Topical delivery of retinyl ascorbate. 3. Influence of follicle sealing and skin stretching.

This influence of skin stretching and hair follicle sealing on the delivery of retinyl ascorbate (RA-AsA) to the epidermis was probed in vitro. Porcine ear skin was subjected to stretching by 2 and 4 mm (3.3 and 6.7%, respectively); the hair follicles of other skin sections were located and painstakingly sealed using adhesive. After mounting in Franz cells the skin was dosed with 100 microl of 2.5 mM RA-AsA in methanol/PBS with water as receptor phase. After 24 h the diffused areas were subjected to tape stripping, and the amount of RA-AsA was determined in 5 groups of 9 strips. Statistical analysis of the resulting depth profiles showed that there was no statistical difference between unstretched skin and the skin that had the follicles sealed across the 5 depth bands. Between 0 and 2 mm stretch there were generally significant differences; between 0 and 4 mm p < 0.001 was obtained at each depth. The data from this limited exercise suggest that in native skin the follicular route does not contribute to dermal absorption, but when disturbed (as when stretched) follicular delivery can substantially increase drug delivery into the skin by up to approximately 20-40%. Skin stretching becomes difficult beyond about 7%.