The mechanical properties of the skin epidermis in relation to targeted gene and drug delivery.

A challenge in combating many major diseases is breaching the skin's tough outer layer (the stratum corneum (SC)) and delivering drugs and genes into the underlying abundant immunologically sensitive viable epidermal cells with safe, practical physical technologies. To achieve this effectively and accurately, design information is needed on key skin mechanical properties when pushing into and through epidermal skin cells. We measure these important mechanical properties by penetrating through the intact SC and viable epidermis (VE) of freshly excised murine skin with a NANO-indenter, using custom tungsten probes fabricated with nominally 5 and 2 microm diameters (with nanoscale tips). We show the skin Young's modulus, storage modulus and stress all dramatically decreased through the SC. Also, for a given penetration depth, decreasing the probe size significantly increases the storage modulus. Biological variation in penetrating the skin was shown. These collective findings advance the rational design of physical approaches for delivering genes and drugs within key cells of the VE.