51 A. Nasir et al. (eds.), Nanotechnology in Dermatology, DOI 10.1007/978-1-4614-5034-4_6, © Springer Science+Business Media, LLC 2013 Over the past decade, the field of nanoscience has grown tremendously because nanomaterials have widespread applications in material sci- ence, engineering, and medicine. Although this work has received a great deal of attention, many challenges must be overcome before nanotech- nology can be routinely applied in nanomedi- cines or in consumer products. Nanomaterials are substances having a physicochemical struc- ture on a scale greater than atomic/molecular dimensions but less than 100 nm, and exhibit physical, chemical, and/or biological character- istics associated with its nanostructure. It is these unique characteristics that made them central components in an array of emerging technolo- gies. Many new companies have emerged to commercialize these products. However, before many of these nanomaterials can be used in bio- logical systems, their toxicology must first be evaluated under realistic environmental, occupa- tional, and medicinal exposure conditions. Engineered nanoparticles (NP) are used in many household products, personal care products including sunscreens and cosmetics, or as drug delivery devices and as contrast imaging agents. The focus of this chapter is to depict how skin can serve as a potential route of exposure to several types of nanomaterials, and will discuss how size, shape, charge, surface properties, and vehicles can be important determinants on the penetration through the rate-limiting lipid bar- rier of the stratum corneum. The transdermal flux of NP has obvious impli- cations to toxicology, therapeutics, and drug delivery, but there is also a need to define the depth and mechanism of NP penetration into the epidermis and dermis after passage through the rate-limiting barrier of the stratum corneum. Penetration into skin without further systemic absorption could modulate immunological func- tion and local toxicity. Consumers or workers often have compromised skin with an altered penetration barrier from conditions such as atopic dermatitis, psoriasis or eczema on their hands or other parts of their body which could influence the rate of penetration and even allow absorption of nanomaterials through skin. Several studies have evaluated the hypothesis that NP may become lodged in the lipid matrix of the skin [1–3] promoting their dermal resi- dence and potential adverse effects. It will be shown how NP size, shape, charge, surface prop- erties, and vehicle as well as animal species will modulate penetration and must be considered when designing studies to assess whether or not NP penetration occurs. There is limited informa- tion on the in vivo dermal penetration of NP in humans; however, there are several published studies in various in vitro model systems and on excised animal and human skins that shed light on this issue. N.A. Monteiro-Riviere, Ph.D. (*) Department of Anatomy and Physiology, CVM, 28 Coles Hall, Manhattan, KS 66506-5604, USA e-mail: monteiro@ksu.edu 6 Skin Penetration of Engineered Nanomaterials Nancy A. Monteiro-Riviere