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The hair thread is a natural fiber formed by keratin, a protein containing high concentration of sulfur coming from the amino acid cystine. The main physical proprieties of the hair depend mostly on its geometry; the physical and mechanical properties of hair involve characteristics to improve: elasticity, smoothness, volume, shine, and softness due to both the significant adherence of the cuticle scales and the movement control (malleability), as well as the easiness of combing, since they reduce the fibers static electricity. The evaluation of these effects on hair may be carried out by several methods, as: optical and electron microscopy, mechanical resistance measuring, shine evaluation and optical coherence tomography (OCT).Uniterms: Hair/properties/evaluation. Hair fibers/properties/evaluation. Hair fibers/characteristics. Optical coherence tomography/analytical application. Optical microscopy/analytical application. Electron microscopy/analytical application O cabelo é uma fibra natural formada por queratina, uma proteína composta por teor elevado de enxofre proveniente da cistina. As propriedades principais do cabelo dependem de sua geometria, estrutura física. Características físicas e mecânicas das fibras capilares envolvem propriedades que melhoram: elasticidade, maciez, volume, maleabilidade, facilidade para o ato de pentear e brilho. A avaliação de tais propriedades do cabelo pode ser obtida por métodos diversos, como: microscopia óptica e eletrônica, mensuração da resistência mecânica, determinação do brilho e tomografia por coerência óptica (OCT).Unitermos: Cabelo/propriedades/avaliação. Fibras capilares/propriedades/avaliação. Fibras capilares/ características. Tomografia por coerência óptica/aplicação analítica. Microscopia óptica/aplicação analítica. Microscopia eletrônica/aplicação analítica.

Hair color change by dye application is a common procedure among women. Hair dyes are classified, according to color resistance, into temporary, semipermanent, demipermanent and permanent. The first two are based on molecules which are already colored. Temporary dyes act through dye deposition on cuticles, but semipermanent may penetrate a little into the cortex and so the color resists up to six washes. Demipermanent and permanent dyes are based on color precursors, called oxidation dyes, and the final shade is developed by their interactions with an oxidizing agent, but they differ from the alkalizing agent used. In oxidation systems, there is an intense diffusion of the molecules into the cortex, what promotes a longer color resistance. Dyes and color precursors present differences related to chromophore groups, hair fiber affinity, water solubility, and photo stability. The aim of this review is to discuss the differences among hair dye products available in the market and their action mechanisms, molecular structures, application methods, and some aspects of formulations.

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