Assessment of human stratum corneum thickness and its barrier properties by in‐vivo confocal Raman spectroscopy

Measurement of water concentration profiles across living human skin by confocal Raman spectroscopy has developed into a powerful tool for a better understanding of distribution and function of water in the epidermis. From the water profile across the epidermis the border between stratum corneum and stratum granulosum can be estimated. This is due to the steep drop in water concentration from the inner to the outer side of the stratum corneum. Water content drops from approximately 70% at the inner stratum corneum to only 30% at the skin surface. This slope of the curve becomes clearly flatter in the stratum granulosum. A second parameter is usually taken from confocal Raman spectroscopy to define the stratum corneum border. This is the content of natural moisturization factor (NMF), which should be present only in the stratum corneum. Located at the depth at which the NMF content levels off and the slope of the water profile curve changes is the stratum corneum border. The goal of this work was to develop stratum corneum thickness detection into a robust and semi-automated measurement relying only on the water profile. Further, the aim was to base the empirical findings of water distribution in the epidermis on a well established theory, Fick’s law of diffusion. A mathematical model was developed to fit the water profile curve for a robust and automated detection of the stratum corneum border. In addition, the new model automatically resulted in an accurately determined slope of the water concentration curve in the stratum corneum. This slope, or more exactly the gradient, is one of two parameters directly related to transepidermal water transport across the stratum corneum.