Hydrolyzed keratin: a versatile excipient for pharmaceutical applications?

Keratins are a broad family of fibrous structural proteins, which form the cytoskeletal intermediate filaments of several epithelial cells. They can be found in keratinocytes in the outer layer of epidermal skin as well as in the skin appendages such as hair or nails in humans and feathers, horns, claws and hooves in animals. Keratins are insoluble in water due to their self-assembling into antiparallel, staggered tetramers, because of the formation of disulfide bridges involving cysteine residues and hydrogen bonds between the carbonyl and imino groups of amino acids [1]. Chemical hydrolysis in an acid or basic environment or enzymatic processes provide peptide water-soluble fractions, known as keratin hydrolysates. These hydrolysates have been found promising applications in the biomedical, cosmetic, agriculture and packaging fields [2-3]. Specifically, these hydrolysates are known in cosmetics as the INCI name of hydrolyzed keratin. This excipient is useful as skin moisturizing, hair repairing, nail hardening and conditioning agent and can be included in the formulations of a large variety of cosmetic products [4]. Despite its large use in cosmetics, there is a lack of information regarding its chemical-physical characterization, which could be helpful to broaden its field of applications, also in the pharmaceutics. The aim of the present work is to characterize the surface-active and thickening properties of hydrolyzed keratin to propose this excipient as a potential natural surfactant and stabilizer for pharmaceutical dispersed systems.