Integrating the dual functionality of plant tannins for natural hair dyeing: A critical review on mechanisms, sustainability and formulation innovation

Abstract

Health and environmental concerns associated with conventional synthetic hair dyes have spurred demand for natural, safe and sustainable alternatives, among which plant tannins (PTs) exhibit significant potential. As widely distributed polyphenolic compounds, PTs possess the unique capacity to perform dual roles in natural hair-dyeing systems: acting as direct dyes to impart colour and serving as mordants to enhance dye fixation and colour fastness. However, a systematic understanding of this integrated dual functionality and its potential synergistic effects remains limited. This review aims to bridge this knowledge gap through a critical and systematic assessment of the dual roles of PTs in natural hair dyeing. We delve into the molecular mechanisms governing PT–keratin interactions and their impact on direct dyeing performance. Furthermore, we systematically elucidate the coordination chemistry, bridging models and efficacy of PTs as mordants, including their inherent mordanting capabilities and synergistic effects with metal ions. This review also examines the sustainability of PT sources, the potential for utilising agro-forestry waste and green extraction strategies. Additionally, we evaluate formulation innovations designed to enhance application performance, such as synergistic co-formulation with other natural ingredients and the application of novel delivery systems. Finally, we discuss current challenges and future research directions, including the use of biotechnology and chemical modification to optimise PT properties. By integrating these key aspects, this review provides a comprehensive framework for understanding the complex behaviour of PTs in hair dyeing, offering theoretical foundations and guiding principles for the development of next-generation efficient, safe and sustainable natural hair-dyeing technologies.