The dynamics of transepidermal water loss (TEWL) from hydrated skin

Background/aims: When an occlusive cover is applied on the skin, the transepidermal water loss (TEWL) is inhibited and the skin moisturises. Upon the removal of the occlusion the water, which accumulated in the skin porosity during the occlusion, evaporates and diffuses outside showing a TEWL rate larger than the basal value, and function of time. The aim of this paper is to develop a physical-mathematical model for simulating the dynamics of Epidermal (free-) Water Evaporation (EWE) and of transepidermal water loss of hydrated skin. Methods/Results: The model, which is based on general results and methods of transport phenomena, has been calibrated and validated by experimental tests, in vivo , with the use of an evaporimeter. Both the theoretical model and the experimental results show that the release of excess water that accumulates in the skin following an occlusion, is a fast transient phenomenon, which has a characteristic time of the order of minutes (the initial decay is very rapid indeed, in less than a minute the TEWL value may decrease by more than 50%). On the other hand, the characteristic measuring time of the evaporimeter may be as large as 30 s. Therefore, as shown in the paper, the use of the evaporimeter for measuring the initial values of TEWL upon immediate removal of the occlusion is not reliable. When a transient phenomenon cannot be fully described by a measuring device that has an initial characteristic response time of the order of the characteristic time of the phenomenon, a mathematical model, which describes the dynamics of the phenomenon must be developed. Then the measurements taken after the measuring device is stabilised may be extrapolated to the initial times by the use of the model, thus allowing a full quantitative description of the phenomenon vs. time. Finally, the experimental results show that a baby's disposable diaper, when loaded with water, behaves like an impermeable occlusive cover on the skin.