This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Hair dyes help people express style and cover gray hair but often contain skin-irritating chemicals. Now, in a paper published in ACS Sustainable Chemistry & Engineering, researchers report on a dye made from plant-derived compounds and zinc ions, a combination that turns hair a black-purple color.
In initial tests, the cinnamon-scented dye stayed in human hair samples longer than a traditional hair dye did and deterred microbial growth. In a preclinical animal trial, the dye didn't irritate skin. Many common hair dyes contain p-phenylenediamine, ammonia or hydrogen peroxide to create lasting, vibrant colors, but all of these chemicals can be skin irritants.
Additionally, ammonia produces a strong, harsh smell. Looking for alternative, less harmful ingredients, Lulu Zhou, Tingting Yan, Detian Xu, Ming Liu and their colleagues saw potential in nanoparticles made from plant-based dyes and metals. These particles displayed rich colors and could bind to proteins but hadn't been tested on human hair, which is primarily a protein called keratin.
So, the team investigated whether a plant-based red dye called alizarin and metal ions could coat and color hair. They added cinnamaldehyde, the main flavor component of cinnamon, for its potential antimicrobial properties and pleasant fragrance. After testing several formulas, the researchers found that the darkest hair coloring came from particles made of cinnamaldehyde added to zinc ions and alizarin mixed in a 2-to-1 ratio.
When the dye was applied to bleached human hair samples, they saw: • Hair treated with the new dye retained around 80% of the black-purple color after 30 washes with a common detergent, compared with 68% for a commercial dye. • Treated hair had lower levels of three microorganisms thought to play a role in causing dandruff compared with bleached hair. • The dyed samples smelled like cinnamon to human testers. In a preclinical trial on live mice, the animals' fur was tinted with the particles and monitored for 14 days. The fur maintained its color, and the mice's skin was unchanged.
In contrast, mice whose fur had been treated with the common hair dye ingredient p-phenylenediamine experienced skin irritation within three days. The researchers also applied solutions of the new pigment to human skin cells, zebrafish and germinating bean sprouts to assess safety for humans, aquatic animals and plants, respectively. Compared with untreated samples, they found no significant differences in cell or zebrafish survival or in bean sprout growth.
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