Delivery of free amino acids into and through the stratum corneum of the skin using micro emulsions and microemulsion-based hydrogels: Formulation, characterization, and ex- vivo permeation studies.

Free amino acids constitute the largest portion (40%) of the natural moisturizing factor. Their level might decline and cause dry skin condition. The treatment strategy involves the replenishment of these components to the skin, and, to our knowledge, there are no reports that involve dermal delivery of free amino acids. The purpose of the current study was therefore to prepare and characterize different micro-emulsions, micro-emulsion-based hydrogels, and hydrophilic creams loaded with free amino acids for dermal delivery. Oil-in-water microemulsions were prepared using carefully selected formulation components. Poloxamer ^® 407 and carbopol ^® 934 were used to prepare the hydrogels. All the formulations were characterized for physico-chemical, permeation and cytotoxicity properties. The results showed that the prepared microemulsions had desired droplet size, size distribution, zeta potential, refractive index, and pH. In the gel preparations, the elastic properties prevailed over the viscous behavior. The hydrogels had non-Newtonian shear-thinning behavior with some thixotropic properties. The free amino acids permeated into the deeper layers of the stratum corneum from the microemulsions, and microemulsion-based hydrogels as compared to conventional hydrophilic cream. The hydrogels were more effective than the microemulsions to deliver the FAAs to the desired site of the skin in a sustained manner. Poloxamer-based hydrogel permeated into deeper skin layers than Carbopol-based hydrogel. Formulations prepared using standard free amino acids and those extracted and purified from oyster mushroom had similar characteristics. All the formulations were stable and safe to be applied topically. In conclusion, microemulsions and microemulsion-based hydrogels can be considered as safe carrier systems for dermal delivery of free amino acids.