# In Vivo Assessment of Water Content, Trans-Epidermial Water Loss and Thickness in Human Facial Skin
## 1 Introduction  
The skin is the largest organ in humans and its primary functions are: protecting the body against external agents (such as micro-organisms, ultraviolet radiation, chemicals and physical insults); maintaining homoeostasis by preventing dehydration and regulating temperature; and enabling tactile feedback by sensing pain and pleasant stimuli [1; 2; 3]. At the macroscopic level the main biophysical characteristics of skin are: water content, thickness, trans-epidermal water loss (TEWL), sebum production, surface pH, melanin index and desquamation [2; 4; 5]. These properties vary significantly depending on the body part. Knowing such variations is important for many practical applications: medical and beauty treatments, for instance, need to adapt to different skin characteristics;likewise, the ability to identify skin location by its properties can be useful in other areas such as forensic analysis. Certain parameters, for instance, thickness, are important for surgical intervention as they may guide skin resection and restoration procedures.  
In the present work we performed an in vivo evaluation of the distribution of water content, TEWL and skin thickness in six facial areas: cheeks, chin, forehead, lips, neck and nose. Water content and TEWL can be regarded, respectively, as the \"stock\" and \"flow\" of water in the human skin. The first (also referred to as \"hydration\") is crucial to maintaining the correct metabolism of the skin. When the amount of water drops below a critical threshold the enzymatic processes can no longer occur properly, resulting in visible changes like dryness, roughens and flaking [6,7]. Trans-epidermal water loss is the flow of water that leaves the body through the skin. It is the most widely used parameter for assessing the skin barrier function [8] and is a good indicator of pathological conditions, such as irritation, atopic eczema, contact dermatitis and psoriasis [2,8,9]. As for skin thickness, we investigated the distribution of this parameter for each of the following layers: stratum corneum (SC), epidermis and dermis.  
The main aim of this work was determining whether the distributions of water content, TEWL and skin thickness are significantly different among the six facial areas investigated, and, conversely, whether there are groups of facial sites (clusters) that are statistically undistinguishable in terms of the parameters considered. The contributions of this paper to the literature are therefore various. On the one hand, we carried out the experiments using innovative measurement devices that enable fast and accurate in vivo analysis of the parameters investigated. In particular, we estimated water content through a full-field capacitance system and skin thickness via an ultrasound scanner: to the best of our knowledge neither of the two methods has been used before for the analysis of the face. Another original contribution of the paper is a statistical analysis of the facial areas for each type of measurement, the goal of which was to identify potential clusters and find phenomenological explanations of the patters observed. To this end we applied a pairwise non-parametric Mann-Whitney U test to check the hypothesis that the measurements in the different facial areas were indistinguishable.  
In the reminder of the paper we first give an overview of earlier works (Section 2) and then describe the materials and methods used in this investigation (Section 3). We present and discuss the results in Sections 4 and 5 and conclude the paper with some final considerations (Section 6).