A novel biomimetic hair colouring system based on the eumelanin precursor 5,6‐dihydroxyindole

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Abstract
Objective: The use of traditional hair dyeing agents to dye hair may cause scalp and hair damage, induce melanocyte stem cell depletion, and in some cases trigger allergic reactions. Therefore, we explored a novel hair colouring approach using 5,6‐dihydroxyindole (DHI), a precursor of eumelanin, to dye grey hair. This study aimed to elucidate the mechanism by which cationic polymer pretreatment enhances the dyeability of DHI, focusing on changes in hair surface charge and the dyeing mechanism of DHI. Methods: Dyeability was assessed by UV–Vis spectrophotometry. The degree of cationization at different pH values was determined by colloid titration, and the hair surface potential was measured using a zeta potential analyser. Adsorption and penetration behaviours were evaluated by confocal fluorescence microscopy with ionic fluorescent dyes. Cross‐sections of DHI‐dyed hair were examined by optical microscopy. Results: Pretreatment with a specific cationic polymer increased the dyeability of DHI‐based formulations. A polymer capable of retaining positive charge under alkaline conditions (pH 10) was identified, and its suitability was confirmed by degree of cationization measurements, zeta potential shifts and fluorescence adsorption imaging. Mechanistic analyses indicated that water‐soluble DHI is retained within the δ‐layer of the cell membrane complex in the cuticle region, where it undergoes air oxidation and polymerization, resulting in fixation within the hair. Conclusion: Cationic polymer pretreatment enhances DHI dyeability and may support the development of DHI‐based formulations as potential alternatives to oxidative hair dyes.

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