On pathways for small molecules into and out of human hair fibers

This paper represents an experimental approach of histology of the human hair fiber in dyeing and diffusion phenomena and its contribution to the interpretation of hair analysis results for drug abuse. Rhodamine B was applied to human hair fibers from either aqueous solution or methanol/ethanol solvent. The experiments were performed on natural hair of different ethnic groups as well as on extensively bleached hair strands. The microscopical study of the pathway of diffusion of rhodamine B into the hair fibers indicated that the reagent had entered the unmodified fibers at the scale edges between the cuticle cells. At the beginning of the diffusion process intercellular diffusion was the preferred route predominantly along the nonkeratinous regions of the cell membrane complex (CMC) and intermacrofibrillar cement. Penetration into the high sulfur regions of the fiber occurred as dyeing in aqueous solution proceeded and resulted in evenly stained cross sections. The dye distribution pattern observed in natural hair exposed to nonaqueous solution showed that rhodamine B did not penetrate the cortex cells as easy as from aqueous solution and selectively stained nonkeratinous regions only. The determination of the amount of dye taken up by the fibers by spectrophotometric analysis demonstrated that samples diffusion generally increased by time and temperature. It also depended on the morphology of the hair sample. The penetration of rhodamine B from aqueous solution was much greater than from methanol/ethanol solvent.