Over the past decade, microneedles have been shown to dramatically increase skin permeability to a broad range of compounds by creating reversible microchannels in the skin. However, in order to achieve sustained transdermal drug delivery, the extent and duration of skin‘s increased permeability needs to be determined. In this study, we used electrical impedance spectroscopy to perform the first experiments in human subjects to analyze the resealing of skin‘s barrier properties after insertion of microneedles. Microneedles having a range of geometries were studied in conjunction with the effect of occlusion to test the hypothesis that increasing microneedle length, number, and cross-sectional area together with occlusion leads to an increase in skin resealing time that can exceed one day. Results indicated that in the absence of occlusion, all microneedle treated sites recovered barrier properties within 2 h, while occluded sites resealed more slowly, with resealing windows ranging from 3-40 h depending on microneedle geometry. Upon subsequent removal of occlusion, the skin barrier resealed rapidly. Longer microneedles, increased number of needles, and larger cross-sectional area demonstrated slower resealing kinetics indicating that microneedle geometry played a significant role in the barrier resealing process. Overall, this study showed that pre-treatment of skin with microneedles before applying an occlusive transdermal patch can increase skin permeability for more than one day, but nonetheless allow skin to reseal rapidly after patch removal.

Purpose Most methods to increase transdermal drug delivery focus on increasing stratum corneum permeability, without addressing the need to increase permeability of viable epidermis. Here, we assess the hypothesis that viable epidermis offers a significant permeability barrier that becomes rate limiting upon sufficient permeabilization of stratum corneum. Methods We tested this hypothesis by using calibrated microdermabrasion to selectively remove stratum corneum or full epidermis in pig and human skin, and then measuring skin permeability to a small molecule (sulforhodamine) and macromolecules (bovine serum albumin, insulin, inactivated influenza vaccine) in vitro. Results We found that removal of stratum corneum dramatically increased skin permeability to all compounds tested. However, removal of full epidermis increased skin permeability by another 1 – 2 orders of magnitude. We also studied the effects of removing skin tissue only from localized spots on the skin surface by covering skin with a mask containing 125-μm holes during tissue removal. Skin permeabilized in this less- invasive way showed similar results. This suggests that microdermabrasion of skin using a mask may provide an effective way to increase skin permeability. Conclusions We conclude that viable epidermis offers a significant permeability barrier that becomes rate limiting upon removal of stratum corneum.

This study sought to determine if microdermabrasion can selectively remove stratum corneum to increase skin permeability. Although, microdermabrasion has been used for cosmetic treatment of skin for decades, no study has assessed the detailed effects of microdermabrasion conditions on the degree of skin tissue removal. Therefore, we histologically characterized the skin of rhesus macaques and human volunteers after microdermabrasion at different conditions. Using mobile tip microdermabrasion, an increase in the number of treatment passes led to greater tissue removal ranging from minimal effects to extensive damage to deeper layers of the skin. Of note, these data showed for the first time that at moderate microdermabrasion conditions selective yet full-thickness removal of stratum corneum could be achieved with little damage to deeper skin tissues. In the stationary mode of microdermabrasion, selective stratum corneum removal was not observed, but micro-blisters could be seen. Similar tissue removal trends were observed in human volunteers. As proof of concept for drug delivery applications, a model fluorescent drug (fluorescein) was delivered through microdermabraded skin and antibodies were generated against vaccinia virus after its topical application in monkeys. In conclusion, microdermabrasion can selectively remove full-thickness stratum corneum with little damage to deeper tissues and thereby increase skin permeability.