Serine is a polar α-amino acid (2-amino-3-hydroxypropanoic acid) used in cosmetics as a skin-identical, water-binding component. It is a prominent constituent of the skin’s Natural Moisturizing Factor (NMF), the mixture of small, hygroscopic molecules concentrated in the stratum corneum that governs water activity, corneocyte plasticity, and orderly desquamation. In leave-on skincare, serine functions as a lightweight skin-conditioning humectant, improving hydration without occlusion, helping maintain a smooth feel, and supporting barrier comfort when paired with compatible osmolytes (e.g., glycerol, PCA, urea). Because serine is neutral at typical formulation pH and highly water-soluble, it integrates readily into toners, gel serums, emulsions, sheet masks, and post-procedure soothers without destabilizing pH-sensitive actives.

In hair/scalp care, serine contributes to antistatic control and conditioning. On damaged fibers, serine’s small size and hydroxyl group support water retention at the cuticle surface and can participate in hydrogen-bonding interactions with keratin. In rinse-off or leave-in systems, serine complements cationic polymers and protein hydrolysates, aiding slip, reducing flyaways, and improving manageability. When used alongside keratin or enzymatic crosslinking strategies, serine can enhance fiber integrity and moisture retention—useful in color-care masks, repair conditioners, and bond-building treatments.