Once the reader accepts a standard definition of nanotechnology, such as that offered by The Royal Society and The Royal Academy of Engineering,1 and then reads my definition of skin delivery available elsewhere (see Two Definitions),2,3 and then realizes that micro- and nanoparticles accumulate in the furrows and ridges on the skin surface where they act as a reservoir, then they are ready to ask the question answered in this article: Do nano-particles penetrate human skin?
Nanoparticles have been defined as single particles with a diameter less than 100 nm,4 which includes titanium dioxide in transparent, inorganic sun care products, and is usually extended to 200 nm to include the zinc oxide in those products. But nanoparticles are only a subset of nanomaterials, which can also include cyclodextrins and liposomes. While cyclodextrins do not represent a nanotechnology in this author's opinion, liposomes do. However, liposomes will not be discussed in this article because unlike nanoparticles that are intended to rest on the skin, liposomes were specifically designed to penetrate the skin. Thus, the nanoparticles addressed in this article are solid particles with a diameter less than 200 nm.