May 2010 in “International Journal of Cosmetic Science” This study found that reduced hair samples showed higher protease degradation than permed hair, which increased with repetitive perming, and degradation was influenced by the type of mercaptan used.
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May 2020 in “International journal of biological macromolecules” In this study, researchers evaluated the binding of keratin-associated proteins and α-keratins to human hair fibers, finding that chemical preactivation significantly enhanced binding to natural hair, while perming altered binding characteristics and increased penetration depth.