Mechanical and Microstructural Properties of Stratum Corneum
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Mechanical and Microstructural Properties of Stratum Corneum Kenneth S. Wu1, William W. Van Osdol2, and Reinhold H. Dauskardt3 1 Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-2205 2 ALZA Corporation, Mountain View, CA 94039-7210 3 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205 ABSTRACT A mechanics approach is presented to study the intercellular delamination resistance and mechanical behavior of stratum corneum (SC) tissue in the direction normal to the skin surface. The effects of temperature and hydration on debonding behavior were also explored. Such understanding, which includes the relationship of mechanical behavior to the underlying SC cellular structure, is essential for emerging transdermal drug delivery technologies. Fracture mechanics-based cantilever-beam specimens were used to determine reproducibly the energy release rates to quantify the cohesive strength of human SC. The debond resistance of fully hydrated SC was found to decrease with increasing temperature, while dehydrated SC exhibited a more complex variation with temperature. Stress-separation tests showed that fracture energies and peak separation stresses decreased with increasing temperature and hydration, although the SC modulus varied only marginally with temperature and hydration. Results are described in terms of microstructural changes associated with hydrophilic regions and intercellular lipid phase transitions. INTRODUCTION As the most externally exposed organ in the human body, the skin provides mechanical protection and a controlled permeable barrier to the external environment to maintain internal homeostasis. The layered construction of the skin represents a composite material in which the components possess specialized functionalities to accommodate a variety of conditions from mechanical stresses to variable ambient moisture and to resist the presence of toxic chemicals, pathogens, and radiation.1 The top layer of the skin, the epidermis, consists of epithelial cells bound together via various cell-adhesion mechanisms including intercellular proteins and lipids. The top most layer of the epidermis, the stratum corneum (SC), consists of layered anucleated cells that mature and subsequently detach in a natural renewing process. The disk-shaped SC cells, composed largely of aligned keratin filaments, create a regular interdigitating structure held together by lipid and protein structures as illustrated in Fig. 1. The SC is the first structure to provide resistance to abrasion and penetration of foreign objects and must provide this protection under highly variable temperature, humidity, and chemical conditions which bodily self-regulation or external environments may induce. In the case of emerging transdermal drug delivery technologies, the application and removal of adhesive drug delivery devices necessitates an appropriate balance between patch adhesive strength and SC cohesive strength in the direction normal to the skin surface. While previous invest
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