Plant-derived dyes’ effect on human hair need more research to justify large-scale production: Chinese review
In recent years, growing global consumer awareness of sustainability and safety in cosmetics production and use has led to increased demand for safer, ‘greener’ and more natural hair colouring agents. This has in turn resulted in the scientific community looking into on natural colourants as a replacement for synthetic hair dyes.
Based on this, researchers at the Beijing Technology and Business University and Beijing University of Chemical Technology conducted a review on the historical applications of a variety of natural plant hair dyes. The review summarised current literature on phytochemicals and their botanical sources, colour chemistry and biological and toxicological properties. It also focused on research into new natural hair dye sources and eco-friendly, cost-effective processing and application technologies, including production and encapsulation.
Compounds in colourants
There are a number of phytochemicals or organic compounds that have been studied as principal colouring components and for their dyeing performance. Certain quinones, for instance, are found in various parts of plants such as rhubarb and aloe.
Generally, they are “naturally abundant colourants with great potential to obtain a broad spectrum of colours on hair, ranging from deep purple to orange and yellow”. However, their present application in commercial hair dyes is limited by their strong odour, poor solubility and susceptibility to light degradation.
Tannins, on the other hand, are commonly found in a wide array of plants and have been reported to be instrumental in providing colour intensity and fastness, while flavonoids are known for their hair dye applications.
Flavonoids are found in a large variety of plants and many have been found to be not only useful in hair colouring but also to possess pharmacological and medicinal properties. However, their colours are easily affected by environmental factors such as light, temperature and oxygen.
For instance, anthocyanin-based preparations have shown efficiency only as semi-permanent hair dye, with dyed colours lasting for up to five washes. At the same time, anthocynanins are unstable when dissolved in water, though adding vitamin E acetate at a low percentage may enhance its stability.
Some flavonoids (such as hematoxylin) are safe and non-irritating as hair dye ingredients, while others (like quercetin) have displayed medicinal bioactivities, including anti-cancer and anti-viral activities. Acacetin has a range of pharmacological properties, such as neuro-protective, cardio-protective, anti-cancer, anti-inflammatory, anti-diabetic, and antimicrobial activities.
Similarly, some studies have reported on indigo’s anti-inflammatory, antioxidant, anti-bacterial and immune regulatory properties. The review stated that indigo was a “sustainable and environmentally friendly natural colourant” that was widely used in commercial hair dyes containing henna for dark colours.
Improving stability and efficiency
Stability is a key consideration when producing hair dyes with natural colourants, resulting in the development of numerous encapsulation systems to protect sensitive compounds or plant extracts from environmental stresses like heat, UV and extreme pH levels. This improves not just the stability but also the dyeing performance of natural colourants in commercial hair dyes.
Two earlier studies by the Beijing Technology and Business University found that encapsulating Chinese gallnut extract significantly improved its “photo- and thermal-stabilities as well as formulation stability in alkaline formulation”, while encapsulating henna extract “remarkedly reduced” its contact toxicity without compromising its hair dyeing qualities.
A more recent area of development for efficient hair dyeing is nanomaterials, which benefit from small particle size, large surface area and nanostructure, and tailorable physiochemical properties. These qualities make nanomaterials ideal carriers of bioactive ingredients for cosmetic applications.
Several inorganic nanocarriers have shown their ability to stabilize plant-derived hair dyes and even enhance their dyeing effects on bleached, pigmented and grey human hair. There have also been reports on the hair dye applications of other abundant, easily obtainable natural clay minerals like kaolin.
The review noted that in general, “inorganic nanomaterials incorporated / functionalized with hydrophilic or hydrophobic dye molecules from natural sources have provided possibilities for the development of novel natural colourant-based hair dye products”.
Conclusion in colour
The review stated that while botanical colourants are advantageous as an alternative to synthetic hair dyes, there are still several challenges to be addressed. These include cumbersome extraction and purification procedures, susceptibility to environmental factors, low dye uptake and poor colour fastness (particularly on unbleached hair), poor colour reproducibility on human hair, and insufficient toxicological data.
At the same time, there should be further investigation into developing efficient encapsulation systems and nanocarriers to enhance the dyeability and colour fastness of natural colourants.
In order to justify the large-scale production of plant-derived hair dye products for consumer use, more studies are needed to comprehend the mechanistic interactions between different plant-derived colourants and human hair.
This will “provide a theoretical basis for the design of efficient plant hair dyes”, said the review, and broaden the sources of plant-derived colourants or engineered microorganisms for large-scale production.
“Recent Advancements in Natural Plant Colorants Used for Hair Dye Applications: A Review”
Authors: Hongyan Cui et al.