Spatial probing of the properties of the human hair surface using Wilhelmy force profiles

The natural surface of human hair (epicuticle) consists of a bilayer of heavily cross-linked proteins toward the individual cuticle cell inside combined with a monomolecular, hydrophobic layer of mixed fatty acids to the outside (F-layer), which is generally assumed to be homogeneous. Wetting force profiles along segments of hair from female test persons with lengths equivalent to about 1 month of growth (approximately 10 mm) are presented. In a multistep analysis, applying curve smoothing as well as Fourier and principal components analysis, for hair lengths comprising daily and weekly growth (2 mm) pronounced systematic changes are observed in the profiles, which show that the wettability curves are nonstochastic in nature and that hair exhibits a strongly nonhomogeneous surface. Specifically, a compound daily rhythm is observed for wettability, which through its typical bimodality can be linked to continuous changes of the hair surface during wake and sleep phases. The data set furthermore suggests systematic monthly changes, which may be related to the menstrual cycle. In consequence, the results not only provide proof for the inhomogeneity of the immediate hair surface but also lead to the hypothesis that it preserves a rather detailed and long-term, individual chronobiological record, through a specific, spatially modulated distribution of hydrophobic (lipids) and hydrophilic (proteins) regions, "written" by the composition of the cell membrane of the cuticle cell prior to apoptosis.