Improved Humidity-Resistant Hair Straightening Using Muconic Acid-Derived Amino Acid Compounds

This study investigated the effects of incorporating muconic acid-derived amino acid compounds (CM) into a reductive hair-straightening formulation on hair shape stability and surface characteristics. Hair tresses obtained from Brazilian donors with moderately wavy to curly hair were treated with a cysteamine-based straightening system, with or without CM, and evaluated under high-temperature, high-humidity conditions (40 °C, 75% relative humidity). CM-treated hair maintained a straighter configuration after 24 h of humidity exposure, whereas the control groups exhibited partial or complete loss of the straightening effect. The mean hair width significantly decreased in the CM-treated group, indicating improved macroscopic shape stability. Furthermore, the coefficient of friction was significantly reduced following CM treatment, and the combing resistance showed a decreasing trend, although the difference was not statistically significant. Scanning electron microscopy (SEM) observations suggested a tendency toward reduced cuticle lifting after repeated thermal and mechanical stress. These findings suggest that preservation of cuticle morphology together with reduced fiber–fiber friction may contribute to enhanced humidity-resistant straightening and improved hair manageability. Overall, CM enhanced humidity-resistant straightening while improving hair surface characteristics. These findings suggest that CM is a promising additive for the development of humidity-resistant hair-straightening formulations.