Evaluation needle length and density of microneedle arrays in the pretreatment of skin for transdermal drug delivery

Solid silicon microneedle arrays with different needle lengths (ranging from 100 to 1100 μm) and needle densities (ranging from 400 to 11,900 needles/cm2) were used to penetrate epidermal membrane of human cadaver skin. After this pretreatment, the electrical resistance of the skin and the flux of acyclovir across the skin were monitored. A linear correlation between the acyclovir flux and the inverse of the skin electric resistance was observed. Microneedle arrays with longer needles (>600 μm) were more effective in creating pathways across skin and enhancing drug flux, and microneedle arrays with lower needle densities (<2000 needles/cm2) were more effective in enhancing drug flux if the microneedles with long enough needle length (>600 μm). In addition, the microneedle arrays were used to penetrate hairless rat skin in vivo, and the trans-epidermal water loss (TEWL) of the rat skin was measured before and after the pretreatment. Treating rat skin with microneedle arrays of lower needle density and longer needle length was more effective in increasing TEWL. Integrity of the stratum corneum barrier of the penetrated rat skin as measured by TEWL recovered back to its base line level within 24 h after the microneedle pretreatment.