Conventional drug delivery using pills or injection is often not suitable for new protein, DNA, and other therapies.¹⁻³ An attractive alternative involves transdermal delivery from a patch, which avoids (i) degradation in the gastrointestinal tract and first-pass effects of the liver associated with oral delivery and (ii) the pain and inconvenience of intravenous injection.⁴⁻⁷ Delivery across skin also offers the possibility to continuously control the delivery rate, in contrast to conventional methods that deliver a large, discrete bolus. Despite these advantages, transdermal drug delivery is severely limited by the poor permeability of human skin; most drugs do not cross skin at therapeutic rates. Chemical,⁸ electrical,⁹ ultrasonic,¹⁰ and other methods have been developed to increase rates of transdermal transport, but have made only limited clinical impact to date.