Triggered release of sensitive active ingredients upon response to the skin's natural pH

Abstract

Active ingredients are one of the key ingredients in modern cosmetic formulations. Apart from various sensory effects, topical applications rely on visible effects that demonstrate the successful battle against signs of skin aging, like wrinkles or sagging skin. Many active ingredients display a challenge for the formulator as they are sensitive to environmental factors, such as heat, light, or pH, or they are not compatible with other components of the formulation. A variety of encapsulation technologies has been developed to encounter these obstacles. However, the existing encapsulation systems are often times not efficient enough, i.e. they possess an uncontrolled release profile or they are not stable during storage in a formulation. These apparent obstacles have been addressed by the development of a novel encapsulation system. A pH-sensitive methacrylate copolymer has been used to generate micron-sized particles that contain a fluorescent dye. The breakdown of the polymer and the subsequent release of the dye was investigated by confocal laser scanning microscopy. It was shown that in the presence a pH 5 buffer solution, which mimics the natural pH of the skin, the microparticles were breaking down, and the fluorescent dye was leaching out of the particle. In addition, the stabilization of a sensitive active ingredient, retinol, was investigated in an oil-in-water formulation. Under accelerated storage conditions, i.e. 3 months at a temperature of 45 °C, the retinol content was determined by HPLC, and the stability of the microparticles was observed. The result shows that during storage the microparticles are stable, protect the incorporated sensitive retinol and stabilize it about sevenfold better in contrast to the free retinol control. Lastly, the bioavailability of retinol has been determined on pig skin. Both free and encapsulated retinol have been incorporated into an oil-in-water formulation. The creams were used in a penetration study using Franz, diffusion cells. The result shows that encapsulated retinol displays a higher penetration profile into pig skin than free retinol. Taken together, we present a novel microparticulate encapsulation system for the stabilization of active ingredients that combines stabilization of the active ingredient in formulation, a controlled pH-triggered release mechanism, and inherent stability of the system.