Abstract:

This study aims to address the deterioration of mechanical properties in chemically treated hair. Conventional organic acid reparatives target weak molecular interactions （e.g., hydrogen bonds）, but exhibit limited efficacy in repairing free sulfhydryl groups generated by disulfide bond cleavage. Herein, we propose leveraging the α, β-unsaturated carbonyl structure of itaconic acid to initiate a Michael addition reaction with free sulfhydryl groups, thereby forming covalent thioether cross-linked networks to enhance the mechanical performance of hair. The reparative mechanism of itaconic acid on reductively damaged hair was systematically evaluated through single-fiber tensile testing （MTT690）, free sulfhydryl quantification （Ellman’s assay）, and cuticle microscopy （HIROX）. The results demonstrate that itaconic acid （w=2%, pH=3.9） maximally restores damaged hair integrity, significantly increasing the breaking load by 21% （P<0.05） compared to the damaged hair. Meanwhile, after multiple itaconic acid treatments, the elastic gradient and breaking load increase by 16.67% and 28.50%, respectively, compared with the damaged hair. These improvements surpass those achieved by succinic acid （4.47%/13.85%） and citric acid （6.08%/15.85%）, underscoring the critical role of covalent cross-linking. Microscopy further reveals that itaconic acid treatment effectively restores cuticle integrity by mitigating lifting and exfoliation, while protecting against transverse fractures induced by cyclic fatigue stress. Overall, these findings elucidate the targeted repair mechanism of itaconic acid on damaged hair, providing a novel strategy for developing high-performance hair reparatives.

Key words: itaconic acid, hair repair, Michael addition, free sulfhydryl groups, keratin, mechanical properties, hair cuticle