Methods Used: Three different batches of Indian Hair, shaved at the scalp, were used for the studies. The hair samples were not chemically treated before and they were washed with SLS solution before the studies. The hair strands with a length of 25-30 cm were divided into separate parts as root, length, and tip respectively along the distance from scalp to the length. These different parts were analyzed with modern analytical techniques like Laser Scanning Microscopy (LSM), SEM, ATR-FTIR, and Streaming Zeta Potential.

Results: LSM and SEM studies showed deposition of lipid like substances in the root hair while proceeding to the tip, no such deposition was observed, and the cuticles are more exposed to weathering. Moving towards the tip, the cuticular damage level increased and finally most of the cuticles were removed at the Tip. Comparative ATR-FTIR studies between root and tip of the hair showed that there were distinct changes in the regions of -CH stretching and -SO3 stretching. More interestingly, the spectra revealed a shift in amide spectra of the tip hair to higher wavenumber compared to that of the root hair. Deconvolution of the amide spectra into smaller components indicated changes in overall protein conformation. The ratio of different conformations α-helix, β-sheet, and random coil were determined, and it is observed that their relative amount changes on proceeding from root to tip. The streaming zeta potential also becomes less negative, and the isoelectric point (IEP) decreased as we moved from root to tip.

Discussion: These studies provided new insights on different properties like protein conformation, porosity/permeability and surface charge of the hair, presence of different biological substances etc. By combining different methods and using natural Indian hair as a model fiber, we established a set of tools that help monitor fiber ageing. LSM and SEM imaging show damage of cuticular structures and removal of the protecting lipids. The damage in the epicuticular layer also leads to rougher surface i.e., the surface roughness increases on moving root to tip due to weathering. ATR-FTIR results supported these facts. In addition, shift in the amide spectra indicated perturbation in the protein structure leading to changes in protein conformation. The contribution of regular conformation like α-helix or, β-sheet decreases and that of random coil increases. The streaming zeta potential measurement also suggested change in water permeability i.e., more water penetration in the tip region than the root due to absence of the protecting layer like lipids and 18-MEA and the changed protein conformation. The changes in IEP also suggest different local environments in root and tip region.

Conclusions: In this study, we observed interesting change of different parameters like protein conformation, streaming zeta potential, water permeability, IEP along the Indian hair surface and their mutual dependence on each other. We developed a set of analytical methods that allows a systematic mapping of these properties along the hair surface. This not only helps to understand the hair surface more clearly but also opens new possibilities to overcome obstacles related to hair color and hair care formulations.