Water diffusivity in porcine stratum corneum measured by a thermal gravimetric analysis technique

Stabilization of Lipid Lamellar Bilayer Structure of Stratum Corneum Modulated by Poly (2-methacryloyloxyethyl phosphorylcholine) in Relation to Skin Hydration and Skin Protection

BACKGROUND

One crucial factor in skin tissue engineering is to understand the hydration and barrier property of skin. We investigated the skin hydration and stabilization strategy of inter-lamellar structure of stratum corneum (SC) using poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC).

METHODS

The unique hydration and stabilization potency of PMPC on the barrier function of the SC examined using freshly excised hairless mouse skin as a model membrane and the relationship between the stabilization of the lipid lamellar bilayer (LLB) and its enhanced water holding capacity was established.

RESULTS

Differential scanning calorimeter based on the phase-transition temperature of lipid domain of SC demonstrated that PMPC stabilized the LLB. The ratio of the heat of lipid phase transition (△H) of SC exposed to water and PMPC for 24 h was 1.51. X-ray crystallography showed the presence of well- organized lipids in intercellular membranes exhibiting short and long periodicity of lamellar phases. The peak at 4.4 nm attributed to the long periodicity phase (LPP) was missing in water-treated SC, where, the presence of 4.2- 4.4 nm peak in PMPC treated SC indicated that PMPC stabilized LPP. Transmission electron microscopy study demonstrated that the LLB structure became more rigid and orderly in PMPC treated SC.

CONCLUSION

The unique ion paired structure of PMPC enhances the barrier function of the SC by stabilizing LLB structure and hydration by inducing weakly bound water. The unique hydration state and stabilization effect from extended water exposure could provide a valuable information to prepare reliable artificial skin matrix and skin tissue.

Mechanism of enhanced dermal permeation of 4-cyanophenol and methyl paraben from saturated aqueous solutions containing both solutes

Dermal permeation through human epidermis and uptake into isolated human stratum corneum (SC) that was and was not delipidized were measured for 2 model compounds, 4-cyanophenol (CP) and methyl paraben (MP), from saturated aqueous solutions containing 1 or both compounds. Because the solutions were in equilibrium with the pure CP and MP, the thermodynamic activity of the compounds was constant. Compared with compounds that are known permeation enhancers, MP and CP would not normally be expected to act as enhancers. Nevertheless, when both compounds were present, the steady-state fluxes through the epidermis increased by factors of 5.2 and 2.6 for MP and CP, respectively. Within the variability of the measurements, this increase in MP flux is consistent with the 6.4-fold increase in the SC uptake, which occurs primarily into the nonlipid regions of the SC. In contrast, the 1.6-fold increase in CP uptake when MP is present is too small to explain the increase in CP flux. These results suggest that CP enhances the skin permeation of MP by primarily increasing the solubility of MP in the SC, especially in the nonlipid regions, while MP increases the skin permeation of CP by enhancing both the solubility and diffusivity of CP in the SC.

Drying stress and damage processes in human stratum corneum

SYNOPSIS

The drying stresses that develop in stratum corneum (SC) are crucial for its mechanical and biophysical function, its cosmetic feel and appearance, and play a central role in processes of dry skin damage. However, quantitative methods to characterize these stresses are lacking and little understanding exists regarding the effects of drying environment, chemical exposures and moisturizing treatments. We describe the application of a substrate curvature technique adapted for biological tissue to accurately characterize SC drying stresses as a function of time following environmental pre-conditioning and chemical treatment in a range of drying environments. SC stresses were observed to increase to stress levels of up to approximately 3 MPa over periods of 8 h depending on pretreatment and drying environment. A unique relationship between the SC stress and water in the drying environment was established. The effect of glycerol on lowering SC stresses and damaging surfactants on elevating SC stresses were quantified. Extensions of the method to continuous monitoring of SC stresses in response to changes in environmental moisture content and temperature are reported. Finally, a biomechanics framework to account for the SC drying stress as a mechanical driving force for dry skin damage is presented.

