Effects of microneedle length, density, insertion time and multiple applications on human skin barrier function: assessments by transepidermal water loss.

Microneedle (MN) arrays have attracted considerable attention in recent years due to their ability to facilitate effective transdermal drug delivery. Despite appreciable research, there is still debate about how different MN dimensions or application modes influence permeabilization. This study aimed to investigate this issue by taking transepidermal water-loss measurements of dermatomed human skin samples following the insertion of solid polymeric MNs. Insertions caused an initial sharp drop in barrier function followed by a slower incomplete recovery - a paradigm consistent with MN-generation of microchannels that subsequently contract due to skin elasticity. While 600 μm-long MNs were more skin-perturbing than 400 μm MNs, insertion of 1000 μm-long MNs caused a smaller initial drop in integrity followed by a degree of long term permeabilization. This is explainable by the longest needles compacting the tissue, which then decompresses over subsequent hours. Multiple insertions had a similar effect as increasing MN length. There was some evidence that increasing MN density suppressed the partial barrier recovery caused by tissue contraction. Leaving MNs embedded in skin seemed to reduce the initial post-insertion drop in barrier function. Our results suggest that this in vitro TEWL approach can be used to rapidly screen MN-effects on skin.