Porous microneedles: Transdermal salt bridge for biomedical device engineering

Microneedles (MNs) are an attractive option as a minimally invasive means to break through the stratum corneum of skin for transdermal drug delivery and the analysis of interstitial fluid. The solid-based porous microneedle (PMN) is a relatively new type of MN with a micro/nanochannel network throughout the whole needle. The PMN filled with electrolyte solution serves as a “salt bridge” to create an ionic pathway across the skin surface layer. This review outlines the advantages of the ionically conductive PMN from the biomedical engineering perspective. After a brief description of the fabrication techniques of PMN, the decrease in transdermal resistance by PMN insertion is quantitatively discussed. In addition, possible applications of PMN-based salt bridge are presented, including the skin potential and resistance measurements, intradermal electrochemical analysis and transdermal molecular transport.