Changes in nano-mechanical properties of human epidermal cornified cells depending on their proximity to the skin surface.

During formation of the stratum corneum (SC) barrier, terminally differentiated keratinocytes continue their maturation process within the dead superficial epidermal layer. Morphological studies of isolated human corneocytes have revealed differences between cornified envelopes purified from the deep and superficial SC. We used atomic force microscopy to measure the mechanical properties of native human corneocytes harvested by tape-stripping from different SC depths. Various conditions of data acquisition have been tested and optimized, in order to obtain exploitable and reproducible results. Probing at 200 nN allowed us to investigate the total stiffness of the cells (at 50 nm indentation) and that of the cornified envelopes (at 10 to15 nm), and lipid envelopes (at 5 to 10 nm). The obtained data indicated statistically significant differences between the superficial (more rigid) and deep (softer) corneocytes, thus confirming the existence of depth and maturation-related morphological changes within the SC. The proposed approach can be potentially used for minimally invasive evaluation of various skin conditions such as aging, skin hydration, and pathologies linked to SC.