Measurement of total water and bound water contents in human stratum corneum by in vitro proton nuclear magnetic resonance spectroscopy

Current views on non-invasive in vivo determination of physiological parameters of the stratum corneum using confocal Raman microspectroscopy

Confocal Raman microspectroscopy is widely used in dermatology and cosmetology for analysis of the concentration of skin components (lipids, natural moisturizing factor molecules, water) and the penetration depth of cosmetic/medical formulations in the human stratum corneum (SC) in vivo. In recent years, it was shown that confocal Raman microspectroscopy can also be used for non-invasive in vivo depth-dependent determination of the physiological parameters of the SC, such as lamellar and lateral organization of intercellular lipids, folding properties of keratin, water mobility and hydrogen bonding states. The results showed that the strongest skin barrier function, which is primarily manifested by the orthorhombic organization of intercellular lipids, is provided at ≈20-40% SC depth, which is related to the maximal bonding state of water with surrounding components in the SC. The secondary and tertiary structures of keratin determine water binding in the SC, which is depth-dependent. This paper shows the technical possibility and advantage of confocal Raman microspectroscopy in non-invasive investigation of the skin and summarizes recent results on in vivo investigation of the human SC.

Review of Modern Techniques for the Assessment of Skin Hydration

Skin hydration is a complex process that influences the physical and mechanical properties of skin. Various technologies have emerged over the years to assess this parameter, with the current standard being electrical probe-based instruments. Nevertheless, their inability to provide detailed information has prompted the use of sophisticated spectroscopic and imaging methodologies, which are capable of in-depth skin analysis that includes structural and composition details. Modern imaging and spectroscopic techniques have transformed skin research in the dermatological and cosmetics disciplines, and are now commonly employed in conjunction with traditional methods for comprehensive assessment of both healthy and pathological skin. This article reviews current techniques employed in measuring skin hydration, and gives an account on their principle of operation and applications in skin-related research.

Simple multimodal optical technique for evaluation of free/bound water and dispersion of human liver tissue

The optical dispersion and water content of human liver were experimentally studied to estimate the optical dispersions of tissue scatterers and dry matter. Using temporal measurements of collimated transmittance [Tc(t)] of liver samples under treatment at different glycerol concentrations, free water and diffusion coefficient (Dgl) of glycerol in liver were found as 60.0% and 8.2×10-7  cm2/s, respectively. Bound water was calculated as the difference between the reported total water of 74.5% and found free water. The optical dispersion of liver was calculated from the measurements of refractive index (RI) of tissue samples made for different wavelengths between 400 and 1000 nm. Using liver and water optical dispersions at 20°C and the free and total water, the dispersions for liver scatterers and dry matter were calculated. The estimated dispersions present a decreasing behavior with wavelength. The dry matter dispersion shows higher RI values than liver scatterers, as expected. Considering 600 nm, dry matter has an RI of 1.508, whereas scatterers have an RI of 1.444. These dispersions are useful to characterize the RI matching mechanism in optical clearing treatments, provided that [Tc(t)] and thickness measurements are performed during treatment. The knowledge of Dgl is also important for living tissue cryoprotection applications.

Keratin-water-NMF interaction as a three layer model in the human stratum corneum using in vivo confocal Raman microscopy

The secondary and tertiary structure of keratin and natural moisturizing factor (NMF) are of great importance regarding the water regulating functions in the stratum corneum (SC). In this in vivo study, the depth-dependent keratin conformation and its relationship to the hydrogen bonding states of water and its content in the SC, are investigated using confocal Raman microscopy. Based on the obtained depth-profiles for the β-sheet/α-helix ratio, the stability of disulphide bonds, the amount of cysteine forming disulphide bonds, the buried/exposed tyrosine and the folding/unfolding states of keratin, a "three layer model" of the SC, regarding the keratin-water-NMF interaction is proposed. At the uppermost layers (30-0% SC depth), the keratin filaments are highly folded, entailing limited water binding sites, and NMF is mostly responsible for binding water. At the intermediate layers (70-30% SC depth), the keratin filaments are unfolded, have the most water binding sites and are prone to swelling. At the bottom layers (100-80% SC depth), the water binding sites are already occupied with water and cannot swell substantially. The hydrogen bonding states of water molecules can only be explained by considering both, the molecular structure of keratin and the contribution of NMF as a holistic system.

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.