Skin membrane electrical impedance properties under the influence of a varying water gradient

Abstract

The stratum corneum (SC) is an effective permeability barrier. One strategy to increase drug delivery across skin is to increase the hydration. A detailed description of how hydration affects skin permeability requires characterization of both macroscopic and molecular properties and how they respond to hydration. We explore this issue by performing impedance experiments on excised skin membranes in the frequency range 1 Hz to 0.2 MHz under the influence of a varying gradient in water activity (aw). Hydration/dehydration induces reversible changes of membrane resistance and effective capacitance. On average, the membrane resistance is 14 times lower and the effective capacitance is 1.5 times higher when the outermost SC membrane is exposed to hydrating conditions (aw = 0.992), as compared to the case of more dehydrating conditions (aw = 0.826). Molecular insight into the hydration effects on the SC components is provided by natural-abundance (13)C polarization transfer solid-state NMR and x-ray diffraction under similar hydration conditions. Hydration has a significant effect on the dynamics of the keratin filament terminals and increases the interchain spacing of the filaments. The SC lipids are organized into lamellar structures with ∼ 12.6 nm spacing and hexagonal hydrocarbon chain packing with mainly all-trans configuration of the acyl chains, irrespective of hydration state. Subtle changes in the dynamics of the lipids due to mobilization and incorporation of cholesterol and long-chain lipid species into the fluid lipid fraction is suggested to occur upon hydration, which can explain the changes of the impedance response. The results presented here provide information that is useful in explaining the effect of hydration on skin permeability.

Figure 1

Dynamic regimes of the PT ssNMR experiments. Theoretical ¹H-to-¹³C polarization transfer efficiency for a CH₂ segment at 11.74 T magnetic field and 5 kHz MAS as a function of correlation time, τ_c, and order parameter, |S_CH|. The input parameters for the calculated intensities are equal to the experimental settings. Color codes are given according to the calculated intensities of the INEPT (red) and CP (blue) polarization transfer schemes. A total absence of signal for both INEPT and CP is represented by white. Adapted from Nowacka et al. (31).

Figure 2

Skin membrane impedance data in logarithmic coordinates after 24 h, 48 h, 72 h, and 96 h with varying water activities in the donor solution (a_w,d). (A) Imaginary part of the impedance as a function of frequency. (B) Effective CPE parameter, Q_eff, as a function of frequency. Solid symbols represent data points used to determine CPE parameters α and Q_eff.

Figure 3

R_mem as a function of time for two skin membranes (A and B) with water gradient as a varying parameter. The water gradient was varied every 24 h by changing the water activity in the donor solution (a_w,d) on top of the skin membrane. The water activity of the receptor solution was constant at a_w,r = 0.992.

Figure 4

C_eff as a function of time for two skin membranes (A and B) with water gradient as a varying parameter. The water gradient was varied each 24 h by changing the water activity in the donor solution (a_w,d) on top of the skin membrane. The water activity of the receptor solution was constant at a_w,r = 0.992.

Figure 5

Molecular dynamics of SC components using DP (gray), CP (blue), and INEPT (red) pulse sequences for signal enhancement of molecular segments in either rigid (CP) or mobile (INEPT) microenvironments. ¹³C spectra of SC equilibrated in solutions with a_w = 0.826 (the peak marked with a dot is from PEG) (A) and a_w = 0.992 (B). Resonance lines from SC lipids and amino acid residues of the SC proteins are labeled in B. The intensity is scaled to give an equal DP signal of the peak centered around 32.8 ppm due to the different amounts of SC sample in each experiment.

Figure 6

SAXS (A) and WAXS (B) data on SC at a_w = 0.826 and a_w = 0.992. Numbers associated with an arrow give the d-spacing (nm). Chol is crystalline cholesterol.