Imaging the distribution of skin lipids and topically applied compounds in human skin using mass spectrometry.

The barrier functions of skin against water loss, microbial invasion and penetration of xenobiotics rely, in part, on the spatial distribution of the biomolecular constituents in the skin structure, particularly its horny layer (stratum corneum). However, all skin layers are important to describe normal and dysfunctional skin conditions, and to develop adapted therapies or skin care products. In this work, time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with scanning electron microscopy (SEM) was used to image the spatial distribution of a variety of molecular species, from stratum corneum down to dermis, in cross-section samples of human abdominal skin. The results demonstrate the expected localization of ceramide and saturated long-chain fatty acids in stratum corneum (SC) and cholesterol sulfate in the upper part of the viable epidermis. The localization of exogenous compounds is demonstrated by the detection and imaging of carvacrol (a constituent of oregano or thyme essential oil) and ceramide, after topical application onto ex vivo human skin. Carvacrol showed pronounced accumulation to triglyceride-containing structures in the deeper parts of dermis. In contrast, the exogenous ceramide was found to be localized in SC. Furthermore, the complementary character of this approach with classical ex vivo skin absorption analysis methods is demonstrated.

Kinetics of precipitation of cellulose from cellulose-NMMO-water solutions.

The regeneration of a solid, crystallized cellulose solution in a N-methylmorpholine-N-oxide (NMMO)-water mixture was studied by measuring the diffusion coefficient of both the water uptake from the regenerating bath and the NMMO outflow to this bath. The diffusion coefficient of water going to the cellulose solution is about 10 times larger than the diffusion coefficient of NMMO leaving the solution. This difference expresses the strongly hygroscopic character of NMMO. None of these coefficients depends on cellulose molecular weight showing that no major rearrangement of cellulose chains occurs at the beginning of the regeneration. The diffusion coefficient of water is not influenced by the cellulose concentration, whereas the diffusion coefficient of NMMO decreases strongly when the cellulose concentration increases. Extrapolating the diffusion coefficient of NMMO versus cellulose concentration to zero shows that the maximal concentration of cellulose in NMMO-water is about 15%. Above this value, undissolved cellulose should be present. From the influence of the NMMO content in the water regenerating bath, it is possible to see that NMMO is removed from the solution if the bath has a NMMO content lower than 60%, to be compared with the 80% NMMO concentration in the solution.