Diffusion modelling of percutaneous absorption kinetics: 4. Effects of a slow equilibration process within stratum corneum on absorption and desorption kinetics

One of the main functions of the skin is to control the ingress and egress of water into and out of the body. The transport kinetics of water in the stratum corneum (SC), the dominant site of resistance in the skin, is normally described assuming a homogeneous membrane model. In the present work, the desorption of water from SC was studied and profiles obtained for amount desorbed versus time profiles that were more consistent with water transport occurring in a heterogeneous membrane. Analysis of the resulting profiles yields a model that is consistent with a slow equilibration/slow binding of water within SC as well as its permeation through the SC. Diffusion model solutions were used to derive the steady-state flux, lag time and mean desorption time for water in SC. The slow binding kinetics of water in the SC are limited and most pronounced in the early transient stages of transport and are not easily discerned using steady-state penetration studies. The practical importance of this work is in its use of desorption experiments to recognise and define the skin reservoir for water and other solutes as well as penetration parameters in defining their transdermal kinetics.

A new technique to assess dermal absorption of volatile chemicals in vitro by thermal gravimetric analysis

Potential health hazards of dermal exposure, variability in reported dermal absorption rates and potential losses from the skin by evaporation indicate a need for a simple, inexpensive and standardized procedure to measure dermal absorption and desorption of chemical substances. The aim of this study was to explore the possibility to measure dermal absorption and desorption of volatile chemicals using a new gravimetric technique, namely thermal gravimetric analysis (TGA), and trypsinated stratum corneum from pig. Changes in skin weight were readily detected before, during and after exposure to vapours of water, 2-propanol, methanol and toluene. The shape and height of the weight curves differed between the four chemicals, reflecting differences in diffusivity and partial pressure and skin:air partitioning, respectively. As the skin weight is highly sensitive to the partial pressure of volatile chemicals, including water, this technique requires carefully controlled conditions with respect to air flow, temperature, chemical vapour generation and humidity. This new technique may help in the assessment of dermal uptake of volatile chemicals. Only a small piece of skin is needed and skin integrity is not necessary, facilitating the use of human samples. The high resolution weight-time curves obtained may also help to elucidate the characteristics of absorption, desorption and diffusion of chemicals in skin.

Phospholipid polymer, 2-methacryloyloxyethyl phosphorylcholine and its skin barrier function

The effect of poly[2-methacryloyloxyethyl phosphorylcholine] (pMPC) on the skin permeation property was investigated by performing in vitro skin permeation study of a model drug, nicotinic acid (NA). Effect of pMPC polymer in donor solution on skin permeation rates was evaluated using side-by-side diffusion cells. Also, the structural alterations in the stratum corneum (SC), inter-lamellar bilayer (ILB) and dermis layers in pMPC-treated and -untreated skin sections were investigated with transmission electron microscopy (TEM). The permeation profile of NA without pMPC in donor solution showed biphasic mode: initial 1st phase and 2nd hydration phase. The sudden, more than 10-fold increase in flux from the initial steady state (43.5 microg/cm2/hr) to the 2nd hydration phase (457.3 microg/cm2/h) suggests the disruption of skin barrier function due to extensive hydration. The permeation profile of NA with 3% pMPC in the donor solution showed monophasic pattern: the steady state flux (10.9 microg/cm2/h) without abrupt increase of the flux. The degree of NA permeation rate decreased in a concentration-dependent manner of pMPC. TEM of skin equilibrated with water or 2% pMPC for 12 h showed that corneocytes are still cohesive and epidermis is tightly bound to dermis in 2% pMPC-treated skin, while wider separation between corneocytes and focal dilations in inter-cellular spaces were observed in water-treated skin. This result suggests that pMPC could protect the barrier property of the stratum corneum by preventing the disruption of ILB structure caused by extensive skin hydration during skin permeation study.