Whereas cosmetic scientists are trying to convince each other, authorities and environmental groups that nanomaterials do not penetrate the skin to the viable layers, there is a second category of nanoparticles that is specifically designed to penetrate skin: liposomes. Specific types of liposomes, such as the elastosomes and at least one proprietary drug delivery technologya, have shown the ability to penetrate human skin, thanks to their flexibility. Figure 1, for instance, shows the influence of flexibility of liposomes on the skin penetration of ketorolac. Significantly more penetrates human skin in vitro from flexible vesicles relative to rigid vesicles.1
For a description of the mechanism of skin penetration enhancement by flexible liposomes, the interested reader is referred to references 2 and 3 that describe different mechanisms of enhancement. Blume describes the osmotic gradient2 whereas Honeywell-Nguyen et al. describe adsorption of the active to the liposomal membrane, penetration of this unit into the stratum corneum where it dissociates and the active subsequently penetrates deeper in the skin.3
Although the exposure to the viable tissues is significantly higher for flexible liposomes relative to particulate nanomaterials, the intrinsic hazard of liposomes has never been a point of concern due to their excellent biocompatibility,2 whereas this may be of some concern with liposomes made of high amounts of surfactants such as the niosomes (nonionic surfactants) and elastosomes.
1. PL Honeywell-Nguyen, GS Gooris and JA Bouwstra, Quantitative assessment of the transport of elastic and rigid vesicle components and a model drug from these vesicle formulations into human skin in vivo, J Invest Dermatol 123 902–910 (2004)
2. G Blume, Flexible liposomes for topical applications in cosmetics, In Science and Applications of Skin Delivery Systems, JW Wiechers, ed, Carol Stream, Ill., USA: Allured Publishing (2008) Chapter 15, pp 269–282
3. PL Honeywell-Nguyen and JA Bouwstra, Vesicles as skin delivery vehicles, In Science and Applications of Skin Delivery Systems, JW Wiechers, ed, Carol Stream, Ill., USA: Allured Publishing (2008) Chapter 12, pp 205–226