The water content of skin has a significant impact on skin properties; sufficient hydration is necessary to keep the skin supple, flexible, and smooth. To understand more completely the water retention properties of the human skin barrier, physical macroscopic properties must be related to the structural organization of the stratum corneum (SC). Water, lipids, and natural moisturizing factor (NMF) influence the molecular structures that affect the properties of SC, including water sorption and binding enthalpy. In this dissertation, a study of water vapor interaction with human adult and neonatal foreskin stratum corneum (SC) and also with hair and nail is presented in detail. This study also focuses on variants of adult human SC, including delipidized and water-washed delipidized SC, to identify the influences of the principal components of SC on water sorption. The procedure consisted of conjoint water vapor sorption and heat flow measurements. The heat of sorption of water in excised human adult cadaveric and surgical SC, neonatal SC, hair and nail at various relative humidities was measured in an isothermal calorimeter at 32°C. Calorimetric measurements, combined with gravimetric water sorption isotherms, were used to calculate the integral and differential enthalpies and entropies associated with binding of water to SC.
Structural and conformational changes in delipidized and water washed delipidized SC in the dried and hydrated states were analyzed using Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy. Structural differences were also analyzed in nails, hair, surgical and neonatal SC. ATR-FTIR results were also interpreted in terms of transformation of keratins from α-helix to β-sheet and random coils, determined using the Amide I band. The thermodynamic properties obtained from calorimetric data were interpreted in conjunction with spectroscopic data.
The magnitudes of the differential enthalpy, the differential free energy and the differential entropy are greatest for adult cadaveric intact SC and least for water-washed delipidized SC and nails. The principal thermal contribution comes from the interaction of water with keratins. The enthalpy and entropy of water sorption in SC are consistent with a picture in which water first binds tightly to polar sites on keratin, replacing keratin-keratin bonds with energetically favorable keratin-water bonds. This process swells and softens the tissue, leading to rapid decreases in the magnitudes of the differential thermodynamic properties and with increasing water content.
Spectroscopic and water sorption results are consistent with the calorimetric results. Spectroscopic results, in conjunction with the calorimetric results indicate that removing the lipid and NMF components alters the conformation of the keratins. Delipidization converts a fraction of keratin α-helixes to turns and random coils, while water sorption converts a fraction of keratin α-helixes to β-sheet, turns, and random coils.
Adult surgical SC is the most appropriate laboratory model to study adsorption and related processes. Cadaveric SC appears to be significantly less ideal, possibly due to changes occurring during preservation and storage. Neonatal SC is less ideal due to differences in ceramide content. Lipids play a key role in maintaining the keratins in a conformation that results in optimum hydration.