Local structure of human hair spatially resolved by sub-micron X-ray beam

While Human hair has been studied by x-ray diffraction and electron microscopy for many years1,2,3,4,5,6,7,8,9, a complete picture has been elusive because hair is weakly scattering, disordered, and heterogeneous. Human hair is a hierarchical structure, largely constituted of intermediate filaments (IFs) that in turn are made up of keratin molecules1,10. Hair has three main regions: (1) the medulla on the central axis, (2) the cortex, and, (3) the cuticle at the exterior of the hair, (Fig. 1A,B). The medulla mainly is filled with a keratin matrix partially composed of IFs that are spatially and orientationally disordered. The majority of the cortex has a dense packing of IFs aligned along the axis of the hair, but are fluid-like in two dimensions in the plane perpendicular to the hair’s axis. The cuticle is well organized in a layered 1D-ordered structure. Figure 1 SEM image of a human hair fiber is shown in (A) with schematic overlays on to the cut face of the hair fiber to emphasize the relative size of the hair and the main hair regions. In (B), we spatially position the SAXS diffraction patterns relative to ... Fraser and colleagues proposed a molecular structural model of α-keratin and the IF in 1959 and 196410,11, but there is still controversy about the exact structure of the IFs that are a major component of hair2,3,12. In most x-ray measurements, many hair fibers were bundled together so as to assure significant signal-to-noise ratio2,7,13. The advent of high brightness synchrotron sources has enabled the usage, in a few experiments, of an x-ray micro-beam on single strands of hair. However, given the typical sizes of x-ray beams (over 2 microns) relative to the diameter of hair (60–140 microns), and the scattering geometry (x- ray perpendicular to hair’s axis), it was difficult to isolate scattering from the different regions5,12,14,15,16. To spatially resolve the hair’s macro regions, we employed a sub-micron x-ray- diffraction beam of 300 nm in the high-resolution scanning direction, and 20 μ in the orthogonal direction. The samples were cross-sectional slices of hair of 30 μ thickness, with a diameter 80 μ; they were studied with the x-ray beam || and ⊥ to the hair’s axis. Our main discussion here will focus on the results of high-resolution SAXS/WAXS. However, we also present our STEM imaging that is complementary to, and provides a context for, the discussion of the SAXS/WAXS data. Further details on the STEM imaging is given at the end of the discussion section. We note that hair and wool have been extensively studied by electron microscopy and one of the first high-resolution electron microscopy studies was reported by Rogers 195917,18, clearly showing the packing arrangement of the IFs in the cortex and cell membrane complex in the cuticle. More recent results have been obtained by TEM and tomography studies8,19.