Abstract
The hair cuticle provides significant protection from external sources, as well as giving rise to many of its bulk properties, e.g., friction, shine, etc. that are important in many industries. In this work, atomic force microscopy-infrared spectroscopy (AFM-IR) has been used to investigate the nanometer-scale topography and chemical structure of human hair cuticles in two spectral regions. AFM-IR combines atomic force microscopy with a tunable infrared laser and circumvents the diffraction limit that has impaired traditional infrared spectroscopy, facilitating surface-selective spectroscopy at ultra-spatial resolution. This high resolution was exploited to probe the protein secondary structures and lipid content, as well as specific amino acid residues, e.g., cystine, within individual cuticle cells. Characterization across the top of individual cells showed large inhomogeneity in protein and lipid contributions that suggested significant changes to physical properties on approaching the hair edge. Additionally, the exposed layered sub-structure of individual cuticle cells allowed their chemical compositions to be assessed. 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, confirming previous findings, and demonstrate the potential of AFM-IR for nanoscale chemical characterization within biological substrates.
Highlights
The primary importance of hair for mammals is in providing thermal insulation
Two less likely possibilities that could lead to the decreased intensity were considered, based on technical details associated with recording atomic force microscopy-infrared spectroscopy (AFM-IR) maps
The first stems from the resonant enhancement that was employed, whereby the quantum cascade lasers (QCLs) laser frequency is tuned to match that of the resonant frequency of the cantilever
Summary
The primary importance of hair for mammals is in providing thermal insulation. Additional benefits include protecting the skin against harmful ultraviolet (UV) radiation and facilitating cooling through perspiration.[1,2,3] understanding the physical and chemical structure of hair fibres is crucial for maintaining its desirable properties and it has been the focus of much research for over 150 years.[4]The hair fiber cross-section consists of three main regions, the medulla, the cortex, and the cuticle.[5]. The primary importance of hair for mammals is in providing thermal insulation. Additional benefits include protecting the skin against harmful ultraviolet (UV) radiation and facilitating cooling through perspiration.[1,2,3] understanding the physical and chemical structure of hair fibres is crucial for maintaining its desirable properties and it has been the focus of much research for over 150 years.[4]. Individual cuticle cells have an internal layered structure consisting of the epicuticle, A-layer, exocuticle and endocuticle, and are separated from each other by cuticular CMC.[3,10,11,12,13] The structure of the cuticle is shown in Fig. 1 where external atomic force microscopy (AFM) mapping across the fiber
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