Transdermal drug delivery offers several advantages over traditional drug delivery systems such as oral delivery and injections (Prausnitz et al .2004). In spite of its advantages, transdermal drug delivery suffers from the severe limitation that the permeability of the skin to all but a few small and hydrophobic molecules is very low. Therefore, it is diffcult to deliver drugs across the skin at a therapeutically relevant rate. Only a handful of low-molecular-weight drugs are clinically administered by this route today. Overcoming skin’s barrier properties safely and reversibly is a fundamental problem that drives innovation in the feld of transdermal delivery.