Therefore, we first studied the effects of microneedle geometry and design on pain in ten healthy volunteers and compared this pain to a 26-gage hypodermic needle. Single microneedles with lengths ranging from 480 to 1450 ƒÝm, widths from 160 to 465 ƒÝm, thicknesses from 30 to 100 ƒÝm and tip angles from 20¢X to 90¢X; and arrays containing 5 or 50 microneedles were inserted into the volar forearms of human subjects, in a double-blinded, randomized study. All microneedles investigated were significantly less painful than the 26-gage hypodermic needle. Microneedle length had the strongest effect on pain, where a three-fold increase in length increased the pain score by seven fold. The number of microneedles also affected the pain score, where a 10-fold increase in the number of microneedles increased pain just over two-fold. Microneedle tip angle, thickness and width did not significantly influence pain.

We also tested the ability of microneedles to generate an immune response via DNA coated microneedles. Microneedles were coated with hepatitis C non-structural - NS3/4A DNA vaccine at a controllable dose of 1.6 ƒÝg per row of five microneedles. Mouse immunization using NS3/4A-coated microneedles resulted in antigen specific cytotoxic T-lymphocyte (CTL) production at low vaccine dose (3 to 8 ƒÝg) and without the use of adjuvants. Chromium release assay demonstrated that the CTL response was robust, while the in vivo tumor challenge assay demonstrated that the CTLs had the ability to recognize and kill cells expressing the antigen, NS3/4A, in vivo. Therefore, this study provided the first evidence that transdermal immunization using DNA-coated microneedles can generate an antigen-specific CTL response in vivo. Further, the CTL response was demonstrated in two commonly used mouse strains, Balb/C and C57Bl/6 mice.

Overall, this study suggests that coated microneedles have the potential to be used for administering DNA vaccines to the skin in a painless manner, which may offer a convenient method for immunization.