H. A. Bruining and G. J. Puppels, The Journal of Investigative Dermatology, Vol. 133, No. 3 March 2001, 434-442;        “Confocal Raman Microscopy for Cosmetic Applications”, published in “Raman Update”, a publication by HORIBA Jobin Yvon, Winter Edition 2005.        
Although some of the above mentioned documents have recognized the usefulness of Confocal Raman Spectroscopy to study the profile of hydration within the Stratum Corneum as a function of depth, as well as a function of time (for example pre- and after-application of a skin moisturization composition), the inventors have made the surprising discovery that, in some cases, the data measured by this advanced technique leads to incoherent results, as is discussed below.
Whilst making measurements of skin hydration of the Stratum Corneum, the inventors found that, unexpectedly, the hydration value as measured by Confocal Raman Spectroscopy at a specific depth within the Stratum Corneum decreased after the application of certain skin care hydration products. These results contradicted visual examination of the surface of the area of the skin tested as well as Corneometer measurements, which indicated an overall increase in skin hydration at the surface of the skin area. After further experimentation and insight, and whilst not wishing to be bound by theory, the inventors have come to the conclusion that certain skin care products can be so beneficial to the health of the skin that they increase the absolute depth of the Stratum Corneum of the users.
As will be discussed in details further below, the inventors have then found that in order to determine the effectiveness of skin care compositions a crucial factor that was previously overlooked is the evolution of the thickness of the Stratum Corneum. In addition to this insight, the inventors have developed a method for determining the thickness of the Stratum Corneum using Confocal Raman Spectroscopy. Traditionally the thickness of the Stratum Corneum has been measured by using biopsies, however this is an inherently destructive process, requiring removal of a section of flesh from the body.