Effective Penetration of the Oxidized Keratin‐Rebonding Materials Into Hair Shafts Based on Charge Conversion

ABSTRACT Materials bearing multiple positive charges reinforce damaged hair fibers through ionic interactions with negatively charged sulfonate groups present in oxidized keratin. However, their strong surface binding often hinders deep penetration into the hair shaft. To address this limitation, we designed β‐carboxylic acid amide moieties that are negatively charged at neutral pH but undergo charge conversion to cationic forms under mild acidification, thereby enhancing the penetration of hair‐rebonding agents. These moieties—engineered with tunable negative charge density, hydrophobicity, and pH‐responsive degradability—are incorporated into O , O' –bis(3–aminopropyl)diethylene glycol (BADG), a representative hair‐rebonding compound. Among the derivatives, the cyclohexane‐1,2,4–tricarboxylic acid (CTA) amide‐modified BADG (CTA2‐BADG) demonstrated the most effective restoration of damaged hair surface morphology. Deep penetration and re‐bonding are validated through fluorescence microscopy, attenuated total reflectance‐Fourier transform infrared (ATR‐FTIR) spectroscopy, and thermogravimetric analysis (TGA). Furthermore, CTA2‐BADG treatment significantly enhanced the tensile strength of damaged hair fibers. These findings highlight charge‐conversion chemistry as a promising strategy for delivering cationic bioactive agents into the deep interior of damaged hair.