Nanoscale Molecular Characterisation of Hair Cuticles using Integrated AFM-IR

The nanometre-scale topography and chemical structure of hair cuticles has been investigated by vibrational spectroscopy and imaging in two spectral regions. The combination of Atomic Force Microscopy with a tuneable infrared laser (AFM-IR) circumvents the diffraction limit that has impaired traditional infrared spectroscopy, facilitating surface spectroscopy at ultra-spatial resolution. The variation in protein and lipid content of the cuticle cell surface approaching its edge, as well as the exposed layered structure of the cell at the edge itself, was investigated. Furthermore, the contribution of cystine-related products to the cuticle layers was determined. The variation of protein, lipid and cystine composition in the observed layers, as well as the measured dimensions of each, correspond closely to that of the epicuticle, A-layer, exocuticle and endocuticle layers of the cuticle cell sub-structure. Statement of Significance Using AFM-IR to analyse the nanoscale cuticle features is both significant and novel in the field. Thus far, the great majority of work on the chemical investigation of the structure of hair has been limited to bulk measurements, or subject to the diffraction limit associated with traditional IR spectroscopies and microscopies. AFM-IR circumvents this diffraction limit and allows nanometre-scale, localised chemical investigation with high surface selectivity. While non-chemical investigations, e.g. those using Transmission Election Microscopy, have previously shown cuticles to have a layered substructure, AFM-IR sheds light on significant chemical variations of protein and lipid compositions within such layers, enabling their quantification.