Mechanical properties of the epidermis and stratum corneum determined by submicron indentation in vitro

The outer skin layers are important drug and vaccine delivery targets in the treatment of diseases. These skin layers possess some important characteristics making them favorable sites for pain-free delivery with minimal damage: a rich population of immunologically sensitive cells as well as the lack of blood vessels and sensory nerve endings [1]. Until today, however, the development of effective cell targeting methods is acquainted with many challenges. A collective shortcoming is a poor understanding of the key mechanical properties of the outer skin layers, e.g. the stratum corneum and epidermis. The anisotropic, dynamic and very complex nature of skin makes it difficult to perform proper mechanical testing as well as to obtain reliable, reproducible data. The stratum corneum is an effective physical barrier of dead cells with a “brick-and-mortar” structure, while the viable epidermis mainly consists of actively migrating keratinocytes constantly undergoing massive morphological and compositional changes. As a consequence, the structure differences among the skin layers lead to significant variations in mechanical properties. Since there is no method available to determine the mechanical behavior of isolated viable epidermis in vivo or in vitro, the mechanical behavior of epidermis and stratum corneum only are investigated here. A commercially available indentation system has been adapted to enable the measurement of these thin soft tissues in an in vitro set up. Combining the outcomes of the two skin layer types leads to an assessment of the contribution of the viable epidermis to the mechanical behavior of skin. To our knowledge, no data have been published yet regarding mechanical bulk properties of (viable) epidermis, while no consistency exists with respect to those of the stratum corneum.