Breaking the skin barrier: achievements and future directions.

Skin delivery is more advantageous for drug administration than other routes since it is more compliant and can avoid the first-pass metabolic effect. More importantly, it can ensure stable blood level of transdermal drugs for a long period of time, avoiding fluctuation and reducing side effects. However, it is restricted by the barrier function of the stratum corneum. Therefore, significant attention has been paid to developing methods to modify the kinetics of skin drug delivery and expand the range of drugs that can be used for transdermal delivery. Novel mechanisms of increasing the intercellular and appendages penetration pathways have also been developed. This review focuses on chemical penetration enhancers, physical permeabilization (sonophoresis, iontophoresis and microneedles) and novel nanocarriers for skin delivery. Recent developments on skin delivery are also discussed.