Quantitative description of human skin water dynamics by a disposition-decomposition analysis (DDA) of trans-epidermal water loss and epidermal capacitance

Background: In vivo water assessment would greatly benefit from a dynamical approach since the evaluation of common related variables such as trans -epidermal water loss or "capacitance" measurements is always limited to instantaneous data. Mathematical modelling is still an attractive alternative already attempted with bi-exponential empirical models. A classical two-compartment interpretation of such models raises a number of questions about the underlying fundamentals, which can hardly be experimentally confirmed. However, in a system analysis sense, skin water dynamics may be approached as an ensemble of many factors, impossible to discretize, but conceptually grouped in terms of feasible properties of the system. The present paper explores the applicability of this strategy to the in vivo water dynamics assessment. Methods: From the plastic occlusion stress test (POST) skin water balance is assessed by modelling trans -epidermal water loss (TEWL) and "capacitance" data obtained at skin's surface. With system analysis (disposition-decomposition analysis) the distribution function, H (t), modelled as a sum of exponential terms, covers only the distribution characteristics of water molecules traversing the skin. This may correspond macroscopically to the experimental data accessed by "corneometry". Separately, the hyperbolic elimination function Q (TEWL) helps to characterise the dynamic aspects of water influx through the skin. Discussion and conclusion: In the observable range there seems to be a linear relationship between the net amount of water lost at the surface by evaporation, and the capability of the system to replenish that loss. This may be a specific characteristic of the system related to what may be described as the skin's "intrinsic hydration capacity" (IHC) a new functional parameter only identified by this strategy. These new quantitative tools are expected to find different applicabilities (from the in vivo skin characterisation to efficacy testing) contributing to disclose the dynamical nature of the skin water balance process