Development of Versatile Hair Dyes Based on Indigofera tinctoria L.: A Contribution to Sustainable Cosmetic Chemistry

Abstract

Plant hair dyes based on Indigofera tinctoria L. show great potential for the development of sustainable and effective hair dye formulations. The enzymatic hydrolysis of the dye precursor indican by β-D-glucosidase in an aqueous environment leads to the formation of indoxyl, which penetrates the hair fiber and oxidizes there to indigo and indirubin. Both dye molecules are insoluble in water and permanently anchored in the hair cortex. The hair structure has a significant influence on the color expression, in particular the subsequent formation of indirubin. This leads from the initial blue tone to a violet color impression. With 5% (w/v) dried and ground indigo leaves in an aqueous solution, a clear color shift of ΔE = 33.0 is evident after 14 days, with permanent color stability over 30 hair washes. Furthermore, the addition of isatin and cysteine or ascorbic acid achieves an immediate and stable indigo-red coloration on the hair for the first time (ΔE day 0 and day 14: = 7.6), in accordance with EU cosmetics guidelines. The combination of indican, β-D-glucosidase and water, essential for color formation from Indigofera tinctoria L., must be taken into account when preparing extracts for product development. A heat-denatured, aqueous indigo extract containing indican offers advantages for product development and enables controlled dye release through the targeted addition of β-D-glucosidase immediately before application. In addition, it shows particularly low color changes on the hair within 14 days (ΔE = 1.6). Furthermore, important factors for the cultivation and processing of the plant in India can be identified in order to ensure dye quality. Finally, based on the totality of investigations, a new color formation pathway of Indigofera tinctoria L. on the hair is presented, starting from the storage form in plants, indican. The results prove that functionality and sustainability are compatible in hair coloring with Indigofera tinctoria L.. The targeted control of color formation enables the development of bio-based hair dyes with high performance and ecological compatibility - a promising alternative to conventional synthetic products.