The micro-sized needles could pierce the skin to deliver drugs effectively in a minimally invasive and painless manner. However, there are only a few reports that identify the invasiveness and painlessness of microneedle (MN), and in vitro characterization studies were conducted to examine the invasiveness of MN in experimental animals and healthy volunteers. First, a fluorescent molecule was applied to show the skin holes according to the application time of MN and then the whitening effect in UV-exposed hairless rats was observed using reflectance spectroscopy according to the application time of MN. The extent of skin irritation by the application time of MN in healthy volunteers was determined from the value of skin redness. Regardless of MN application time, skin redness occurred and then disappeared 30 min after removal of MN; this phenomenon was insignificant with the application time of MN. Thus, if the MN was applied, a skin hole appeared, skin redness was observed and then the skin redness disappeared 30 min after removal of MN. Taken together, polymer MN might be a suitable tool for safe transdermal drug delivery of small molecules.
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