Controlling the hydration of the skin though the application of occluding barrier creams

Abstract

The skin is a barrier membrane that separates environments with profoundly different water contents. The barrier properties are assured by the outer layer of the skin, the stratum corneum (SC), which controls the transepidermal water loss. The SC acts as a responding membrane, since its hydration and permeability vary with the boundary condition, which is the activity of water at the outer surface of the skin. We show how this boundary condition can be changed by the application of a barrier cream that makes a film with a high resistance to the transport of water. We present a quantitative model that predicts hydration and water transport in SC that is covered by such a film. We also develop an experimental method for measuring the specific resistance to water transport of films made of occluding barrier creams. Finally, we combine the theoretical model with the measured properties of the barrier creams to predict how a film of cream changes the activity of water at the outer surface of the SC. Using the known variations of SC permeability and hydration with the water activity in its environment (i.e. the relative humidity), we can thus predict how a film of barrier cream changes SC hydration.

Figure 1.

(a) The influence of RH on the water flux (TEWL) and (b) the thickness for SC and a model membrane composed of stacked bilayers where one side of the membranes faces a physiological solution (correspond to 99.6%RH), and the other side faces an environment that is defined by its RH. Solid lines, fit to experimental data obtained for intact SC [2]; dashed line, calculated data for a responding membrane [3]; dotted line, calculated data for a non-responding membrane [4]. In figure (b), the calculated data are presented as the change in thickness relative to the fully swollen membrane (a_out = a_in) (left axis). The experimental data for SC are given as measured thickness (right axis).

Figure 2.

Sorption data for porcine SC [32] for SC from two different animals (solid and dotted line). Water uptake (g_water/gsc) as a function of RH.

Figure 3.

Experimental data for transport of water across a film of the model W/O emulsion (thickness 73 μm, 42%RH).

Figure 4.

The effect of an occluding barrier cream on the water activity at the skin surface, skin hydration, skin permeability and TEWL. (a–d) Calculated profiles for skin covered by 20 μm thick films of different specific diffusive resistance (ρ = 100 (dashed line), 500 (dotted line) and 5000 (dashed-dotted line) m h^–1 g⁻¹) and for bare skin surface (solid line, no film). (a) Water activity a_out as a function of RH, (b) hydration in the very upper layer of SC (wt% water in SC) as a function of RH, (c) water permeability of SC as a function of RH and (d) water flux across SC (TEWL) as a function of RH.

Figure 5.

Calculated data for how film thickness influence water activity at the outer surface of the SC and TEWL for the model emulsions investigated (one O/W emulsion with measured ρ = 58 m h⁻¹ g⁻¹, and one W/O emulsion with measured ρ = 423 m h⁻¹ g⁻¹. (a) Water activity as a function of film thickness. (b) TEWL as a function film thickness.