Epidermal powder immunization (EPI) and particle-mediated epidermal delivery (PMED) utilize helium gas to deliver powdered proteins, polysaccharides, inactivated pathogens, or DNA-coated particles into the epidermis at supersonic speeds (Weniger and Papania 2008) (Fig. 2c) . Companies involved in developing this technology include Powderject, PowderMed (acquired by Pfizer in 2006), and Iaculor Injection. It is not known if this device technology class is still in active development (PATH 2009 ). Conventional protein antigens must be specially formulated for delivery by EPI, and are spray dried into powders of suitable density and size (20-70 lm). A clinical trial has been conducted evaluating delivery of a powdered inactivated influenza vaccine by EPI injection, which found that immunogenicity was comparable to standard delivery by IM needle and syringe . EPI has also shown efficacy in preclinical studies with hepatitis B and HIV vaccines (Chen et al. 2002; Osorio et al. 2003) .