Repeatable Perming via Thiol–Michael Click Reaction: Using Amide Derived from Maleic Acid and Cystine

This study presents a new perming method using N,N′-bis(maleoyl)-l-cystine (MA2-CySS) that employs a thiol–Michael click reaction to achieve repeatable perming without oxidative damage. Unlike traditional oxidative perming, MA2-CySS preserves hair's structural integrity, maintaining superior mechanical properties and reducing color change and cuticle damage. The perming efficiency was comparable to oxidative methods, with over 1300% efficiency across three cycles, and showed no cytotoxicity up to 1000 μg/mL, indicating it is safer for hair and scalp applications. Optimal conditions for perming were found to be pH 9, 2 wt%, and 40 °C for 30 minutes, achieving an SH% of approximately 82%. The study highlights the potential of MA2-CySS as a promising alternative to conventional perming methods, offering similar performance with less damage and better preservation of hair's natural properties.