Surface Area, Pore Structure and Density of Hair Fibers
Hair is an abundant natural resource and a prime target of cosmetic and biotechnological industries. Key structural characteristics that define hair properties and help improve the understanding of the nature and application of human hair fibers include BET surface area, pore size distribution, and skeletal density. BET surface areas and pore size can be greatly reduced or enhanced through systematic chemical or radiation treatments.
From an industrial perspective, hair presents many appealing features. Human hair in particular is an evolutionary simple form of animal skin cells, compared to wool, scales, or feathers. Human hair is an abundant renewable natural resource, a popular target of the cosmetics industry, and an attractive substrate for biotechnological applications [1, 2]. In essence, hair is a ubiquitous biomaterial made up of protein filaments. Hair filament cross-sections (Figure 1) show three distinct regions: the cuticle, cortex, and medulla. The cuticle is a thin outer layer of flat cells spread in the fashion of roof shingles, the cortex is a thick layer that contains rod-like keratin molecules and melanin pigment cells, and the medulla is a loosely packed area at the center.
Lipid molecules spread over the cuticle impart hydrophobic properties to hair filament surfaces. This is important because the exposed surface area of the cuticle controls the friction, wetting, adsorption, and diffusion of active ingredients used for hair treatments such as bleaching, coloring, and conditioning. Indeed, cuticle damage through mechanical (hair brushing, blow drying) and chemical (bleaching, sunlight) means can expose and develop pore structure. This has significant implications for the mechanical properties and overall health of hair. Fundamental hair structural parameters, specifically BET surface area, pore size distribution, and density have been measured on hair fibers and are reported below.
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- J.G. Rouse, M.E. Van Dyke. Materials, 2010, 3, 999-1014.
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