Vitamin A in the form of all-trans retinol is a popular component of cosmetic anti-aging creams due to its effects in rejuvenating skin, smoothing wrinkles and enhancing elasticity. Although retinol aids in the creation of stronger, healthier skin,1, 2 its incorporation into dermal creams is problematic due to the highly fragile nature of the molecule. Retinol is a naturally occurring hydrophobic compound with a polyolefinic structure that is subject to facile isomerization to cis-isomers, with lowered biological activity,3 upon exposure to light. Moreover, the photo-sensitive molecule is also rapidly degraded by oxygen and elevated temperatures. Chemists handling retinol in the laboratory employ inert atmosphere, e.g., nitrogen or argon, in the absence of light since significant degradation occurs after several hours of exposure to air.4 Derivatives such as retinyl acetate, propionate and palmitate esters are less susceptible to decomposition but are also less active, requiring an additional hydrolysis step to release retinol in the skin.2
For practical handling purposes, it would therefore be advantageous to stabilize retinol by encapsulating it in a carrier, which is also beneficial in that it would allow for the incorporation of stabilizers such as butylated hydroxytoluene (BHT) and vitamins E and C, widely used as antioxidants. Furthermore, the use of a carrier could help to address the limited aqueous solubility of retinol. Indeed, there has been considerable research interest in developing methods for stabilizing retinol in a range of host materials. Examples include liposomes,5 polymers,6 solid lipid nanoparticles,7 chitosan8 and silica microparticles9—even a combination of materials to give a multiply stabilized system.10
Clearly, however, the necessity to protect retinol from the surrounding environment must be balanced with the ease by which the molecule can later be released from the host matrix. There is considerable evidence for improved stability in the encapsulated form, as evidenced by the number of stabilized retinol/carrier products that are available commercially. In response, the author’s company has developed a process for encapsulating retinol as part of a wider technology for the encapsulation of hydrophobic molecules into organosilica microparticles. The synthetic procedures employed are sol-gel based and conducted at an ambient temperature and benign pH to form ceramic particles in which the active molecules are fully encapsulated. This technology minimizes the release of the active in aqueous conditions but rapidly releases it under lipophilic conditions.