Notes: Background/alms: Evaporimetry and transepidermal water loss (TEWL) evaluations are among the most important means of approaching basic cutaneous physiology in terms of its epidermal barrier function. However, physiological skin complexity, together with the loose specifications currently available for analytical purposes, justify the need for further investigating and identifying eventual covariates and limiting factors with respect to both the epidermal barrier function issue and evaporimetry itself.Methods: The relevant operating procedures were investigated with respect to the anatomical and functional equivalence of contralateral sites as they are often used in TEWL determinations and topically applied substance efficacy testing. The study was conducted on human volunteers using five different sites in both forearms, using a standard evaporimetry apparatus. Special attention was put on the right/left forearm difference regarding normal daily life motoric dominance reported by each individual.Results: A consistent difference between TEWL values measured at the most active arm was detected when compared with the readings from the opposite limb. Also, those sites placed nearer the extremities of the forearm behave differently from the inner ones.Conclusions: All information thus gathered suggests that using contralateral sites in study protocols for TEWL determinations, particularly when one side is intended to act as control, requires awareness with respect to the physiological and functional differences of the sampling sites chosen. In any case, a proper randomisation and balancing of the experimental design is crucial.

Notes: 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.

Notes: Background/aims: The evaluation of transepidermal water loss (TEWL) is one of the methods most frequently used in studies involving skin water dynamics. However, TEWL does not provide a direct measurement of epidermal barrier function, being rather a surrogate effect of it. In particular, when external stimuli change cutaneous water balance, these stimuli must be taken into account in order to achieve a rigorous interpretation of the results. Since TEWL is primarily attributed to the water flux from the deepest layers of the skin, through the epidermis, and towards the external environment, this whole process is reasonably compliant with Fick's first law of diffusion. Within this perspective, the aim of this work was to develop a two compartment mathematical model capable of quantitatively describing cutaneous water mass balance over time and thus to provide practical and objective comparable parameters that are particularly useful for studies critically depending on a precise evaluation of TEWL.Methods: The theoretical basis for the proposed model was tested with data collected from different protocols that use TEWL as the main indicator of induced cutaneous alterations. Those alterations involved either external inputs or intrinsic changes on the cutaneous pathway capable of changing the whole water balance across the skin, either altering epidermal cohesion or promoting a significant influx of external water.Results: Both the evaporation and the hydration equilibrium processes, merged within the TEWL curve profiles, were thoroughly described in a quantitative fashion using a mathematical model. Direct parameters were calculated, such as kinetic rate constants (khydr and kevap) and half-life times (t1/2 hydr and t1/2 evap) chosen to describe the hydration and evaporation processes. Secondary parameters, such as time to reach the maximum and an area under the curve derived parameter, the dynamic water mass (DWM), were also calculated, providing additional data particularly useful for comparisons.Conclusion: Modelling TEWL experimental data in a compartmental analysis framework provides valuable parameterized information for characterising and comparing results. Numerical parameters derived on solid theoretical assumptions are reliable indicators for many situations, as long as those assumptions are not violated. The proposed bicompartmental model may also be suitable for other cutaneous water balance related variables, such as capacitance or conductance, although eventually based on distinct model assumptions. In any case, this modelling conceptualisation seems to render valuable insights into the kinetic description of the main processes involved in the net evaporation and water distribution within the complex human skin hydration process.

Notes: Background: Eventual relationships between the vascular function and transepidermal water loss (TEWL), in vivo, have not been entirely explored. By promoting local perfusion alterations through a well-known challenge test, the ‘tourniquet-cuff occlusion’ manoeuvre, the present study searches for other dynamical factors influencing the cutaneous barrier, further exploring the applicability of these flow-related variables in dermatological research.Methods: By applying the tourniquet-cuff manoeuvre to a group of healthy volunteers (n=20), transcutaneous (tc) gases (pO2–pCO2), LDF (laser doppler flowmetry) and TEWL were considered as representing the dynamical aspects under study and measured non-invasively.Results and conclusion: An haemodynamical relationship between tcpO2 and LDF in the post-occlusive period was clearly identified, defining the autoregulatory index as a numerical descriptor of the local metabolic-flow adjustment under stress. TEWL was also significantly affected by the manoeuvre, especially during the post-occlusive period, although no significant relationships between TEWL and other tc variables could be found. The present findings seem to suggest that, under the present experimental conditions, local haemodynamics may also influence TEWL measurements and the skin barrier.

Notes: Abstract:  The objective of the present study was to test the discriminative capacity of the mathematical modeling of the transepidermal water loss (TEWL) curves that result from a plastic occlusion stress test (POST) to variations in the skin barrier – insults inflicted to the skin or differences in two distinct anatomical regions. This study was exclusively performed in the arm. On the first part of the work, three different insults to the skin barrier were assessed: tape stripping, lipid extraction with ether : acetone, and skin-surface biopsy. Anatomical differences were studied in the mid-forearm and in the wrist. All sites were submitted to a POST, after which the desorption curves were recorded. The mathematical model was adjusted to the TEWL data points. Results indicate differences in the parameters obtained in the control and treated sites, which suggests differences in the water dynamics after the damage was inflicted and shows that the method is valid for the objectives proposed. There were also significant differences in the parameters obtained in the wrist and in the volar forearm, which indicates that the method is also sensitive to variations in skin histology and anatomy.