Protein hair mask conditioners penetrate the hair to the ______

Protein hair mask conditioners are specifically designed to penetrate deeper into the hair structure to provide strength and repair damage. The cortex is the middle layer of the hair, and it plays a crucial role in determining the hair's strength, elasticity, and overall health. When protein conditioners penetrate the cortex, they can effectively replenish lost proteins and moisture, thereby helping to rebuild and strengthen the hair from within. This leads to improvements in texture and resilience, making the hair less prone to breakage.

In contrast, the cuticle is the outermost layer and acts as a protective barrier, while the medulla, which is often absent in finer hair types, is the innermost layer with no specific role in cosmetic treatments. The surface primarily refers to the outer appearance of the hair, which can be influenced by products but does not involve deep penetration or structural change. Thus, the cortex is the correct answer as it is where the significant repair and fortification of the hair occurs when using protein treatments.